GHG quantification

General GHG quantification rules can be found in the Riverse Standard Rules.

Calculations of GHG emissions for the baseline and project scenarios shall follow a robust, recognized method and good practice guidance. The overall methodological approach is a comparative life cycle assessment (LCA) at the project-scale, based on .

Functional unit

The functional unit shall be 1 tonne of crushed rock/mineral applied to the soil.

System boundary

Net CDR shall be calculated for each reporting period as the following, where $P$ signifies the project scenario/treatment plots, and $B$ signifies the baseline scenario/control plots:

$$\textbf{(Eq.1)}\ Net_{CDR} = Project_{CDR}- Baseline_{CDR} -Project_{emissions}$$

Where,

• $Net_{CDR}$ represents the net carbon removals caused by project in the reporting period, in $tCO_2eq$, and equals the amount of removal RCCs to issue.

• $Project_{CDR}$ represents the net CDR from the Project Scenario, in $tCO_2eq$, calculated in Equation 2.

• $Baseline_{CDR}$ represents the net CDR from the Baseline scenario, in $tCO_2eq$, calculated in Equation 10.

• $Project_{emissions}$ represents the induced emissions caused by the project, in $tCO_2eq$, is calculated in Equation 3.

Project scenario

The net CDR from the project scenario is determined by measuring the CDR gains in the (NFZ) ($NFZ_{P,\ net\ removal}$), and subtracting the CDR losses in the (FFZ) ($FFZ_{P,\ loss}$) and the GHG emissions from upstream and onsite activities ($Project_{emissions}$), using the following equations:

$$\textbf{(Eq.2)}\ Project_{CDR} = NFZ_{P\, net\ removal}-FFZ_{P\, loss}$$

Where,

• $Project_{CDR}$ was described in Equation 1.

• $NFZ_{P\, net\ removal}$ represents the net gain in carbon removal measured in the project/treatment area's NFZ in $tCO_2eq$, detailed in sections Method 1: Direct measurement of export and Method 2: Mass balance. Project Developers shall choose one of the the two methods shall be used to quantify the amount of CDR occurring in the NFZ. It is calculated in Equation 4.

• $FFZ_{B\, loss}$ represents the loss/reversal of carbon removal in the project FFZ in $tCO_2eq$, detailed in the section Project FFZ loss.

$$\textbf{(Eq.3)}\ Project_{emissions} = E_{extraction}+E_{processing}+E_{transport}$$

Where,

• $Project_{emissions}$ represents the induced emissions caused by the project, in $tCO_2eq$.

• $E_{extraction}$ represents the induced emissions caused by extraction of feedstock in the Project Scenario, in $tCO_2eq$, detailed in the Project induced emissions section, and calculated in Equation 7.

• $E_{processing}$ represents the induced emissions caused by processing of feedstock in the Project Scenario, in $tCO_2eq$, detailed in the Project induced emissions section, and calculated in Equation 8.

• $E_{transport}$ represents the induced emissions caused by transport of feedstock in the Project Scenario, in $tCO_2eq$, detailed in the Project induced emissions section, and calculated in Equation 9.

NFZ Method 1: Direct measurement of export

NFZ removal is calculated by measuring the export of carbonate alkalinity exceeding a counterfactual baseline, integrated across the duration of the reporting period.

Specifically, this is calculated by combining alkalinity concentrations—derived from a time series or in-situ measurements of porewater or drainage water—with water flux measurements at the NFZ boundary of the project/treatment plot, and comparing them to corresponding measurements from a representative baseline/control plot.

This is achieved through aqueous-phase measurements in each reporting period

What to measure

• carbonate system parameters (e.g. alkalinity, DIC)

• major ion concentrations (e.g. base cations $Ca^{2+},\ Mg^{2+}$, major anions)

See the Measurements and data sources section for details on which measurements may be used.

Where to measure

Either of the measurements listed on the left can be measured in:

• soil porewater at the end/depth of the NFZ

• drainage or catchment waters beyond the NFZ

The specific NFZ boundary is defined by the site hydrology, detailed in the Site Characterization section.

Sampling requirements for aqueous phase samples are described in the Number of aqueous samples per strata section.

Both project and baseline (i.e. treatment and control plot) NFZ removal ($NFZ_{net\ removal}$) shall be calculated according to the following equations.

Calculations: NFZ removal Method 1: Direct measurement of export

All terms have a spatial scope of the NFZ and a temporal scope of one reporting period, and have units of tCO$_2$eq.

$\textbf{(Eq.4)}\ NFZ_{P,\ net\ removal} = \sum_{time} CDR_{export,\ NFZ}$

Where

• $NFZ_{P,\ net\ removal}$ is described in Equation 2, and represents the net CDR occurring in the NFZ during the reporting period for the project scenario (i.e. treatment plots).

• $CDR_{export,\ NFZ}$ represents the sum of measured alkalinity export from the NFZ, time-integrated over the reporting period. Further details on how to measure each component are presented in the Measurements and data sources section.

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NFZ Method 2: Mass balance

In this method, the potential maximum CDR from alkalinity release from the feedstock is measured in each reporting period, along with adjustments to net CDR based on changes in soil inorganic carbon stocks. These adjustments account for:

• Cation uptake in biomass

• Cation retention on sorption sites

• Secondary carbonate formation, and

• Alkalinity that is not charge balanced by bicarbonate.

NFZ removal is calculated using Equation 5, following these steps in each reporting period:

1

Measure base cation release

Base cations released from feedstock dissolution are measured during the reporting period.

2

Calculate potential CDR

The cation release measurements are used to calculate the potential CDR associated with the present reporting period (CDR gains).

3

Adjust by inorganic carbon changes

Calculated potential CDR is adjusted by considering changes in each potential carbon loss term relative to the previous reporting period. These could result in either CDR loss or additional CDR (positive or negative signs).

After accounting for each carbon loss term, the remaining alkalinity from feedstock dissolution reflects the additional alkalinity that remains in solution and is transported beyond the NFZ into the FFZ, where further CDR loss terms are applied (see Project FFZ loss section).

This method is based mostly on solid soil measurements, but may also include some porewater measurements (see the Feedstock dissolution and Inefficient conversion of alkalinity to CDR sections).

Calculations: NFZ removal Method 2: Mass balance

All terms have a spatial scope of the NFZ and a temporal scope of one reporting period, and have units of tCO$_2$eq. Details for how to measure each component are presented in the Measurements and data sources section.

$\textbf{(Eq.5)}\ NFZ_{P,\ net\ removal} = CDR_{FD}-CDR_{biomass}-CDR_{Alk\ inefficiency}-CDR_{sorption}-CDR_{carbonate\ precip}-CDR_{silicate\ precip}$

• $NFZ_{P,\ net\ removal}$ represents the net CDR occurring in the NFZ during the reporting period. This is used for both project (treatment) and baseline (control) scenarios. It accounts for both removal and loss of carbon via CDR. It represents the same term as in Equation 5, but is calculated using a different method.

• $CDR_{FD}$ represents the increase in CDR as the potential theoretical maximum CDR associated with the measured release and loss of base cations from feedstock dissolution during that reporting period.

• $CDR_{biomass}$ represents the permanent decrease in CDR from cation uptake into biomass.

• $CDR_{Alk\ inefficiency}$ represents the permanent decrease in CDR from generated alkalinity that is not charge balanced by bicarbonate, for a variety of reasons, ensuring permanent CDR. This is due notably to pH-dependent carbonic acid system speciation; non-carbonic acid weathering from sulfuric, nitric or organic acids; and acid buffering.

• $CDR_{sorption}$ represents the temporary decrease in CDR from base cation sorption on soil exchange sites or reductions in exchangeable acidity. This value notably may be positive or negative, suggesting that in a given reporting period there may be a net de-sorption i.e. addition of base cations to the soil.

• $CDR_{carbonate\ precip}$ represents the change in CDR from the precipitation of secondary carbonate minerals in the NFZ.

• $CDR_{silicate\ precip}$ represents the permanent decrease in CDR from the formation of secondary silicate minerals. Its measurement is not required in this methodology, since it is assumed to be covered by $CDR_{FD}$ when taking measurements sufficiently deep in the NFZ (see the Silicate precipitation section below for details).

Changes in soil organic carbon (SOC) are currently not required for carbon credit issuance due to high uncertainty, yet may be important for gains or losses in CDR. Riverse is actively monitoring research on this topic, and may add SOC measurement requirements in future versions of this methodology.

Project FFZ Loss

FFZ losses shall be considered in surface water and surface oceans (see Equation 6). The project site's specific hydrology, with expected flow paths and residence times, shall be taken into account, according to the Site Characterization Report.

FFZ losses are expected to occur over thousands of years; however, for the purposes of RCC issuance, they must be estimated upfront based on the total potential CDR and proportionally allocated across reporting periods according to the amount of CDR reported and credited. These losses shall be amortized and accounted for using the same approach applied to upstream project emissions, as described below.

If Project Developers can prove that weathering products will not pass through surface water, and will instead travel straight to groundwater and then the ocean, then the surface water CDR loss may be omitted.

As detailed in the Site characterization section, Project Developers shall describe the expected final CDR reservoir, indicating whether carbon is ultimately stored as bicarbonate ions in the ocean or as carbonate minerals within the watershed. If carbonate minerals are a significant reservoir, developers must assess the risk of strong acid weathering of this carbonate; if the risk is high, it must be accounted for as a deduction in the project's CDR through the Reversal Risk assessment.

Calculations: FFZ loss

All terms have a temporal scope of 1000 years, and are allocated to reporting periods proportionally to the amount of CDR reported. All terms have units of tCO$_2$eq. Details for how to measure each component are presented in the Measurements and data sources section.

$\textbf{(Eq.6)}\ FFZ_{P,\ loss} = CDR_{groundwater}+CDR_{surface\ water}+CDR_{surface\ ocean}$

Where

• $FFZ_{loss}$ represents the total losses of CDR expected for the entire amount of spread rocks.

• $CDR_{groundwater}$ represents the CDR loss due to permanent alkalinity sinks in groundwater/in the lower vadose zone. Its measurement is not required in this methodology, due to modeling and monitoring limitations (see the Groundwater section below for details)

• $CDR_{surface\ water}$ represents the CDR loss due to permanent alkalinity sinks and CO$_2$ evasion in surface waters, such as rivers and lakes (see the Surface water FFZ section below for details).

• $CDR_{surface\ ocean}$ represents the CDR loss due to permanent alkalinity sinks and carbonic acid system re-equilibration in the ocean (see the Surface ocean FFZ section below for details).

Project induced emissions

System boundary

The project system boundary shall include the following processes:

Feedstock extraction

Mining and extracting feedstock. This shall be omitted if it is proven that the feedstock is waste from other mining activities. Feedstock shall be considered waste if it has no economic value and would not have been used otherwise. Emissions from the following activities shall be included:

• Electricity production

• Fuel production and combustion

• Water provisioning

• Material production and waste treatment

• Equipment production and waste treatment

Feedstock processing

Feedstock crushing and storing, including emissions from:

• Electricity production

• Fuel production and combustion

• Water provisioning

• Material production and waste treatment

• Equipment production and waste treatment

Transport and spreading

Transporting feedstock:

1. from the mine to the project processing site and

2. from the processing site to the spreading areas.

Emissions from the following activities shall be included:

• Energy production and use

  • Fuel production and combustion

  • Electricity production

• Embodied emissions

  • Vehicle production and end of life

  • Road production and maintenance

See the Transportation module from the BiCRS methodology for more details and equations, which shall also be applied here. Project Developers may choose between the Fuel amount, Fuel-efficiency or the Distance-based approach for monitoring and calculating transport energy use emissions.

Any process that is shown during a screening LCA to have contribute than 1% of the total induced emissions and removals may be excluded, up until the cumulative excluded processes exceed 3% of total induced emissions and removals.

Data sources

The required primary data for project emissions calculations are presented in Table 1. These data may be estimated for ex-ante validation and updated with real production values and proof for ex-post verification, depending on the data type, and shall be made publicly available. Additional data sources for processes not explicitly listed should be added if they are within the 1% cutoff threshold mentioned above.

Table 1 Summary of primary data needed from projects and their source for initial project certification and validation. Several data points need to be updated annually during verification if the upon a successive spreading events (see Monitoring Plan section).

Parameter	Unit	Source
Detailed process diagram with included/excluded processes	Flow chart	Internal process documents
Waste status of feedstock	Text description	Contract with feedstock provider, receipts, invoices
Energy amount and type for feedstock extraction	kWh, MJ, liters	Operating records, machinery/equipment tracking, invoices, bills, receipts
Energy amount and type for feedstock processing	kWh, MJ, liters	Operating records, machinery/equipment tracking, invoices, bills, receipts
Transport data	-	See the Transportation module from the BiCRS methodology for more details
Type of other input/emission*	Text description	Internal process documents
Amount of other input/emission*	kg, liter, kWh, MWh, GWh, m$^3$	Meter readings, bills, internal tracking documents, invoices, contracts, gas analyzers or sensors on pyrolysis equipment, calculated using conversions from other primary project data

The version 3.10 (hereafter referred to as ecoinvent) shall be the main source of emission factors unless otherwise specified. Ecoinvent is preferred because it is traceable, reliable, and well-recognized. The ecoinvent processes selected are detailed in Appendix.

If the available emission factors do not accurately represent the project, a different emission factor may be submitted by the Project Developer, and approved by the Riverse Certification team and the VVB. Any emission factor must meet the data requirements outlined in the Riverse Standard Rules, and come from traceable, transparent, unbiased, and reputable sources.

Temporal allocation of project emissions

Project emissions shall be allocated across the reporting periods. This allocation shall be done in a way that ensures that all upstream emissions are accounted for within the first 50% of potential CDR (as modeled from Potential CDR over the project lifetime calculations, described in the Feedstock characterization section). The distribution may be done proportionally to the amount of CDR completed/credits issued in each reporting period, or may be done more upfront to incur the induced emissions early on to reduce uncertainty later.

For example, if the project's estimated CDR potential is 1,000 tCO$_2$eq over 10 years, with the following repartition:

• Year 1: 100 tCO$_2$eq

• Year 2: 300 tCO$_2$eq

• Year 3: 200 tCO$_2$eq

• Year 4: 100 tCO$_2$eq

• Year 5-10: 50 tCO$_2$eq/year

Then the 50% mark is 500 tCO$_2$eq, which the project is expected to reach in reporting period of year 3.

The project's upstream emissions represent 100 tCO$_2$eq.

These 100 tCO$_2$eq shall be allocated across the first 500 credits issued, expected to occur within the first 3 years of the project. They may be allocated proportionally over the amount of CDR reported in that period, following:

• Year 1: 100/500 = 20% of induced emissions allocated to this reporting period = 20 tCO$_2$eq

• Year 2: 300/500 = 60% of induced emissions allocated to this reporting period = 60 tCO$_2$eq

• Year 3: 100/500 = 20% of induced emissions allocated to this reporting period = 20 tCO$_2$eq

The actual rate and amount of CDR may differ from the initial estimates. The allocation of upstream emissions shall always aim to be counted within the first 50% of estimated CDR, and the actual year in which this is realized may vary, coming earlier or later than initially estimated. Project Developers should anticipate this and incur their induced emissions accordingly/conservatively.

Calculations- Project emissions

Project induced emissions shall be calculated using Equation 4 above. Each term in Equation 3 is calculated using the following

$\textbf{(Eq.7)}\ E_{\text{Extraction}} = \sum (\text{Amount}_i \times EF_i)$

Where,

• $E_{\text{extraction}}$ represents the emissions from all relevant processes related to feedstock extraction.

• $Amount_i$ represents the amount of the input/emission of type $i$, in the same units as the emission factor.

• $EF_i$ represents the emission factor for the input/emission of type $i$ in kg CO$_2$eq per given unit from ecoinvent (see Appendix 1 for ecoinvent database options).

$\textbf{(Eq.8)}\ E_{\text{processing}} = \sum (\text{Amount}_i \times EF_i)$

Where,

• $E_{\text{processing}}$ represents the emissions from all relevant processes related to feedstock processing.

• $Amount_i$ and $EF_i$ are described in Equation 7.

$\textbf{(Eq.9)}\ E_{\text{transport}} = E_{transport\ energy}+E_{transport\ embodied}$

Where,

• $E_{\text{transport\ energy}}$ is calculated using the equations in the Transportation module from the BiCRS methodology, using either the Energy amount, Energy efficiency or Distance based approach.

• $E_{transport\ embodied}$ is calculated using the equations in the Transportation module from the BiCRS methodology, from the Embodied emissions section.

Baseline scenario

The baseline scenario shall represent the conditions or practices that would occur in the absence of the project. Only removals are considered in the baseline scenario, not induced emissions, to ensure that the project is only credited for removals it causes beyond business-as-usual removals that would have happened anyway. This includes changes in CDR due to:

• use of pH adjusting products on agricultural fields where feedstock is spread (e.g. agricultural lime)

• cropping and tillage on agricultural fields where feedstock is spread

• fertilizer use on agricultural fields where feedstock is spread

• irrigation on agricultural fields where feedstock is spread

• weathering in waste feedstock piles

This is calculated using:

$$\textbf{(Eq.10)}\ Baseline_{CDR} = NFZ_{B\, net\ removal}+Feedstock_{B,\ removal}-FFZ_{B\, loss}$$

Where,

• $NFZ_{B\, net\ removal}$ represents the net gain in carbon removal measured in the baseline/control area's NFZ in $tCO_2eq$, measured using the same approach as in the Project Scenario, detailed in section Baseline NFZ.

• $Feedstock_{B,\ removal}$ represents the net gain in carbon removal measured in the business as usual management of the rock feedstock used in the project (if it is waste rock), in $tCO_2eq$, detailed in section Baseline Feedstock CDR.

• $FFZ_{P\, loss}$ represents the loss/reversal of carbon removal in the baseline FFZ in $tCO_2eq$, detailed in the section Baseline FFZ loss.

By default, the overall structure of the baseline scenario for a given project is valid for the entire crediting period. This may change if the Project Developer informs Riverse of a material change in their operations or in baseline conditions, and/or if the methodology undergoes revisions that change the baseline scenario. Note that the actual values in the baseline scenario are updated in each reporting period.

Baseline NFZ

Spreading of pH adjusting products (e.g. agricultural lime) to agricultural fields can result in either CDR gains, losses, or neutral effects, or several different effects at different times in the process (e.g. short-term losses but long-term gains). ERW projects shall only be issued removal credits for the removals they cause beyond any baseline CDR gains.

To account for this, control plots are managed and monitored to measure CDR gains ($NFZ_{B,\ net\ removal}$) that would have occurred in absence of the project, to deduct from the project CDR. Quantification of baseline CDR in the NFZ on fields shall be done by:

• NFZ in a BAU control plot: applying NFZ method 1 or NFZ method 2 to the baseline/control plots to measure and calculate $NFZ_{B,\ net\ removal}$. The same Method must be chosen for both the control and treatment plots. and

• NFZ in a negative control plot: assume that all agricultural lime that would have been used generates CDR at 100% efficiency with nearly no CDR loss

See the Control plot section for more details on how to set up, justify, and monitor control plots.

Baseline NFZ results shall only be included in the project net CDR quantification if they result in CDR gain (i.e. if the value is negative). If the use of agricultural lime in the baseline scenario is determined to be a net source of emissions, it's value shall be considered 0 tCO2eq, to ensure avoided emissions from liming are not counted towards project CDR.

If negative control plots are used, it shall be conservatively assumed that all agricultural lime that would have hypothetically been used (e.g. using regional statistics or historical records) dissolves and generates CDR at 100% efficiency, with negligible carbon loss terms.

Only loss from non-carbonic weathering adjustments may be considered, using the same assumptions and calculations as in the project scenario/treatment plots.

This baseline CDR should be subtracted from the project CDR in the reporting period when the agricultural lime spreading would have occurred.

Baseline FFZ loss

Loss of CDR from processes in the FFZ ($FFZ_{B,\ loss}$) shall be calculated by applying the same models, calculations and assumptions from the Project FFZ loss section to the control plots.

Baseline feedstock CDR

The baseline scenario shall also consider CDR from the alternative fate of feedstock ($Feedstock_{B,\ removal}$), if the project uses waste feedstock e.g. from mining, that would have otherwise been stored and exposed to the atmosphere and driven CDR.

Project Developers shall

• Describe the alternative fate of feedstock.

• If waste, characterize the feedstock's inorganic carbon content, mineral and waste handling practices, and expected storage conditions.

• Explain assumptions around if/how this storage leads to baseline CDR.

• Justify any expectations of zero ambient weathering from the mineral feedstock.

If baseline weathering is not assumed to be zero, Project Developers shall model CDR from baseline weathering for the surface layer of feedstock, considering the same factors described in the Feedstock Characterization section, plus the environmental conditions where feedstock is stored (e.g. rainfall, temperature...). If it is estimated based on a preliminary LCA and modeling results to be <1% of emissions, it may be omitted, per the Riverse Standard Rules.

Measurements and data sources

Measurement approaches may differ by project and must be clearly described in the PDD.

Final values selected for use in the specified CDR term must adhere to composite sampling and homogenization requirements outlined in the Sampling protocol section. For CDR quantification, 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), or

• the 10th percentile of a posterior distribution (for Bayesian models).

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

Alkalinity export ($CDR_{export,\ NFZ}$)

The following measurements 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. Riverse 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 Riverse 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 ($CDR_{FD}$)

The following measurements 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:

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

• comparing results between the treatment and control plots, measuring base cations directly (i.e. when immobile tracers are not present in the feedstock).

A decrease in measured base cations represents a loss of base cations from the solid phase, which 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.

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 ($CDR_{biomass}$)

Biomass cation uptake 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 ($CDR_{Alk.\ inefficiency}$)

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 (see Table 2 for a summary):

• 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 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 measure the following:

  • directly measure the flux of nitrate, sulfate, chloride, and dissolved phosphorus ions in the aqueous phase from the NFZ , or

  • if it can be proven that nitric acid from nitrification is the main source of non-carbonic acid weathering, as opposed to the other mentioned ions, 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.

• 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 ($CDR_{sorption}$)

The temporary changes in base cation availability due to adsorption onto soil particle surfaces 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 ($CDR_{carbonate\ precip}$)

The precipitation and formation of secondary carbonates 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 ($CDR_{silicate\ precip}$)

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.

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 ($CDR_{groundwater}$)

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. 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 Riverse 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.

Validation requirements

• qualitative discussion of expected flow paths and residence times through the lower vadose zone and groundwaters in the Site Characterization Report.

Verification requirements

• none

Surface water FFZ ($CDR_{surface\ water}$)

The following sources of CDR loss in downstream surface waters 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.

• In the primary river system of the deployment catchment, specifically the highest-order river segment within the expected hydrological flow area.

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.

Additionally, Project Developers shall provide a qualitative justification that the site and its hydrology will not lead to substantial organic carbon destabilization downstream, which can occur e.g. in tropical peatlands (in the Site Characterization Report). Although this may be a substantial source of FFZ CDR loss, tools do not yet exist to reliably quantify it.

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 ($CDR_{surface\ ocean}$)

CDR loss due to outgassing from DIC system equilibration in surface ocean waters 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 modeling results.

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.

Assumptions

• When calculating $CDR_{FD}$ (the increase in CDR as the potential theoretical maximum CDR associated with the measured release and loss of base cations), it is assumed that all base cations released through feedstock dissolution are fully charge-balanced by bicarbonate (HCO₃⁻). This is later adjusted in the Inefficient conversion of alkalinity to CDR term.

• Organic acids are assumed to degrade after reacting with silicate minerals, producing dissolved inorganic carbon (DIC) equivalent to that from carbonic acid weathering.

• It is assumed that no net CDR gains occur in the FFZ; only potential losses are considered, a conservative approach that may underestimate CDR in acidic soils where feedstock dissolution is high.

• Secondary silicate and carbonate precipitation are already accounted for through integrated weathering DIC export measurements in NFZ Method 1 and full-depth solid-phase soil assessments of feedstock dissolution in NFZ Method 2.

Uncertainty assessment

See general instructions for uncertainty assessment in the Riverse Standard Rules. The outcome of the assessment shall be used to determine the percent of avoided emissions to eliminate with the .

The following assumptions have low uncertainty:

• Calculating $CDR_{FD}$ assumes that all base cations released through feedstock dissolution are fully charge-balanced by bicarbonate (HCO₃⁻)

• Organic acids are assumed to degrade after reacting with silicate minerals, producing dissolved inorganic carbon (DIC) equivalent to that from carbonic acid weathering.

• Secondary silicate and carbonate precipitation are already accounted for through integrated weathering DIC export measurements in NFZ Method 1 and full-depth solid-phase soil assessments of feedstock dissolution in NFZ Method 2.

The following assumptions have high uncertainty:

• It is assumed that no net CDR gains occur in the FFZ. This is a very conservative assumption that leads to under-crediting.

The baseline scenario selection guidance in this methodology has low uncertainty because it is based on project-specific information and is well known.

The equations presented in this methodology have low uncertainty because they consist of basic operations. The uncertainty in equations used at the project-level to convert measurements into CDR shall be assessed for each project.

No estimates or secondary data are used as a default for projects in this methodology.

Uncertainty of models and measurements shall be assessed at the project level. Project Developers shall justify the statistical methods used to assess uncertainty, such as frequentist approaches, Bayesian approaches, or error propagation, and transparently disclose assumptions, equations and results.

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), or

• the 10th percentile of a posterior distribution (for Bayesian models).

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

The minimum uncertainty at the methodology level is estimated to be low. This translates to an expected discount factor of at least 3% for projects under this methodology. Project-specific factors may introduce higher uncertainty and justify higher discount factors for given projects.

Appendix

Appendix 1: Ecoinvent processes

The table below presents a non-exhaustive selection of Ecoinvent activities that may be used in the GHG calculations for this module. Additional activities may be used for any project, if the following selection does not cover all relevant activities.

Table A1 List of ecoinvent 3.10 processes used in the GHG reduction quantification model, all processes are from the cutoff database

Input	Ecoinvent activity name
grid electricity	market for electricity, low voltage market for electricity, medium voltage
onsite solar electricity	electricity production, photovoltaic, 570kWp open ground installation, multi-Si
diesel fuel material	market for diesel, low-sulfur market for diesel
diesel burning	diesel, burned in agricultural machinery diesel, burned in diesel-electric generating set, 18.5kW
natural gas burning	natural gas, burned in gas turbine
heat, from steam	market for heat, from steam, in chemical industry
heat, from municipal incineration	heat, from municipal waste incineration to generic market for heat district or industrial, other than natural gas
heat, from biomethane burning	market for heat, central or small-scale, biomethane
heat, from straw burning in a furnace	heat production, straw, at furnace 300kW
heat, from natural gas	market for heat, district or industrial, natural gas market for heat, central or small-scale, natural gas
water	market for tap water market for water, decarbonised market for water, deionised
non-hazardous landfill	market for process-specific burdens, slag landfill market for process-specific burdens, sanitary landfill market for process-specific burdens, inert material landfill
hazardous waste treatment	market for hazardous waste, for incineration market for hazardous waste, for underground deposit