Biomass Storage in Subsurface Mines

1.0 Introduction

This Module (Independent components of Isometric Certified Protocols which are transferable between and applicable to different Protocols.) outlines the requirements for the storage (Describes the addition of carbon dioxide removed from the atmosphere to a reservoir, which serves as its ultimate destination. This is also referred to as “sequestration”.) of biomass in subsurface mining operations for the purpose of carbon dioxide removal (The term used to represent the CO₂ taken out of the atmosphere as a result of a CDR process.) (CDR). A net decrease in atmospheric CO₂ will occur when biomass is processed and stored in a way to substantially slow or arrest microbial degradation and prevent decay products migrating to the atmosphere. When these carbon cycle interventions are combined with engineered controls and favorable site characteristics to limit disturbance, this stored carbon may persist for long periods of time.

While this Module provides specific requirements on how biomass can be stored to significantly delay the degradation of biomass and contain decay products, it is noted that adherence to the storage provisions outlined in this Module does not guarantee durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) of carbon storage. Given the significant diversity in the chemical characteristics of biomass, biomass processing prior to storage, storage environments, and other site-specific considerations, the durability claims associated with removals utilizing this Module must be evidenced with direct, time-series evidence and a detailed, quantitative description of how those storage conditions will be maintained throughout the Project (An activity or process or group of activities or processes that alter the condition of a Baseline and leads to Removals or Reductions.) lifetime. Additionally, this Module describes the requirements of monitoring and reporting of fugitive gasses. Any biomass processing and carbon quantification must be conducted in accordance with other Isometric Protocols (A document that describes how to quantitatively assess the net amount of CO₂ removed by a process. To Isometric, a Protocol is specific to a Project Proponent's process and comprised of Modules representing the Carbon Fluxes involved in the CDR process. A Protocol measures the full carbon impact of a process against the Baseline of it not occurring.) and Modules.

2.0 Structure and Future Versions

Biomass is inherently labile with respect to degradation by fungi, microorganisms and other biogeochemical redox reactions. While there are many biomass processing technologies (e.g., desiccation, biochar, bio-oil (A mixture of water, organic acids, aldehydes, ketones, sugars, phenols, and other organic compounds derived from the thermal breakdown of biomass. Thermal breakdown of biomass is achieved via thermochemical processes, such as pyrolysis, which heat biomass in low- or no-oxygen environments to high temperatures (~e.g. 350-650°C). Bio-oil is often also referred to as pyrolysis oil or bio-crude.)) and storage options (e.g., burial, injection, sinking) present in the market today, scientific literature has not yet reached consensus on the carbon storage durability of different processing and storage techniques.

Given this current state of understanding of subsurface biomass burial for CDR, Isometric takes a restrictive approach to Project eligibility. Specifically, Isometric will assess the current state of scientific understanding and consensus on subsurface biomass burial technologies on an individual Project basis. Once such technologies have demonstrably met a reasonable burden of scientific evidence, they will be added to the Eligible Biomass Processing and Storage Options listed below as the Module is updated. Isometric will update this list as frequently as necessary, guided by scientific research and market learnings.

3.0 Eligible Biomass Sourcing and Associated Storage Options

3.1 Biomass Sourcing

All Projects must have biomass feedstocks (Raw material which is used for CO₂ Removal or GHG Reduction.) that meet all the applicable eligibility requirements outlined in the Isometric Biomass Feedstock Accounting Module including sustainability, market leakage (The increase in GHG emissions outside the geographic or temporal boundary of a project that results from that project's activities.), and counterfactual (An assessment of what would have happened in the absence of a particular intervention – i.e., assuming the Baseline scenario.) storage criteria and quantification.

3.2 Mine Requirements

The eligibility of subsurface mining operations for subsurface biomass storage will be assessed by Isometric on a Project by Project basis.

3.2.1 Applicable Mining Operation Types

Operational Subsurface Mines

• Active underground mining operations where biomass storage can be integrated into mined-out areas without interfering with ongoing mineral extraction.
• Operations must demonstrate that biomass storage does not extend the economic life of mine (LOM) or incentivize continued/increased mining operations.
• Storage of feedstocks must be in areas permanently undisturbed by future mining activities (The steps of a Project Proponent’s Removal or Reduction process that result in carbon fluxes. The carbon flux associated with an activity is a component of the Project Proponent’s Protocol.).

Closed or Closing Subsurface Mines

• Closed underground mines that are currently in care and maintenance status.
• Mines in the process of completing closure plans but not yet fully remediated.
• A closed mining operation where infrastructure remains accessible and in serviceable condition.

The following mining operation types will not be considered as applicable biomass storage Project locations under this Module:

Mining Operations Developed for Carbon Storage

• Operational or planned subsurface mining operations that have been opened or planned specifically for the purpose of carbon storage.

Mining Operations with Life-of-Mine Extensions

• Operational subsurface mining operations that have extended or are likely to extend the LOM as a direct result of carbon removal activities.

Note: Where a mining operator has extended the LOM for reasons not related to carbon removal activities, a signed affidavit may be provided by the operator as evidence.

Fully Closed and Remediated Mines

• Closed mining operations, including surface and subsurface, that have completed closure plans and have been remediated (in line with The Project closure plan).

Surface Mining Operations

• Open pit surface mining operations of any type.
• Operational mines with active groundwater flow through storage areas that cannot be diverted or adequately managed.

4.0 Monitoring Requirement Risk Categories

The permanence of CO₂ storage through biomass burial in subsurface mines depends on preventing or minimizing potential leaks out of the storage area which could lead to stored CO₂ could be re-emitted to the atmosphere or mobilized to other subsurface environments. Within this Module ‘storage area’ is defined as the chamber or sealed area within which biomass is placed for the purpose of permanent burial, as well as the surrounding area of expected diffusion. In this context, leaks refers to any release of GHGs (Those gaseous constituents of the atmosphere, both natural and anthropogenic (human-caused), that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect, whereby heat is trapped in Earth’s atmosphere (CDR Primer, 2022).) that originate from the stored biomass, including CO₂ and methane generated by anaerobic decomposition of that biomass after burial and storage.

This section details the potential risks and leak pathways (A collection of Removal or Reduction processes that have mechanisms in common.) in subsurface biomass storage Projects. Leak pathways represent routes through which CO₂ stored in biomass could be lost from storage reservoirs (A location where carbon is stored. This can be via physical barriers (such as geological formations) or through partitioning based on chemical or biological processes (such as mineralization or photosynthesis).), through gaseous emissions (The term used to describe greenhouse gas emissions to the atmosphere as a result of Project activities.) (e.g., CO₂, CH₄), aqueous transport (dissolved organic/inorganic carbon (DOC (A carbon removal pathway that captures and durably stores carbon from seawater, which induces additional uptake of atmospheric carbon dioxide in the ocean.)/DIC (The concentration of inorganic carbon dissolved in a fluid.)) or particulate organic carbon (POC) and/or physical displacement (subsidence, seismic activities) ¹.

Potential risks to expected durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) are site specific, but may include:

Risk A: Gas phase formation and migration out of the storage area

Subsurface environments host diverse microbial communities capable of degrading organic matter under both aerobic and anaerobic conditions. If residual oxygen is present within the storage chamber at the time of closure, aerobic heterotrophic microorganisms will oxidize organic carbon to CO₂, consuming oxygen until anoxic conditions are established. Once oxygen is depleted, anaerobic metabolic pathways become dominant, including fermentation, sulfate reduction, iron reduction, and methanogenesis. Even under anoxic conditions designed to minimize decomposition, some fraction of stored biomass may undergo anaerobic degradation, producing GHGs ². The gas phase formed by biomass degradation may leak from the storage area and provide the most direct pathway for GHGs release to the atmosphere. This is especially important for coal bearing mines that have natural sources (Any process or activity that releases a greenhouse gas, an aerosol, or a precursor of a greenhouse gas into the atmosphere.) of CH₄ emissions. Such GHGs migration occurring via:

• Migration through anthropogenic infrastructure (such as mine shafts or boreholes) - Storage areas (chambers) in subsurface mines are typically sealed using engineered barrier systems designed to prevent gas migration through mine infrastructure and boreholes. These barriers may include: mud slurries, bulkheads (concrete (A composite material composed of aggregate, cement, sand and water that cures to a solid over time.) walls), rock seals, and combinations of the three. However, horizontal gas migration may still occur depending on design and material quality ³. For example, the effectiveness of mud slurries as a barrier can be comprised by consolidation, shrinkage, thickness, gaps and cracks, exposure to mine ventilation and erosion. For concrete walls leaks may occur when there is insufficient bonding to host rocks, degradation from sulfate attack (particularly in coal mines) or other geochemical processes, as well as structural failure if inadequately designed. Abandoned boreholes from exploration, water wells, or mining operations may provide direct vertical pathways if not properly sealed. These anthropogenic features are not always fully documented in historical records, requiring careful investigation during site characterization.
• Migration along geological features - Gases may also migrate upward through the overlying rock mass via natural geological discontinuities. These pathways include geological faults, fracture zones, soluble bedrocks (e.g., karst topography). Geological faults represent zones of crustal weakness where rock displacement has occurred. Active or recently active faults may create high-permeability conduits for gas migration. The potential for migration through faults depends on factors such as fault activity, orientation, zone characteristics, and hydrological connectivity. Fracture zones are rock masses containing networks of smaller-scale discontinuities (joints, bedding planes, cleavage) that can collectively provide migration pathways. The connectivity and permeability of these networks are influenced by rock type, burial depth, stress history, and the degree of mineralization or cementation. Soluble bedrocks (such as karst topography found in limestone, dolomite, or evaporites) are susceptible to dissolution, creating voids, caves, and preferential flow paths. While less common in typical mining lithologies, these features must be evaluated when carbonate or evaporite units are present in the overlying stratigraphy.

Risk B: Injected biomass interacts with fluids, rock, and minerals within and surrounding the storage area, especially dissolution/migration with an aqueous phase

CO₂ storage through biomass burial in subsurface mines can be mobilized through aqueous pathways in the form of dissolved organic carbon (DOC) (The concentration of organic carbon dissolved in a fluid.), dissolved inorganic carbon (The concentration of inorganic carbon dissolved in a fluid.) (DIC), or particulate organic carbon (POC) and dissolved gases (CH₄), this can then migrate out of the storage area. Water may intrude into the storage chamber as a result of:

• Groundwater infiltration - Changes in subsurface conditions could result in groundwater flow through or around storage chambers which could mobilize carbon, where water contacts with biomass, rocks and minerals can leach soluble organic and inorganic compounds, transporting stored CO₂. If substantial groundwater flow occurs within the storage area, this could represent a significant long-term leak mechanism. Additionally, groundwater flow could transport dissolved oxygen, electron acceptors (e.g., sulfate, nitrate, ferric iron), and nutrients into storage chambers, accelerating aerobic or anaerobic decomposition processes. Finally, if there is high groundwater fluxes, it could physically erode, suspend, or transport particulate organic matter, further promoting leaks.
• Surface water intrusion through mine infrastructure during flood events - Surface water intrusion through mine infrastructure could introduce large volumes of oxygenated water, suspended sediment, and nutrients, creating conditions conducive to rapid aerobic decomposition. Additionally, surface water can physically mobilize and transport biomass particles. Projects utilizing this Module are unlikely to be impacted by surface water intrusion, due to the implementation of leak/seepage safeguards and minimum depth requirements. It is the expectation that engineered sealing barriers employed between storage chambers will be designed to restrict/block surface water intrusion.
• Leachate generation and water-rock-biomass geochemical interactions - Biomass contains soluble organic compounds (sugars, phenolics, tannins, organic acids) that are rapidly mobilized upon contact with water, generating leachate with elevated DOC concentrations and reduced pH. Acidic leachate can promote dissolution of carbonate minerals (calcite, dolomite) common in mine host rocks, releasing Ca²⁺, Mg²⁺, and DIC into solution ⁴. This can alter local pH and carbonate equilibria, potentially triggering CO₂ degassing if the system becomes oversaturated. Conversely, in some mineral assemblages, dissolution of silicate minerals under acidic conditions may promote secondary carbonate or clay precipitation, potentially altering porosity and permeability. Organic ligands in leachate can also complex with dissolved metals (Fe, Mn, Al), influencing their mobility and potentially precipitating metal-organic phases that alter pore structure ⁵.

To avoid monitoring of risk associated with aqueous phase, the Project Proponent must demonstrate that no water will infiltrate the storage area. This demonstration must be supported by site-specific hydrogeological evidences confirming the absence of groundwater flow and surface water infiltration pathways at the operating depth of the Project. This evidence must be provided in the Project Design Document (PDD) and must include, at minimum:

• Hydrogeological site characterization data (e.g., borehole logs, hydraulic conductivity measurements, groundwater level monitoring records).
• A site-specific groundwater flow assessment, and documentation of engineered sealing barrier design and integrity.

Note: Where applicable, the Project Proponent must also demonstrate that the storage depth is sufficient to preclude surface water ingress under extreme precipitation or flood scenarios.

Risk C: Physical disturbance, such as structural failures and ground movement caused by seismic activity within the storage area, could result in leaks and the release of stored carbon

• Seismic activity and ground displacement - The likelihood and severity of seismic impacts depends on regional seismicity, site-specific ground motion amplification, and the design robustness of engineered systems. In seismically active areas, earthquakes and associated ground displacement can compromise storage integrity through several mechanisms: cracking or displacement of engineered barriers (e.g., concrete bulkheads) due to seismic shaking, creating new pathways for gas and/or water migration; fault activation, generating new fracture permeability and vertical migration pathways; tunnel or chamber collapse in mine workings, potentially disrupting containment systems or exposing biomass to the atmosphere; alteration of groundwater flow regimes due to changes in fracture connectivity, potentially increasing water infiltration into storage chambers; and, in extreme cases, surface-rupturing earthquakes creating direct connections between the storage depth and the surface.
• Mine subsidence and structural failure - As biomass decomposes, it undergoes volume reduction, which could cause settling, compaction, or collapse of biomass within storage chambers. This physical change may alter the distribution of void space, affect the mechanical loading on roof spans and pillars, and create preferential flow pathways for gas or water. In extreme cases, significant volume loss could contribute to localized subsidence. Additionally, if decomposition products (e.g., biofilms, precipitates) accumulate on fracture walls or engineered barriers, they could alter permeability in ways that are difficult to predict. Subsurface mines can also experience long-term subsidence as rock pillars yield or roof spans collapse. Relevant subsidence mechanisms that could result in leaks include: tensile fracturing extending from mine depth toward the surface, creating gas migration pathways; compression or shearing of storage chambers, which may breach seals or create new openings; and alteration of surface drainage patterns and groundwater flow, potentially directing water toward storage areas.

Note: Requirements that align with the specific risk categories (Risk Categories A, B and C) outlined within this Section can be identified within squared brackets throughout this Module.

5.0 Evidence of Biomass Durability

• This must include a microbial inoculum representative of the storage site. Including a microbial inoculum during testing provides a conservative (Purposefully erring on the side of caution under conditions of Uncertainty by choosing input parameter values that will result in a lower net CO₂ Removal or GHG Reduction than if using the median input values. This is done to increase the likelihood that a given Removal or Reduction calculation is an underestimation rather than an overestimation.) scenario forecast of expected durability.

5.1 Biomass Placement

5.2 Engineered Barriers

Documentation must include the following:

Project Proponents must submit the required evidence of biomass durability to the Isometric and the VVB for review and approval prior to crediting.

6.0 Permit and Site Characterization

6.1 Permitting

6.2 Site Characterization

6.2.1 Storage Depth

The rationale for this requirement is based on considering the zone of active surface weathering and soil formation (approximately <10m) ⁶_,⁷, seasonal temperature and moisture fluctuations (approximately <50m)⁸_,⁹, reducing the risks to durability of stored carbon [A,B,C]. Below this 150m there is also minimal risk of impact from potential surface disturbances and land use activities [C]. The minimum depth requirement is established to ensure:

• Geological Isolation - where there is sufficient overburden to provide multiple barriers between stored biomass and surface disturbances, such as surface water infiltration through mine infrastructure, isolation from seasonal temperature fluctuations and buffer from surface land use changes. In addition, isolation provides an additional protection against potential migration out of the targeted storage reservoir [A,B] ¹⁰.
• Atmospheric Stability - at >150m depth, natural ventilation exchange with surface is minimized, temperature and humidity conditions are more stable, less susceptible to seasonal atmospheric variations and reduced the risk of oxygen ingress from surface.
• Physical Protection - deep storage provides enhanced protection from surface erosion and mass wasting, agricultural or construction activities, wildfire or other surface disturbances and human excavation, unauthorized access or disturbance [C].

Note: Isometric may accept Projects with a minimum depth of <150m, where a Project Proponent provides justification for a reduction in minimum depth. Reductions in minimum depth requirements will be assessed on a case by case basis by Isometric, in consultation with The Project VVB.

6.2.2 Storage Area

6.2.2.1 Mine Geometry, Geological Configuration and Ventilation Conditions

Mine Geometry

Mine Geological Configuration

Mine Ventilation Configuration

6.2.2.2 Mine Infrastructure, History, Hydrological, Geomicrobiological, and Thermal Environment

Mine Infrastructure

Mine History

Hydrological Characterization

Such assessments should include:

• 1,000-year storm modeling for the Project location.
• An assessment of potential surface-to-subsurface hydrologic connections through fractures, old workings, or subsidence features.
• A monitoring plan for water accumulation in storage chambers and implement immediate pumping/drainage if water exceeds certain time residence criteria.

The following are recommended for all Projects when undertaking site characterization studies:

Geomicrobiological Characterization

Thermal Environment Characterization

6.2.2.3 Gas Transport

Diffusivity biomass and expected migration distances must be calculated in line with Section 6.2.2.3.1 and used to define the Project storage area boundary.

6.2.2.3.1 Gas Diffusion Calculations

To allow such quantification, Project Proponents must:

If appropriate, include biogeochemical consumption/mineralization terms that may further limit migration. The relationship between time and distance in a diffusive system is described by the characteristic diffusion timescale:

[math: t = \frac{L^2}{D_{\mathrm{eff}}}]

(Equation 1)

Where:

• L = The length of the diffusion path (meters). In the context of containment, this represents the thickness of the confining layer.
• [math: {D_{\mathrm{eff}}}] = The effective diffusivity of the material (m²/s)
• t = The characteristic time (seconds) required for gas to migrate across distance (L).

To determine the potential migration distance of gas over a specific Project lifespan, this formula can be rearranged to solve for the length of the diffusion path (i.e., confining layer thickness) (L):

[math: L = \sqrt{t \cdot D_{\mathrm{eff}}}]

(Equation 2)

Using the relationship in Equation 2, the time (t) can be set to 1000 years (converted to seconds), and [math: {D_{\mathrm{eff}}}] varies based on the grain size and material properties. This allows the calculation of the distance GHGs produced from biomass will diffuse during the lifespan of The Project, and determine whether GHGs are expected to reach the surface ¹⁹.

To calculate amount of instantaneous diffusive flux, Fick’s laws of diffusion can be used:

[math: J = -D_{\mathrm{eff}} \cdot \frac{dC}{dx}]

(Equation 3)

Where:

• J = flux (mol m^-2 s^-1).
• [math: {D_{\mathrm{eff}}}] is the effective diffusion coefficient in the medium m² s^-1.
  • Note: Where there are multiple mediums, such as slurry and rock, the calculation must be carried out for each medium.
• [math: \frac{dC}{dx}] is the concentration gradient mol (m^-3 · m^-1).
  • Note: The minus sign means diffusion goes from high concentration to low concentration.

This allows the estimation of diffusive flux out of the storage area per unit area, either as a conservative (Purposefully erring on the side of caution under conditions of Uncertainty by choosing input parameter values that will result in a lower net CO₂ Removal or GHG Reduction than if using the median input values. This is done to increase the likelihood that a given Removal or Reduction calculation is an underestimation rather than an overestimation.) maximum (J_max) or as a time dependent flux J(t). By further multiplying this value with the disposal footprint area A (i.e., vertical area of the containment layer) (m²), the calculation allows the estimation of the mass transfer rate (mol s^-1) across the containment layer. As part of the modelling exercise, the transient diffusion should be modeled with appropriate boundary conditions and reporting time dependent flux J(t), including the peak flux J_max reporting and cumulative mass transferred over the assessment period ²⁰.

6.2.2.4 Geotechnical and Seismic Hazards Assessment

Geotechnical Characterization

Project Proponents must conduct and provide a geotechnical evaluation of long-term mine stability, including:

Where Project Proponents can provide evidence from a partnered mining operator that satisfies the above requirements, the Project Proponent will not be required to undertake additional geotechnical evaluations. In such instances, the Project Proponent shall provide evidence and documentation that the mining activities and storage chambers are in compliance and certified by the relevant engineering methodologies and regulations.

Seismic Hazards Assessment

7.0 Monitoring Requirements

Note: Requirements that align with the specific risk categories (Risk Categories A, B and C) outlined within Section 4 can be identified within squared brackets.

Subsurface biomass burial is a nascent storage technology for carbon dioxide removal, therefore addressing potential risks to durability is important for ensuring robust quantification and monitoring of CO₂ removals.

Monitoring frequency must be defined based on monitoring phases, monitoring frequency must be the highest during the post-closure phase where chambers are sealed and anoxic environments are established.

7.1 Operational Monitoring

7.1.1 Monitoring of the Emplaced Biomass Slurry Mixture

7.1.2 Storage Chamber Monitoring

• Placement of gas probes in mine airways between storage chambers and mine entries.
• Monitoring of adjacent chambers or pre-existing boreholes for elevated CO₂/CH₄.

7.1.3 Engineered Backfill

Where the project activities necessitate excavations that would not have occurred in the counterfactual scenario, Project Proponents must account for any reductions in soil organic carbon due to these excavations. This must be accounted for by measuring the drop in concentration of soil organic carbon compared with a representative control site. Increases in soil organic carbon are not considered creditable removals under this Module.

7.1.4 Seismic Monitoring

As specified in the Section 6.2, for Projects that take place within active seismic zones, Project Proponents must perform a site-specific study of regional seismicity to establish a baseline of seismic hazards [C] ²².

• The monitoring system should include deeper wireline or cemented subsurface geophones for microseismic monitoring. This should ideally be combined with at/near ground level stations as part of an integrated detection strategy.
• The objective of the monitoring is to determine the presence or absence of: Induced micro-seismic activity associated with facility wells (if applicable); Activity near subsurface discontinuities, faults, or fractures; Any seismic activity within the Area of Review (AOR) (The area surrounding an injection well described according to the criteria set forth in the U.S. Code of Federal Regulations § 40 CFR.146.06, which, in some cases, such as Class II wells, the project area plus a circumscribing area the width of which is either 1⁄4 of a mile or a number calculated according to the criteria set forth in § 146.06.) of the facility and the storage reservoir of Magnitude 2.7 or greater.

7.1.5 Groundwater Monitoring

7.1.6 Seal Monitoring

These barriers must ensure there are no leaks through the barriers over relevant timescales (1,000+ years) under site representative conditions and characteristics (e.g., expected pressure gradients, hydrostatic head, geochemical exposure, and mechanical stress).

7.2 Post Emplacement Monitoring

• Detection and measurement of greenhouse gasses, including CO₂, CH₄, N₂O, in any open chambers or at the sealing layer/barrier of the last accessible chamber.
  • Note: Where the monitoring of N₂O is not suitable for a specific feedstock, the Project Proponent must provide justification of why the monitoring of N₂O is not relevant. Such omissions will be assessed on a Project by Project basis by Isometric.
  • Projects may utilize tracing techniques (e.g., isotope tracers) in order to quantify how much of the gas sampled is the result of biomass decomposition or mining activities. If no such methods are used, all increases in greenhouse gas will be considered reversals. Detection of reversals must be immediately reported to Isometric and will be handled according to Isometric’s reversal policy (See the Isometric Standard for more details).
• Pressure and airflow outside of storage chambers.
• Physical conditions of the site, including regular inspections of confining materials (e.g., synthetic liners).
• Other site-specific parameters mutually agreed upon by the Project Proponent and Isometric such as:
  • Temperature and humidity.
  • Gaseous control measurements (e.g., background CH₄ production) using representative control sites/chambers.
  • Crack/strain gauges.
• Oxygen concentration.

8.0 Site Closure

• An estimate of the maximum amount of possible hazardous additives (if used) kept on site during the facility’s operating life.
• A detailed description of closure methods.
• A description of any other required steps, such as groundwater monitoring and leachate management.
• A schedule of closure activities, including closure dates for each unit and the entire facility.
• A description of how the management of each hazardous additive (if used) will be performed.

To limit long term environmental impacts from storage facilities, Project partner operators (where relevant) must formalize closure plans in accordance with local regulations and standards.

8.1 Future Human Activities

Note: The suitability of an operator affidavit in lieu of a distinct assessment will be assessed on a Project by Project basis by Isometric and the Project VVB (Third-party auditing organizations that are experts in their sector and used to determine if a project conforms to the rules, regulations, and standards set out by a governing body. A VVB must be approved by Isometric prior to conducting validation and verification.).

9.0 Risk of Reversal

Projects utilizing this storage Module must assess the Risk of Reversal according to the Isometric Standard Risk Assessment Questionnaire in consultation with Isometric. The Risk of Reversal may vary due to Project specific considerations. The Risk of Reversal will be reassessed every 5 years, aligning with the Crediting Period, or when new scientific research and knowledge are produced that highlights significant risks to the durability of the stored carbon.

When quantifying reversals, the Project Proponent must use a 100-year time horizon when selecting GWP. For this Module, GWP100 values provided in the IPCC 6th Assessment Report ²⁴ (for CH₄, N₂O, and other relevant non-CO₂ gasses must be used, or most recent updates. For methane, biogenic and non-biogenic GWP differentiation can be applied, where appropriate.

10.0 Calculation of CO₂e_Storage

[math: CO_{2}e_{Storage}] represents the amount of CO₂ (stored as organic carbon, C) that is buried and stored in the mine. This is the gross amount stored for each batch burial and does not account for reversals of storage from the storage formation.

Equations and storage calculation requirements for [math: CO_{2}e_{Storage}], including considerations for monitoring activities, are set out in Section 7.3.3 of the Subsurface Biomass Carbon Removal and Storage Protocol and are not repeated in this Module.

11.0 Extractive Industry Specific Safeguards

The Project Proponent is responsible for collecting and submitting Project data, including data generated by a partner operator, where a Project is undertaken within an active extractive operation or quarry. Such data may be used for both environmental monitoring, site characterization and carbon removal quantification, as long as the data is judged as suitable for such purposes by Isometric and the Project VVB. Where data submitted has been generated, or collected, by a partner operator, the Project Proponent must submit information related to uncertainty analysis, as well as any standard operating procedures (SOP) used by the partner operator.

Note: It is the sole responsibility of the Project Proponent to retrieve, manage and submit all data and information required to meet the requirements of this Module and relevant Protocols, even in instances where data has been generated by a partner operator.

12.0 Legal Framework to Ensure Permanence

The Project Proponent must demonstrate that the restrictive agreement provides legal protection against biomass disturbance for either 1,000 years or a restriction that is enforceable in perpetuity. The purpose of such a legal mechanism is to prevent excavation of and/or the interference with the storage facility for the entirety of the Project lifetime, at a minimum.

Table 1 Land Durability Requirements

13.0 Record Keeping

14.0 Land Security and Removal Durability

There is no single credible mechanism that can ensure, without uncertainty, that biomass buried in the subsurface will remain undisturbed in perpetuity given the relative nascency of such legal mechanisms relative to the time horizons required in the Isometric Standard. Land durability claims are subject to social and political factors and are thus different in nature from claims regarding physical or geologic durability. Isometric has developed a set of land security eligibility criteria that align with current best practices for legal strategies to restrict future uses of land. ThisThe Protocol and this Module also considers the risk associated with land ownership as a risk factor in determining the Risk of Reversal and corresponding buffer pool.

Where the Project Proponent has partnered with a mining operator, for the purpose of biomass storage, evidence is required to demonstrate intended land use following Project cessation and subsequent mine closure. Such evidence may take the form of the operations closure plan or any relevant permitting documents. Where closure plans and remediation schemes have been updated, or amended, to take into consideration the storage of biomass within the mines subsurface, such documents must be submitted to Isometric and the Project VVB for review.

15.0 Acknowledgements

Isometric would like to thank following contributors to this Module:

• Rebecca Tyne, Ph.D.
• Nicholas Ashmore, Ph.D.
• Josh Steinberg (Rewind Earth)
• Nitai Amiel (Rewind Earth)
• Eitan Brettler (Rewind Earth)

Isometric would like to thank the following reviewers of this Module:

• Hannah Murnen (Graphyte)

16.0 Definitions and Acronyms

Independent components of Isometric Certified Protocols which are transferable between and applicable to different Protocols.

Describes the addition of carbon dioxide removed from the atmosphere to a reservoir, which serves as its ultimate destination. This is also referred to as “sequestration”.

The term used to represent the CO₂ taken out of the atmosphere as a result of a CDR process.

The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.

An activity or process or group of activities or processes that alter the condition of a Baseline and leads to Removals or Reductions.

A document that describes how to quantitatively assess the net amount of CO₂ removed by a process. To Isometric, a Protocol is specific to a Project Proponent's process and comprised of Modules representing the Carbon Fluxes involved in the CDR process. A Protocol measures the full carbon impact of a process against the Baseline of it not occurring.

A mixture of water, organic acids, aldehydes, ketones, sugars, phenols, and other organic compounds derived from the thermal breakdown of biomass. Thermal breakdown of biomass is achieved via thermochemical processes, such as pyrolysis, which heat biomass in low- or no-oxygen environments to high temperatures (~e.g. 350-650°C). Bio-oil is often also referred to as pyrolysis oil or bio-crude.

Raw material which is used for CO₂ Removal or GHG Reduction.

The increase in GHG emissions outside the geographic or temporal boundary of a project that results from that project's activities.

An assessment of what would have happened in the absence of a particular intervention – i.e., assuming the Baseline scenario.

The steps of a Project Proponent’s Removal or Reduction process that result in carbon fluxes. The carbon flux associated with an activity is a component of the Project Proponent’s Protocol.

Those gaseous constituents of the atmosphere, both natural and anthropogenic (human-caused), that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect, whereby heat is trapped in Earth’s atmosphere (CDR Primer, 2022).

A collection of Removal or Reduction processes that have mechanisms in common.

A location where carbon is stored. This can be via physical barriers (such as geological formations) or through partitioning based on chemical or biological processes (such as mineralization or photosynthesis).

The term used to describe greenhouse gas emissions to the atmosphere as a result of Project activities.

A carbon removal pathway that captures and durably stores carbon from seawater, which induces additional uptake of atmospheric carbon dioxide in the ocean.

The concentration of inorganic carbon dissolved in a fluid.

Any process or activity that releases a greenhouse gas, an aerosol, or a precursor of a greenhouse gas into the atmosphere.

A composite material composed of aggregate, cement, sand and water that cures to a solid over time.

The concentration of organic carbon dissolved in a fluid.

A standards organization that develops and publishes voluntary consensus international standards.

Purposefully erring on the side of caution under conditions of Uncertainty by choosing input parameter values that will result in a lower net CO₂ Removal or GHG Reduction than if using the median input values. This is done to increase the likelihood that a given Removal or Reduction calculation is an underestimation rather than an overestimation.

A worldwide federation (NGO) of national standards bodies from more than 160 countries, one from each member country.

The organization that develops and/or has overall legal ownership or control of a Removal or Reduction Project.

A process for evaluating and confirming the net Removals and Reductions for a Project, using data and information collected from the Project and assessing conformity with the criteria set forth in the Isometric Standard and the Protocol by which it is governed. Verification must be completed by an Isometric approved third-party (VVB).

A conservative adjustment applied to estimated greenhouse gas (GHG) emission reductions or carbon removals to account for uncertainties, risks, or variability in measurement, permanence, or effectiveness of the credited activity. It reduces the amount of carbon credits issued to ensure environmental integrity and avoid over-crediting.

A set of data describing pre-intervention or control conditions to be used as a reference scenario for comparison.

The period of time over which a Project Design Document is valid, and over which Removals or Reductions may be Verified, resulting in Issued Credits.

A lack of knowledge of the exact amount of CO₂ removed by a particular process, Uncertainty may be quantified using probability distributions, confidence intervals, or variance estimates.

A common and recognized insurance mechanism among Registries allowing Credits to be set aside (in this case by Isometric) to compensate for Reversals which may occur in the future.

Underground Injection Control

The area surrounding an injection well described according to the criteria set forth in the U.S. Code of Federal Regulations § 40 CFR.146.06, which, in some cases, such as Class II wells, the project area plus a circumscribing area the width of which is either 1⁄4 of a mile or a number calculated according to the criteria set forth in § 146.06.

for open systems, biogeochemical and/or physical interactions which occur during the removal process that decrease the CO₂ removal .

Third-party auditing organizations that are experts in their sector and used to determine if a project conforms to the rules, regulations, and standards set out by a governing body. A VVB must be approved by Isometric prior to conducting validation and verification.

The escape of CO₂ to the atmosphere after it has been stored, and after a Credit has been Issued. A Reversal is classified as avoidable if a Project Proponent has influence or control over it and it likely could have been averted through application of reasonable risk mitigation measures. Any other Reversals will be classified as unavoidable.

The document that clearly outlines how a Project will generate rigorously quantifiable Additional high-quality Removals or Reductions.

Activities that remove carbon dioxide (CO₂) from the atmosphere and store it in products or geological, terrestrial, and oceanic Reservoirs. CDR includes the enhancement of biological or geochemical sinks and direct air capture (DAC) and storage, but excludes natural CO₂ uptake not directly caused by human intervention.

A systematic and independent process for evaluating the reasonableness of the assumptions, limitations and methods that support a Project and assessing whether the Project conforms to the criteria set forth in the Isometric Standard and the Protocol by which the Project is governed. Validation must be completed by an Isometric approved third-party (VVB).

A publicly visible uniquely identifiable Credit Certificate Issued by a Registry that gives the owner of the Credit the right to account for one net metric tonne of Verified CO₂e Removal or Reduction. In the case of this Standard, the net tonne of CO₂e Removal or Reduction comes from a Project Validated against a Certified Protocol.

17.0 References