Direct Ocean Capture and Storage

8.3.3

Transportation Emissions Accounting

The GHG Accounting Module v1.0 provides requirements on transportation emissions must be calculated for The Project so that they can be subtracted in the net CO2e removal calculation.

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

• transportation of consumables to the DOCS location
• transportation and shipping related to collection and analysis of samples for environmental monitoring
• transportation of compressed gaseous or liquid CO₂ or CO₂-containing injectant (such as a carbonate slurry) or carbonated minerals, via freight transportation services, such as rail, truck, or maritime transport
• transportation of samples for lab analysis

9.0 CO₂ Storage

9.1 Durability and Reversal Risks

There are two storage reservoirs for DOCS projects: (1) CO₂ that is extracted from seawater must be stored in a durable reservoir, and (2) CO₂ removed from the atmosphere through air-sea equilibration is durably stored in the ocean as dissolved inorganic carbon (DIC). Only the storage in reservoir (2) is credited, but storage in reservoir (1) is a prerequisite for the Project to be net-negative and allow for credit issuance.

For example, if a project removes 10t CO₂ from the ocean and stores it in a geological reservoir (1), and after air-sea equilibration the ocean (2) absorbs 9t CO₂, then Credits would be issued based on the 9t CO₂ removed from the atmosphere. However if the 10t that was removed from the ocean and stored in geological reservoir (1) ends up being released to the atmosphere after a few years, the net effect of the Project is a 1t emission of CO₂ (10t emitted and 9t removed through air-sea equilibration). Thus, the durability of DOCS removals depends heavily on the durability of both storage reservoirs. The durability for DOCS Credits will be based on the storage reservoir with the shortest durability.

9.1.1 Storage of CO₂ removed from seawater

CO₂ removed from seawater must be stored in a high durability (> 1,000 year) storage reservoir. This Protocol provides multiple options for durable storage of CO₂. The Project Proponent can choose from available options when submitting their Project for validation:

The above storage Modules include requirements on permitting, monitoring, risk of reversal and calculation of GHG emissions related to monitoring of the storage reservoir.

9.1.2 Storage of CO₂ removed from atmosphere

The long term storage reservoir of ocean CO₂ uptake is as Dissolved Inorganic Carbon (DIC) in the ocean. The durability and reversal risks of this storage reservoir are discussed in the following Module:

9.2 Buffer Pools

As outlined in Section 2.5.9 of the Isometric Standard, the Buffer Pool is a mechanism used to insure against Reversal risks associated with a storage reservoir. Each storage reservoir that is used by a DOCS project must have its own separate buffer pool, so that the total buffer pool for a project is the sum of all the buffer pools. For example, in the case of a hypothetical project:

• The CO2 removed from seawater is stored in a saline aquifer, which has a 2% buffer pool
• Storage of DIC as a result of DOCS does not require an additional buffer pool contribution because the Project is only restoring DIC after removing it, so therefore no additional carbon storage relative to baseline is occurring that would warrant an additional buffer pool contribution
• Therefore, the total buffer pool for the DOCS project is 2% of the final Credits issued.

Details of buffer pools for specific reservoirs are described below. For more details on Reversals, refer to Sectionsthe 2.5.9Risk of Reversal and 5.6Buffer Pool sections of the Isometric Standard. Risk of reversal information is given in Appendix 4: Risk of Reversal Questionnaire, with further information provided within the relevant storage module.

9.2.1 Buffer pool for storage of CO₂ removed from seawater

Reversals in the storage of CO₂ removed from seawater and stored in geologic reservoirs may be detected during post-sequestration monitoring, and the buffer pool size and procedures for how to attribute detected reversals are described in the relevant storage Modules. Note that the buffer pool is a percentage of final Credits issued, and not a percentage of CO₂ sequestered.

9.2.2 Buffer pool for ocean storage of CO₂ removed from atmosphere

Regarding the CO₂ removed from the atmosphere, based on the present understanding, reversals in the global ocean Dissolved Inorganic Carbon (DIC) reservoir are not directly observable with measurements and attributable to a particular project. Reversals of DIC in the ocean is instead a system wide storage uncertainty that should be addressed through further scientific research, with no possibility of observable reversals. Thus, this storage reservoir has aNo Very LowObservable Risk Level of Reversal according to the Isometric Standard Risk Assessment Questionnaire. The buffer pool corresponding to this lowest risk score is 20% and is meant as an additional precaution against unknowns.

Following the Isometric Standard Section 2.5.9, storage uncertainty for open systems should primarily be accounted for as part of the removal quantification. The approach for quantifying the reversal risk and the stability of the ocean DIC reservoir will be reassessed and updated as new research and understanding arises.

9.2.3 Worked example of buffer pool contributions

For example, in the case of a hypothetical project:

• The CO₂ removed from seawater is stored in a saline aquifer, which has a 1% buffer pool
• Storage of DIC as a result of DOCS does not require an additional buffer pool contribution because the Project is only restoring DIC after removing it, so therefore no additional carbon storage relative to baseline is occurring that would warrant an additional buffer pool contribution. Additionally, as an open system, there are no observable reversals from DIC.
• Therefore, the total buffer pool for the DOCS project is 1% of the final Credits issued.

10.0 Pre-Deployment Requirements

All pre-deployment requirements must be described in the PDD, as outlined in Section 5.

The requirements related to the ocean site are as follows:

• Project Proponents must receive official permitting for discharging effluent from the DOCS process from all relevant authorities of the water body of the Project site and affected vicinity.
• Project Proponents must provide a site description including the dominant circulation and modes of variability, as well as the frequency of various environmental conditions. At minimum, information must be provided on seasonal patterns, predominant currents in relation to tidal ranges, atmospheric forcing (winds and air temperature), periods of maximum stratification, periods of upwelling or downwelling and periods when receiving water quality conditions may be more critical for ecological health. For some sites, it may also be necessary to include discussion on riverine inputs, groundwater inputs, waves, sediment transport and ice presence.
• Project Proponents must disclose other marine CDR activities which are co-located at the site. Currently, there is minimal research on co-located marine CDR projects. Due to nonlinear interactions, attribution and prediction of air-sea CO₂ uptake and environmental impacts for co-located projects is more challenging. Project co-location includes projects with overlapping mixing zones, spatially and temporally, and may extend to projects with overlapping regions of air-sea CO₂ uptake. Collaboration on quantification and environmental monitoring efforts is recommended for co-located projects. The level of interaction between projects is expected to decrease as the CDR perturbation becomes more dilute away from the Direct Ocean Capture facility. Guidance on robust quantification and responsible environmental safeguarding for multiple projects operating in the same area will be updated with the latest research.
• Project Proponents must develop a conceptual model of plume transport and dispersion, and produce a mixing zone model (which is usually a standard requirement in ocean discharge permits, see Appendix 3). The mixing zone model will estimate initial dilution and inform the spatial extent of monitoring efforts. Sensitivity analysis is required to demonstrate adherence to water quality limits at the boundary of the mixing zone under varying environmental conditions. Retention time and advection or ejection events away from the DOCS location may also be relevant.
• Project Proponents must conduct pre-deployment ocean monitoring, including carbonate chemistry parameters, following Section 11.4.
• Project Proponents must select CO₂ extraction rates that result in effluent pH that adhere to permitted thresholds. The DIC-depletion of seawater should gradually ramp up to the intended rate for continuous discharge.
• Project Proponents must have sufficient plans for restoring effluent water quality prior to discharge, including but not limited to recovering alkalinity and dissolved oxygen concentrations.
• Project Proponents must identify and provide detailed information on the storage and disposal of any hazardous by-products.

The requirements related to the CO₂ storage site are as follows:

• Suppliers must identify CO₂ storage locations and adhere to the requirements of the applicable storage Module. For example, storage locations must obtain relevant permits and conduct site characterization as required by the applicable storage Module.

11.0 Measurement and Monitoring Requirements

A monitoring plan must be established prior to project activities and described in the PDD. The aims of the monitoring plan are to:

• quantify net CDR,
• validate models used in net CDR quantification,
• demonstrate permit compliance,
• monitor environmental conditions,
• monitor for reversals in storage, and
• establish processes for adaptive management to ensure that projects stop if negative impacts are identified.

The monitoring plan must include details about monitoring duration, frequency, location, sample collection methods, analytical methods, thresholds (if applicable), data reporting and quality assurance/quality control. Analysis and reporting of monitoring data and measurement uncertainties should occur for every Reporting Period (see Section 8.1 for definition of a Reporting Period). Monitoring guidance and requirements will be updated in alignment with advances in sensor technology which enhance the capacity for measurement and monitoring.

11.1 Monitoring locations

This Protocol requires and recommends monitoring to occur at the following locations, which are illustrated in Figure 3:

1. CO₂ stream: after extraction of CO₂ from seawater, measurements of the captured CO₂ stream must be taken immediately after CO₂ extraction to confirm the amount of CO₂ captured from seawater, and immediately before sequestration into a storage reservoir to quantify any fugitive emissions.
2. Seawater influent: the intake pipe where seawater is drawn in. Measurements taken here must be before any pre-treatment of the seawater and before temporary acidification or CO₂ extraction occurs.
3. Seawater effluent: the outflow pipe immediately before discharge into the ocean. These measurements must be taken after CO₂ extraction and alkalinity restoration.
4. Edge of mixing zone: the mixing zone is the area near the discharge where initial dilution takes place and is a regulatory concept that describes the area where water quality criteria can be exceeded. Beyond the mixing zone, it may be difficult to detect measurable signals from the DOCS projects, so in-ocean measurements are concentrated at the edge of the mixing zone.
5. Deployment area: certain measurements, such as for monitoring ecological impacts, may occur outside the mixing zone. Monitoring that occurs outside the mixing zone but still in the vicinity of the Project is referred to as within the “deployment area”.

In the PDD, when describing the monitoring plan, Project Proponents must provide a description of their monitoring locations. Note that additional monitoring is required depending on the storage Module used for the captured CO₂, which is described in Section 8.2.3.42 and the relevant Storage Modules for the Project.

[Image: Figure 3]

Figure 3 Plan view showing an example monitoring map for a project using geologic CO₂ storage, where T = temperature, S = salinity, TA = Total Alkalinity, DO = Dissolved Oxygen, TSS = Total Suspended Solids, and Chl-a = chlorophyll-a. Bold text indicates required measurements for all DOCS projects, additional measurements are recommended and are project-and-site specific. See Table 2 for justification and more details of environmental monitoring measurements. *Alkalinity may be calculated from measurement of a third carbonate system variable e.g. pCO₂, with routine bottle samples of alkalinity to check alkalinity calculations are consistent with measurement.

11.2 CO₂ stream measurements

For the captured and sequestered fluid, both the concentration of CO₂, and the mass of fluid, must be measured for quantification of CO₂Capture,RP in Section 8.2.13.1.1.

The concentration of CO₂ in the gaseous, dissolved or supercritical CO₂ stream must be:

• measured immediately downstream from the DOCS process and upstream from the point of sequestration; and
  • where CO₂ streams from multiple projects are sequestered into a single storage location, total CO₂ mass input from the Project and of the total CO₂ stream may be measured at the location of transfer from the DOCS facility into the combined stream. The weight fraction of CO₂ determined at this point may be used to calculate the CO₂ injected as measured immediately upstream from the sequestration point
• measured using a continuous inline analyzer for CO₂ concentration, such as NDIR, TDL, or similar, which satisfies the below requirements:
  • CO₂ analyzer must have an accuracy of 2% of full scale or better, with limited drift specification (<2%)⁴³
  • recorded at a frequency of 1-minute intervals at minimum and output averages at 1-hour intervals at most,
  • must be calibrated in accordance with and at a frequency which meets or exceeds manufacturer calibration requirements, but which in any case must be no less than annual, and
    • calibration gasses must be traceable to national standards and a certificate of analysis provided indicating so; and
  • raw data must be made available upon request.

The mass of captured and sequestered fluid (𝑚𝐼𝑛𝑗) is measured via a calibrated mass flow meter or a volumetric flow meter and density measurements over a defined time interval (Δt). Preference is for high-accuracy flow meters such as coriolis or thermal mass flow meters, although other metering solutions are allowable. Flow metering must meet the following requirements:

• be provided with a factory calibration for the specific gas or injectant composition expected;
• be subject to inspection;
• have calibration traceable to national standards;
• adhere to manufacturer specifications for calibration and maintenance;
• be installed in accordance with manufacture installation guidelines, including, for example, minimum distances up or downstream of piping disturbances required to ensure accurate flow measurement;
• be selected and installed for the expected and observed operating range of the injection system;
• have accuracy specification <2% full scale, with preference for meters with accuracy of 1% or better; and
• have sampling time of less than one minute, however meters may use averaging and provide data outputs on 1-minute to 1-hour averaged frequencies.

11.2.1 Procedure for handling missing data in CO₂ stream measurements

In general, the Project Proponent must notify Isometric and the VVB when data gaps or missing calibration data occur and must clearly explain the approach taken and document the missing data within the GHG statement. For parameters where frequent, sub-hourly measurements are required (notably CO₂ concentration measurements in the CO₂ stream, and the measurement of mass of CO₂ injected), the Project Proponent must adhere to the following procedure for handling missing data.

• Where there are data gaps in measurement of the relevant parameter of up to 30 minutes, the Project Proponent may claim using an average quantity, based on the measurements proceeding and following the data gap.
• Where there are such data gaps of longer than 30 minutes, the Project Proponent may apply this approach for up to a 30 minute period within the duration of the data gap, but no more than this. For the remainder of the period of the data gap, i.e. in excess of 30 minutes, the CO₂ stream measurement must be treated as zero, due to a lack of data. In addition, data gaps must account for less than 5% of the data used for the removal calculation within a given Reporting Period, any missing data above this is also not creditable.
• Where a calibration is missed, one must be completed as soon as this is noticed. For data collected between when the calibration was required and when it actually took place, a conservative estimate should be used agreed between the VVB, Project Proponent and Isometric.

11.3 Seawater influent and effluent

Project Proponents must monitor effluent characteristics prior to ocean discharge through ongoing measurement of pH, alkalinity, temperature and salinity of the effluent. Temperature and salinity are required for determining physical characteristics of the effluent, and pH and alkalinity are monitored to demonstrate compliance with the effluent discharge limits allowed under official permitting and described in the PDD. Carbonate system measurements must also be used to inform and validate the amount of carbon removed and extent of carbonate equilibrium in the effluent.

Alkalinity is generally determined in the lab from bottle samples (see Appendix 1, Section 14.1 for more guidance), which may be difficult to sustain ongoing high-frequency sampling. The frequency of alkalinity measurements may decrease once a project can demonstrate it is operating at a steady state. There are also new sensors being developed which can continuously measure TA in situ ^44,45 (see Appendix 1, Section 14.2 for more guidance on sensors), which when paired with continuous pH measurements, enables high frequency monitoring of the carbonate system with smaller errors than if pH and pCO₂ are used.

It is a known limitation of using pH and pCO₂ measurements to constrain the full carbonate system that uncertainty can be high ⁴⁶, and uncertainties may be even higher for DOCS projects when the pCO₂ is near 0. Thus, it is generally not recommended to calculate alkalinity and DIC in the influent and effluent streams from pH and pCO₂ measurements. If a Project Proponent wishes to calculate alkalinity and DIC from pH and pCO₂, then routine bottle samples are still required to check that derived values of alkalinity and DIC are consistent with directly measured values. Project Proponents are responsible for ensuring that the effluent monitoring location provides a representative and well-mixed characterization of the effluent.

It is also recommended that Project Proponents measure a third carbonate system variable to assess measurement errors and the initial state of carbonate system disequilibrium at the point of discharge. When reporting calculated carbonate chemistry variables in the PDD, justification of state of equilibrium, equilibrium constants used in carbonate chemistry speciation software and number of carbonate chemistry parameters used for calculations must be disclosed.

Effluent must adhere to the pH safety threshold allowed under official permitting. The safety threshold must be met at the location indicated on permit allowances (such as at the end of the pipe prior to, or in the ocean after initial mixing). Regardless of where the safety threshold is applied, Project Proponents should calculate the corresponding in-pipe threshold, such that ongoing compliance can be monitored.

A complimentary set of measurements of the seawater influent are required to fully constrain the impact of the DOCS process on the carbonate system and water quality. pH and DO levels must be restored to environmentally safe levels before discharge to the ocean. Where applicable, seawater must be recombined with any alkaline solids removed during pre-treatment.

In intake pipes, Project Proponents must take actions to mitigate risk of adversely harming marine biota originally contained in the seawater, and assess the realized ecological impacts of impingement and entrainment in the intake pipe due to pumping, filtering, and treatment of seawater prior to and during the DOCS process. Such impacts can be determined through demographic or conditional mortality approaches.

Projects Proponents must check that the difference in DIC between the influent and effluent is equivalent to the CO₂ capture stream.

11.4 Ocean monitoring

Ocean monitoring must span the following general locations listed below. This Protocol does not prescribe exact ocean monitoring locations. Project Proponents are responsible for determining appropriate monitoring locations. It is recommended that Project Proponents use models to plan and optimize sampling design ⁴⁷. A diagram of the monitoring locations, including the location in the water column, must be included in the monitoring plan submitted in the PDD.

11.4.1 Mixing Zone

The area where initial dilution takes place is often called the mixing zone (or "a reasonable mixing zone" or "zone of initial dilution"), typically on the scale of O(100m), depending on the scale of discharge. A mixing zone is a regulatory concept, which describes a spatial area surrounding the discharge infrastructure where water quality criteria can be exceeded ⁴⁸. Permits which allow for a mixing zone require water quality criteria to be met at the edge of a mixing zone. A mixing zone is a simplified representation of initial mixing, the true spatial extent of initial mixing changes dynamically with environmental conditions.

Detection of a measurable signal may be difficult beyond the mixing zone, especially for small-scale deployments. Increased sampling efforts may be necessary for accurate assessment within the mixing zone due to heterogeneous effects of turbulence. Beyond the mixing zone, receiving waters will display less turbulent variability, which can provide more representative measurements of the impact of the CDR intervention. Thus, environmental monitoring should be focused on the edge of the mixing zone. A spatially distributed array of sensors may provide more comprehensive coverage of the mixing zone and plume transport.

Project Proponents must monitor temperature, salinity, two carbonate chemistry parameters (pH, TA, DIC, pCO₂), dissolved oxygen (DO), turbidity, and total suspended solids (TSS) at the edge of the mixing zone. Temperature and salinity are necessary for determining physical conditions, and should be collocated with other measured quantities. Two carbonate chemistry parameters (pH, TA, DIC, pCO₂) must be monitored to characterize the carbonate system, for the purpose of monitoring water quality, determining local carbonate saturation state and informing loss term estimates. An additional third carbonate system measurement is recommended to assess the local state of carbonate disequilibrium. DO, turbidity and TSS are water quality indicators. Action thresholds are placed on pH, TSS and DO (see Section 11.4.5: Determination of Action Thresholds). Monitoring Chl-a and dissolved inorganic nutrients are also recommended.

Burst sampling is recommended for parameters monitored with autonomous sensors within the mixing zone. By sampling at a high frequency, burst sampling can provide insights into the turbulent mixing processes and contributes to greater statistical certainty in the temporal variability and distributions of the monitored parameter.

11.4.2 Biological and ecological monitoring in deployment area

Tailored biological and ecological monitoring for specific projects and sites must be determined by conducting an environmental risk assessment and mitigation strategy (see Section 6).

Project Proponents are recommended to conduct periodic ecological surveys to monitor phytoplankton community abundance and composition, benthic community abundance and diversity (if the discharge plume interacts with the seafloor), and presence and potential interactions with species of special concern (IUCN listed, commercially exploited or keystone species). Functional diversity should be considered in addition to taxonomic diversity. Survey sites should include the immediate vicinity of the discharge pipe and specific benthic habitats within the affected area of the DOCS project activities to be identified and determined on a site-by-site basis.

Action thresholds are not imposed on ecological indicators due to the difficulty of establishing ecological baselines and attributing ecological changes. Data collection remains imperative for establishing a foundation for future analysis and enabling the assessment of cumulative and chronic impacts of DOCS. Ecological data must be made publicly available, and the location of where data is stored publicly must be included in the PDD.

11.4.3 Measurements for model validation and model inputs

Widespread global and regional oceanographic measurements are essential for characterizing the dynamic ocean baseline and better understanding the background ocean carbon cycle. Oceanographic observations and datasets that have wide spatial and temporal coverage are crucial for model validation, which must be demonstrated prior to using the model used to quantify air-sea CO₂ uptake (see Appendix 3 Mixing Zone and Coastal Dynamics Modeling, and Section 3.6 in Air-Sea CO₂ Uptake Module v1.1). Furthermore, ocean data (such as winds, currents, tides, waves, turbulent mixing etc.) are used as inputs to the model used to quantify air-sea CO₂ uptake. This data may originate from data collected at the field site by the Project, or data made available from other sources such as government agencies or academic institutions. Data used from third-party sources must be spatially and temporally relevant to the domain.

Maintaining and expanding regional and global ocean observations is necessary for ensuring robust quantification of marine CDR projects. It is not feasible for a Project Proponent to carry out these efforts alone, so this must be part of a wider collaborative effort between government, academia, industry, and NGOs.

It is recommended that Project Proponents measure air-sea carbon flux to validate modeled carbon flux. At this time, there are no existing sensors which can directly measure CO₂ flux. Methods for integrating proxy and/or tracer measurements into a quantification for CO₂ uptake should be validated via intercomparison. Methods may include gradient method, eddy covariance, flux chambers or dual tracer regression, and modeled uptake.

11.4.4 Ocean monitoring frequency and duration

Monitoring must span pre-deployment, throughout project operations and post-project operations. For pre-deployment environmental monitoring, Project Proponents must provide a baseline characterization of monitored water chemistry and ecological parameters at the site, which should be sufficiently long and well-resolved to characterize background ocean variability. The exact duration and frequency of monitoring will depend on the parameters being measured, as well as site and project specific factors. Listed below are considerations for determining and justifying the monitoring frequency and duration, and some illustrative examples can be found in Appendix 2.

The duration of pre-deployment monitoring should consider:

• the timescales of the risks identified in the environmental risk assessment (Section 6)
• duration of CO₂ capture operations
• site-specific residence time of the receiving water
• timescales of natural variability and availability of historical data

Similarly, post-deployment environmental monitoring should consider:

• the timescales of the risks identified in The Projects environmental risk assessment
• site-specific residence time

The monitoring frequency should reflect that which is necessary to provide sufficient information about the effluent and receiving waters. Some factors which may influence the frequency and intensity of the monitoring plan include:

• operational capacity of the Project. The monitoring frequency may need to be increased at times where the discharge is nearing operational capacity.
• location of the discharge. The monitoring frequency may be more intensive depending on the sensitivity of receiving waters.
• duration and intensity of project operations. The monitoring frequency should differ depending on the duration of project operations and more intense monitoring should occur during ramp-up or operational changes vs. steady state operation.
• seasonality. For projects with seasonally varying action thresholds, more frequent monitoring during seasons with greater environmental risks may be required.

11.4.5 Ocean monitoring for ecosystem safety

Project Proponents must conduct an environmental risk assessment and develop a mitigation strategy (see Section 6). Thresholds on ocean monitoring parameters are used to determine safe limits for discharge, mitigate environmental risks and identify negative environmental impact.

Two types of thresholds are used:

1. Safety thresholds are imposed on the effluent characteristics and controlled prior to discharge.
2. Action thresholds are measured at the edge of the mixing zone. Parameters measured are indicators of water quality and/or unintended environmental changes. Action thresholds can be absolute or relative, and can be determined by regulatory limits, relative to natural variability based on historical data or relative to a control site. For example, the action threshold on pH could be pH < 9 or ±0.2 relative to natural variability or to a control site, whichever is more conservative. Action thresholds may vary seasonally, and should be consistent with monitoring requirements under official permitting.

Effective implementation of these measures must also be accompanied by a robust monitoring plan to detect negative impacts and stop projects when necessary.

Control site guidance

Establishing a control site can help isolate the impacts of the CDR project activities from baseline variability. However, finding a control site is challenging due to various factors. Firstly, an ideal control site will have a clear separation between perturbation and control, where the only difference between the two locations is the CDR project activities. Spatially, this means that the correlation scale between relevant parameters should exceed the separation distance (distance between CDR site and control site), while simultaneously, CDR mixing zones must be smaller than the site separation.

However, environmental noise introduces variability, challenging the clear separation of CDR and control sites. Furthermore, balancing the size of CDR activities with instrument detection limits is crucial; large perturbations risk contaminating the control site, while smaller ones may be challenging to detect amidst background noise. These challenges underscore the difficulty in validating a control site. Thus, for this Protocol, deploying instruments or collecting samples at a control site is determined on a site by site basis as outlined in the PDD and is not universally required.

11.4.5.1 Threshold Violations

Violations are determined based on the threshold type, and specificities of the site, permit and intended activities. For example, exceedance of safety thresholds may be determined by daily mean or max values and exceedance of action thresholds in the mixing zone may be determined by weekly and/or monthly mean or max values. All violations need to be reported, and gaps in ocean monitoring data must be justified.

11.4.5.2 Enforcement actions

Credits cannot be issued for time periods without sufficient measurements to demonstrate compliance with action and safety thresholds. The handling of data gaps must be reported in the PDD. Removal activity from discharges that occur during time periods of safety threshold violations will not be eligible for crediting. Action threshold violations must trigger adaptive management plans. Failure to adequately address environmental risks or remediate any harm will lead to a project being subject to Credit cessation.

11.4.6 Missing data and outliers in ocean data

In some cases, circumstances may occur that result in measurements or data that are missing, incomplete or out of line with expectations given the Project design or previous measurements. Similarly, disruptions to the Project area and ongoing monitoring (e.g., extreme weather events or equipment failure) may result in missing data, outliers or unexplained results. For the purposes of this Protocol, outliers are defined as data that are more than three standard deviations from the mean (or equivalent percentiles for non-normal distributions). In such instances, the Project Proponent may seek clarification on how the data should be handled. When such instances occur, the details must be reported to the VVB and Isometric as quickly as possible after identification. In such situations, and on a case-by-case basis, Isometric will work to remedy the situation in consultation with the Project Proponent and VVB. Examples of possible remedies include omitting outliers that represent a highly improbable result or replacing missing data with a conservative estimator of a group of samples collected from a nearby and likely representative area

11.5 Modular requirements

The Modules associated with this Protocol have their own set of required parameters that need to be monitored. Please refer to the following Sections of the Modules to see a complete list of all requirements:

11.6 Summary of monitored parameters

Table 2 below summarizes the required and recommended monitoring parameters. Required measurements indicate the minimum common set of measurements needed for all projects under this Protocol. Additional required measurements may be necessary based on specifics of the Project, site and environmental risk mitigation plan. The exact monitoring plan must be described in the PDD.

Use of calibrated in situ sensors for continuous ocean monitoring with discrete water samples for validation is recommended (see Appendix 2). Analytical methods for discrete bottle samples must follow the Guide to Best Practices for Ocean CO₂ Measurement (Dickson et al. 2007) for carbonate chemistry parameters. Guidance for measurements and sampling strategy will be updated as innovations in measurement develop.

Table 2. Summary of required and recommended measurements for monitoring. Required measurements indicate the minimum common set of measurements needed for all projects under this Protocol. Additional required measurements for ocean monitoring may be necessary based on specifics of the Project, site, and environmental risk mitigation plan. The exact monitoring plan must be described in the PDD.

12.0 Acknowledgements

Isometric would like to thank the following reviewers of this Protocol and relevant Modules:

• Dustin Carroll, Ph.D.
• Kay Suselj, Ph.D.
• Kalina Grabb, Ph.D.
• Matt Eisaman, Ph.D.

13.0 Definitions and Acronyms

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.

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

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

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 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).

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 concentration of inorganic carbon dissolved in a fluid.

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

The amount of carbon exchanged between two or more Reservoirs over a period of time.

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

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.

Certificates are issued to the Certificate Account of a Project Proponent with whom Isometric has a Validated Protocol after an Order for Verification and Certificate Issuance services from a Buyer and once a Verified Removal or Reduction has taken place.

A calculation, series of calculations or simulations that use input variables in order to generate values for variables of interest that are not directly measured.

The process by which all emissions associated with a Project's Removal or Reduction process, including leakages, are accounted for.

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).

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.

Lowering future GHG releases from a specific entity.

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).

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

The amount of CO₂ emissions that would cause the same integrated radiative forcing or temperature change, over a given time horizon, as an emitted amount of GHG or a mixture of GHGs. One common metric of CO₂e is the 100-year Global Warming Potential.

​​Considering impacts at each stage of a product's life cycle, from the time natural resources are extracted from the ground and processed through each subsequent stage of manufacturing, transportation, product use, and ultimately, disposal.

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).

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

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

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.

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.

An acceptable difference between reported Removals/emissions or Reductions/emissions and what an auditor determines is the actual Removal/emissions or Reduction/emissions.

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 document submitted alongside Claimed Removals and/or Reductions that details the calculations associated with a Removal or Reduction, including the Project's emissions, Removals, Reductions and Leakages, presented together in net metric tonnes of CO₂e per Removal or Reduction.

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

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

Improperly allocating the same Removal or Reduction from a Project Proponent more than once to multiple Buyers.

An evaluation of the likelihood that an intervention—for example, a CDR Project—causes a climate benefit above and beyond what would have happened in a no-intervention Baseline scenario.

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

Resources provided to projects that are generating, or are expected to generate, greenhouse gas (GHG) Emission Reductions or Removals.

Products that have a significant market value and are planned for as part of production.

Materials of value that are produced incidentally or as a residual of the production process.

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

An estimate of the emissions intensity per unit of an activity.

An analysis of how much different components in a Model contribute to the overall Uncertainty.

A community resource where Project Proponents publish and visualize their early processes, Removal and Reduction data and Protocols – enabling the scientific community to share feedback and advice.

The document, written by a Project Proponent, which records key characteristics of a Project and which forms the basis for Project Validation and evaluation in accordance with the relevant Certified Protocol. (Also known as “PDD”).

An entity that purchases Removals, Reductions, or Environmental Attributes, often with the purpose of Retiring Certificates to make a Removal, Reduction, or Environmental Attribute claim.

A database that holds information on Verified Removals and Reductions, and reviewed EACs, based on Protocols. Registries Issue Certificates, and track their ownership and Retirement.

Any person or entity who can potentially affect or be affected by Isometric or an individual Project activity.

A United States Government agency that protects human health and the environment.

The natural processes and interactions that occur within an ecosystem, including the flow of energy and materials through biotic and abiotic components, encompassing activities like nutrient cycling, primary production, and habitat provision, which collectively maintain the balance and stability of the ecosystem.

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

Any process, activity, or mechanism that removes a greenhouse gas, a precursor to a greenhouse gas, or an aerosol from the atmosphere.

GHG sources, sinks and reservoirs (SSRs) associated with the project boundary and included in the GHG Statement.

Life cycle GHG emissions associated with production of materials, transportation, and construction or other processes for goods or buildings.

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

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

Reporting Period

The escape of CO₂ to the atmosphere after it has been stored, and after a Certificate 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.

A regulatory concept describing the spatial area surrounding the discharge infrastructure where water quality criteria can be exceeded.

Defined as an excess of proton acceptors over proton donors, which functionally describes the ability of a solution to neutralize acids to the CO₂ equivalence point.

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

The term used to describe allocation of Project emissions to multiple Removals or Reductions.

Any emissions that occur to compensate for biomass that was previously serving another purpose and is now being used for carbon removal or GHG reduction. For example, if agricultural waste was previously left on a field to decompose - fertilizer production to replace those nutrients need to be accounted for.

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.

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

A measurement which correlates with but is not a direct measurement of the variable of interest.

The use of satellite, aircraft and terrestrial deployed sensors to detect and measure characteristics of the Earth's surface, as well as the spectral, spatial and temporal analysis of this data to estimate biomass and biomass change.

A satellite-based navigation system.

The stretch of tidally influenced river where the river and ocean meet. In this protocol, this region is bounded by the head of tide and the seaward limit of estuarine influence in the ocean.

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

Occur when a Sink – created or enhanced by human activity – removes greenhouse gasses from the atmosphere (CDR Primer, 2022).

14.0 Appendix 1: Guidance for High Quality Marine Data Collection

This Appendix is general for mCDR approaches: OAE, DOCS

While quantification of mCDR often relies heavily on ocean models, field measurements play an important role in:

• ensuring sufficient and up to date baseline marine data
• providing data for calibration and validation of ocean models
• monitoring environmental health during and after project activity

Broadly, field measurements may include moored in situ sensors, ship-based or autonomous transecting and profiling, discrete bottle sampling, field surveys, aerial imaging and remote sensing. There is significant variability in the specific instruments and field methods that may be used to collect data.

Project Proponents must provide thorough documentation on planned and completed field campaigns, including details on the campaign design (spatial and temporal coverage, sampling density, data collection techniques, instrument resolution), metadata, data processing, analytical and statistical techniques, instrument calibration and data quality control. In the sections that follow, we highlight some recommended references and best practices to ensure high quality data generation from field measurements.

Collecting discrete bottle samples alongside continuous autonomous sensor measurements is recommended. Both measurement types offer advantages, for example autonomous sensors can offer wider temporal coverage, while bottle samples are crucial for ground truthing absolute values. Additionally, bottle samples can be used to calibrate drift of long term autonomous sensor deployments.

To facilitate accessibility, transparency and interoperability of research relevant data, data collected from field campaigns must adhere to FAIR data principles⁹. Project Proponents should follow the data standards and controlled vocabularies described in Guide to Best Practices in Ocean Alkalinity Research: Chapter 13–Data reporting and sharing for ocean alkalinity enhancement research ⁴⁹. Metadata should be complete according to the ⁵⁰.

Note on alkalinity and pH measurements

Alkalinity is used in multiple fields that may have different terminology and definitions. For example, total or titration alkalinity is widely used in oceanography, whereas charge balance alkalinity is commonly used in freshwater systems and by geologists⁵¹. This Protocol uses the oceanographic community terminology when referring to measurements taken in the ocean, where total alkalinity (TA) for seawater is defined following Dickson 1981 based on acid titration to the CO₂ equivalence point⁵². However, alternative definitions of alkalinity that may be more appropriate for specific projects (e.g. for non-seawater systems, such as effluent from a wastewater stream) should be specified in the PDD.

There are also different scales used for pH measurements which may differ by over 0.1⁵³. For example, the National Bureau of Standards (NBS) scale is suitable for freshwater systems and waters with low ionic strength, while for seawater, the total hydrogen ion scale is more commonly used⁵⁴. This Protocol recommends the total hydrogen ion scale for seawater pH measurements, following the SOPs below in the Guide to Best Practices for Ocean CO₂ Measurements. Care should be taken to account for differences in pH scales, such as when comparing pH measurements against regulatory thresholds.

14.1 Specific Guidance for Bottle Samples

For discrete bottle samples, we refer Project Proponents to the Best Practice Data Standards for Discrete Chemical Oceanographic Observations ⁵⁵. Additional considerations for discrete bottle sampling of carbonate chemistry are discussed in Guide to Best Practices in Ocean Alkalinity Research: Chapter 2 (Seawater carbonate chemistry considerations for ocean alkalinity enhancement research: theory, measurements and calculations) ⁵⁶.

Recommended analytical methods for bottle samples:

14.2 Specific Guidance for in situ sensors

For autonomous instruments, Project Proponents are recommended to follow manufacturer provided user manuals for instrument preparation, maintenance, calibration and quality control. Environmental and deployment factors which may affect the quality and interpretation of data, (e.g. instrument orientation, interference, presence of bubbles, sediment, and biology) must be described.

Adoption of innovative sensor technologies is encouraged. For novel sensors, additional information that would typically be available from a manufacturer would also have to be provided. These include detection range, resolution, accuracy, performance under different environmental conditions (e.g. temperature ranges, depths) and response time. The expected measurement conditions must be within the sensor’s range.

14.2.1 Pre-deployment

Project Proponents must perform instrument calibration before deployment, and recalibrate instruments at an interval recommended by the manufacturer. Calibration curves or equations for the instrument or sensor must be documented. Post-deployment calibration is recommended to account for instrument drift over the deployment duration.

14.2.2 Maintenance During Deployment

Project Proponents must develop a maintenance plan for moored instruments deployed longer than 30 days. This should include plans to clean or replace sensors that may be impacted by biofoul, such as optical sensors, conductivity cells or flow through sensors.

14.2.3 Data Post-processing

Data processing techniques must be documented, including quality control, filtering, despiking and statistical analysis. The recommended quality control process differs between instruments. Data should be reported using quality control flags consistent with International Oceanographic Data and Information Exchange (IODE)⁵⁷.

The quality control manuals for autonomous instruments prepared by the Integrated Ocean Observing System are a recommended resource.

Quality control checks could include:

• completeness of observations
• ordering of data
• check against physically valid limits
• check data obeys physical laws
• checking statistical tests
• checking regularities of space and time variations

Data Analysis Methods for Physical Oceanography is a recommended resource for statistical analysis methods⁵⁸.

14.3 Specific Guidance for Tracer Studies

Tracer studies track effluent using a tracer that is either naturally present or added. They can be useful for mCDR projects to:

• characterize DIC-deficient or alkalinity-enhanced plume transport
• validate physical transport in the ocean model via tracking plume movement
• validate alkalinity uptake via dual tracer regression for OAE projects (Project Proponents are not expected to perform dual tracer regression)

Ideal tracers are not harmful to the environment, have near zero background concentration, are conservative or decay very slowly in relation to the duration of field work, mix freely and can be measured in the field at low concentrations. As such, examples of tracers that may be used in DOCS are fluorescent dyes (i.e. rhodamine dye) and gas tracers (SF₆, CH₃, SF₅, ³He).

Fluorescent dye tracers, such as rhodamine dye, are preferable because they can be detected from aerial imaging or high frequency in situ sensors. Rhodamine dye degrades in days to weeks, and can be helpful for characterizing near-field mixing. Gas tracers have a lifespan on the order of years, and can be useful for large scale experiments, however, they have high global warming potential.

Tracers can be injected into the effluent to increase the trackability of the effluent. Alternatively, the tracer dye can be released directly into the water column as a slug release, to help constrain localized dispersion characteristics for coastal models.

If a tracer study is conducted to characterize pCO₂-plume transport, the sampling plan must include vertical profiling along transects. A sufficient number of studies are required to produce a sensitivity analysis. Sampling plan, data analysis and interpretation will be evaluated on a case by case basis. See the LOC-NESS project for an example of an OAE tracer study design and accompanying sensing suite.

The US EPA Operating Procedure ⁵⁹ and USGS Procedure for Dye Tracer Measurements⁶⁰.

The following checklist can be referenced for tracer release studies using rhodamine dye to validate an ocean model or characterize plume transport.

Study planning:

• duration of release should give sufficient time for the field team to complete sampling (this will be site specific)
• required tracer quantity should be based on sensor detection limit, effluent flow rate, duration of test and site specific dilution capacity

Materials preparation:

• prepare dye mixture, ensuring the correct units of dye concentration are used
• dye injection pump should be calibrated in lab to confirm desired volumetric dosage rate
  • include release rate at different battery levels and temperature
• prepare instrument and site specific calibration curve between dye concentration and effluent concentration

Release:

• if released with effluent, released at a rate proportional to effluent release rate
• if released with effluent, tracer must be added at an adequate mixing length before final discharge
• if released with effluent, sample from an access valve to measure concentration prior to final discharge
• density and velocity matching for effluent or slug release are considered
• no additions after the injection point

Sampling (ship-based or underway system):

• prior to release, conduct spatial salinity and temperature profiling.
• for ship based sampling, sensor should be placed off the bow, firmly braced and clear of bow waves to minimize disturbance due to wake of the vessel.
• positioning via GPS or triangulation should be +/- 2.5m to ensure sampling station. positions and boat tracks are accurate
• for all measurements, record metadata including sample location, time, depth, sensor probe and other visual observations.
• note areas of high concentrations beyond the mixing zone, as this can signal accumulation or recirculation, which cause the effluent to resurface elsewhere.

Data quality and interpretation:

• consider accuracy of positioning data, variation in discharge flow rate, variability in environmental conditions, calibration curve and measurement method in the interpretation of results.

14.4 Ocean Monitoring Examples

Below are some hypothetical examples that are for illustrative purposes only, to demonstrate some of the site and project considerations when determining the appropriate monitoring duration and frequency for a particular parameter and use case.

Example 1

An DOCS project is monitoring for changes in ocean pH near the discharge site to make sure the change in pH does not exceed a threshold of +/-0.2 of the natural variability of pH. Their pilot testing entails 1 week of CO₂ extraction, and the residence time of the region where pH sensors are deployed is 1 day. At this site, it is known from previous studies and published literature that the dominant temporal modes of variability for pH are diurnal and seasonal, due to biological activity.

Monitoring duration: Because the activity and operating period are much shorter than the seasonal variability in this case, it is not necessary for the baseline characterization to span a full year to resolve the seasonal cycle. Instead, the baseline monitoring for a few days before deployment is sufficient. There is ongoing monitoring during the 7 day CO₂ extraction period, and the post-CO₂ extraction monitoring for pH should additionally span at least the residence time. To be conservative and to collect additional safety data though, the Project decides to extend post-CO₂ extraction monitoring for 1 week.

Monitoring frequency: To resolve the diurnal cycle variability, sampling should be conducted at a higher frequency than daily, e.g. sampling at minimum every 6 hours would result in at least 4 measurements per day and would be sufficient here. The Project uses continuous pH sensors, which allows them to sample much more frequently to fully resolve the diurnal cycle.

Example 2

An environmental risk of a mineral OAE project that the local community is concerned about is bioaccumulation of metals in a nearby oyster farm. To address this, the Project is monitoring the concentration of trace metals in oyster tissues before and after the Project activity. The Project is dosing feedstock particles into the ocean for 1 week, but some of the feedstock will accumulate on the seafloor and it can take up to 3 months for the particles to fully dissolve. The farm grows oysters in the ocean for 1 year before harvesting.

Monitoring frequency: The accumulation of any metals in oysters will take some time, especially because it will take a few months after project dosing completes for the feedstock particles to dissolve. Thus it was decided to sample the oysters once a month for the first 3 months following the Project since that is the period during which particle dissolution is expected to occur, and then once a season after that.

Monitoring duration: In this case, the community would like to know the impacts on the oysters that are being harvested for food. So the post-deployment monitoring lasts for 1 year post-dosing (taking seasonal samples) to span the oysters growth cycle and to ensure that metal concentrations do not exceed the local food safety regulatory limits. A control sample was collected from a number of oysters pre-deployment.

15.0 Appendix 2: Mixing Zone and Near-field Modeling for ocean outfalls

This Appendix is general for mCDR approaches: OAE, DOCS

15.1 Introduction

For point source discharges, characterization of plume transport and spread is required in the immediate vicinity of the outfall, which can be referred to as the mixing zone and the near-field domain. Modeling in the mixing zone is required by standard permitting processes to demonstrate adherence to water quality criteria and environmental safeguarding. The output of the mixing zone model can be used in a near-field model for upscaling the CDR intervention to a time and depth-dependent CDR forcing used in the model for quantifying air-sea CO₂ uptake.

15.2 Background Concepts

Mixing is a multi-scale phenomenon, and a model or series of models may be required to upscale a near point source discharge from a CDR project into a CDR forcing function which can be applied to the model used to quantify air-sea gas CO₂ fluxes.

In the initial dilution region (or mixing zone), transport and mixing is dominated by the differences in velocity and density between the discharge and the receiving water. Typically, effluent is discharged at a higher velocity than ambient currents, which generates turbulent mixing. Ocean outfalls tend to be at depth and discharge less dense effluent than the receiving water (ie. freshwater discharging into marine waters). The relative density difference affects the rise rate, where the discharge surfaces or the position of the plume in the water column if trapped beneath the surface. Figure A2-1 shows the cross-section of the initial jet motion of a submerged buoyant jet discharged into stratified water ⁶¹. Ocean outfalls may also be near the surface or above the surface.

Initial dilution continues until energy dissipation brings the plume velocity to match the receiving water, and the plume is at a depth where it has the same density as the receiving water. Once this point is reached, the effluent plume typically moves horizontally with the receiving waters and ambient turbulence causes further dispersion.


Figure A2-1 Initial transport and dispersion of buoyant plume discharged at depth into stratified waters. Inspired by Jirka et al. (1979)

Dilution Capacity

Dilution capacity of a receiving water is the effective volume of receiving water available to dilute the discharge. Dilution capacity can be variable due to environmental conditions. For example, stratification reduces the effective volume of receiving water by constraining vertical mixing, and tides affect the total volume of water in the receiving body. For environmental safeguarding, the worst-case scenario for mixing must be evaluated with a sensitivity analysis. This typically coincides with low tide and strong stratification, although it can be significantly altered by other environmental factors.

Relevant Environmental Factors in Receiving Waters

The marine environment is dynamic, with stratification, atmospheric forcing, tidal flows and freshwater inputs, varying on a range of timescales from diurnal to seasonal. In the marine environment, coastal circulation is often dominated by local wind and waves interacting with bathymetry. In shallow waters, ocean waves also affect mixing and dilution. Buoyancy-driven currents can also exist along coastlines. Estuaries are further impacted by freshwater inflow, tidal activity, stratification, groundwater and benthic activities.

15.3 Near-field Modeling

The majority of projects will have a density difference between effluent and marine receiving waters, and as such it is advised to couple a mixing zone model with a near field model.

A mixing zone model represents the discharge plume when it is dominated by its own momentum and buoyancy, whereas the near-field model represents the plume's transport and dispersal once it is dominated by ambient dynamics. The near-field model should bridge the spatial and temporal scales between the mixing zone model and the larger domain model used to quantify air-sea CO₂ fluxes.

15.3.1 Mixing zone model

Commercial mixing zone models, such as CORMIX or Visual Plumes, can be used to determine initial plume dispersion. We note that mixing zone models assume sufficient dilution capacity of the receiving water such that the ambient conditions are not affected by the discharge. The tracer concentration from a mixing zone model can be used as an input in the coastal model.

15.3.2 Near-field model

At minimum, the near-field model must accurately reproduce the hydrodynamic processes of the study area and behavior of a non-reactive tracer. Opting to do a simpler model such as 1D or 2D is only permissible in certain cases and must be thoroughly justified. A summary of acceptable models are included in Table A3-1. Commercially available modeling software can be used. Biogeochemistry is not required for near-field modeling, due to the relatively short time-scales of the near-field model compared to time-scales of air-sea gas exchange.

Table A3-1. Options for near-field model

15.3.3 Model set up

The model domain should be large enough to ensure that boundary condition influences are negligible in the vicinity of the plume. The model resolution should be chosen to appropriately resolve near-field dynamics, particularly those that might not be well-represented in the model used to quantify air-sea CO₂ fluxes .

Table A3-2 and Table A3-3 summarizes the typical input data required for the mixing zone model and near-field model respectively. Project Proponents are required to report all inputs to model runs.

If projects are using existing ocean outfalls, the outfall specifications must be determined (Table A3-2). For new infrastructure, modeling should be done to optimize outfall and diffuser design to maximize initial dilution and/or minimize environmental impact.

Since field campaigns span a limited time period, distributions of observed quantities are unlikely to represent historical maxima and minima. It is advised that inputs for environmental variables are based on statistical analysis of all historical data available.

Any parameterizations used to represent physical dynamics in the model must be disclosed in the PDD.

Table A3-2. Mixing Zone inputs

Table A3-3. Near-field model inputs

15.4 Calibration and Validation

Near-field models must demonstrate capability to represent hydrodynamic processes of the site through comparison with independent field data. The model can be calibrated using a subset of field observations, and validated by applying the calibrated model to one or more historical events, and demonstrating a reasonably high level of agreement between model and observations. Project Proponents should refer to Section 4.1 in the Air-Sea CO₂ Uptake Module v1.1 for guidance on model validation.

In addition to validating baseline physical dynamics, it is advantageous to ensure that the model can replicate the CDR intervention. Evaluating the physical transport of the CDR intervention (ie. DIC-depleted seawater release or alkalinity-enhanced seawater) can be accomplished through a tracer study, such as dye injection into effluent. Validating the biogeochemical response to the CDR perturbation involves tracking the spatial extent and relative changes in carbonate chemistry parameters. Achieving such validation would necessitate starting with a validated baseline model. Next, this validated baseline would be utilized to design an efficient sampling plan. Following that, a CDR perturbation with a discernible signal above background environmental variability must be produced. Additionally, sufficient instrumentation and sampling resources would be required to provide the necessary spatial and temporal coverage for observing the evolution of carbonate chemistry parameters following an CDR perturbation. This is an iterative cycle which consists of models and measurements informing each other. Although recommended, validation of the biogeochemical response at the Project-level is not required, with the anticipation that ongoing scientific inquiry and field trials will advance a comprehensive system-wide validation of biogeochemical modeling for CDR. Where possible, it is recommended that individual processes (such as carbon flux) are validated through comparison with measurements collected from the field.

See Appendix 2 for guidance on ocean sampling and high quality data generation.

15.5 Sensitivity Analysis

A sensitivity analysis is required to develop a distribution of CDR forcing functions. If the near-field model is not able to replicate realistic atmospheric and tidal (if applicable to project site) forcing that is representative of the Reporting Period, then the sensitivity analysis must span a wider variety of historical conditions and the following parameters should be varied in ensemble scenarios:

• tidal variations, if applicable to project site (including spring-neap and semi-annual tides)
• meteorological conditions (wind speed and direction, air temperature)
• seasonal variability (in temperature, salinity, currents, wind speeds etc)

The result of the sensitivity analysis should be a range of expected, best and worst case conditions for the distribution of CO₂-depleted or alkalinity-enhanced water within the water column (from the near-field model), and water quality conditions (from the mixing zone model).

15.6 Reporting

All stages of modeling from conceptualization to results interpretation must be reported.

• for full transparency and reproducibility, all model inputs, model configuration, and model code, should be made available.
• model outputs and analysis code must be shared so that the results are reproducible
• written report describing:
  • model set up
  • configuration
  • input data sets
  • model scenarios
  • results of model calibration and validation
  • justification for choices made
  • documentation of any tracer study conducted
  • model results depicting sensitivity of CDR forcing functions
  • discussion of results and confidence levels

Model runs must be retained and reported in the GHG Statement submitted for verification.

16.0 Appendix 3: Supplementary Figures

This Appendix contains supplementary figures to further clarify parts of the Protocol.

Figure A3-1 illustrates some of the quantification options described in Section 7.4.1. All projects must measure the seawater carbon capture rate at the effluent (Step 1), and all projects must quantify the net CO₂ removal as a result of air-sea gas exchange in an appropriate ocean model (Step 3). To upscale the point source effluent measurements into a 3-dimensional CDR forcing that can be applied to the model used to quantify air-sea CO₂ uptake, Project Proponents have flexibility to choose from a number of options (Step 2).

[Image: Figure A3-1]

Figure A3-1 Example of quantification options described in Section 7.4.1.

17.0 Appendix 4: Risk of Reversal Questionnaire

This risk assessment identifies the pathway specific risk factors relevant to a carbon removal project. The relevant risk factors identified as part of a risk assessment are included in the monitoring plan requirements for the project, with details included in the Project Design Document. Project specific risk factors inform the required duration of monitoring along with the monitoring requirements set out in the Protocol and the requirements set out in the Monitoring Section of the Isometric Standard.

If Reversals are not directly observable (i.e., all storage is as DIC in an open system), the Project's Risk of Reversal is automatically "No observable risk." Such Projects do not need to complete this questionnaire, but must still maintain a monitoring plan in accordance with the requirements of the relevant Protocol.

The risk score, as determined by the Risk of Reversal Questionnaire, will determine a project’s buffer pool contribution. Projects must re-assess their reversal risk at the renewal of each crediting period, or if monitoring identifies a reversal-related risk, or if an actual reversal event takes place. In any event, projects should reassess their reversal risk at a minimum every 5 years.

The Risk of Reversal Questionnaire questions that pertain to this protocol, drawn from the programme-level Risk of Reversal Questionnaire defined in Appendix B: Risk Reversal Questionnaire of the Isometric Standard, include the following:

Note the Risk Score at any step cannot be negative.

Risk Score Categories:

• 0: Very Low Risk Level (21% buffer)
• 1-2: Low Risk Level (5% buffer)
• 3-4: Medium Risk Level (7% buffer)
• 5+: High Risk Level (10-20% buffer)

Project specific risk factors will depend on the form of carbon being stored (i.e., organic vs. inorganic), the method of storage (e.g., mineralization, encapsulation), the location of carbon storage (e.g., subsurface, ocean), and the proximity of that carbon to potential agents of reversal.

For projects with carbon storage as inorganic carbon, the presence of the following risk factors must be reflected in the risk score corresponding to question 10:

• Acidic fluid
• Alkaline fluid (if stored as dissolved inorganic carbon)
• Temperatures in excess of 800 degrees celsius

For projects with any form of subsurface carbon storage, the presence of the following risk factors must be reflected in the risk score corresponding to question 10:

• Seismicity
• Subsurface migration

18.0 Relevant Works