Enhanced Weathering in Agriculture

This Protocol provides the requirements and procedures for the calculation of net carbon dioxide equivalent (CO₂e) Removal from the atmosphere via Enhanced Weathering (EW) in agricultural settings. Agricultural EW is considered a subcategory of EW approaches.

Silicate weathering naturally sequesters approximately 0.1-0.3 Gt of CO₂ per year^{1, 2, 3} and is thus a critical feedback mechanism in global climate regulation on geologic timescales. In this process, atmospheric CO₂ dissolved in surface water reacts with silicate rocks and is converted to dissolved inorganic carbon or carbonate minerals, both of which constitute stable sinks for CO₂ on geologic timescales^{4, 5}. Natural weathering can be accelerated by applying crushed silicate rock to agricultural land, where the increased reactive surface area of the rock and damp conditions in the soil lead to elevated reaction rates. Alkaline species produced through weathering reactions allow for the increased storage of CO₂ in the aqueous phase, typically as dissolved bicarbonate. This dissolved inorganic carbon is eventually exported to the ocean where it can be stably stored for millennia⁶. This process is known as enhanced weathering and has been proposed as an effective natural Carbon Dioxide Removal (CDR) technique, in accordance with the Intergovernmental Panel on Climate Change (IPCC)⁷, to mitigate potential impacts of anthropogenic climate change⁸. Given that agricultural land covers approximately 38% of land surface on Earth, EW in agricultural settings has large scalability potential with relatively small changes in farm management. Scalability is aided by existing infrastructure of rock spreading as a soil amendment. Current estimates suggest that EW in `cropland has the capacity to remove on the order of 1-2 Gt of CO₂ per year⁸. In addition to CO₂ removal, EW can potentially provide co-benefits to farmers in the form of increased crop yield and resiliency^{9, 10}, as well as replenishment of soil nutrients^{11, 12} and reduction of nitrogen loss ^{9, 13}.

This Protocol accounts for the quantification of the gross amount of CO₂ removed via agricultural EW, as well as all cradle-to-grave, life-cycle Greenhouse Gas (GHG) emissions associated with the process.

The Protocol ensures:

• consistent and accurate procedures are used to measure and monitor all aspects of the EW process required to enable accurate accounting of net CO₂e removals
• consistent system boundaries and calculations are utilized to quantify net CO₂e removal for agricultural EW projects
• requirements are met to ensure the CO₂e removals are Additional
• evidence is provided and verified by independent third parties to support all net CO₂e removal claims

In addition to CO₂ removal, dissolution of silicate rocks in agricultural settings may provide co-benefits through the addition of nutrients, alkalinity and silica in soils. Potential co-benefits include:

• Combating soil acidification through addition of alkalinity⁹
• Increased soil nutrients, including K, P, Mg and Ca^14,15
• Increased crop yields^14,15,16
• Enhanced crop resistance to common pests and disease^17,18,19
• Increased crop resilience to drought²⁰
• Decreased nitrogen loss, leading to reductions in fertilizer use, eutrophication and nitrous oxide emissions^18,13
• Microbial diversification and resilience^21,22

Increased soil nutrients and crop resilience by EW may lead to increased yields. EW also has the potential to further mitigate emissions via reduction of nitrous oxide (N₂O, NO) fluxes in cropland, which is currently thought to represent 50-80% of global anthropogenic N₂O emissions¹³.

This Protocol mainly utilizes and is intended to be compliant with the following Standards and Protocols:

• Isometric Standard
• ISO, EN, 14064-2: 2019 - Greenhouse Gases - Part 2: Specification with guidance at the project level for quantification, monitoring, and reporting of greenhouse gas emission reductions or removal enhancements

Additional reference standards that inform the requirements and overall practices incorporated in this Protocol include:

• ISO 14064-3: 2019 - Greenhouse Gases - Part 3: Specification with Guidance for the verification and validation of greenhouse gas statements
• ISO 14040: 2006 - Environmental Management - Lifecycle Assessment - Principles & Framework
• ISO 14044: 2006 - Environmental Management - Lifecycle Assessment - Requirements & Guidelines

Additional standards, methodologies and Protocols that were reviewed, referenced or for which attempts were made to align with or leverage during development of this Protocol include:

• Global Rock C-sink, Guidelines for the Certification of Carbon Sinks created by Enhanced Rock Weathering in Croplands, v0.9, Carbon Standards International, October 2022
• Criteria for High-Quality Carbon Dioxide Removal, Carbon Direct, Microsoft, 2023
• BS EN 15978:2011 Sustainability of construction works - Assessment of environmental performance of buildings - Calculation method
• Foundations for Carbon Dioxide Quantification in Enhanced Rock Weathering Deployments, Cascade Climate, 2024

This Protocol was developed based on the current state of the art, publicly available science regarding EW in agriculture. Because EW is a novel CDR approach, with limited published literature, this Protocol incorporates requirements that may be more stringent than some current relevant regulations or other Protocols related to EW for CDR.

Future versions of this Protocol may be altered, particularly regarding requirements for demonstrating Durability of EW, as the stability of CO₂ captured by EW from Feedstock dissolution in agricultural soils is better demonstrated and documented and the overall body of knowledge and data regarding all processes, from feedstock supply to conversion and to permanent storage, is significantly increased.

This Protocol applies to Projects or processes which:

• Utilize crushed rock or mineral feedstock applied to agricultural land to capture CO₂.
  • Rock or mineral feedstock is defined as rock containing alkaline earth and alkali metals (i.e. Ca, Mg, K, and Na) which converts CO₂ to aqueous bicarbonate when applied in sufficient quantities to agricultural land. This includes but is not limited to silicate and carbonate rocks and waste materials such as steel slag.
  • Agricultural land includes all arable land and permanent cropland, as defined by the United Nations Food and Agriculture Organization (FAO). Similarly-managed lands that do not meet this definition will be considered on a case-by-case basis.
• Export alkalinity generated through weathering reactions from soils to the ocean via riverine transport (Isometric will address alternative drainage pathways in future versions of this Protocol).

This Protocol applies to projects and associated operations that meet all of the following project conditions:

• the Project characterizes feedstock prior to usage according to the Rock and Mineral Feedstock Characterization Module v1.2, to ensure eligibility of feedstock selection and ecological suitability.
• the Project provides a net-negative CO₂e impact (net CO₂e removal) as calculated in the GHG Statement, in compliance with Section 8.
• the Project does not disproportionately harm underserved or marginalized communities, in compliance with the Environmental and Social Impacts Section of the Isometric Standard and Section 5 of this Protocol.
• the Project is considered additional, in accordance with the requirements of Section 6.4.
• the Project provides long duration storage (1000 year estimated) of CO₂ in seawater and/or soil.

Projects that are explicitly ineligible include the following:

• Projects that lead to a sustained and substantial net decrease on crop yields.

As previously stated, per the UN FAO definition of permanent and arable crop land, meadows and pastureland are eligible for EW projects. This Protocol will refer to monitoring requirements specific to row cropland, such as monitoring crop yield, health, and resiliency. Such measurements may not be applicable to projects taking place on pastureland and therefore may not be required in the Project Design Document (PDD). Justifications to omit measurements in such instances are allowable in the PDD.

The Project must consider environmental and social impacts at all Project locations, including the mine or quarry where feedstock is sourced, during feedstock transportation, and at the deployment site. Appropriate measures must be implemented to identify and eliminate potential risks to terrestrial and aquatic ecosystems and biodiversity. Where risks cannot be eliminated, the Project Proponent must identify measures to monitor ecosystem health and mitigate adverse effects through a project-specific mitigation plan. Refer to Environmental and Social Impacts Section of the Isometric Standard for further guidelines.

Credits issued under this Protocol are contingent on the implementation, transparent reporting and independent verification of comprehensive safeguards. These safeguards encompass a wide range of considerations, including environmental protection, social equity, community engagement and respect for cultural values. The process mandates that safeguard plans be incorporated into all major project phases, with detailed reports made accessible to stakeholders. Adherence to and verification of environmental and social safeguards, in accordance with the Environmental and Social Impacts Section of the Isometric Standard, is a condition for all crediting projects.

Ongoing environmental assessment must be completed in accordance with the Isometric Standard to identify potential risks, followed by the development of tailored mitigation plans by subject matter experts where necessary. Project Proponents must first strive to avoid negative environmental impacts. In cases where adverse environmental effects take place, the Project Proponent must develop plans to minimize and rectify them. Effective implementation of these measures must also be accompanied by a robust monitoring plan to ensure efficacy. Project Proponents must demonstrate active stakeholder engagement throughout this process, in accordance with the Stakeholder Input Process outlined in the Isometric Standard. All mitigation strategies must align with local and international environmental laws and contribute to sustainable project outcomes.

Enhanced Weathering of rock or mineral feedstock may be associated with the release of Potentially Toxic Elements (PTEs) such as nickel (Ni) and chromium (Cr) or other harmful contaminants such as asbestiform minerals, which may pose an environmental risk. To prevent or mitigate such risks, the Project Proponent must take the following measures:

• Comprehensive analyses must be conducted in accordance with Isometric's Rock and Mineral Feedstock Characterization Module v1.2. Project Proponents should select rock or mineral feedstocks that minimize the risk of soil and groundwater contamination, e.g. by selecting feedstocks that do not contain PTEs in concentrations that may lead to exceeding regulatory limits. The Metal Accumulation Calculator provided by Cascade Climate is a useful tool that can be used to determine risks of metal accumulation with a chosen feedstock.
• A robust monitoring system must be established to regularly check for PTEs in soil and groundwater for projects with high contamination risk, as determined by the feedstock used. This will likely involve periodic sampling and analyses alongside field monitoring for removals and must be determined on a project basis. The concentration of contaminants in soil and water must not exceed the limits established by the local authority where the project is located. In the absence of local regulations, the Project Proponent must adhere to standards set by the European Union (EU) or the United States Environmental Protection Agency (US EPA). Justification behind the regulatory body selection must be provided in the PDD. An environmental monitoring plan is required for projects in which there is a significant risk of exceeding local regulatory limits of PTEs (e.g. Ni, Co, Cr) or asbestiform minerals at the selected feedstock application rate. We note that in many contexts the risk of PTEs reaching groundwater may be managed through soil monitoring and remediation efforts. All Project Proponents must submit a description of contamination risks in the PDD and, where applicable, a description of the environmental monitoring plan.
• If pre-existing PTE concentrations exceed applicable regulatory limits or guidance (as identified in the baseline scenario), the Project may still be considered for crediting against this Protocol. This is contingent on the Project Proponent providing evidence of existing elevated PTE concentrations. To qualify, the Project must undertake specific remediation strategies to mitigate further contamination. Further contamination could occur by increasing the concentration of potential contaminants in the project area or by spreading contamination to new areas. Remediation strategies could include altering the amount of feedstock applied or source material. Any project with pre-existing elevated PTE concentrations which further aggravates soil contamination will not meet the criteria for this Protocol.

The Project Proponent must carefully consider and implement measures where necessary for the following potential impact areas before proceeding with an EW project. Appropriate measures must be implemented to identify and eliminate potential risks to human health and cultural rights at all project locations.

Maintaining agricultural productivity is critical to the environmental and social sustainability of EW projects. The Project Proponent must document how the project will monitor agricultural productivity and soil quality, including which productivity and soil characteristics will be tested and the frequency of testing. If justified, the Project Proponent may use proxy variables in lieu of direct testing or measurement.

All environmental and social safeguards will be verified to be implemented at all locations in the EW process, including at the feedstock source, transportation, and distribution sites.

The Project Proponent must regularly assess the combined environmental impact of EW, which may include (but is not limited to) heavy metal concentrations in soil, crops, and groundwater. This may involve collecting data on soil and water quality, biodiversity indicators or agricultural productivity. The cadence of monitoring will vary based on the parameter. Accumulation of heavy metals in soils must be assessed at the end of the first Reporting Period, and should be assessed in subsequent Reporting Periods. Accumulation of heavy metals in crops is recommended in most cases, but required if soil or feedstock testing indicates potential for bio-accumulation. It is recommended that aqueous measurements be assessed for heavy metals.

The Project Proponent must use the collected data to inform ongoing management of EW practices. This data must be shared with the public through Isometric's platform, in accordance with Section 6.6 of this Protocol. The Project Proponent must be prepared to adjust the EW strategy based on monitoring results and feedback from environmental studies and community engagement.

The following topics are covered briefly in this Protocol due to their inclusion in the Isometric Standard, which governs all Isometric Protocols. See in-text references to the Isometric Standard for further guidance.

For each specific Project to be evaluated under this Protocol, the Project Proponent must document Project characteristics in a Project Design Document (PDD) as outlined in the Documentation Section of the Isometric Standard. The PDD will form the basis for project verification and evaluation in accordance with this Protocol, and must include consideration of processes unique to each project, such as:

• detailed feedstock characterization (see Rock and Mineral Feedstock Characterization Module)

  • Project Proponents utilizing mafic or ultramafic rock feedstocks are not required to perform elemental characterization of Hg provided they provide both of the following in the PDD:
    • results from XRD analysis, as required by the Rock and Mineral Feedstock Characterization Module, confirming that mercury bearing phases (eg. sulfides) are not present in the feedstock
    • citations from peer reviewed literature on the same or similar formation to the feedstock source that Hg concentration is below the more stringent of: regional regulations or the threshold set by the USEPA

• geographical designations of control plot(s) (see Section 10.1.1)

• description of measurement methods for all required analyses, cross-referenced with relevant standards where applicable

• description of any geochemical models used to quantify processes relevant to the calculation of CO₂ removal that are not directly measurable (see Section 8.2)

• a comprehensive sampling plan in accordance with Section 10, including climatic monitoring and field management plan

Projects must be validated and the Project GHG Statement (net CO₂e removal) verified by an independent third party consistent with the requirements described in this Protocol as well as in the Isometric Standard.

The Validation and Verification Body (VVB) must consider following requisite components:

1. Verify that feedstock adheres to the requirements listed in the Rock and Mineral Feedstock Characterization Module v1.2.
2. Verify that the quantification approach and monitoring plan adheres to requirements of Section 8, including demonstration of required records.
3. Verify that the Environmental & Social Safeguards outlined in Section 5 are met.
4. Verify that the project is compliant with requirements outlined in the Isometric Standard.

As EW Projects scale, a Project may expand its operations after initial validation by adding new feedstocks or Removal Areas. Project expansion does not generally require an entirely new validation, as only the new additions (e.g. feedstocks, Removal Areas) need to be validated by the VVB. A site visit is not always necessary for project expansion, but may be conducted if deemed necessary by the VVB, in consultation with Isometric. This decision is based on factors such as whether the expansion materially changes the project design, introduces new risks, affects additionality or LCA boundaries, or introduces new permitting requirements. In some cases, a remote site visit (e.g., remote interviews, photo or video evidence) may be sufficient. 

The threshold for Materiality, considering the totality of all omissions, errors and mis-statements, is 5%, in accordance with the Materiality Threshold Section of the Isometric Standard.

Verifiers must also verify the documentation of uncertainty of the GHG Statement as required by the Uncertainty Section of the Isometric Standard. Qualitative Materiality issues may also be identified and documented, such as²³:

• control issues that erode the verifier's confidence in the reported data
• poorly managed documented information
• difficulty in locating requested information
• noncompliance with regulations indirectly related to GHG emissions, removals or storage

Project Validation and Verification must incorporate site visits to project facilities, namely agricultural fields being used as control, treatment or deployment plots, in accordance with the requirements of ISO 14064-3:6.1.4.2. This is to include, at a minimum, site visits during the first Validation or Verification of a Project, to the project site(s). Validators should, whenever possible, observe project operation to ensure full documentation of process inputs and outputs through visual observation and validation of instrumentation, measurements, and required data quality measures.

All quarries typically need to be visited, but where multiple quarries are operated by the same company and located in a similar area, a representative subset may be sufficient. There is no fixed amount or percent of fields that need to be visited. Isometric and the VVB will determine the appropriate number of fields based on site specific characteristics such as cropping system and MRV approach.

A site visit must occur at least once during Project Validation. Additional site visits may be required if there are substantial changes to field operations over the course of a Project's Validation period, or if deemed necessary by Isometric or the VVB.

Verifiers and validators must comply with the requirements defined in the Validation and Verification Requirements Section of the Isometric Standard. In addition, teams must maintain and demonstrate expertise associated with the specific technologies of interest, including soil sampling, analysis and data processing.

CDR via EW in agriculture is a result of a multi-step process including quarrying, transporting and spreading rock, with activities in each step potentially managed and performed by a different operator, company or owner. When there are multiple parties involved in the process a single Project Proponent must be specified contractually as the sole owner of the Credits to avoid double counting of CO₂e removals. Contracts must comply with all requirements defined in the Ownership Section of the Isometric Standard.

The Project Proponent must be able to demonstrate additionality through compliance with the Additionality Section of the Isometric Standard. The Baseline scenarios and Counterfactual utilized to assess additionality must be project-specific, and are described in Sections 8 and 10 of this Protocol.

Additionality determinations must be reviewed and completed at the time of initial Verification as well as following significant changes to project operating, including but not limited to:

• regulatory requirements or other legal obligations for project implementation change or new requirements are implemented
• project finance indicate Carbon Finance is no longer required, potentially due to:
  • increased tipping fees for waste feedstocks
  • sale of co-products that make the business viable without Carbon Finance
  • reduced rates for capital access

Any review and change in the determination of additionality will not affect the availability of Carbon Finance and Carbon Credits for the current or past Crediting Periods, however, if the review indicates the Project has become non-additional, this will make the Project ineligible for future Credits²⁴.

The uncertainty in the overall estimate of the net CO₂e removal as a result of the Project must be accounted for. The total net CO₂e removed for a specific Reporting Period, $RP$, $CO_2e_{Removal,\ RP}$, must be conservatively determined in accordance with the requirements outlined in the Uncertainty Section the Isometric Standard. See Section 10.1.4 for more details on uncertainty requirements for this Protocol.

Projects must report a list of all input variables used in the net CO₂e removal calculation and their uncertainties, including:

• required measurements in Appendix 2 (e.g., soil and porewater measurements)
• data used to model and estimate riverine losses and marine losses
• emission factors utilized, as published in public and other databases used
• values of measured parameters from process instrumentation, such as truck weights from weigh scales, electricity usage from utility power meters and other similar equipment
• laboratory analyses, including analysis of rock or mineral feedstocks

More detailed uncertainty information should be provided if available, as outlined in the Uncertainty Section of the Isometric Standard. Uncertainty must be quantified at the project scale, which may include multiple fields within a region.

In addition, a sensitivity analysis that demonstrates the impact of each input parameter's uncertainty on the overall net CO₂e removal uncertainty must be provided. Details of the sensitivity analysis method must be provided so that the results are reproducible. Input variables may be omitted if they contribute to a $\leq$ 1% change in the net CO₂e removal.

In accordance with the Isometric Standard, all evidence and data related to the underlying quantification of CO₂e removal and environmental and social safeguards monitoring will be available to the public through Isometric's platform. That includes:

• Project Design Document
• GHG Statement
• Measurements taken and supporting documentation, such as calibration certificates
• Emission factors used
• Scientific literature used
• Proof of approval for necessary permits

The Project Proponent can request certain information to be restricted (only available to authorized Buyers, the Registry and VVB) where it is subject to confidentiality. This includes emissions factors from licensed databases. However, all other numerical data produced or used as part of the quantification of net CO₂e removal will be made available.

The scope of this Protocol includes GHG sources, sinks and reservoirs (SSRs) associated with an EW CDR project.

A cradle-to-grave GHG Statement must be prepared encompassing the GHG emissions relating to the activities outlined within the system boundary.

GHG emissions associated with the Project may be as direct emissions from a process or storage system or as indirect emissions from combustion of fuels, electricity generation, or other sources. Emissions must include all GHG SSRs within the system boundary, from the construction or manufacturing of each physical site and associated equipment, closure and disposal of each site and associated equipment, and operation of each process, including embodied emissions of equipment and consumables used in the project. The Project Proponent is responsible for identifying all sources of emissions directly or indirectly related to project activities.

Any emissions from sub-processes or process changes that would not have taken place without the CDR Project must be fully considered in the system boundary. Any activity that ultimately leads to the issuance of Credits should be included in the system boundary.

The system boundary must include all SSRs controlled by and related to the Project, including but not limited to the SSRs in Figure 1 and Table 1. If any GHG SSRs within Table 1 are deemed not appropriate to include in the system boundary, they may be excluded provided that robust justification and appropriate evidence is provided in the PDD.

Figure 1. Process flow diagram showing system boundary for EW projects

Table 1. Systems boundary and scope of activities to be included for EW projects

Miscellaneous GHG emissions are those that cannot be categorized by the GHG SSR categories provided in Table 1.
The Project Proponent is responsible for identifying all sources of emissions directly or indirectly related to project activities and must report any outside of the SSR categories identified as miscellaneous emissions.

Emissions associated with a project's impact on activities that fall outside of the system boundary of a project must also be considered. This is covered under Leakage in Section 8.5.4.

In line with the GHG Accounting Module v1.1, The Project must:

• Consider all GHGs associated with SSRs, in alignment with the United States Environmental Protection Agency’s definition of GHGs, which includes: carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂0) and fluorinated gasses such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF₆) and nitrogen trifluoride (NF₃). For CO₂ stored, only CO₂ will be included as part of the quantification. For all other activities all GHGs must be considered. For example, the release of CO₂, CH₄, and N₂O is expected during diesel consumption;
• Quantify emissions in tonnes CO₂ equivalent (t CO₂e) using the 100-year Global Warming Potential (GWP) for the GHG of interest, based on the most recent volume of the IPCC Assessment Report (currently the Sixth Assessment Report); and
• Consider Materiality of SSRs in line with Isometric requirements.

In some instances, EW project activities may be integrated into existing activities. Activities or portions of activities that were already occurring in the baseline and would have continued to occur without the EW project may be omitted from the system boundary, subject to the conditions set below.

For the purpose of this provision, an "activity" may refer to an operational sub-unit, such as an individual trip leg, a single pass of field equipment, or a discrete processing step, where such sub-units can be cleanly delineated by equipment, timing, and physical scope. Where a pre-existing activity is partially modified by the project (for example, a transport trip whose route is extended to include project-specific stops, or a field operation whose pass is lengthened to include project inputs), the activity must be partitioned into:

• portions that are materially unchanged by the project, which may be excluded from the system boundary under this provision; and
• portions that are new, extended, or altered as a result of the project, which must remain within the system boundary and be quantified in the relevant sub-section in Section

An activity, or portion of an activity, may only be excluded where the Project Proponent can demonstrate all of the following:

• it was occurring as part of routine operations prior to project activities;
• it would have continued to occur in the absence of the EW project; and
• its scope, frequency, equipment, route, payload, and intensity are not materially altered as a result of project activities.

Evidence supporting these conditions must be provided in the PDD. This must include either:

• Historic records documenting the activity prior to project start. Acceptable records include management logs, operational records, applicator invoices, or equivalent documentation; or
• A signed affidavit from the relevant operator (e.g., farmer, land manager, applicator, or equivalent party) confirming the activity was part of routine operations prior to the project.

And the following:

• A signed affidavit from the relevant operator (e.g., farmer, land manager, applicator, or equivalent party) confirming:
  • the equipment, route, timing, cadence, and intensity of the activity are not materially changed as a result of the EW project; and
  • the activity would have continued at a comparable level absent the project.

Where these conditions are met, only the emissions associated with the activity as it would have occurred in the baseline may be excluded. Any incremental emissions attributable to the project must remain within the system boundary and be accounted for in the relevant emissions sub-section.

The baseline scenario for EW projects assumes the activities associated with the EW project do not take place, no new infrastructure is built and business as usual agricultural practices occur.

The counterfactual is the CO₂ that would have been removed from the atmosphere and durably stored as a result of natural weathering or pre-existing land practices. This is determined through the use of a control plot, as described in Section 8.4, with detail on monitoring requirements described in Section 10.4.3.

EW in agriculture typically consists of an application of a characterized rock or mineral feedstock followed by discrete sampling periods to quantify CO₂ removals. Rock or mineral feedstock must be sourced, which may include quarrying, processing (e.g., crushing) and transportation, before it is spread over agricultural land. A combination of soil, aqueous and other geochemical measurements are then used to quantify the total CO₂ removals over a period of time.

The Reporting Period for EW represents an interval of time over which removals are calculated and reported for verification. Monitoring of CO₂ removals for EW may include a combination of discrete sampling (e.g., soil sampling) and continuous sampling. The equations used to calculate net CO₂e removals will pertain to all GHG emissions and CO₂ removals occurring over a Reporting Period. In most cases, this Reporting Period will be an interval of time bounded by sampling events.

GHG emission calculations must include all emissions related to the project activities that occur within the Reporting Period. This includes:

• any emissions associated with project establishment allocated to the Reporting Period (See Section 8.5.1);
• any emissions that occur within the Reporting Period (See Section 8.5.2);
• any anticipated emissions that would occur after the Reporting Period that have been allocated to the Reporting Period (See Section 8.5.3); and
• leakage emissions that occur outside of the system boundary that are associated with the Reporting Period (See Section 8.5.4).

Total net CO₂e removal is calculated for each Reporting Period, and is written hereafter as ${CO}_{2}^{}e_{Removal,\ RP}^{}$. The final net CO₂e removal quantification must be conservatively determined, giving high confidence that at a minimum, the estimated amount of CO₂e was removed.

In line with the Isometric Standard, this Protocol requires that Removal Credits are issued ex-post.

Net CO₂e removal for EW in agriculture for each Reporting Period, $RP$, can be calculated as follows. The final net CO₂e removal quantification must be conservatively determined, giving high confidence that at a minimum, the estimated amount of CO₂e was removed (refer to Section 6.5 for details). Net CO₂e removal is calculated as:

$${CO}_{2}^{}e_{Removal,\ RP}^{}\  = \ {CO}_{2}^{}e_{Stored,\ RP}^{}\ –\ {CO}_{2}^{}e_{Counterfactual,\ RP}^{}–\ {CO}_{2}^{}e_{ Emissions,\ RP}^{}$$

(Equation 1)

Where:

• ${CO}_{2}^{}e_{Removal,\ RP}^{}$ - the total net CO₂e removal for the Reporting Period, $RP$, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Stored,\ RP}^{}$ - the total CO₂ removed from the atmosphere and stored as inorganic carbon in the solid or aqueous form in the treatment and deployment (in 3-plot approach) plots for the $RP$, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Counterfactual,\ RP}^{}$ - the total counterfactual CO₂ removed from the atmosphere and stored as inorganic carbon in the solid or aqueous form for the $RP$, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Emissions,\ RP}^{}$ - the total GHG emissions for the $RP$, in tonnes of CO₂e

Note: Reversals that occur after Credits have been issued are not included in this equation. See the Reversals and Buffer Pools Section of the Isometric Standard for further information.

The total amount of CO₂ stored from an EW project conceptually includes the following terms:

$${CO}_{2}^{}e_{Stored,\ RP}^{}\  = \ \sum_{}^{}{CO}_{2}e_{Drawdown,\ RP} - \sum_{}^{}{CO}_{2}e_{Losses,\ RP}$$

(Equation 2)

Where:

• ${CO}_{2}^{}e_{Drawdown,\ RP}^{}$ - CO₂ removed from the release of base cations for Reporting Period, $RP$, in tonnes of CO₂e. These base cations may derive from newly weathered rock or mineral feedstock, net dissolution of carbonate minerals over the previous Reporting Period, or desorption of base cations that had previously come out of solution by surface sorption.
• ${CO}_{2}^{}e_{Losses,\ RP}^{}$ - Inorganic carbon lost due to soil column (bio)geochemical processes for the Reporting Period, $RP$, as well as downstream riverine and marine losses, in tonnes of CO₂e. These losses include plant uptake of base cations, secondary silicate mineral formation, carbonate precipitation, non-carbonic acid neutralization, sorption of base cations to cation exchange sites, re-equilibration of the carbonate system in rivers and oceans and any other relevant processes.

This quantification framework considers the terms given in Equation 2 relative to two distinct zones, referred to as the Near-Field Zone (NFZ) and the Far-Field Zone (FFZ). The NFZ represents the portion of the upper soil column in which the weathering reaction and subsequent processes (e.g. soil and biomass uptake, secondary mineral formation) must be directly tracked and, for practical purposes, is defined as the depth of deepest soil sampling. The FFZ is defined as the transport path for the weathering products from the boundary of the NFZ to the final storage reservoir (typically the ocean). This includes the deep soil column, groundwater network, riverine transport and ocean discharge.

In this mass/charge balance approach, we explicitly account for both: 1) permanent losses of base cations (and corresponding CO₂ removals) that happen after base cations are released from a rock or mineral feedstock through weathering and 2) the possibility that base cations may be temporarily, but not permanently, rendered ineffective for removals.

Losses considered to be permanent include:

• Plant uptake of base cations by harvested biomass
• The formation of secondary silicates (e.g., clays)
• Non-carbonic acid neutralization (e.g., neutralization of acid produced from nitrification of ammonia fertilizers)
• Degassing due to re-equilibration of the dissolved inorganic carbon system

Losses considered to be temporary include:

• The formation of carbonate minerals
• Base cation sorption to cation exchange sites

One particularly consequential aspect of these CO₂ removal loss terms is the potential time lag associated with cation sorption. A base cation may undergo many generations of sorption and desorption to cation exchange sites while migrating through the soil column, delaying the CO₂ removal effect until that cation is sufficiently deep in the soil column or more permanently transitions to the aqueous phase. There is currently no widely held scientific consensus on the best practices for modeling these cation sorption dynamics.

The sorption of base cations to cation exchange sites will cause re-equilibration of dissolved inorganic carbon species to maintain charge balance, which will result in degassing of CO₂ into soil pore space. If this degassing occurs sufficiently deep in the soil column, the resulting transient CO₂ may be sufficiently isolated from the atmosphere as to be considered a removal while the cation is still migrating through the soil column.

We also note that while this Protocol treats non-carbonic acid neutralization as a loss, from a systems-level perspective, alkalinity consumed by non-carbonic acid neutralization in the NFZ may reduce CO₂ degassing in the far-field zone (FFZ). Robust quantification of this effect and its counterfactual remains an active area of research, and Isometric will consider incorporating FFZ-level accounting as the science and available tools mature.

This Protocol defines removal relative to the primary quantification medium (e.g. solid or aqueous measurements). Where soil is used for CDR quantification, this Protocol considers removal to have occurred when base cations are exported from the NFZ. This Protocol currently recommends the depth of the NFZ to be the deeper of

1. 20 centimeters below the surface, or
2. the depth of tillage plus a buffer of 5 to 10 centimeters ^{25, 26}.

A shallower definition of the NFZ may be used in circumstances where meeting these criteria is not feasible (e.g., shallow water table which cannot be accessed through conventional sampling methods). Such deviations must be reported and justified in the PDD. Where aqueous phase measurements are used for quantification, base cations can be credited within the NFZ once they have entered the porewater.

All Crediting Projects must design a sampling plan that directly measures the initial weathering of rock or mineral feedstock. Once a statistically significant amount of feedstock weathering has occurred, the Project Proponent may be eligible for Credits. This Protocol currently allows for two primary quantification approaches for determining the carbon stored from an EW project: 1) soil-based quantification and 2) porewater-based quantification. The determination used for crediting must be designated in the PDD. The details of quantification requirements are addressed in the following sections.

This Protocol includes a list of quantification approaches that can be used to determine the amount of carbon stored. There are a number of other technologies at various stages of development that may prove useful for quantifying enhanced weathering in an agriculture setting (e.g., reactive transport models, ion exchange resin). The Isometric science team will regularly assess the state of scientific consensus around new and emerging technologies for quantifying CDR via Enhanced Weathering. In addition to quantification approaches, this Protocol also includes a list of allowable optional validation approaches that may be used by Projects. The use of a validation approach is recommended.

Project Proponents must quantify the amount of carbon stored using one of the quantification approaches in List 1. The quantification approach must be conducted within an appropriate sampling framework, as discussed in Section 10.1 (e.g., 2-plot, 3-plot). Additionally, Project Proponents are recommended to select and implement at least one validation approach in List 2. This validation is meant to be a localized, independent sense check on the quantification approach. If the validation approach is conducted, the validation approach must be conducted within both a treatment and control area that meets the minimum percentages of the project area (see Equation 9). The same approach cannot be used for both quantification and validation.

The quantification and validation approaches currently accepted for Crediting Projects are:

List 1: Quantification Approaches

• Project scale soil-based quantification
• Project scale porewater-based quantification

List 2: Validation Approaches

• Local soil-based validation (so long as solid phase is not used for quantification)
• Local porewater-based validation (so long as aqueous phase is not used for quantification)
• Local ion exchange resin validation

In the event that a Project Proponent does not select soil-based quantification or validation from List 1 or 2, the Project Proponent is still required to take soil agronomic measurements at the project scale for baseline sampling, and at 10% of porewater sampling locations in subsequent sampling events. Refer to Table 4 for recommended and required soil agronomic measurements.

Projects are recommended to conduct an independent check on the primary quantification approach using a secondary quantification method from List 2. The quantification check is considered passed if the median of the secondary approach is greater than or equal to the 30th percentile of the primary approach, i.e.

$$P_{50}(secondary) \geq P_{30}(primary)$$

(Equation 3)

Where:

• $P_{50}(secondary)$ - the median value from the secondary method (List 2)
• $P_{30}(primary)$ - the 30th percentile value from the primary quantification approach (List 1)

The optional validation check should assume the same NFZ depth as the quantification approach. In most cases this will be 20 cm. Projects unable to maintain parity between the quantification and validation approaches must justify this deviation in the PDD and describe how the difference in depth will be conservatively reconciled.

See Appendix 4 for the rationale behind making the validation check recommended instead of required.

To account for uncertainty in CDR quantification, Credits are issued at a lower percentile of the distribution produced by the primary quantification approach (see Uncertainty), rather than at the median. The percentile depends on whether a validation approach has been implemented and passed:

• $P_{30}$ (default): Projects that do not implement a validation approach, or whose validation approach does not meet the pass criterion defined in Section 8.3.1.1, Credit at the 30th percentile of the primary CDR distribution. This corresponds to approximately half a standard deviation downward adjustment under a normal distribution.
• $P_{40}$ (passed optional validation): Projects whose validation approach meets the pass criterion Credit at the 40th percentile of the primary CDR distribution. This corresponds to approximately a quarter of a standard deviation downward adjustment under a normal distribution.

See Section 10.1.4 for more details on CDR and uncertainty quantification.

It is anticipated that new technologies and methods for quantifying and validating Enhanced Weathering in agriculture will emerge in the coming years. These new technologies will be regularly assessed by the Isometric science team for inclusion in this Protocol. New technologies and methods that have some demonstrated efficacy when compared to conventional soil and porewater approaches will be included in List 2 and may be used as a validation approach. New technologies and methods that have demonstrated efficacy and have gathered wide scientific consensus will be included in both Lists 1 and 2 (may be used for either quantification or validation).

Particularly, we anticipate that reactive transport models or other suitable models will improve significantly in the coming years from large amounts of calibration data. Isometric will continue to evaluate reactive transport models for inclusion in Lists 1 and 2 in future iterations of this Protocol.

The exact requirements that must be met for a novel method to be used for either validation or quantification will depend on both the method itself and the parameter it is being used to quantify. In general, methods being used for quantification will have a higher burden of proof than those being used for validation.

At a minimum, all proposed novel methods for validation or quantification must submit the following for consideration:

• Peer-reviewed scientific articles evaluating use of the method or technique in relevant settings
• A description of the method or technique and how it is applied in the Project
• Standard operating procedures including all aspects of the method or technique (e.g. installation, sample collection and preparation, analysis)
• Any code or data analysis tools used in normal operation of the method/technique
• Any data demonstrating efficacy of the technique within the Project, including bench-scale experiments, mesocosms and field trials.

In addition to the above, methods proposed for quantification must submit the following:

• Data demonstrating efficacy of the method or technique in all extremes of operational conditions
  • Where the method or technique is implemented in the field, this is likely satisfied by one full year of deployment.
  • Where the method or technique is implemented in laboratory analysis, this includes assessment of the method or technique for all sample types with which it will be used. For example, if the technique is a novel soil preparation for cation analysis, efficacy must be demonstrated across the range of soil types taken from the project area.
• Comparison with conventional methods or techniques for the measured parameter(s)
  • The exact nature of the comparison will depend on the method or technique proposed.
  • For example, if the proposed technique is a novel method for sampling the aqueous phase, the comparison must include data from both conventional porewater sampling methods (e.g. lysimeters, rhizons) and soil measurements.
  • The proposed method and the conventional method must agree within the propagated error of the measurements.

The total amount of carbon stored from an EW project can be determined from soil measurements according to:

$${CO}_{2}^{}e_{Stored,\ RP}^{} = ({CO}_{2}^{}e_{Weathering, RP}^{}  - {CO}_{2}^{}e_{BiomassLoss,\ RP}^{}  - {CO}_{2}^{}e_{NetNewCarbonate,\ RP}^{} \\ -\ {CO}_{2}^{}e_{NetNewSilicate,\ RP}^{}- {CO}_{2}^{}e_{NetSorption,\ RP}^{} - {CO}_{2}^{}e_{NonCarbonicNeut, RP}^{})*\eta_{RiverOcean}$$

(Equation 4)

Where:

• ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ - CO₂ removed from the release of base cations from feedstock for the Reporting Period, $RP$, in tonnes of CO₂e.
• ${CO}_{2}^{}e_{BiomassLoss,\ RP}^{}$ - Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone by the uptake of base cations by harvested plant biomass (annual crops) or new plant growth (perennial crops) for the $RP$, in tonnes of CO₂e.
• ${CO}_{2}^{}e_{NetNewCarbonate,\ RP}^{}$ - Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone from the net formation of new carbonate minerals in the soil column for the , in tonnes of CO₂e. This will typically lead to a 50% decrease in the removal efficiency for silicate (100% for carbonate) feedstocks over aqueous phase export.
• ${CO}_{2}^{}e_{NetNewSilicate,\ RP}^{}$ - Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone from the formation of new silicate minerals in the soil column for the $RP$, in tonnes of CO₂e. This term is included for completeness and is not required to be quantified at this time.
• ${CO}_{2}^{}e_{NetSorption,\ RP}^{}$ - Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone from the net sorption of base cations to cation exchange sites in the NFZ for Reporting Period $RP$, in tonnes of CO₂e. We note that, in some cases, this value may be negative, indicating a net release of cations that had accumulated in previous Reporting Periods.
• ${CO}_{2}^{}e_{NonCarbonicNeut,\ RP}^{}$ - Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone from neutralization of acids other than carbonic acid for the $RP$, in tonnes of CO₂e.
• $\eta_{RiverOcean}$ - Total retention factor for all relevant processes in rivers and the ocean that lead to a reduction of CO₂ that ends up durably stored in the ocean. This factor is dimensionless with values between 0 and 1.

See Section 10 for specific requirements for the quantification of each term.

Note: ${CO}_{2}^{}e_{NetNewSilicate,\ RP}^{}$ is the average net change in silicate mineral (e.g., clay mineral) content between the start and end of the Reporting Period. There is no widely accepted and operationally feasible quantitative method for determining modest changes in secondary silicate mineral content in soils; clay content of soils is typically expressed as a percentage of clay-sized particles, and mineralogy is determined by X-ray diffraction. Thus, detection of new secondary clays requires either significant enough clay formation to shift the percentage of clay in the soil column or quantitative formation of clay mineralogies that are distinct from the initial assemblage (unlikely for modest pH change in soils with similar moisture retention; see Wilson (1999)²⁷ for an overview of the parameters controlling clay formation in soil). This term is included for completeness; Isometric will not explicitly require measurement of this parameter at this time.

Project Proponents pursuing soil-based quantification must consider all terms listed in Equation 4, but alternative methods may be appropriate for rigorous quantification of CDR. For example, some Project Proponents may choose to monitor the loss of alkalinity in a project area by performing full acid digests on soil samples with limited pre-processing (e.g. removal of carbonates, extraction of the soil exchangeable fraction). In this instance, weathering, cation sorption, carbonate formation and clay formation will all be integrated into a single measurement. Such deviations may be appropriate and will be considered on a project by project basis. In such cases, the Project Proponent must provide a detailed description of the methods and describe how the chosen analyses map onto the terms in Equation 4 in the PDD.

For aqueous phase quantification, we make the simplifying assumption that all of the major soil column processes, including the release of base cations from weathering and soil losses, are accounted for in the aqueous geochemistry of water that has infiltrated to some depth. At this time, we are recommending this depth coincide with the depth of the NFZ, typically 20 cm. Therefore, the total amount of carbon stored from an EW project can be determined from porewater measurements in the top 20 cm of soil. Alternatives to 20 cm may be justified for similar reasons described in the previous sections. Well-defined catchment waters may be used in place of porewaters where the project area is contained entirely within such well-defined catchments. In the context of this Protocol, a catchment is considered well-defined if:

• Groundwater runoff from the Project area drains at known points, as determined by hydrologic maps, and
• Groundwater runoff from the Project area is isolated from other water sources, and
• The Project Proponent can demonstrate, given feedstock application rate, anticipated weathering rate, average annual water fluxes in the Project area and distance from the Project to the drainage point that the alkalinity flux from the Project is is resolvable at a proposed measurement cadence.

In instances where catchment waters are used, the Project Proponent must provide supporting documents detailing site-specific hydrogeology including details on characterization, time accounting, attribution, measurement and sampling as well as loss accounting.

$CO_2e_{Stored, \ RP}$ can be calculated from the aqueous phase via the following equation:

$${CO}_{2}^{}e_{Stored,\ RP}^{} = {CO}_{2}^{}e_{Aqueous,\ RP}^{} * \eta_{RiverOcean}$$

(Equation 5)

Where:

• ${CO}_{2}^{}e_{Aqueous,\ RP}^{}$ - CO₂ removed as determined from the infiltration of carbonate alkalinity to the depth of the NFZ, typically 20 cm, in the Reporting Period, $RP$, in units of tonne CO₂e.
• $\eta_{RiverOcean}$ - Total retention factor for all relevant processes in rivers and the ocean that lead to a reduction of CO₂ that ends up durably stored in the ocean. This factor is dimensionless with values between 0 and 1.

See Section 10.5 for specific requirements for the quantification of each term. It is important to note that, although ${CO}_{2}^{}e_{BiomassLoss,\ RP}^{}$ is not explicitly included in aqueous phase quantification, alkalinity may still be taken up by plants below the observation window. Project Proponents using aqueous phase quantification for Credits must still consider and quantify plant uptake if roots extend below the depth of porewater sampling.

Project Proponents pursuing aqueous phase quantification must consider all terms listed in Equation 5, but alternative methods may be appropriate for rigorous quantification of CDR. Such deviations may be appropriate and will be considered on a project by project basis. In such cases, the Project Proponent must provide a detailed description of the methods and describe how the chosen analyses map onto the terms in Equation 5 in the PDD.

Synthetic ion exchangers, more commonly referred to as ion exchange resins, have been employed in soil science for many years to constrain nutrient fluxes in the soil column (e.g. Lehman et al., 2001²⁸, Lang & Kaupenjohann, 2004²⁹, Johnson et al., 2005³⁰, Predotova et al., 2011³¹, Grahmann et al., 2018³²). Typically, monitoring will consist of a vessel filled with a mixture of ion exchange resin beads and quartz sand that is buried in the soil column. Soil water then flows through the device and ions sorb to the resin beads, accumulating within the device. At the end of a sampling period, the accumulated ions are analyzed.

Ion exchange resins are a novel technique for EW measurements, with limited data on their use in EW deployments. For this reason, ion exchange resins may be used as a validation medium (List 2 in Section 8.3.1) under this Protocol. As more data becomes available, the use of ion exchange resins as a quantification medium will be revisited. Calculation of $CO_2e_{Stored, \ RP}$ can be calculated as a validation check using ion exchange resins following Equation 5. Project Proponents using ion exchange resins for validation must provide detailed information on:

• The resin(s) used, including documentation that demonstrates the resin(s) target one or more of the following ion classes relevant to the Project's quantification approach:

  • Base cations, including Ca²⁺, Mg²⁺, K⁺ and Na⁺;
  • All non-carbonic acid sources, which may include NO₃^-, PO₄^3- and SO₄^2-;
    • Ion exchange resins that employ sulfur-based functional groups (e.g., sulfonic acid groups in strong acid cation resins) are susceptible to sulfur leaching during extraction, which can produce positively biased sulfate (SO₄²⁻) estimates. Where resin-derived sulfate concentrations are used in the non-carbonic acid correction, Project must demonstrate that sulfate values are not artifacts of resin leaching, acceptable approaches include:
      • Comparison of resin-derived sulfate against independent porewater sulfate measurements;
      • Use of resin blanks (unexposed devices processed identically) to quantify and subtract the sulfur contribution from functional group leaching;
      • Use of resins with non-sulfur-based functional groups for anion capture.
  • Dissolved inorganic carbon (HCO₃⁻, CO₃²⁻), using weak acid resins or equivalent approaches.

• Total Ion Exchange Capacity (IEC) of each resin device deployed, expressed in milliequivalents (meq) or millimoles of charge (mmol_c);

• Selectivity coefficients for the resin(s) used, particularly the relative affinity for target weathering products (Ca²⁺, Mg²⁺) versus competing ions (e.g., Al³⁺, Fe²⁺, Na⁺).

• Description of the installation procedure, particularly focusing on:

  • Methods used to mitigate disturbance of pre-existing soil structure, including soil horizon integrity and macropore networks;
  • Deployment period, including the equilibration period following installation but before feedstock is applied. Where installation disturbs soil structure, a minimum settling period of 1–2 weeks is recommended to allow soil structure to re-stabilize and water flow patterns to normalize ^33,34. This settling period may be reduced or waived where subsequent field management practices (e.g., tillage, cultivation) disrupt soil structure to a comparable or greater extent than the installation procedure itself;
  • Depth of installation relative to the application zone and typical rooting depth;
  • Spatial distribution of devices across the project area to ensure representativeness.

• Description of the extraction and measurement procedures used, including:

  • Extraction solution composition, volume, and contact time;
  • Analytical methods for quantifying extracted ions;
  • Quality assurance and quality control (QA/QC) procedures, including blanks and standards;
  • Analysis of the complete cation suite, including but not limited to: Ca²⁺, Mg²⁺, K⁺, Na⁺, Al³⁺, Fe²⁺, and relevant trace metals. This allows for assessment of competitive sorption effects.

The use of selective ion resins (i.e., resins targeting a single ion class) is permitted, provided the Project Proponent demonstrates that weathering-relevant ions not captured by the resin are independently constrained. Mixed-bed or multi-resin configurations that capture multiple ion classes simultaneously are also permitted.

Project Proponents must evaluate whether ion exchange devices approached saturation during the deployment period. Capacity utilization should be calculated for each device by calculating the percentage of total IEC (ion exchange capacity) occupied by recovered ions using the following relationship:

$$Capacity\ Utilization (\%) = \frac{\sum \text{recovered cations (meq)}}{\text{Total IEC (meq)}} \times 100$$

(Equation 6)

If devices approach saturation, the Project Proponent should consider reducing deployment duration in subsequent monitoring periods.

Further, to ensure that non-target ions have not disproportionately occupied exchange sites at the expense of target weathering products, the molar ratio of target base cations (Ca²⁺ and Mg²⁺) to total recovered cations should be calculated. Ion exchange resin devices installed in the vadose zone are subject to localized flow divergence or convergence, where soil water may bypass the device or preferentially funnel through it. This can lead to under- or over-estimation of dissolved ion fluxes, respectively. Project Proponents should document the measures taken to ensure representative water contact with the resin (e.g., device design, backfill procedures, installation orientation). Where feasible, comparison of resin-derived fluxes against independent porewater or lysimeter measurements is recommended to assess the magnitude of any flow bias.

In addition, the fraction of total recovered charge attributable to Al³⁺ and Fe²⁺ should be reported, as elevated concentrations of these ions may indicate competitive displacement of target cations or preferential resin saturation.

Project Proponents should adjust deployment duration to ensure:

• Sufficient ion accumulation for accurate quantification, demonstrated by recovered ion concentrations that are statistically distinguishable from field blanks
• Avoidance of saturation
• Alignment with the quantification method monitoring schedule

In regions subject to freeze/thaw cycles, Project Proponents should consider the potential for physical damage to resin devices (e.g., bead fracturing, loss of hydraulic contact with the surrounding soil) and for remobilization of sorbed ions. Deployment periods should be planned to avoid or account for freeze/thaw events, and any devices recovered after freeze/thaw exposure should be inspected for physical integrity before extraction and analysis.

As ion exchange resins quantitatively remove dissolved ions from porewaters, the derived estimates of removals should utilize Equation 5, and should otherwise follow the recommendations and requirements pertaining to aqueous phase sampling wherever applicable.

${CO}_{2}^{}e_{Counterfactual,\ RP}^{}$ describes the CO₂ that would have been removed from the atmosphere and durably stored in the baseline scenario as a result of pre-existing land management practices, natural background soil weathering, or weathering of excess feedstock fines.

$${CO}_{2}^{}e_{Counterfactual,\ RP}^{} = {CO}_{2}^{}e_{Counterfactual,\ LMP}^{} + {CO}_{2}^{}e_{Counterfactual,\ Background}^{}+ {CO}_{2}^{}e_{Counterfactual,\ feedstock fines}^{}$$

(Equation 7)

Where:

• ${CO}_{2}^{}e_{Counterfactual,\ LMP}^{}$ is the counterfactual weathering attributed to business as usual land management practices targeting pH adjustment, discussed in Section 10.1.1.3
• ${CO}_{2}^{}e_{Counterfactual,\ Background}^{}$ is the natural weathering in soil that would occur regardless of project activities or land management practices
• ${CO}_{2}^{}e_{Counterfactual,\ feedstock\ fines}^{}$ is the weathering of feedstock fines that would have occurred in the counterfactual scenario where the mine or quarry stockpiled excess fines from existing operations

In practice, ${CO}_{2}^{}e_{Counterfactual,\ LMP}^{}$ and ${CO}_{2}^{}e_{Counterfactual,\ Background}^{}$ are not calculated distinctly and together are referred to as the control plot correction, ${CO}_{2}^{}e_{Counterfactual,\ Control Plot}^{}$, which monitors the decrease in base cations in the control plot. ${CO}_{2}^{}e_{Counterfactual,\ Control Plot}^{}$ and ${CO}_{2}^{}e_{Counterfactual,\ feedstock\ fines}^{}$ are subtracted from ${CO}_{2}^{}e_{Stored,\ RP}^{}$ in Equation 1. The control plot correction must be assessed for all projects, regardless of whether lime is applied in the business as usual scenario, as it is indistinguishable from background soil weathering.

${CO}_{2}^{}e_{Counterfactual,\ Control Plot}^{}$ is calculated using a business as usual control plot. The control plot ensures that any removals associated with business as usual agricultural liming or natural weathering of the soil are not attributed to project activities. The control plot correction will only be applied if there has been a statistically significant change in alkalinity in control plots in a given Reporting Period (see [Section 10.1.3](Statistical Requirements for Crediting) for more details). The control plot will serve as the baseline in modeling with respect to measured soil and porewater parameters.

${CO}_{2}^{}e_{Counterfactual,\ Control Plot}^{}$ is determined using the same equations (Equations 4 and 5) as the previous section for ${CO}_{2}^{}e_{Stored,\ RP}^{}$ in which all the encompassed terms are determined using a control plot of land as described in Section 10.1.1. $CO_2e_{Stored}$ in the control plot will be calculated as the change in concentration of base cations in the NFZ within a Reporting Period. The calculation of $CO_2e_{Stored}$ in the counterfactual term does not distinguish between weathering from BAU lime application and background soil weathering.

For instances where the Project Proponent has discretion as to which methods can be used to determine a particular weathering or loss term, the methods used must be the same for both the area over which feedstock is applied and the control plot.

Where a business as usual control plot is maintained and lime is applied between the beginning and end of the Reporting Period, but samples are not taken immediately post lime application, Project Proponents may estimate the cation addition attributed to lime application using the operational record of the lime application rate.

In some cases, the counterfactual land management practice may have been net emitting, for example if quicklime was used as the liming agent. In this scenario, ${CO}_{2}^{}e_{Counterfactual,\ RP}^{}$ may be taken as zero on a case-by-case basis in consultation with Isometric and the VVB. To claim this scenario, Project Proponents must submit clear justification, including:

• Proof that the high-emissions liming agent would have been used in the counterfactual scenario (e.g. through signed affidavits of agricultural partners and records demonstrating that the net-emitting practice was standard for at least 5 years preceding the onset of project activities);
• All calculations and emissions factors used to determine that the counterfactual practice is net-emitting. This includes: (1) historical quicklime application rates, (2) quicklime manufacturing emissions, and (3) the CDR drawdown potential of the quicklime assuming 100% dissolution. If (2) exceeds (3), then the counterfactual is considered net-emitting and may be eligible to be set to zero.

For further information on why the counterfactual may be set to zero in a net-emitting scenario, please see Appendix 3 of the Wastewater Alkalinity Enhancement Protocol v1.2.

Project Proponents that establish that the counterfactual land management practice is net emitting should not maintain that particular practice in a BAU control plot. Instead, these Project Proponents should maintain an unamended control plot to account for ${CO}_{2}^{}e_{Counterfactual,\ Background}^{}$.

In addition to counterfactual CO₂ related to business as usual farming practices, in some instances it may be appropriate to consider weathering of feedstock that would have occurred without project intervention. For example, if the feedstock used constitutes a waste product that was not mined or quarried specifically for project activities and was stored in open-air conditions, some degree of surficial weathering could be expected over timescales relevant to a project lifetime. Project Proponents using these feedstocks must account for counterfactual weathering if the feedstock does not undergo additional processing prior to deployment. Please see the Rock and Mineral Feedstock Characterization Module v1.2 for requirements related to quantifying counterfactual feedstock weathering.

${CO}_{2}^{}e_{Emissions,\ RP}^{}$ is the total quantity of GHG emissions from operations and allocated embodied emissions for each Reporting Period $RP$. This can be calculated as:

$${CO}_{2}^{}e_{Emissions,\ RP}^{}\ = \ {CO}_{2}^{}e_{Establishment,\ RP}^{}\ + \ {CO}_{2}^{}e_{Operations,\ RP}^{} + \ {CO}_{2}^{}e_{End-of-life,\ RP}^{}+ \ {CO}_{2}^{}e_{Leakage,\ RP}^{}$$

(Equation 8)

Where:

• ${CO}_{2}^{}e_{Emissions,\ RP}^{}$ - the total GHG emissions in a Reporting Period, $RP$, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Establishment,\ RP}^{}$ - the total GHG emissions associated with project establishment in a $RP$, in tonnes of CO₂e, see Section 8.5.1
• ${CO}_{2}^{}e_{Operations,\ RP}^{}$ - the total GHG emissions associated with operational processes in a $RP$, in tonnes of CO₂e, see Section 8.5.2
• ${CO}_{2}^{}e_{End-of-life,\ RP}^{}$ - the total GHG emissions that occur after the RP and are allocated to the $RP$, in tonnes of CO₂e, see Section 8.5.3
• ${CO}_{2}^{}e_{Leakage,\ RP}^{}$ - represents the GHG emissions associated with the project’s impact on activities that fall outside of the system boundary of a project, over a given Reporting Period, in tonnes of CO₂e, see Section 8.5.4

The following sections set out specific quantification requirements for each variable. It is anticipated that most emissions associated with EW projects will occur during the Calculation of ${CO}_{2}^{}e_{Establishment,\ RP}^{}$ phase.

GHG emissions associated with $CO_2e_{Establishment,\ RP}$ should include all historic emissions incurred as a result of project establishment, including but not limited to the SSRs set out in Table 1.

Project establishment emissions occur from the point of project inception to after the spreading event has taken place. GHG emissions associated with project establishment may be allocated in one of the following ways, with the allocation method selected and justified by the Project Proponent in the PDD:

• As a one time deduction to the first Reporting Period(s)
• Allocated over the first half of the anticipated project lifetime (meaning the point at which 50% of the feedstock weathering potential is realized) as annual emissions
• Allocated per output of product (i.e., per tonne CO2 removed) so long as establishment emissions are fully accounted by the time 50% of the weathering potential has been realized.

If a Project Proponent Credits a Removal Area once, ammortizes establishment emissions, but does not Credit that Removal Area again, these amortized emissions must be accounted for within 2 years of when 50% of the feedstock weathering potential will be realized, by either Crediting that field again or by deducting the Credits from other removals.

Requirements for monitoring, reporting, reviewing, and adjusting amortization schedules are outlined in Section 7 of the GHG Accounting Module v1.1.

GHG emissions associated with $CO_2e_{Operations\, RP}$ should include all emissions associated with operational activities including but not limited to the SSRs set out in Table 1. For EW projects, the Reporting Period begins after the spreading event on agricultural land has occurred and ends once the weathering potential has been realized and MRV activities have ceased.

$CO_2e_{Operations,\ RP}$ emissions occur over the Reporting Period for the deployment being Credited and are applicable to the current deployment only. $CO_2e_{Operations,\ RP}$ emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted on a case by case basis in agreement with Isometric.

$CO_2e_{End-of-life,\ RP}$ includes all emissions associated with activities that are anticipated to occur after the Reporting Period, but are directly or indirectly related to the Reporting Period. For example this could include ongoing sampling activities for MRV for the specific deployment (directly related) if applicable, or end-of-life emissions for project facilities (indirectly related to all deployments).

GHG emissions associated with $CO_2e_{End-of-Life,\ RP}$ may occur from the end of the Reporting Period onwards, and typically through to completion of project site deconstruction and any other end-of-life activities.

GHG emissions associated with activities that are directly related to each deployment must be quantified as part of that Reporting Period. GHG emissions associated with activities that are indirectly related to all deployments may be allocated in the same ways as set out in $CO_2e_{Establishment,\ RP}$.

$CO_2e_{Leakage,\ RP}$ includes emissions associated with a project's impact on activities that fall outside of the system boundary of a project.

It includes increases in GHG emissions as a result of the project displacing emissions or causing a knock on effect that increases emissions elsewhere. This includes emissions associated with activity-shifting, market leakage and ecological leakage.

It is the Project Proponent's responsibility to identify potential sources of leakage emissions. For an EW project, feedstock replacement must be considered as part of the leakage assessment, at a minimum.

$CO_2e_{Leakage,\ RP}$ emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted in certain instances, on a case by case basis in consultation with Isometric.

GHG emissions accounting must be undertaken in alignment with the GHG Accounting Module v1.1, which ensures a consistently rigorous standard in how GHG emissions are quantified and reported between different CDR Projects and approaches. This includes:

• Requirements for data quality, including a detailed data quality hierarchy for activity data and emission factors;
• Consideration of materiality in emissions accounting;
• Emissions amortization requirements;
• Co-product allocation requirements;
• Waste input accounting relating to inputs to the process that are wastes, for example fine rocks from mining operations that are stockpiled; and
• By-product accounting relating to inputs to the process that are by-products, for example fine rocks from mining operations where this is not considered a waste.

The Energy Use Accounting Module 1.3 provides requirements on how energy-related emissions must be calculated for The Project so that they can be subtracted in the net CO₂e removal calculation. It sets out the calculation approach to be followed for intensive facilities and non-intensive facilities and acceptable emission factors.

Energy emissions are those related to electricity or fuel usage. They may include, but are not limited to:

• Electricity consumption for equipment for drying rock or mineral feedstock after milling
• Electricity consumed in laboratories for feedstock characterization
• Fuel used in handling equipment, such as fork trucks or loaders
• Fuel consumption of agricultural machinery for spreading, tilling, and sampling

The GHG Accounting Module v1.1 provides requirements on how transportation and embodied emissions must be calculated for the Project so that they can be subtracted in the net CO₂e removal calculation.

Embodied emissions are those related to the life cycle impact of equipment and consumables. They may include, but are not limited to:

• Rock or mineral feedstock and associated production, processing, treatment and transportation equipment
• Sampling equipment and consumable materials such as augers and storage containers
• Raw materials and equipment used in the fabrication, assembly and construction of agricultural machinery for spreading, tilling, and sampling

Transportation emissions are those related to transportation of products and equipment. They may include, but are not limited to:

• Transportation of feedstock to agricultural site
• Transportation of rock from quarry to crushing site
• Transportation and shipping related to collecting samples for environmental monitoring

The primary storage reservoir of the CO₂ removed through Enhanced Weathering is Dissolved Inorganic Carbon (DIC) in the ocean. The durability and reversal risks of this storage reservoir are discussed in the Dissolved Inorganic Carbon Storage in Oceans Module.

As outlined in the Reversals and Buffer Pools Section of the Isometric Standard, the Buffer Pool is a mechanism used to insure against Reversal risks that may be observable and attributable to a particular project through monitoring.

In Enhanced Weathering, carbon is stored as dissolved inorganic carbon (DIC) in the ocean, a vast and well-mixed reservoir where storage durability is expected to be over 10,000 years. As this constitutes an open system with no directly observable reversals, the Isometric Standard classifies Projects operating under this Protocol as “No observable risk” for the purposes of the Risk of Reversal assessment, and such Projects are not required to complete a Risk of Reversal questionnaire. This results in a 0% buffer pool for Projects using this Protocol. However, if a Project identifies project-specific factors which impact the Risk of Reversal, they are required to complete the Risk of Reversal Questionnaire. Notwithstanding this classification, Projects must maintain a monitoring plan that meets the requirements set out in Section 10 of this Protocol.

This section outlines the monitoring approach that Project Proponents must take in Crediting Projects. Several of the monitoring requirements described below include measurements that will be used in the quantification of rock spread and determination of removals (see Section 8.2).

A Removal Area is a collection of fields over which CDR quantification is conducted across consecutive Reporting Periods. A field is defined as a contiguous plot of land subject to the same field management practices, including crop rotation. A Project may consist of one or more Removal Areas, and Project Proponents may add new Removal Areas over time as feedstock is spread on new fields. The total Project area is the sum of all Removal Areas. All deployments and sampling events within a Removal Area must occur on similar time frames. For the purposes of CDR quantification, a Removal Area may be further subdivided into individual fields or groups of similar fields, referred to as Units collectively, or Control Units, Treatment Units and Deployment Units more specifically. Field similarity and time frame requirements are discussed in more detail in Section 10.1.2.

In all projects, the area of control and treatment plots (where applicable) should each total at least a minimum percentage of the total Project area. The minimum percentages, $p$, are as follows:

$$p = \begin{cases} 
2.5\%, & A \le \text{1000 ha} \\ 
2.5\% - 1.5\% \times \log_{10}\!\left(\frac{A}{1000}\right), & \text{1000 ha} \lt A \lt \text{10,000 ha} \\ 
1\%, & A \ge \text{10,000 ha} 
\end{cases}$$

(Equation 9)

Where:

• $p$ is the minimum percentage of the Project area that must be allocated to control plots, and a similar area allocated to treatment plot if applicable
• $A$ is the total Project area (sum of all Removal Areas), in hectares

There is no minimum or maximum Project area, however, projects must designate at least one control, treatment and (if using the 3-plot model) deployment plot per 2,000 hectares. Project areas exceeding 2,000 hectares but less than 4,000 hectares will thus need to designate a minimum of 2 control, treatment and/or deployment plots; Projects exceeding 4,000 hectares but less than 6,000 hectares will need to designate a minimum of 3 control, treatment and/or deployment plots; and so on. This lower limit is set primarily to avoid heterogeneity in factors that will influence weathering rate (e.g., precipitation).

Project Proponents with Project areas distributed over large distances must maintain a set of plots for each region even if the area criteria listed above do not require it. This may include additional sets of units per climatic and/or geo-political region, in consultation with Isometric. Difference in field and climatic characteristics (e.g., soil type, cropping system, precipitation, temperature range, etc.) may also necessitate the maintenance of additional treatment and control sites.

Control, treatment and/or deployment plots will in many cases be contiguous, but this is not a requirement. Projects may contain non-contiguous fields, managed by different farmers or landowners, so long as all plots are representative of each other, in accordance with Section 10.1.2.3.

Where applicable, analytical methods must be cross-referenced with appropriate standards (e.g., ISO, EN, BSI, ASTM, EPA) or standard operating procedures (SOP). Where a project utilizes a non-standardized methodology or SOP for the determination of a listed parameter, the Project Proponent is required to outline the relevant method within the PDD submitted to the Validation and Verification Bodies (VVB).

Project Proponents have the choice between two monitoring frameworks in Crediting projects: the 2-plot and the 3-plot approach.

Figure 2

Schematic of in-field monitoring approaches, illustrating the relative size of the control, treatment, and deployment plots in the 2- and 3-plot approaches, as well as CO₂ drawdown from the atmosphere.

Table 2. Summary of Removal Area. $p$ represents the minimum percentage given by Equation 9.

The 2-plot approach for quantifying removals from EW in agriculture calls for the designation of the Project area into one of two categories:

• Control - area representing CO₂ removal under business as usual practices
• Treatment (2-plot) - area representing CO₂ removal from application of rock or mineral feedstock

The Treatment area is used to directly observe the CO₂ removal resulting from EW in agriculture, while the control is used to directly observe counterfactual (or business as usual) CO₂ removal. The 2-plot approach should be generally thought of as a flat, per-area sampling scheme.

The 3-plot approach allows for intensive, high-resolution data collection and monitoring of EW projects and their counterfactuals at smaller scales, while monitoring the remainder of the Project at lower resolution.

In this construction, the project area must be divided into three sections that will be designated as the control, treatment and deployment areas defined as follows:

• Control - densely monitored area representing CO₂ removal under business as usual practices
• Treatment (3-plot) - densely monitored area representing CO₂ removal from application of rock or mineral feedstock
• Deployment - less densely monitored area representing CO₂ removal from application of rock or mineral feedstock

Unless otherwise specified, the measurements collected in the control, treatment or deployment plots will be used to quantify removal terms contained within that particular area (i.e. deployment samples are used to quantify removals in the deployment plot). There may be some cases where measurements collected in the treatment area may be extrapolated to the deployment area (e.g., quantification of biomass uptake of base cations), although measurements for quantifying gross weathering cannot be extrapolated. It is recommended to take the same number of samples from the control, treatment and deployment plots within a given Removal Area.

The purpose of a control plot is to quantify the removal that would have otherwise occurred without the application of rock or mineral feedstock (see Section 8.4.1). Most notably, this includes any CO₂ removal effect of agricultural liming that would have occurred in the absence of project activities. The control area must represent a minimum of percentage of the total project area as given by Equation 9. The control area must be maintained using a continuation of historical agricultural practices to represent what would have occurred in the absence of the CO₂ removal project. This includes, for example, liming if EW is replacing current liming practices. Note that all loss terms will also be accounted for in counterfactual removals. If the Project Proponent is unable to maintain a business as usual control plot concurrent with the project deployment, the Project Proponent may opt to do the following:

• Provide a written explanation detailing why it was not possible to maintain a control plot.
• Provide liming records of the project area over the last 10 years.
• Assume as counterfactual that liming would have occurred at the average liming rate for the past 10 years and assume either a 100% efficient CO₂ removal processes (i.e., all lime is exported in the aqueous phase as bicarbonate with no losses) or that the loss terms measured in the rest of the project area also would have occurred in a liming counterfactual.

In instances where the last 10 years of liming records cannot be obtained, the Project Proponent must work in consultation with Isometric to identify the most plausible counterfactual land management scenario. Examples of possible sources of information include signed affidavits from knowledgeable local individuals or aggregated county level records. The Project Proponent must detail the sources of this information in the PDD.

Project Proponents that establish that the counterfactual land management practice is net emitting, such as through the application of quicklime, should not maintain this practice in the control plot. Further guidance on establishing a net emitting counterfactual scenario may be found in Section 8.4.

The treatment (2-plot) encompasses the project area on which rock or mineral feedstock is applied. The treatment area will be subject to monitoring as prescribed by the sampling plan. Application of feedstock to the treatment area must be uniform unless otherwise stated (see Section 10.1.1.6.1: Variable Application Rates for further details).

The treatment (3-plot) must represent a minimum percentage of the total project area given by Equation 9. If a Project Proponent opts to maintain control and treatment areas that are larger than the minimum percentage, it is recommended that these two areas maintain parity in both size and sampling density. The treatment area will be subject to intensive monitoring as prescribed by the sampling plan. Application of feedstock to the treatment area must be at the same feedstock volume per unit area as the deployment area.

The deployment area encompasses the remaining project area that is not within the control or treatment plots. In most cases, this will represent (100-2p)% of the project area (Table 2).

In some cases, Project Proponents may vary the feedstock application rate within a single deployment due to project-specific constraints, such as heterogeneity in soil characteristics or regional agricultural practice, or for research purposes. These Projects are still be eligible for Crediting under this Protocol.

As described in Section 10.4.4, this Protocol requires a soil-based determination of feedstock application to quantify the maximum Project CDR potential. Project Proponents utilizing variable application rates are required to submit application rate maps for the Project area. These maps can be generated from either recommended application rates supplied to the applicator (typically from farmer-supplied soil data) or directly from the applicator. This data must be used to design soil sampling plans such that the full range of application rate is captured.

Projects seeking Credits with variable application rates are required to use the 3-plot model and designate treatment areas that correlate with different application rates in the Project Area. For example, if application rate in a particular field varies from 1-4 tonnes/ha, the Project Proponent must subdivide this into at least two ranges of 1-2 tonnes/ha and 3-4 tonnes/ha, each with separate treatment plots. $CO_2e_{Stored,\ RP}$ will be determined for areas with similar application rates, which will then be used to calculate a weighted average of $CO_2e_{Stored,\ RP}$ for the full Project area.

The Project Proponent must designate one or more control areas that can be reasonably considered representative of the Removal Area. Project Proponents should consider a range of climatic, soil, environmental, topographic, agronomic and hydrological properties contained within the Removal Area when determining the representative plots. More details on how to assess representativeness can be found in Section 10.1.2.3. Project Proponents may use publicly available data sets to inform plot designation. All data and derived metrics used for designating control plots must be provided in the PDD. Where no data is available, the Project Proponent must undertake a soil survey to characterize relevant soil characteristics. Control plots based on the data provided in the PDD should capture both the central tendency and dynamic range of the listed control variables.

Some Project Proponents may choose to designate a contiguous area for the control area, however, this is not a requirement. The Project Proponent may designate two or more smaller areas that, in aggregate, represent the minimum percentage of the project area (see Equation 9) for either the control or treatment area. This might be done if field conditions do not readily allow for a single representative control and treatment areas.

In the 2-plot approach, the treatment area will consist of the remaining project area, which in many cases will be 97.5% of the total project area.

Project Proponents must report the boundaries of the 2-plot designations, including project area maps, soil pH maps and soil texture maps with clearly demarcated boundaries and the GPS coordinates for the boundaries. Where control plots are adjacent to treated plots, it is recommended that the Project Proponent designates "buffer zones" near the edges of the control plots wherein samples will not be collected due to an elevated risk of feedstock migration. Geographical details on such designations must be reported.

The criteria for designating the treatment and control in the the 3-plot approach are the same as those described for the control in the 2-plot approach. Project Proponents must report the boundaries of the 3-plot designations as described in the 2-plot approach above.

Projects exceeding 1,000 hectares are recommended to maintain research plots for the purpose of furthering scientific understanding of outstanding research question in EW projects. Research plots should include a control and treated plot, and may be located within the control, treatment or deployment plots established for Crediting. Project Proponents may choose the location and design of the research plot pursuant to specific scientific questions. It is recommended that research plots are designated such that maximum variability is captured. Project Proponents should share results from the research plot publicly.

Individual treatment and deployment fields must be split into Removal Areas according to the feedstock applied and the season where spreading occurred, as determined by climatic conditions in the region and extreme precipitation events. Control fields must be assigned to Removal Areas. Control field baseline samples must be collected no more than six months before or one month after baseline samples in the treated Removal Areas. It is strongly recommended that control baseline samples be collected no more than one month before Removal Area baseline samples. If lime was applied to some but not all fields in the two years prior to baseline sample collection, this must also be considered when allocating fields to Removal Areas. Project Proponents must document and provide justification for the definition of each Removal Area, with reference to relevant climate data and farming records.

Site conditions impacting the weathering rate can vary at the field level. Each field in a Removal Area must be characterized according to a baseline soil-geochemical fingerprint, which will be used to assess field similarity for stratification and control plot representativeness. The fingerprint should draw from the following pre-application soil properties:

• Soil pH
• Soil Inorganic Carbon (SIC) 
• Soil texture (sand %, silt %, clay %)
• Cation Exchange Capacity (CEC)
• Base saturation of cations eligible for Crediting
• Bulk soil composition, including base cation concentrations and immobile tracer concentrations, if applicable

If distinct application rates, irrigation rates or cropping systems apply to different fields, this should also be considered when constructing fingerprints.

Fingerprint similarity must be quantified using a multivariate distance metric. The Mahalanobis distance is recommended, as it accounts for correlations between fingerprint variables and differences in variable scale. The Mahalanobis distance between fields $i$ and $j$ is defined as:

$$D_M(i,j) = \sqrt{(\mathbf{x}_i - \mathbf{x}_j)^T \mathbf{S}^{-1} (\mathbf{x}_i - \mathbf{x}_j)}$$

(Equation 10)

where:

• $\mathbf{x}_i$ and $\mathbf{x}_j$ are the fingerprint vectors for fields $i$ and $j$
• $\mathbf{S}$ is the pooled sample covariance matrix estimated from all fields in the Removal Area

This distance metric requires stable estimation of the covariance matrix, which generally requires at least $n \geq 10p$ observations across all fields, where $p$ is the number of fingerprint variables. Where this condition is not met, Project Proponents should either reduce the number of fingerprint variables or justify an alternative distance metric in the PDD. The same distance metric must be used consistently for both stratification and the control plot representativeness test.

Project Proponents must document and provide justification in the PDD for: 

• the exact subset of variables used to define the fingerprint, acknowledging that the most relevant variables will vary by Removal Area and quantification approach. Variables where the observed range is unlikely to reflect meaningful differences in weathering behavior between fields should be excluded.
• how each variable is robustly scaled prior to computing the distance metric. Variables with known non-linear relationships to weathering rate should be transformed accordingly before inclusion. For example, a log transformation may be appropriate for pH.
• the distance metric selected to compute similarity and why it is appropriate given the variable types and sample size, along with any distance-based inclusion or exclusion thresholds.

To ensure that weathering is quantified over areas that are sufficiently homogeneous for signal resolvability while maintaining adequate statistical power for uncertainty quantification, Project Proponents must determine if it is appropriate to pool control, treatment and deployment fields (independently) into Units for CDR quantification using the fingerprint similarity distance metric defined above. Units may consist of the full set of fields of a given type in the Removal Area, a single contiguous field or a group of similar non-contiguous fields, depending on the sampling design and field characteristics.

A power analysis should be used to determine if a Unit has a sufficient number of samples to detect the expected weathering signal. In some cases the Project Proponent may need to collect more than the recommended number of samples due to factors such as the in-field heterogeneity observed at the end of the Reporting Period. The minimum number of samples needed to detect the expected feedstock weathering can be calculated as follows assuming normally distributed data ^{35, 36, 37, 38}:

$$N_x\  \geq \ \frac{2\sigma^{2}_{x}(z_{1-\frac{\alpha}{2}} + z_{1-\beta})^2}{{\Delta_{expected,\ x}}^{2}}$$

(Equation 11)

where:

• $N_x$ is the number of samples needed to detect feedstock weathering in Unit $x$
• $\sigma_x$ is the standard deviation in the relevant measure of alkalinity (base cation concentrations for soils, carbonic acid system parameters or cation concentrations for waters) at baseline in Unit $x$
• $z_{1 - \frac{\alpha}{2}}$ is the z-score associated with the chosen two-tailed significance level (1.96 for ɑ = 0.05)
• $z_{(1 - \beta)}$ is the z-score associated with the target statistical power (0.84 for 80% power, or $\beta$ = 0.2)
• $\Delta_{expected,\ x}$ is the estimated change in the relevant measure of alkalinity between the beginning and the end of the Reporting Period

Project Proponents are advised to use a conservative estimate of weathering rate or a safety factor above the calculated $N_x$ when using this equation to determine the number of samples needed to achieve statistical significance.

A rarefaction analysis should be conducted for each baseline variable that is an input to the bootstrap mean estimate of Unit-level CDR (see Section 10.1.4.3 on sampling uncertainty quantification below) to determine whether a Unit has sufficient statistical power for uncertainty quantification:

1. Compute the bootstrap mean of each variable using progressively larger random subsets of the available baseline samples, from a minimum of 5 samples up to the full sample set
2. Repeat each subsample size at least 200 times to generate a distribution of bootstrap means at each sample size
3. Identify the largest sample size $n^*$ (across variables) at which the width of the 95% confidence interval of the bootstrap mean changes by less than a justified percentage (5% recommended) of the overall observed range of the variable for all subsequent increments. The proposed Unit sample size should be at least $n^*$

Where the Unit sample size cannot satisfy both criteria independently, fields should be pooled until both are satisfied using the distance metric to evaluate fingerprint similarity defined above. Where pooling cannot achieve both criteria even at the Removal Area scale, the parametric fallback described in Section 10.1.4.3.2 should be used.

This approach to stratification scales with the lifetime of the project. Over the course of the Crediting Period, Project Proponents may aggregate Control, Treatment, and Deployment Units within the same Removal Area to form new Units with sufficient sample size.

Project Proponents may propose alternative, reproducible approaches for pooling fields in consultation with Isometric.

Project Proponents must verify that at least one Control Unit is sufficiently representative of each Treatment Unit using the distance metric chosen for defining fingerprint similarity.

The following are recommended approaches for determining control plot representativeness. Project Proponents may propose and justify alternative formulations.

Where a Treatment Unit consists of a single field, a Control Unit is considered sufficiently representative of a Treatment Unit if its distance from the Treatment Unit in fingerprint space, $D_{CT}$, satisfies:

$$D_{CT} \leq k \cdot \bar{D}_{within,\ T}$$

(Equation 12)

where:

• $\bar{D}_{within,\ T}$​ is the mean pairwise distance between individual baseline sample points within the Treatment field, computed using the same metric
• $k$ is a coverage factor set to 2 by default. Project Proponents may justify an alternative value of $k$ in the PDD

Where a Treatment Unit consists of multiple pooled fields, a Control Unit is considered sufficiently representative of a Treatment Unit if $D_{CT}$ satisfies:

$$D_{CT} \leq k \cdot \bar{D}_{T}$$

(Equation 13)

where $\bar{D}_{T}$​ is the mean pairwise distance between fields within the Treatment Unit computed using the same metric, and $k$ is defined as above.

Where no Control Unit satisfies the criterion for a given Treatment Unit, the Project Proponent must either designate an additional representative Control Unit, or work in consultation with Isometric to determine the most appropriate conservative approach, which may include using the nearest available Control Unit with an additional uncertainty penalty applied to the control correction distribution, ${CO}_{2}^{}e_{Counterfactual,\ Control Plot}^{}$.

$CO_2e_{Stored}$ must be computed for each Treatment Unit. A control correction must be applied to each Treatment Unit as the weighted average of ${CO}_{2}^{}e_{Counterfactual,\ Control Plot}^{}$ in each representative Control Unit with a statistically significant change in alkalinity (see Section 10.1.3), where weights are derived from fingerprint similarity and sum to unity for each Treatment Unit.

Project Proponents using the 3-plot design should match each Deployment Unit to the most similar Treatment Unit using the same distance metric and demonstrate that the matched Treatment Units are sufficiently representative. Where representativeness is demonstrated and the Treatment Unit exhibits lower variance than the Deployment Unit, the statistical precision gained from denser Treatment Unit sampling can be used to reduce the downside adjustment applied to the Deployment Unit median for Crediting, rewarding investment in more intensive monitoring where it is genuinely representative. This may be demonstrated through one of the following: 

1. at both the start and end of the Reporting Period, the inputs to weathering calculations (bulk soil composition, including base cation and applicable immobile tracer concentrations for soil-based quantification, and either major and trace element composition or two carbonic acid system parameters for aqueous-phase quantification) are not statistically significantly different between the Treatment Unit and the Deployment Unit; or
2. the distributions of $CO_2e_{stored}$ per hectare ($\rho_{CDR}$) in the Treatment Unit and the Deployment Unit are not statistically significantly different.

For this comparison, a two-sided Mann-Whitney U test should be used at a significance level of 0.05, as it is sensitive to differences in distributional shape beyond the central tendency and does not require assumptions of normality. Where the Project Proponent can demonstrate that the distributions in both the Treatment Unit and Deployment Unit are approximately normally distributed, a two-sample t-test may be used as an alternative. The statistical test used, input data, and results must be documented and reported.

Where similarity is demonstrated, the Credited $\rho_{CDR}$ for the Deployment Unit may then be calculated as:

$$\rho_{CDR,\ D,\ credit} = \rho_{CDR,\ D,\ p50} - \Delta_T$$

(Equation 14)

Where:

• $\rho_{CDR,\ D,\ p50}$ is the median $CO_2e_{stored}$ per hectare of the Deployment Unit
• $\Delta_T = \rho_{CDR,\ T,\ p50} - \rho_{CDR,\ T,\ credit}$ is the downside adjustment of the Treatment Unit
  • $\rho_{CDR,\ T,\ p50}$ is the median $CO_2e_{stored}$ per hectare of the Treatment Unit
  • $\rho_{CDR,\ T,\ credit}$ is the $CO_2e_{stored}$ per hectare at the Crediting percentile for the Treatment Unit

In this case, the Deployment Unit should inherit the control correction computed for the matched Treatment Unit. A single Treatment Unit may be matched to multiple Deployment Units and its downside adjustment ($\Delta_T$) may be applied independently to each matched Deployment Unit.

If the statistical test fails to demonstrate similarity, or if $\rho_{CDR,\ T,\ credit}$ is negative, the Treatment Unit and Deployment Unit must be treated independently for Crediting purposes i.e., the Deployment Unit must be Credited based on its own distribution of $CO_2e_{stored}$ per hectare and the same Control Unit representativeness tests apply independently.

The Project Proponent must demonstrate a statistically significant weathering signal in each Treatment and Deployment Unit. If a project is utilizing soils-based quantification methods, a statistically significant decrease in the concentration of base cations in the NFZ must be observed between the beginning and end of a Reporting Period. If a project is utilizing waters-based quantification methods, a statistically significant export of alkalinity must be observed. We recommend using a one-tailed t-test to demonstrate this statistical significance³⁶. Alternative statistical tests may be appropriate and may be justified based on characteristics of the data distribution (e.g., use Mann-Whitney U-test if data is not normally distributed). The Project Proponent may alternatively use a two-dimensional interpolation framework to estimate the spatially integrated weathering over the project area (with uncertainty explicitly quantified). In such cases, the interpolation scheme, uncertainty quantification routine and an appropriate test for statistical significance must be described in detail in the PDD.

A significance level of 0.05 must be used for all statistical tests, where the the null hypothesis that there is no change in alkalinity at the end of the Reporting Period compared to the earliest time point. The statistical test used, the input data and the result of this statistical test must be reported.

For soils-based quantification, all statistical tests must be performed on base cation concentrations (e.g., mol/kg or g/kg) or base cation concentrations converted to units of equivalents of charge per kilogram of soil (e.g., eq/kg). In cases where Project Proponents opt not to measure the relative concentrations of cations in soil immediately after deployment, the mean and uncertainty distribution of cation abundance at the start of the corresponding Reporting Period may not be known. For the purposes of demonstrating statistical significance, the Project Proponent may estimate the uncertainty of cation abundance immediately after spreading to be the same as the uncertainty observed at the end of the Reporting Period. This estimation can only be done for the first Reporting Period after feedstock is applied.

Project Proponents must also apply the same statistical test to determine if there has been a statistically significant change in alkalinity in control plots. If a project is utilizing soil-based quantification methods, this test must be applied independently to the concentrations of each base cation in each Control Unit. Where a statistically significant change is detected in any base cation-Unit pairing, ${CO}_{2}^{}e_{Counterfactual,\ Control Plot}^{}$ must be quantified and incorporated into weathering calculations (see Section 10.1.4 for more details).

Uncertainty in the calculation of $CO_2e_{stored}$, including losses, must be quantified for each Removal Area using Monte Carlo methods. Use of analytic variance propagation will not be permitted for uncertainty quantification of this term.

Uncertainty in weathering quantification originates from several distinct sources, each of which must be rigorously quantified and propagated through to the final net CDR calculation. These sources include analytical uncertainty, parameter uncertainty, sampling uncertainty, and model uncertainty. Analytical, parameter, and sampling uncertainties are propagated jointly via Monte Carlo simulation. Model uncertainty is evaluated through comparative model runs and incorporated via conservative selection or ensemble aggregation.

Analytical uncertainty is the uncertainty arising from the precision and accuracy of laboratory instruments and analytical methods, such as ICP-MS/OES for measuring cation and tracer concentrations. For each measured variable used in weathering calculations, the uncertainty must reflect the full analytical method, including sample preparation and instrument precision, and must separate random error from any identified systematic bias. Systematic bias must be corrected for, and the residual uncertainty documented.

After correction for bias, random analytical error must be characterized using the greater of: (i) the relative standard deviation of laboratory duplicate analyses, or (ii) the error derived from certified reference material (CRM) recovery. The resulting distribution must be documented and used to parameterize a mean-zero perturbation around the measured value in Monte Carlo simulations.

Parameter uncertainty is the uncertainty in any non-measured inputs used in weathering calculations, including constants, conversion factors, or parameters derived from literature values rather than direct measurement. Some examples include the inputs to water flux calculations and carbonic acid system calculations in aqueous-phase quantification (see Section 10.5).

Similar to analytical uncertainty, a plausible error distribution must be constructed, documented and justified for each input parameter to sample from in Monte Carlo simulations. In all cases, the chosen distribution must demonstrably not underestimate uncertainty. Distributions should be informed by published literature wherever possible, such as the use of an error envelope of $\pm$ 20% around the crop coefficient $K_c$ used to compute evapotranspiration losses in Equation 33. Where no suitable literature exists, Project Proponents must derive the distribution from first principles or empirical data collected as part of the project, and provide a clear justification for why the chosen range is conservative. Where parameters are correlated, Project Proponents must preserve the correlation structure in sampling (e.g., through joint sampling), or justify assuming independence.

Sampling uncertainty is the uncertainty arising from the finite number and spatial distribution of samples collected within each Unit and sampling event. Project Proponents must document and justify their approach to sampling uncertainty quantification in the PDD. Isometric may require an additional conservative uncertainty penalty in cases where sampling uncertainty cannot be reliably estimated.

Project Proponents must determine if there is significant spatial autocorrelation between observations at the field- or Unit-level. If significant spatial autocorrelation is present, this must be accounted for when quantifying sampling uncertainty.

Spatial autocorrelation should be assessed using Global Moran's I computed directly on the change in alkalinity between the baseline and the end of the Reporting Period (e.g. changes in base cation concentrations for soils-based quantification) using distance-based weights:

$$I = \frac{n}{W} \times \frac{\sum_{i=1}^{n} \sum_{j=1}^{n} w_{ij}(\Delta_i - \bar{\Delta})(\Delta_j - \bar{\Delta})}{\sum_{i=1}^{n}(\Delta_i - \bar{\Delta})^2}$$

(Equation 15)

Where:

• $\Delta_i$ is the change in alkalinity between the baseline and the end of the Reporting Period at location $i$
• $\bar{\Delta} = \frac{1}{n} \sum_{i} \Delta_i$ , where $n$ is the number of measurement locations
• $w_{ij} = \frac{1}{d_{ij}}$ , where $d_{ij}$ is the Euclidean distance between locations $i$ and $j$
• $w_{ii} = 0$ (no self-weight)
• $W = \sum_{i=1}^{n} \sum_{j=1}^{n} w_{ij}$

Weights must be row-normalized such that $\sum_j w_{ij} = 1$ for each location $i$. If different numbers of samples were collected at the baseline and the end of the Reporting Period, Project Proponents should use the locations from the sampling event with the lower sample count to compute $I$. $\Delta_i$ should be computed using the values at these locations for that sampling event and the values from the closest location (calculated using the Euclidean distance) for the sampling event with the higher sample count.