Site characterization and monitoring

Scope of the site

The compilation of sites where spreading and monitoring occurs is called the project area. Each project registered with Riverse shall have one project area which is described in the initial Site Characterization Report contained in the validated PDD, plus any additional appendix Site Characterization Reports when new sites are added for successive spreading events. The project area is composed of fields/sites which will be statistically analyzed together with similar:

• Administrative oversight and jurisdiction

• Geographic area

• Timing of spreading (less than 5 years between consecutive spreading events)

The depth of the NFZ shall be 15 centimeters, or the tillage depth plus a 10 centimeter buffer, whichever is deeper. Deeper or shallower NFZs, related to e.g. the depth of the water table or of plant roots, may be considered on a case by case basis if the Project Developer provides sufficient justification based on site hydrology or agronomic practices.

For advancement of ERW scientific understanding, it is recommended that on a small representative subset of the deployment area, samples are taken beyond the depth of the NFZ (60-100 centimeters, or the end of a relevant soil horizon), in order to research inner-NFZ transformations such as secondary mineral precipitation and cation sorption.

Site characterization report

Project Developers shall submit a Site Characterization Report as an appendix to the PDD during project validation, before any rock spreading occurs, that:

• justifies why the project area is appropriate for ERW under baseline conditions before any rock spreading occurs, and is expected to lead to measurable CDR

• delineates the spatial extent and field areas contained within the project area

• describes any stratification approach used and the stratification results

• details the number, location and management strategy of control plots

• describes the site sampling plan (number of samples to take and sampling approach)

• provides results and sources for all components listed in Table 1 for the site, soil and FFZ.

The characteristics listed in Table 1 shall be reported in the Site Characterization Report with evidence. This evidence may include measurements from , data from farmers, secondary databases, remote sensing, or other well documented, rigorous and reliable sources.

These characteristics shall be used to justify the modeling estimates, the control/treatment structure, sampling protocol, and the measurement plans (e.g. where to install measurement infrastructure, at what depth, frequency of measurements...).

Table 1 The characteristics outlined in the table shall be included in the Site Characterization Report, prepared by the Project Developer for project validation.

Site

The following characteristics apply to the Near Field Zone (NFZ).

Characteristic	Description
GPS coordinates	GPS coordinates and map of extent of site
Cropping system	Crops, rotation schedule
Root depth	Maximum root depth of crops
Fertilization practice	Frequency and amount of agronomic pH control and fertilizer use in previous 3 years, and expected use during the crediting period
Tilling practices	Frequency and depth of tilling practices in previous 3 years, and expected use during the crediting period
Irrigation practices	Frequency and amount of irrigation in previous 3 years, and expected use during the crediting period
Local climate	Temperature, rainfall, humidity, annual and monthly average
Soil horizons	Depth and types of different soil horizons, especially changes related to vertical infiltration of water (e.g. hardpans, plow pans, caliche layers...)

Soil

All items below shall include measurements of both the baseline composition and the spatial and temporal variability. They shall include averages and distributions (standard deviation, variance...). Those with an asterisk (*) are optional.

The following characteristics apply to the Near Field Zone (NFZ).

Characteristic	Description
Soil type	Indicates many other soil characteristics and the particle size distribution and which affects water retention and mineral dissolution rates
Soil and porewater pH and buffer pH	Determines the acidity of the environment, as lower pH enhances mineral dissolution and CO₂ capture potential.
Base saturation	Reflects the proportion of exchangeable base cations (e.g., Ca²⁺, Mg²⁺) available, which influences soil's ability to neutralize acidity and facilitate weathering.
Bulk density	Determines soil compaction, influencing water infiltration and the interaction of crushed rock particles with soil and water.
Mineralogy	Identifies existing weatherable mineral concentrations in the soil, determining baseline weathering
Cation exchange capacity (CEC)	Measures the soil's ability to hold and exchange cations, affecting the retention of weathering products like Ca$^{2+}$ and Mg$^{2+}$
Total alkalinity	Indicates the baseline ability of soil and porewater to neutralize acids and CO$_2$ sequestration capacity
Porewater DIC concentration	Represents the baseline dissolved inorganic carbon in the NFZ, used for tracking CO₂ capture and movement through the NFZ
Soil organic carbon concentration*	Influences microbial activity and CO₂ production or storage, and is an important supplementary indicator to measure (for co-benefits or ESDNH)
Soil inorganic carbon concentration*	Baseline measurement of pre-existing carbonates, important for assessing the net carbon removal potential of ERW

FFZ

The following characteristics apply to the Far Field Zone (FFZ).

Characteristic	Description
Hydrologic flow paths	Expected water flow paths, expected near-term carbon fate (groundwater, watershed, rivers, ocean entry point, aquifer), potential horizontal movement
Final carbon storage reservoir	Carbonate minerals, DIC in groundwater, DIC in oceans
Groundwater system durability	If "Final carbon storage reservoir" is DIC in groundwater, the fluids passing through the groundwater enter a flow path with high durability
CDR in vadose zone	What processes could cause CDR increases or losses below the depth of the NFZ (e.g. secondary precipitates)
Water table depth	Average and variability across deployment area
Surface water organic carbon	Sites must not have expected substantial organic carbon destabilization in downstream surface waters

Project Developers shall describe the sampling protocol used for taking pilot measurements and justify why it was appropriate. This description must consider the number of sub-samples and composite samples, location of sampling, secondary sources used, compositing and homogenization steps, and statistical analyses done on samples (e.g. average and distribution).

Stratification

Project Developers should perform to group the plots within a deployment area according to their key characteristics that influence CDR.

Stratification must combine key climate and soil properties to delineate strata that are relatively homogeneous in factors influencing ERW. If used, a stratification approach shall be developed during project validation, prior to rock spreading, although this may be revised at later auditing events. The purpose is to:

• designate treatment and control plots per strata (1 control plot per strata), reducing variability and improving representativeness of control plots, and

• may be used for spatial extrapolation.

If Project Developers do not plan on pursuing a spatial extrapolation option, and prefer to use the default control plot density rates, this step may be skipped.

Method and tools

Stratification shall be done using evidence from, data from farmers, secondary databases, remote sensing, or other well documented, rigorous and reliable sources.

It is recommended that Project Developers perform this step in GIS software as a multi-criteria analysis that overlays layers representing each characteristic, but other approaches, methods and software can be considered on a case by case basis.

Statistical methods for grouping

Categorical variables (e.g. soil type) should be treated as separate values, unless the Project Developer can justify why multiple types should be combined. Continuous variables (e.g. pH, soil bulk density...) should be grouped using one of the following methods and justified by the Project Developer. A Multivariate Analysis of Variance (MANOVA) test must be done to validate that the differences between the final groups are statistically significant.

Statistical methods for grouping continuous variables

• Standard deviation intervals

• Equal intervals

• Quantiles

• Domain-specific thresholds (e.g. acidic, neutral and alkaline soils)

• Natural breaks/gaps

• Statistical clustering models (e.g. hierarchical or K-means clustering)

Stratification Variables

The suggested climate and soil properties to delineate strata are listed below. The four properties in bold are required for stratification, but Project Developers are encouraged to use as many properties as reasonably possible to increase the likelihood of statistical significance. Additional properties not listed here may be considered if justified as relevant.

• Soil or porewater pH

• Soil type

• Feedstock application rate

• Type of crop grown

• Soil moisture

• Soil texture

• Topographic Wetness Index (TWI)

• Temperature

• Precipitation

• Slope

• Wind exposure

• Cation Exchange Capacity (CEC)

• Soil organic carbon

Steps

The steps include:

• Define the variables for stratification within the project area (see minimum required variables above)

• Statistically assess the variables to establish strata and . Summarize the cutoff values for criteria.

• Provide a summary report listing each stratum, its value for each variables, and names, GPS coordinates and map outline of each parcel.

• Assign at least one treatment and control plot to each stratum, ensuring representativeness and minimizing bias (see more details on control plots below).

Stratification results

The final stratification approach shall be described in the PDD and include:

• a GIS generated map showing the extent of the total project area and the location of different strata.

• list of the different strata types identified, with the number of disparate strata plots and the total area covered by each strata.

For example, a project's deployment area may contain 1,000 ha. The project selects the minimum four stratification criteria plus Topographic Wetness Index (TWI), and group them using the following approach:

• pH:

  • Low pH: < 5.5 (Acidic soils)

  • Moderate pH: 5.5–7.0 (Neutral soils)

  • High pH: > 7.0 (Alkaline soils)

• Soil type

  • Vertisols: Clay-rich soils with high swelling/shrinking capacity, high CEC (cation exchange capacity), and moderate ERW potential.

  • Ultisols: Highly weathered acidic soils, low in base cations, high ERW potential due to nutrient-poor status.

  • Mollisols: Organic-rich, fertile soils with high base saturation and moderate ERW potential.

• Feedstock application rate: same across entire project area

• Type of crop grown:

  • Wheat

  • Sugarcane

This generates the following strata:

• S1: Low pH, Vertisols, Wheat, area 100 ha

• S2: Moderate pH, Vertisols, Wheat, area 300 ha

• S3: High pH, Mollisols, Sugarcane, area 200 ha

• S4: Low pH, Ultisols, Sugarcane, area 400 ha

The MANOVA tests show significant differences for pH (𝑝 < 0.001 p<0.001), soil type (𝑝 < 0.001 p<0.001), and TWI (𝑝 < 0.01 p<0.01), supporting the stratification approach.

The Project Developer generates a map of the deployment area showing outlines of the strata, and provides the Stratification results in the Site characterization report.

Sampling protocol for monitoring

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

During monitoring, Project Developers shall provide their ex-post Sampling Procedure as part of their Monitoring Report, documenting the actual sampling approach that was used ex-post.

Sampling Plan components

The Sampling Plan is made ex-ante before rock spreading, and 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...)

• 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

• any sampling components that are not fixed, and may/will deviate from the original plan as the project continues operations and gathers more data and revisions are made

• 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. Project Developers shall consider site-specific hydrological factors such as precipitation patterns and irrigation events when determining and explaining the sampling schedule.

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.

Sampling procedure components

The Sampling Plan described above is developed ex-ante during validation and outlines the intended sampling approach. During monitoring and ex-post verification, Project Developers must provide a Sampling Procedure, described in the Monitoring Report, which documents the actual sampling approach that was implemented.

Ideally, the Sampling Procedure should align exactly with the Sampling Plan. However, given real-world challenges that may arise during monitoring, deviations are expected. The purpose of documenting the Sampling Procedure ex-post is to ensure transparency by capturing any adjustments made to the original plan.

The Sampling Procedure shall include all elements listed in the Sampling Plan components section.

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 shall be time-paired to reduce temporal variability, i.e. samples taken in the control and treatment plots should be taken no more than 3 hours apart from one another.

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. Riverse 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).

Plot types

Results from sampling and validation-stage measurements (before any spreading occurs) shall be used to designate treatment and control plots within the project area.

Treatment plots

Treatment plots are the areas where Project Developers have spread feedstock. The sum of all treatment plots is the treatment area. The treatment area shall be at least 97.5% of the area of the given strata.

If the Project Developer pursues Spatial Extrapolation, treatment plots can be split into original plots and extrapolated plots.

Control plots

Control plots are used to measure baseline weathering and CDR that would have occurred without the project intervention. This CDR is subtracted from the project's CDR, to only issue credits for CDR that occurs beyond business as usual processes.

Control plots shall be selected to be representative of the project area and avoid contamination of weathering material from treatment plots. Each control plot shall correspond to one specific , and statistical analyses are done on these control-treatment pairs. The characteristics that shall be measured in control plots are presented in Table 2.

For projects using Method 1: Direct measurement of export to calculate CDR in the NFZ, if the sampling point at the end of the NFZ is catchment or drainage waters, as assessment of the site hydrology shall ensure that catchment or drainage waters of the treatment and control plots remain separate. This is to avoid collecting water that mixes signals from both plots, as this would compromise the comparison.

Table 2 The characteristics to measure in control plots and the measurement frequency.

Characteristic	Frequency	Note
Baseline CDR from counterfactual weathering	Each reporting period	Using the same NFZ measurement method as the treatment area, and FFZ deductions (see GHG quantification section for more details)
Soil organic carbon changes	At least once after rock spreading during the crediting period	Measurement shall be taken during the same reporting period for the treatment area/project scenario
Crop yields	At least once after rock spreading during the crediting period	Measurement shall be taken during the same reporting period for the treatment area/project scenario

Control plot number and size

For projects using stratification, all projects must have at least one control-treatment pair per stratum, or 3 control-treatment pairs per project if there are fewer than 3 strata.

The control plot shall be at least 2.5% of the area of the given strata.

Control plot management

Control plots should include business as usual (BAU) practices (e.g. continued use of pH management/agricultural lime on agricultural fields at pre-deployment rates). Where this is not possible, negative control plots can be used instead. Negative control plots include no pH management, but continue other BAU agronomic practices (e.g. cropping, tilling, fertilizer...).

If a negative control plot is used instead of a BAU control plot, it shall be conservatively assumed that all agricultural lime dissolves and generates CDR at 100% efficiency, with negligible carbon loss terms. See the Baseline scenario GHG quantification section for more details.

BAU plots shall maintain the following counterfactual practices where relevant:

• Liming

• Crop selection

• Tilling

• Fertilization

• Irrigation

Project Developers shall justify the amount, frequency, type, and any other relevant information for each BAU practice. The hierarchy of evidence from most to least preferred is:

1. records of historical or recent management/agricultural practices, using a trend/projection of recent practices, unless the Project Developer can justify that the trend is not representative

2. records of historical or recent management/agricultural practices, using a different value within the range of recent practices, e.g. if recent trends are not representative

3. records of average local/regional practices

4. recommended practices by local agronomists or extension agents

Control plot representativeness

Control plots must be representative of the strata to which they belong. A control plot is considered sufficiently representative if the standardized mean difference (SMD) between treatment and control plots across the quantified soil and site characteristics listed in Table 1 is less than 10%.

Alternative justifications for representativeness may be accepted on a case-by-case basis, subject to review. This requirement is expected to be readily met following proper stratification.

Spatial extrapolation of measurements

Spatial extrapolation allows Project Developers to expand their project area by adding new sites and spreading events with fewer monitoring requirements. Projects become eligible for this option only after meeting rigorous statistically backed conditions, ensuring low variability and reliable results under lower-density sampling.

The intent is to support a flexible project design—where intensive, high-density sampling is performed in selected treatment and control plots, while broader operational areas can be monitored at lower density.

This approach is referred to as spatial extrapolation because it involves applying the results from one geographic location (the high-density sampling area) to similar areas within the same strata. Note that it is distinct from temporal extrapolation, which would involve projecting results forward in time, which is not covered in this methodology.

Conditions for extrapolation

After the first round of monitoring (e.g., one year), Project Developers shall evaluate the variance of measured CDR within each stratum. The objective of this step is to determine whether the observed CDR variability within the stratum is sufficiently low to justify spatial extrapolation to new plots within the same stratum type.

Project Developers shall calculate variance ($\sigma^2$) in measured ex-post CDR within each strata: if the variance of CDR is less than 10% of the mean measured CDR outcome for the stratum, then the strata is eligible for spatial extrapolation.

If the 10% variance threshold is not met, Project Developers may revise the stratification approach by dividing heterogeneous strata into smaller, more uniform sub-strata, whose results may be eligible for extrapolation. Each new sub-stratum must be assigned its own treatment–control plot pair.

If a stratum does not meet the low-variability threshold required for extrapolation eligibility, Project Developers may still expand activities to identified new sites (see Successive spreading events section). However, these sites will not qualify for the reduced sampling requirements of extrapolation. Instead, they must be treated as standard treatment plots.

New sites added in this manner must be documented in Site Characterization Reports and can proceed with additional rock spreading. The number of required samples shall be determined using a power analysis, as detailed in the Number of aqueous samples per strata and Number of soil samples per strata sections.

Extrapolation requirements

Once eligibility for extrapolation is confirmed, Project Developers may establish extrapolation plots, that share the same characteristics as the validated strata, by either:

• Option 1: Create new sites: add new sites to the project area, within the same strata type, by spreading rock on new fields. A Site Characterization Report shall be submitted for the new sites, and specific monitoring requirements apply when a new site is added.
• Option 2: Reduce sampling on existing sites: designate some existing plots in eligible strata—where rock has already been applied and CDR variability is low—as extrapolation plots. In subsequent years, these plots may undergo reduced sampling frequency.

Extrapolation plots differ from original plots, which are monitored using full-density sampling for verification.

All extrapolation plots shall undergo at least one round of full-density sampling to confirm that their CDR results are consistent with those from the original plots.

• For Option 1: Create new sites, this means conducting one round of sampling and measurement on the new extrapolation plot to confirm alignment with the original site's CDR outcomes.

• For Option 2: Reduce sampling on existing sites, this requirement is considered fulfilled by the first round of sampling which is used to validate the strata as low variability.

Extrapolation plots do not require separate control plots. Instead, the control plot from the corresponding original plot is assumed to be representative of the extrapolation plot's treatment area. This assumption is based on the validation process, which confirms that both sites share the same key characteristics relevant to ERW. The size of the original control plot may remain the same (at least 2.5% of the area of the given strata), even if the total size of the strata increases with the addition of new extrapolation plots.

Sampling within the extrapolation plot remains mandatory. At least three samples per extrapolation plot shall be collected. Extrapolation plots are not required to be contiguous with one another, or with the original plot.

When calculating CDR for the treatment area of a given stratum that includes both original and extrapolation plots, all measurements from both plot types shall be included. All samples shall be weighted equally in the calculations, even if samples from the extrapolation plot represent a larger area.