Site characterization

Scope of the site

The compilation of sites where spreading and monitoring occurs is called the project area. Each project registered with Rainbow 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 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 should be at least 20 centimeters, or the tillage depth plus a 10 centimeter buffer, whichever is deeper. Deeper or shallower NFZs may be considered on a case by case basis if the Project Developer provides sufficient justification based on site hydrology or agronomic practices, including but not limited to the depth of the water table, depth of plant roots, or disturbance from installing sensors in non-tilled soil layers).

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, is expected to lead to measurable CDR

• demonstrates that spreading feedstock in the project area meets the Environmental and Social Do No Harm requirements, is not expected to cause adverse effects on the environment, and is not upstream of peatlands

• delineates the spatial extent,, and various sites 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 for each site in the project area. Sources may include measurements from , data from farmers, secondary databases and soil maps, remote sensing, or other well documented and reliable sources.

These characteristics shall be used to justify the modeling estimates, the control/treatment structure, stratification, and sampling protocol (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).

Some agronomic characteristics listed below cannot be reliably projected multiple years in advance, because farmers may make such decisions annually. Project Developers should provide their best estimates of agronomic practices upon project validation, and actual practices shall be documented and updated throughout the crediting period.

Characteristic	Description
GPS coordinates	GPS coordinates and map of extent of site
Cropping system	Past and expected crop types and rotation schedule
Root depth	Maximum root depth of expected 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 according to World Reference Base for Soil Resources (WRB)	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
Heavy metal content	Characterize the starting conditions of the soil, assess the risk of exceeding safe pollutant thresholds
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, e.g. from being upstream of peatlands

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, timing 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. The 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 (at least 1 control plot per strata), reducing variability and improving representativeness of control plots, and

• (optional) used for spatial extrapolation.

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

There is no minimum requirement for the number of strata in a project. For small or homogeneous project areas, the entire area can be grouped into a single stratum, with one representative control plot assigned to it.

Similarly, there is no minimum number of strata or variance level within strata, so Project Developers may choose to effectively skip this step, by grouping all fields into one poorly-defined heterogeneous strata.

Statistical methods and grouping variables

The suggested climate and soil properties to delineate strata are listed below. The properties in bold are strongly recommended 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 and timing

• Feedstock type

• Type of crop grown

• Soil moisture

• Soil texture

• Topographic Wetness Index (TWI)

• Temperature

• Precipitation

• Slope

• Wind exposure

• Cation Exchange Capacity (CEC)

• Soil organic carbon

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 approaches listed below, and justified by the Project Developer:

• 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)

The Project Developer shall provide a statistical approach and results for grouping plots that are sufficiently similar to one another into strata.

Steps and implementation

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).

• The stratification results may be revised and plots reassigned throughout the project if new sites are added, if successive spreading events alter the representativeness of sites, or as a result of general learning and improvements. Such modifications should be submitted to Rainbow for approval before changing the monitoring and sampling approach. Such modifications are allowed as long as all areas that are to be issued credits are covered directly by high-density sampling, or are designated an extrapolated area with at least 12 months of results corroborating with a high density sampling plot.

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.

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.

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

Treatment plots can be further split into high-density and low-density sampling plots (a "3-plot approach"), where small and highly sampled fields are used to primarily collect NFZ data, and large low-density sampling areas are allowed to have fewer measurements, have their CDR count, and apply measurements from their corresponding high-sample and control plots.

Project Developers shall take a sufficient number of samples in the low-density area to confirm that the range of results falls within those of the high-density sampling area. It is recommended that this sampling density does not fall below 1 sample/15 ha, i.e. 0.0667 samples/ha. The reduced sampling density on the low-density area shall be defined in relation to the density of sampling on the original area, which shall be determined using a power analysis or other justification that the sampling plan will lead to statistically significant results between the treatment and control.

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 , 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, an 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 management

Control plots should include business as usual (BAU) practices, including but not limited to 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 may 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 number and size

Each treatment plot shall have a corresponding control plot that is representative of the treatment plot. The number and size of control plots shall be large enough to capture the baseline variability of the treatment area, and to ensure statistically significant comparisons with the treatment plot.

At least one control-treatment pair is required per stratum. If control plots are large, it is recommended to split them into multiple smaller plots, rather than maintaining a single, sparsely-sampled large plot. Control plots do not need to be contiguous with one another, or with their corresponding treatment plots. Small projects with only one stratum may contain only one control plot.

Control plots shall represent at least 1% of the area of a given stratum, with a larger percentage recommended for smaller strata, up to 5%. Based on these guidelines, the following decay function was interpolated (shown in Figure 1) and is recommended to help guide Project Developers in determining the appropriate control plot area as a function of stratum area (in hectares, $x$):

$$\textbf{(Eq. 1)}\ \%\ Control\ area= 4.13 \times e^{-3.14e-4x}$$

Project Developers shall describe in the PDD the number and relative area of control plots in each stratum, considering the size of the stratum, baseline variability, sampling density, and statistical power.$$

Figure 1 The recommended control plot size is plotted against the corresponding stratum size.

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 the stratification steps.

Spatial extrapolation of measurements

old

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 at least 12 months of monitoring, 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 assess the representativeness of the high-density sampling area compared to the extrapolated area, using robust statistical methods. Any of the following approaches are allowed to demonstrate that the strata is eligible for spatial extrapolation.

• 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

• if identified extrapolation plot results are within an 80% confidence interval of the mean of the original/high density sampling area

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.

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 at least 12 months of monitoring, 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 assess the representativeness of the high-density sampling area compared to the extrapolated area, using robust statistical methods. Any of the following approaches are allowed to demonstrate that the strata is eligible for spatial extrapolation.

• 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

• if identified extrapolation plot results are within an 80% confidence interval of the mean of the original/high density sampling area

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.