Sampling and measurements

Measurement approaches may differ by project and must be clearly described in the PDD. Measurements shall be performed by third-party independent labs, and follow a recognized and standardized measurement technique (e.g. ISO 10304 for non-carbonic acid concentration).

Final values selected for use in the specified CDR term must adhere to composite sampling and homogenization requirements outlined in the Sampling protocol section. It is recommended that the values selected for CDR quantification represent either:

• the lower bound of a two-sided 80% confidence interval (for frequentist approach, e.g. bootstrapping), or

• the 10th percentile of a posterior distribution (for Bayesian models, e.g. Monte Carlo simulations).

If these are not used, higher uncertainty is assumed and a larger discount factor should be applied.

Carbon removal measurements

Alkalinity export

The following measurements of alkalinity export ($CDR_{export,\ NFZ}$) are required for projects using Method 1: Direct measurement of export for NFZ removal quantification.

If carbonate system parameters are used, Project Developers shall measure at least two, and ideally three, from the following:

• pH

• Total alkalinity

• Dissolved Inorganic Carbon (DIC)

• [$CO_2$] (Dissolved $CO_2$ concentration)

• [$HCO_3^-$] (Bicarbonate concentration)

• [$CO_3^{2-}$] (Carbonate concentration)

The following adjustments shall be made to direct measurements of alkalinity export, where applicable:

• If total alkalinity is used, Project Developers shall assess and discuss any potential contribution from organic alkalinity.

• If base cation concentration or total alkalinity are used, Project Developers shall account for carbonic acid system speciation to correctly convert these values to DIC (note that in NFZ Method 2: Mass balance, this is accounted for in the term $CDR_{Alk.\ inefficiency}$).

• If carbonate minerals are present, Project Developers shall differentiate weathering sources by identifying if weathering products come from silicate weathering or carbonate mineral dissolution (from feedstock or fertilizers). Net CDR calculations shall be adjusted to remove CDR from weathering of carbonate minerals.

The use of in-situ continuous soil measurements, such as ion exchange resins, to directly measure alkalinity export is a promising area of development. However, these methods must be validated across diverse field conditions to be deemed generally applicable. Rainbow is closely monitoring advancements in this field and may permit the use of in-situ continuous soil measurements under the following conditions:

• General approval for all projects if a scientific consensus is established regarding the representativeness of this approach.

• Case-by-case approval if a Project Developer demonstrates accuracy in field conditions that are similar to the project site, via measurements undertaken directly by the Project Developer, or via secondary sources/existing measurements. This requires comparing novel in-situ measurement results with standard methodologies currently approved in this framework.

Validation may be conducted during the first year of project monitoring, with the option to transition to in-situ methods in later years, subject to approval by the Rainbow Certification team and the VVB.

Validation requirements

• ex-ante sampling plan

• identification of carbonate system parameters/DIC to measure

• baseline pre-spreading concentrations and variability of carbonate system parameters/DIC

• analysis/justification of signal resolvability of given sampling/measurement plan

• plan to adjust results accounting for organic alkalinity, carbonic acid system speciation, and non-carbonic acid weathering

• estimated potential CDR

Verification requirements

• ex-post sampling procedure

• measurement and extraction methods

• measurement of carbonate system parameters and/or major ion

• final adjustments accounting for organic alkalinity, carbonic acid system speciation, and non-carbonic acid weathering

• calculated CDR

Feedstock dissolution

The following measurements for feedstock dissolution ($CDR_{FD}$) are required for projects using NFZ Method 2: Mass balance.

Feedstock dissolution shall be measured via solid-phase soil-based mass balance measurements, comparing the concentration of soluble base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) in soil samples at the beginning and end of the reporting period (or for the first reporting period, directly after feedstock application and at the end of the reporting period). Such measurements may be done:

• Option 1: within the treatment plot, relative to an immobile tracer element that does not dissolve (e.g., Zr, Ti, Nb), or

• Option 2: comparing results between the treatment and control plots, measuring base cations directly (i.e. if immobile tracers are not present or abundant).

A decrease in measured base cations represents a loss of base cations from the solid phase, which suggests weathering is occurring, and represents the potential maximum increase in CDR during that reporting period.

The potential maximum increase in CDR is calculated by first converting solid-phase base cation loss to equivalent bicarbonate formation (according to the base cation charge), and finally converting bicarbonate to CO$_2$eq assuming a 1:1 replacement ratio on a molar basis (although this 1:1 ratio is adjusted later in the Inefficient conversion of alkalinity to CDR term).

Decreasing base cations from the solid phase of the NFZ only suggest CDR because they may be:

• dissolving into the aqueous phase into porewater and successfully driving CDR, or

• going somewhere else accounted for in the other terms such as biomass uptake.

If Option 2 is used to measure feedstock dissolution without an immobile tracer, Project Developers shall justify their approach for addressing the following elements, which are otherwise controlled by the use of an immobile tracer:

• potential decreases in base cation concentrations caused by physical processes (such as wind or water erosion) that may remove base cations from the solid-phase measurement zone, and

• variability in feedstock application rates.

Due to the lack of certainty in interpreting decrease of base cations (i.e. whether it causes CDR, is tracked in other terms, or is untraceable), it is strongly recommended to measure base cation concentration in soil porewater, particularly in cases where significant uncertainty exists regarding the fate of base cations (e.g., high potential for physical transport or secondary mineral formation). If porewater measurements show discrepancies with solid-phase losses, additional verification may be required to adjust CDR estimates accordingly.

Validation requirements

• ex-ante sampling plan

• identify immobile tracer and base cations

• estimate feedstock application rate

• measure baseline soil concentrations and variability of immobile tracer

• analysis/justification of signal resolvability of given sampling/measurement plan

• expected mass-balance equation

• estimated potential CDR

Verification requirements

• ex-post sampling procedure

• measurement and extraction methods

• concentration of immobile tracer and base cation/s (solid phase required, aqueous phase optional)

• calculated potential CDR

Biomass uptake

Biomass cation uptake ($CDR_{biomass}$) shall be measured only in the biomass is that removed from the field. This shall be done by sampling plant tissues and measuring base cation content. All base cations that may contribute to weathering shall be measured.

• For annual crops, this includes measuring base cation concentration of all harvested biomass. Base cation concentration is multiplied by the total mass of biomass removed to obtain total base cations removed.

• For perennial crops, this includes measuring base cation concentration of all new growth biomass. Base cation concentration is multiplied by the total mass of new growth biomass to obtain total base cations removed.

Total base cations removed in the project/treatment fields are compared to total base cations removed in the baseline/control fields, to determine the net loss of base cations from biomass uptake. Base cation uptake from biomass is converted to alkalinity loss which is converted to CDR in tCO$_2$eq.

These measurements are required for all projects using Method 2: Mass balance. For projects using Method 1: Direct measurement of export, this shall only be included if the end of the NFZ, and therefore the depth of weathering product export measurements, is shallower than the root depth.

Validation requirements

• ex-ante sampling plan

• identify base cations to measure

• crop description (annual vs perennial, crop type)

• maximum crop root depth

• expected measurement method

Verification requirements

• ex-post sampling procedure

• measurement and extraction methods

• measured concentration of base cation/s

• total biomass removed (annual crops) or new growth (perennial crops)

• calculated potential CDR loss

Inefficient conversion of alkalinity to CDR

The potential maximum increase in CDR from feedstock dissolution measurements assumes that all base cations released from feedstock are charge balanced by bicarbonate, contributing to the most efficient CDR outcome. This assumption does not account for several sources of inefficiency in base cation release driving CDR, which must be corrected through adjustments to the following ($CDR_{Alk.\ inefficiency}$):

• pH-dependent speciation: carbonic acid system speciation (i.e. ratio of of $CO_2aq$, $HCO_3^-$, and $CO_3^{2-}$) depends on pH. In high pH soils, more carbonate than bicarbonate is present, and when base cations react with carbonate it leads to less CDR than if they had reacted with bicarbonate (1 mole of CO$_2$ removed rather than 2 moles of CO$_2$, respectively). To account for this, Project Developers shall measure the following:

  • Measure at least two carbonate system parameters in the aqueous phase from the list provided in the Alkalinity export measurement section

  • Use a carbonate speciation model (e.g. PHREEQC) to assess the distribution of $CO_2aq$, $HCO_3^-$, and $CO_3^{2-}$.

  • Compare the modeled DIC:Alkalinity ratio to the ideal ratio (typically close to 1 at moderate pH) to determine how much DIC formation is “lost” to high-pH speciation.

  • Alternatively, if direct measurements are not available, apply a conservative correction/loss term by 1) estimating strong acid addition to, or production in, the NFZ, 2) assuming that all of the previously estimated acidity leads to CDR loss

• Non-carbonic acid weathering: weathering by sulfuric, nitric or organic acids instead of carbonate, which releases base cations but does not generate alkalinity and lead to CDR. To account for this, Project Developers shall use one of the following approaches:

  • Directly measure the flux of major anions (nitrate, sulfate, chloride, and dissolved phosphorus ions) in the aqueous phase from the NFZ.

  • If nitric acid from nitrification is the main source of non-carbonic acid weathering, as opposed to the other mentioned anions, then non-carbonic acid weathering can be estimated using documented ammonia fertilizer application rates and as assumed 100% nitrification of ammonia. This may be adjusted with measurements of nitrogen-use efficiency from plant biomass with sufficient proof.

  • If carbonate system parameters cannot be directly measured, Project Developers may use a conservative proxy correction factor to estimate the proportion of weathering driven by non-carbonic acids, as a function of soil pH, from .

• Acid buffering: acidity released from soil exchange sites (exchangeable or bound acidity), which may react with bicarbonate and reverse CDR. To account for this, Project Developers shall measure the following:

  • Measure bound acidity in soil samples and calculate changes over the reporting period

  • Calculate lost CDR assuming a 1:1 molar ratio of bound (or total) acidity neutralized to moles of CO₂ released (exchangeable acidity is already accounted for in the Base cation sorption measurement section below)

These measurements are required for all projects using Method 2: Mass balance. For projects using Method 1: Direct measurement of export, these measurements are not necessary since they are already inherently accounted for in export measurements.

Validation requirements

• ex-ante sampling plan (notably frequency/management of porewater samples, if used)

• planned measurement methods

• pH-dependent speciation: choose direct porewater measurements or conservative deduction

  • If porewater measurements, estimated water volume infiltrated through NFZ soil and two carbonate system parameters to measure

  • If conservative deduction, estimated source and amount of strong acid addition to or production in the NFZ

• Non-carbonic acid weathering: choose direct porewater measurements or nitric acid from fertilizer simplification

  • If porewater measurements, estimated water volume infiltrated through NFZ soil and chosen anions to measure

  • If nitric acid simplification, justification that that nitric acid from nitrification is the main source of non-carbonic acid weathering, and estimated amount of ammonia fertilizer to apply

• Acid buffering: estimated bound acidity in the NFZ

• from all categories, estimated magnitude of potential CDR loss

Verification requirements

• ex-post sampling procedure

• measurement and extraction methods

• pH-dependent speciation

  • If porewater measurements, measured water volume infiltrated through NFZ soil and two carbonate system parameters results

  • If conservative deduction, measured source and amount of strong acid addition to or production in the NFZ

• Non-carbonic acid weathering

  • If porewater measurements, measured water volume infiltrated through NFZ soil and anion concentration results

  • If nitric acid simplification, proof that that nitric acid from nitrification is the main source of non-carbonic acid weathering, and proven amount of ammonia fertilizer applied

• Acid buffering: measured bound acidity in the NFZ

• calculated potential CDR loss

Base cation sorption

Any temporary changes in base cation availability due to adsorption onto soil particle surfaces ($CDR_{sorption}$) must be accounted for in each reporting period. In any given reporting period, this may result in a net adsorption (base cations becoming bound and unavailable) or net desorption (base cations become available again to drive alkalinity generation and CDR), and therefore a net gain or loss of CDR.

This is measured via changes in the stock of base cation in the exchangeable fraction in the NFZ. The exchangeable fraction refers to the base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) that are loosely held on soil particle surfaces (e.g. clay minerals and organic matter) and can be readily exchanged with the soil solution (aqueous phase). Changes in base cation stock are calculated by multiplying changes in cation exchange capacity (CEC) and .

These measurements are required for all projects using Method 2: Mass balance. For projects using Method 1: Direct measurement of export, these measurements are not necessary since they are already inherently accounted for in export measurements.

Validation requirements

• ex-ante sampling plan

• planned extraction and measurement method

• base cations to be measured

• estimated results and magnitude of potential CDR loss

Verification requirements

• ex-post sampling procedure

• measurement and extraction methods

• base cations measured

• base saturation and CEC at the beginning and end of the reporting period

• calculated change in CDR from adsorption/desorption of base cations

Carbonate precipitation

The precipitation and formation of secondary carbonates ($CDR_{carbonate\ precip}$) within the NFZ can:

• Decrease optimal ERW CDR efficiency because base cations are tied up in carbonate minerals instead of remaining in solution to support bicarbonate (HCO₃⁻) export, which is the most effective and preferred pathway for long-term CO₂ removal in ERW, because this pathway results in a 2:1 CO₂ removal ratio (2 moles CO₂ removed per mole of Ca$^{2+}$/Mg $^{2+}$)

• Still contribute to some CDR if the carbonates remain stable over long timescales, as they store CO$_2$ in mineral form, creating a long-term carbon sink. This pathway is less effective because it results in a 1:1 CO₂ removal ratio (1 mole CO₂ removed per mole of Ca$^{2+}$/Mg $^{2+}$)

Secondary carbonate formation can be treated in ERW projects by:

• Method 1 Direct measurement of export:

  • CDR decreases from secondary carbonate formation are already accounted for in the integrated measurements of DIC export, since the corresponding base cations are not measured as being exported.

  • CDR increases from long term CO₂ removal and storage in carbonates are not accounted for in this method, because they could dissolve and carbon removal would be reversed. Plus, upon dissolution and export from the NFZ, they would be measured and counted as CDR. This would result in double counting: CDR from a given base cation can't be counted once as temporary storage in carbonates and a second time as export.

• Method 2 Mass balance:

  • CDR decreases from secondary carbonate formation are already accounted for in the feedstock dissolution measurements, which shall be taken to the depth of the NFZ to fully account for secondary carbonate formation (and any potential dissolution). Projects shall ensure carbonate phases are retained during soil sample processing (e.g., avoid ammonium acetate or acid rinses that remove carbonates).

  • CDR increases from net CO₂ removal and permanent storage in carbonates may be optionally proven using soil inorganic carbon (SIC) measurements, comparing increases in SIC in the treatment and control plots. Such measurements shall prove that newly formed secondary carbonates are driven by ERW. Project Developers shall distinguish newly formed carbonates from background SIC using one of the following methods:

    • Stable isotope analysis (δ¹³C) to confirm that new carbonate formation is derived from atmospheric CO₂.

    • Sequential SIC sampling over time to track ERW-driven changes in carbonate content.

    • Depth-resolved SIC profiling to check if carbonates form at expected ERW-reactive depths.

    • Microscopic mineral analysis (XRD, SEM-EDS) to confirm carbonate crystal morphology and formation process.

See Foundations for Carbon Dioxide Removal Quantification in ERW Deployments for more details and explanation.

Project Developers shall assess how application of agricultural lime to control and treatment plots affects the measurements and how it is accounted for.

These measurements are required for all projects using Method 2: Mass balance, and excluded from projects using Method 1: Direct measurement of export.

Validation requirements

• choice whether to measure CDR increase in the NFZ from secondary carbonate formation.

  • If no, no further requirements.

  • If yes, the following are required:

    • ex-ante sampling plan accounting for baseline variability of SIC and agricultural lime application

    • planned extraction and measurement method

    • estimated results and magnitude of potential CDR loss

Verification requirements

• choice whether to measure CDR increase in the NFZ from secondary carbonate formation.

  • If no, no further requirements.

  • If yes, the following are required:

    • ex-post sampling procedure

    • measurement and extraction methods

    • Newly formed SIC concentration at beginning and end of reporting period in treatment and control plots

    • calculated change in CDR from secondary carbonate precipitation

Silicate precipitation

CDR decreases when dissolved base cations (Ca$^{2+}$, Mg$^{2+}$, K$^{+}$, Na$^{+}$) and silica (SiO$_2$) precipitate into secondary mineral phases, such as clays, amorphous silica, or Fe/Al oxyhydroxides, instead of remaining in solution to drive alkalinity export ($CDR_{silicate\ precip}$).

Projects are not required to measure secondary silicate and other secondary phase formation separately, since its impact on CDR is already accounted for under the two approved measurement methods:

• Method 1: Direct measurement of export: Any decrease in net CDR due to secondary phase formation is already reflected in reduced alkalinity flux at the NFZ outflow

• Method 2: Mass balance: Feedstock dissolution is measured at the depth of the NFZ, reflecting only base cations that are exported from the NFZ (i.e. excluding any that are precipitated into secondary mineral phases in more shallow parts of the NFZ)

Therefore, there are no additional measurement requirements related to this term. It is described here for completeness.

Groundwater FFZ

Similar biogeochemical processes in the NFZ may continue in the lower vadose zone, sometimes stretching meters below the depth of the NFZ, and in groundwaters, leading to CDR loss ($CDR_{groundwater}$). Due to that there are not sufficient models or monitoring tools to assess these processes, these are excluded from CDR calculations. This is an active topic of research that Rainbow is closely monitoring.

Nonetheless, Project Developers shall assess the site hydrology, and provide a qualitative discussion of expected flow paths and residence times through the lower vadose zone and groundwaters in the Site Characterization Report.

During ongoing monitoring and verificatin, there are no additional measurement requirements related to this term.

Surface water FFZ

The following sources of CDR loss in downstream surface waters ($CDR_{surface\ water}$) should be accounted for:

Outgassing from DIC system equilibration

When alkalinity from rock weathering enters the ocean, CO$_2$ may be released into the atmosphere (outgassed) as the two meeting bodies of water adjust their carbonate balance, especially when mixing with water that has different chemistry.

How to calculate:

Justify expected CO$_2$ outgassing using CO$_2$ flux equations for water-air gas exchange, using

1. An average annual or seasonal pH, based on either direct measurements from the project or on reliable databases, and

2. Either direct measurements of surface water temperature and DIC/pCO$_2$, or a conservative estimate of carbonate system parameters, and

3. Assume that water is in full equilibrium with the atmosphere.

Where to measure:

Calculate for the following two locations, and apply the CDR loss result that is greater from the following two calculations:

• In the immediate discharge zone, where the weathering products from a deployment drains into the first surface water system. This zone should already be identified in the Site Characterization Report, based on the regional hydrology.

• In the primary river system of the deployment catchment, specifically the highest-order river segment within the expected hydrological flow area. This river segment should already be identified in the Site Characterization Report, based on the regional hydrology.

Carbonate mineral burial

If surface waters become oversaturated with calcite, there is a higher likelihood that dissolved calcium and carbonate ions will precipitate as carbonate minerals, causing a loss of CDR from alkalinity that does not reach long-term CO₂ storage sinks like the ocean.

Project Developers shall evaluate calcite saturation in the immediate discharge zone to assess the risk of carbonate precipitation (see instructions below). If the discharge waters have a baseline or a post-DIC maximum calcite saturation index (SI) of

• SI > 1, then Project Developers shall model CDR loss from downstream carbonate precipitation

• SI < 1, then this term may be omitted/assumed to be zero.

Project Developers may optionally also measure this for other downstream surface waters before the ocean, but only an assessment at the immediate discharge basin is required.

Project Developers shall describe their sampling plan and justify why it is temporally and spatially representative and sufficient.

Project Developers shall conservatively account for only carbonate precipitation and not its dissolution.

Calculating SI: saturation index shall be calculated using the following equation

$$SI = \log_{10} (\frac{[{Ca}^{2+}] [{CO}_3^{2-}]}{K_{sp}})$$

Where

• $SI$ is the saturation index

• $[Ca^{2+}]$ and $[CO_3^{2-}]$ represent calcium and carbonate ion concentrations, respectively

• $K_{sp}$ represents the solubility product constant of calcite, which is temperature dependent and shall be calculated using a temperature-adjusted solubility equation.

Modeling carbonate burial: If the calculated SI > 1, Project Developers shall use process-based models to model carbonate precipitation and hydrological models to model fluid flow. Model structure, data inputs, key assumptions, model uncertainty, and sources shall be clearly defined, and should be publicly available.

Project Developers shall demonstrate that the model can accurately reproduce relevant background variations, including spatial and temporal fluctuations in carbonate system parameters.

Validation requirements

Outgassing from DIC system equilibration:

• water pH value and data source (description of measurement or secondary source)

• direct measurements of surface water temperature and DIC/pCO$_2$, or a conservative estimate of carbonate system parameters

• identification of the immediate discharge zone and the primary river system (name, GPS coordinates)

• calculated CO$_2$ outgassing for the immediate discharge zone and the primary river system

• overall calculated CDR loss value to apply

• plan of how to account for this loss in upcoming reporting periods, following the requirements outlined in the Temporal allocation of project emissions section

Carbonate mineral burial

• identify the immediate discharge zone (name, GPS coordinates)

• sampling plan and results for calcium and carbonate ion concentrations

• calculated solubility product constant of calcite ($K_{sp}$)

• value of calcite saturation index (SI). If

  • SI < 1: no further requirements

  • SI > 1: description of and results from process-based model to model carbonate precipitation and hydrological model to model fluid flow

  • overall calculated CDR loss value to apply

  • plan of how to account for this loss in upcoming reporting periods, following the requirements outlined in the Temporal allocation of project emissions section

Justification that the site and its hydrology will not lead to substantial organic carbon destabilization downstream

Verification requirements

Outgassing from DIC system equilibration:

• amount of initially estimated CDR loss applied to the reporting period

• any additional CDR loss to consider from successive spreading events during the reporting period

• update of adherence to the requirements outlined in the Temporal allocation of project emissions section

Carbonate mineral burial

• If initial SI results were >1

  • amount of initially estimated CDR loss applied to the reporting period

  • any additional CDR loss to consider from successive spreading events during the reporting period

  • update of adherence to the requirements outlined in the Temporal allocation of project emissions section

Surface ocean FFZ

CDR loss due to outgassing from DIC system equilibration in surface ocean waters ($CDR_{surface\ ocean}$) shall be accounted for. This may be done using models, such as those described for modeling carbonate burial, or using conservative assumptions and thermodynamic storage efficiency calculations.

Such calculations shall assume complete equilibration between the surface ocean carbonic acid system and atmospheric CO$_2$, at representative temperature, salinity, and current atmospheric pCO$_2$ at the time of calculation. These calculation parameters should be obtained from reliable secondary sources for the specific ocean basin where weathering products are expected to flow into, based on hydrological measurements or modeling (e.g. from the database, or output from the model).

One option for estimating the equilibration factor between the surface ocean and atmosphere is to apply the approach from , which suggests a factor of 1.4 to 1.7 for divalent cation sequestration. When adjusted for CO$_2$ equivalence—given the 2:1 ratio of bicarbonate to CO$_2$—this results in a factor range of approximately 0.7 to 0.85. A default value of 0.85, reflecting the global average and commonly used in practice, may be applied where no more region-specific data are available.

This factor, represented as η in the Steinour equation, accounts for the fraction of CO$_2$, ultimately sequestered following equilibration in the surface ocean.

Validation requirements

• identified specific ocean basin into which weathering products are expected to flow

• choice of modeling or thermodynamic storage efficiency calculations approach

• if modeling, description of model and CDR loss from outgassing results

• if calculations, justification and source of calculation parameters, and CDR loss from outgassing results

• overall calculated CDR loss value to apply

• plan of how to account for this loss in upcoming reporting periods, following the requirements outlined in the Temporal allocation of project emissions section

Verification requirements

• amount of initially estimated CDR loss applied to the reporting period

• any additional CDR loss to consider from successive spreading events during the reporting period

• update of adherence to the requirements outlined in the Temporal allocation of project emissions section

Models

Models may be used for several components in this methodology. The uses of models include:

• Feedstock dissolution for ex-ante calculations, estimating provisional credit volumes, and creating expected timeline of weathering and crediting (required)

• Groundwater flow path and residence time models (required)

• Hydrological flow path, determining which ocean basin weathering products will end up in (required)

• FFZ loss models (rivers and oceans)

  • surface water carbonate mineral burial (required if maximum in immediate discharge basin is > 1)

  • ocean outgassing (optional, may be conservatively calculated using simple conversions instead)

Models used shall be transparently described in the PDD, including a description of the overall structure of the model, key sources/references, assumptions, input data, and secondary/fixed data used.

The use of models beyond the requirements and outside the purpose of crediting (e.g. reactive transport models) is encouraged for the advancement of the scientific field, and to facilitate model use in MRV in the future, but is not required for carbon credit issuance. See the co-benefits section on how this work is accounted for.

Sampling plan for monitoring

Project Developers shall describe their ex-ante Sampling Plan for monitoring the measurements that are used in GHG quantification. This shall be prepared during project validation, before any rock is spread. It shall reference the described above to justify the sampling protocol will ensure signal resolvability, representativeness, and minimize bias. Secondary sources and desk research may also be used as sources to justify the sampling protocol.

Project Developers may revise the Sampling Plan and/or Monitoring Plan throughout the crediting period, based on initial measurement results, or as scientific knowledge and best practices evolve. Any proposed changes should be submitted to Rainbow before implementation. Depending on the scope of the revisions, updates may require auditing and approval by the VVB. Failure to obtain prior approval may result in measurements being rejected for credit verification and issuance, due to substantial deviations from the audited and agreed-upon Monitoring Plan.

Ideally, sampling events should align exactly with the Sampling Plan. However, given real-world challenges that may arise during monitoring, some deviations are expected. Project Developers shall note any deviations from the Sampling Plan during monitoring, addressing all points listed in the Sampling Plan components section below. Any deviations shall be documented in the Monitoring Report during verification.

Sampling Plan components

The Sampling Plan shall include descriptions of:

• field area coordinates of sampling/monitoring sites (center and radius for sub-samples)

• CDR quantification approach (Method 1: Direct measurement of export or Method 2: Term balance gain-loss) and a list of each measurement that will be used/required by the project

• number of samples taken per strata (see Number of samples section below)

• stratification approach and results

• sampling pattern approach (random, grid, transect, targeted...), with a justification of why the pattern is suitable for the element being measured, the characteristics of the project area, and the monitoring objectives

• sampling steps e.g. depth, coring technique, laboratory techniques, storage, compositing, instruments/methods, approach for reducing/determining analytical error

• averaging, compositing and grouping of data, and plan for handling missing data

• any sampling components that are not fixed, and are anticipated to deviate from the original plan as the project continues operations and gathers more data (approval of such contingencies here may avoid the need to audit changes to the Sampling Plan later)

• frequency of sampling

The Sampling plan shall include a detailed description of the sampling frequency for each stage of the project. At a minimum, the following time points must be addressed:

• Pilot sampling: Conducted well before rock spreading to support site characterization and inform stratification. (Mandatory)

• Baseline sampling: Performed immediately prior to rock application to document pre-spreading conditions. (Mandatory)

• Post-spreading sampling: Conducted shortly after rock application (e.g. within 30 days) to capture early-phase responses. (Optional)

• Ongoing monitoring: Sampling conducted over the course of the project to track changes and quantify CDR over time. (Mandatory – at least once per reporting period; recommended frequency is annually or more frequently depending on site conditions and project design)

For aqueous-phase samples (e.g. porewater or drainage water), the sampling frequency must be specifically justified, considering site-specific hydrological factors such as precipitation patterns and irrigation events.

Projects using Method 2: Solid phase mass balance for CDR quantification are required to perform sample compositing. This means that for one monitoring site, one sample is sent for laboratory analysis, but that sample is composed of many sub-samples (between 6-20). These sub-samples are taken from a small geographical area, within a defined radius of the center monitoring site point, and are homogenized to form one sample.

Number of aqueous samples per strata

The following sampling requirements apply to projects pursuing Method 1: Direct measurement of export for CDR quantification.

Project Developers shall justify the following in the Site Characterization Report, accounting for site hydrology and temporal and spatial variability of measurements of weathering product concentration and water flux through the NFZ:

• total number of samples

• statistical power of sample number

• spatial placement of sampling points per strata

• frequency of sampling

• temporal and spatial interpolation methods

It is recommended that this justification be based on power analysis of baseline variability of dissolved species being measured (e.g. alkalinity, base cation concentration...), measured in sampling before any rock is spread, but other justifications will be considered on a case by case basis. Alternatively, this could be justified using the variability of other factors that affect hydrology and weathering rates such as topography, soil and buffer pH, base saturation, soil texture...

The number of samples shall be sufficient to establish a statistically significant time-integrated export of carbonate system parameters (e.g. alkalinity, DIC) and/or major ion concentrations (e.g. base cations $Ca^{2+},\ Mg^{2+}$, major anion) at the end of the NFZ, between the treatment and control plots. It is in the Project Developer's best interest to ensure enough samples are taken to obtain a statistically significant result, otherwise no significant CDR will be detected and no credits issued (see CDR verification and credit issuance section for credit issuance requirements).

Treatment and control plot samples must be time-paired to minimize temporal variability. Samples from control and treatment plots shall be collected within a timeframe that ensures comparable environmental conditions across all samples.

If no porewater can be extracted from the soil due to dry conditions, it shall be assumed by default that no CDR is occurring. This may be modified given sufficiently justified temporal interpolation methods.

The use of process-based models for temporal interpolation is an active area of research, aiming to predict changes in soil water chemistry and alkalinity concentration based on variations in water flux. However, due to high uncertainty, current models are considered insufficient by scientific experts for reliable predictions. Rainbow is closely monitoring advancements in this field and may permit the use of models once a scientific consensus is established.

Number of soil samples per strata

The following sampling requirements apply to projects pursuing Method 2: Mass balance for CDR quantification.

The necessary sampling density to obtain a statistically significant result is dependent on the baseline variability of the soil. Therefore, no fixed sampling density can be recommended, and this must be determined separately for each stratum in each project. The number of samples per stratum should be defined using a power analysis based on the baseline mean and variance of base cation concentration.

The following approach is recommended, but Project Developers may propose and justify an alternative approach if it is more relevant for their project-specific conditions.

Treatment plots: The number of samples needed per treatment plot per stratum should be determined using a paired T-Test power analysis on the expected mean difference, effect size, or minimum detectable change.

Control plots: The control plots may use the same sampling density as the treatment plot in the corresponding stratum, or a power analysis considering the absolute or relative allowable error may be used to determine the number of samples needed for control plots.

It is in the Project Developer's best interest to ensure enough samples are taken to obtain a statistically significant result, otherwise no significant CDR will be detected and no credits issued (see CDR verification and credit issuance section for credit issuance requirements).