Quantifying enhanced weathering

Below, you can explore what kinds of quantitative methods might play a role in estimating net carbon removal from enhanced weathering. Read more in the accompanying explainer.

QUICK LOOK

A tool for exploring methods used to quantify enhanced weathering outcomes

Variable	Method	Rock application	Initial weathering	Field processes	Watershed transport	Ocean storage
Bulk density	Various (e.g., Geoprobe sampling, core method, clod method, excavation method)	N/A	Secondary	Primary	N/A	N/A
Rock application N/A Initial weathering Secondary Field processes Primary Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts N/A Notes Bulk density refers to the mass of dry soil per unit volume and is influenced by many factors, including soil type and field management. Bulk density measurements are required to rigorously extrapolate soil-based concentration measurements (see e.g., "Soil total carbon," "Soil inorganic carbon," "Soil organic carbon," or "Trace metals") to field-scale stock estimates. Comments Various methods exist to estimate bulk density, including the core method, excavation method, and clod method. Equivalent soil mass accounting is a less widely used, but more rigorous method for reporting soil stock measurements based on soil mass instead of soil volume. References Sharma et al., 2020 Al-shammary et al., 2018
Cation exchange capacity (CEC) and exchange complex cations	NH₄OAc, BaCl₂ or appropriate extraction for given soil	N/A	Primary	Primary	N/A	N/A
Rock application N/A Initial weathering Primary Field processes Primary Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts Crop health, Ag inputs Notes Cation exchange capacity (CEC) refers to the negatively charged sites on soil solid surfaces (e.g., clay minerals, organic matter) where cations are attracted, held, and exchanged with soil pore water — collectively called the soil exchange complex. Interactions between cations produced from weathering reactions and the soil exchange complex can produce a delay between initial weathering and detectable changes to cation or dissolved inorganic carbon concentrations in soil pore water or field runoff. Characterizing the CEC and changes in base saturation (the type of cations present on the exchange complex) can be important for interpreting other weathering signals. Comments Changes are most likely to be observed in base saturation/exchange complex cations. However, cation exchange capacity (CEC) itself could change if secondary clay minerals form or if the system pH changes as a result of weathering, which would impact the fraction of CEC that is pH-dependent. References Matilla & Rajala, 2021
Cations or anions	ICP-AES, ICP-MS, AAS, IC	N/A	Primary	Primary	N/A	N/A
Rock application N/A Initial weathering Primary Field processes Primary Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Water Impacts N/A Notes Direct measurements of ions in soil water or downstream export can provide evidence of weathering at the field or watershed scale. These measurements require samples from soil pore water (e.g., from a lysimeter) or downstream waters, specific sampling techniques (e.g., sample acidification with nitric acid), and analysis using tools such as inductively coupled plasma atomic emission spectroscopy or mass spectroscopy (ICP-AES or ICP-MS), atomic absorption spectropcopy (AAS), or ion chromatography (IC). Comments Measuring cations can provide the basis for estimating weathering or potential carbon removal. In contrast to total alkalinity or dissolved inorganic carbon (DIC), the cations associated with rock weathering (e.g., Na+, Ca2+, Mg2+) are conserved and not subject to the same dynamic shifts as DIC. Cation or anion measurements can also be used to estimate alkalinity and interpret total alkalinity data, and certain anions (e.g., NO3-) can be useful for considering effects of strong acids. Ion flux as a result of enhanced weathering is a continuous process and can be highly variable over time. As a result, there are major challenges associated with taking intermittent measurements and trying to integrate them to characterize overall ion flux. Isolating the effects of rock weathering on ion fluxes requires a rigorously constructed baseline and counterfactual scenario. References Campbell et al., 2023 Calabrese et al., 2022 Weihermüller, et al., 2007
Crop yield	Record keeping, weighing	N/A	N/A	Extra	N/A	N/A
Rock application N/A Initial weathering N/A Field processes Extra Watershed transport N/A Ocean storage N/A Transient No Type Meaurement, Records Category Crops Impacts Crop health Notes Changes in yield due to rock application are possible and are generally presumed to be positive, for example by increasing the availability of additional micronutrients or increasing pH. These effects could also potentially be neutral or negative, for example via pH changes that negatively impact nutrient availability. Yield may be determined from farm records of industrial-scale harvesting or using smaller-scale hand harvest approaches. Comments Yield impacts are less likely if rock composition is relatively benign (see "elemental composition"), the soil is relatively fertile to begin with, and no changes to fertilizer regime are made (see "field management"). Tracking yield changes may not be necessary if a clear positive or neutral relationship is established for a given weathering material, soil type, and crop type. References Shapiro et al., 1989 Trout and DeJonge, 2017 Swoboda et al., 2022
Dissolved inorganic carbon (DIC)	DIC analyzer	N/A	Primary	Primary	N/A	N/A
Rock application N/A Initial weathering Primary Field processes Primary Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Water Impacts N/A Notes Increases in total dissolved inorganic carbon (DIC) can provide evidence of weathering and be used to estimate carbon dioxide removal at the field or watershed scale. DIC measurements require samples from soil pore water (e.g., from a lysimeter) or downstream water and specific sampling techniques, plus a DIC analyzer. Comments Dissolved inorganic carbon (DIC) is subject to fluctuations as pH and other system parameters change, altering carbonate speciation. Therefore, DIC measurements are context-dependent and do not necessarily represent lasting outcomes. Continuous monitoring of DIC would be required to capture the total changes as a result of enhanced weathering. However, this poses a challenge given realistic sampling constraints and equipment interference with farming operations. Care must be taken that other field management changes are not responsible for observed shifts in DIC (e.g., changing irrigation source or rate), which emphasizes the importance of establishing a dynamic baseline rather than solely monitoring DIC prior to rock application. Note that there may be a delay between initial weathering and a DIC signal as a result of interactions between weathered cations and the soil exchange complex (see "Cation exchange capacity (CEC) and exchange complex cations"). Changes to DIC would also be reflected in total alkalinity measurements (see "Total alkalinity"), and it is typically more challenging to measure DIC directly. References Sigfusson et al., 2006 Weihermüller, et al., 2007
Electrical conductivity	EC probe, sensors	N/A	Primary	N/A	N/A	N/A
Rock application N/A Initial weathering Primary Field processes N/A Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Water Impacts N/A Notes Electrical conductivity (EC) is typically correlated with total alkalinity. EC may be measured alongside alkalinity in water samples, or in lieu of total alkalinity if the correlation is well established for a given system. Measurements may be made in situ using sensors or on collected water samples. Comments Measuring changes to electrical conductivity (EC) using bulk soil sensors has been suggested as an easy, low cost approach to estimating changes to total alkalinity (see "Total alkalinity") as a result of enhanced weathering. However, this approach has not been validated in the field and is subject to significant variability based on environmental conditions such as field moisture conditions and soil texture. In general, care should be taken to consider processes besides weathering that might impact EC. References Amann and Hartmann, 2019
Field management	Record keeping	N/A	Essential	Essential	N/A	N/A
Rock application N/A Initial weathering Essential Field processes Essential Watershed transport N/A Ocean storage N/A Transient No Type Records Category Management Impacts Ag inputs Notes Field management affects the field system, and should inform estimates of initial weathering and other field-scale processes. In addition to records characterizing the application of the weathering material (see "Rock application"), it is important to track more field management practices such as the application of agricultural lime and fertilizer, tillage patterns, and crop type and rotation. Comments Tracking liming practices is especially important for understanding if the application of the weathering material substitutes for the application of agricultural lime, and how that potential substitution may impact net carbon removal. Tracking fertilizer application is important for understanding the prevalence of fertilizer-derived ions and the likelhood that applied rock is weathered by strong acids rather than carbonic acid. A system for record keeping should be established up front, as commerical farm record keeping priorities may differ from enhanced weathering needs. References Oh and Raymond, 2006 Larkin et al., 2022
Field outgassing	Default discount factor	N/A	N/A	Primary	N/A	N/A
Rock application N/A Initial weathering N/A Field processes Primary Watershed transport N/A Ocean storage N/A Transient N/A Type Other Category N/A Impacts N/A Notes Default factors may be used to account for leakage from field processes that result in CO₂ outgassing (e.g., carbonate precipitation or secondary mineral formation). These default factors could be established, for example, by estimating a worst case scenario given a certain set of eligibility criteria and applying it across the board. Comments If large enough, default factors may result in adequate discounting for leakage from field processes. However, applying default factors rather than explicitly quantifying leakage does not support learning nor a geographically-tailored understanding of enhanced weathering deployment.
Greenhouse gas fluxes	Various (e.g., chamber, Eddy covariance flux tower, remote sensing)	N/A	Primary	Primary	N/A	N/A
Rock application N/A Initial weathering Primary Field processes Primary Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Gas Impacts N/A Notes Greenhouse gas fluxes may change in response to rock weathering at various points in the system (field, watershed, ocean). Gas flux measurements could inform understanding of the effects of enhanced weathering on the total carbon budget of the ecosystem, or indicate changes to other gas fluxes such as N₂O emissions, which may be reduced as a result of pH increases from enhanced weathering. Such fluxes may be measured directly at field scale and should be compared to areas without rock application. Comments Gas fluxes can be highly variable over time and in response to environmental conditions. Measurement and modeling approaches based on gas flux measurements must take into account the potential for cycles of absorption and outgassing happening on different timescales, as well as changing environmental conditions such as temperature and moisture. Ideally, automated chambers are used to capture temporal variability. This approach is likely to be expensive and labor intensive, and should be used alongside other quantitative methods that bound weathering outcomes. References Almaraz et al., 2022 Blanc-Betes et al., 2020 Carbon Drawdown Initiative, 2023
Irrigation	Flow meter, record keeping	N/A	Essential	Essential	N/A	N/A
Rock application N/A Initial weathering Essential Field processes Essential Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement, Records Category Management Impacts N/A Notes Tracking irrigation data provides context for assessing the water inputs available for weathering reactions. Determining a water balance for the field is important for converting measurements expressed as concentrations (mass / volume) into weathering or carbon removal fluxes (mass / time). In addition to tracking irrigation rates, timing, and methods, keeping records of irrigation water sources and characteristics is also important, particularly because groundwater may be a significant source of alkalinity inputs to the system. This information may be obtained from records, flow meter monitoring of irrigation water, and / or water sample analysis. Comments A system for record keeping should be established up front, as commerical farm record keeping priorities may differ from enhanced weathering needs. References Maher and Chamberlain, 2014
Isotopic tracers	Various (e.g., MC-ICP-MS)	N/A	Primary	N/A	N/A	N/A
Rock application N/A Initial weathering Primary Field processes N/A Watershed transport N/A Ocean storage N/A Transient Maybe Type Measurement Category Rock, Soil, Water Impacts N/A Notes Changes in the concentrations of isotopic tracers (e.g., strontium or neodymium) could be used to constrain the initial dissolution of rock dust. Isotopic tracers play a key role in our understanding of weathering dynamics over geologic time, including identifying source rocks and estimating weathering and erosion rates. Applying this approach to characterize field-scale enhanced weathering requires clearly characterizing baseline isotopic ratios of the rock as well as the soil, soil solution, or downstream export, as applicable. Comments Sufficient differences in the isotopic signature between the soil and rock amendment are required. References Campbell et al., 2023 Paytan et al., 2021
Mobile / immobile element ratios	Geochemical mass balance	N/A	Primary	N/A	N/A	N/A
Rock application N/A Initial weathering Primary Field processes N/A Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Rock, Soil Impacts N/A Notes Geochemical mass balances allow for determination of chemical weathering rates by comparing the concentrations of immobile elements like titanium (Ti) or ziconium (Zr) — which originate in the weathering material and remain in the soil as weathering progresses — to concentrations of mobile elements, like silicon (Si), calcium (Ca), sodium (Na), or magnesium (Mg), which are progressively lost. Applying this technique to enhanced weathering can theoretically allow for constraint of initial rock weathering and is time-integrated, alleviating the need for continuous monitoring. Comments See Reershemius et al, 2023 for discussion of potential limitations of applying a mass balance approach to EW, including the need for appropriately robust analytical precision. Sufficient differences in immobile element concentration between the soil and rock amendment are also required. References Reershemius et al., 2023 (preprint)Beerling et al., 2023 (preprint)
Ocean outgassing	Default discount factor	N/A	N/A	N/A	N/A	Primary
Rock application N/A Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage Primary Transient N/A Type Other Category N/A Impacts N/A Notes Default factors may be used to account for leakage from ocean processes that result in CO₂ outgassing (e.g., carbonate precipitation or secondary mineral formation). These default factors may be established via a wide range of processes, for example, by estimating a worst case scenario given a certain set of eligibility criteria and applying it across the board. Comments If large enough, default factors may result in adequate discounting for leakage from ocean processes. However, applying default factors rather than explicitly quantifying leakage does not support learning nor a geographically-tailored understanding of enhanced weathering deployment.
Ocean outgassing	Process or empirical models	N/A	N/A	N/A	N/A	Primary
Rock application N/A Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage Primary Transient N/A Type Model Category N/A Impacts N/A Notes Process models of ocean circulation and ocean equilibration with the atmosphere can be used to estimate additional ocean outgassing of CO₂ that occurs as a result of enhanced rock weathering. In contrast, empirical models are based on statistical analysis of observed data, and do not contain a mechanistic representation of ocean processes. Comments It is unlikely that smaller-scale projects will be able to estimate ocean leakage based on direct measurement given the large spatial scale and long temporal scale of these processes. However, few ocean models exist today that are tailored to a consideration of ocean outgassing that happens as a result of enhanced weathering. Developing and validating models of these ocean processes is an area of active research. Models used to understand the long-term fate of weathering products in the ocean should be comparable across CDR pathways that account for similar ocean dynamics (e.g., ocean alkalinity enhancement). References Kanzaki et al., 2023 Harrington et al., 2022 Fennel et al., 2023 (preprint)
pCO₂	Sensors, infrared gas analyzers	N/A	Secondary	N/A	N/A	N/A
Rock application N/A Initial weathering Secondary Field processes N/A Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Soil, Water Impacts N/A Notes The partial pressure of CO₂ (pCO₂) in the soil or watershed influences initial weathering rates, provides information about carbonate equilibria in a system, and may change in response to weathering reactions. Comments Fluctuations in the partial pressure of CO₂ (pCO₂) could be driven by a number of factors, including changes in pH, microbial activity, or other environmental factors. Enhanced weathering could plausibly increase or decrease pCO₂. This may introduce challenges for using pCO₂ measurements as an indicator of weathering. References Dietzen et al. 2018
pH	pH meter, sensors	N/A	Secondary	Primary	N/A	N/A
Rock application N/A Initial weathering Secondary Field processes Primary Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Soil, Water Impacts Crop health, Heavy metals, Soil biology, Freshwater biology, Ag inputs Notes Farmers rely on understanding and managing soil pH to ensure healthy crop growth and nutrient availability. Because silicate weathering is a proton-consuming reaction, an increase in pH is expected and could signal that weathering has occured. The magnitude of this change depends on the weathering rate, the starting pH of the system, and the buffering capacity of the soil. Increasing pH may cause wide-ranging impacts, from altering nutrient availability and trace metal mobility (positively or negatively) to reducing N₂O emissions or shifting management practices. Monitoring pH also provides important information for constraining carbon removal — for example, as an environmental factor that determines carbonate speciation and therefore the relationship between cations released by weathering and bicarbonate formation. pH is a routine soil measurement and may be measured on soil or water samples collected for other purposes. Comments Enhanced rock weathering could shift watershed pH in ways that affect freshwater ecosystems and geochemical processes. For example, changes in system pH as a result of enhanced weathering might also shift the mobilization and bioavailability of potentially toxic trace metals in the environment. Monitoring watershed pH could thus be an important dimension of tracking environmental impacts at scale. References Edwards et al., 2017 Neina, 2019
Plant cation uptake	ICP-AES, ICP-MS, AAS, record keeping	N/A	N/A	Extra	N/A	N/A
Rock application N/A Initial weathering N/A Field processes Extra Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Crops Impacts N/A Notes If crops engage in luxury consumption of certain nutrients following rock application, this could lead to higher rates of cation removal at the field scale. Changes in yield could also alter cation concentrations in exported biomass. This loss of cations would reduce the potential carbon dioxide removal of the associated enhanced weathering. Plant cation uptake can be measured using plant biomass samples from treated and control plots, which can then be processed via an appropriate digestion, followed by various forms of spectroscopy, such as inductively coupled plasma atomic emission spectroscopy or mass spectroscopy (ICP-AES or ICP-MS) or atomic absorption spectroscopy (AAS). An appropriate sampling scheme and measurement protocol is required. Comments Tracking plant cation uptake may not be necessary unless there is evidence of luxury consumption or changes in yield. Depending on the rock mineralogy and elemental composition, it's also possible that plants might take up toxic trace metals which would need to be monitored with similar measurements (see "Rock mineralogy," "Rock elemental composition," and "Trace metals"). References Campbell et al., 2023 ten Berge et al., 2012 Ramos et al., 2022 Calabrese et al., 2022
Precipitation	Local weather station, record keeping, or other	N/A	Essential	Secondary	N/A	N/A
Rock application N/A Initial weathering Essential Field processes Secondary Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement, Records Category Environment Impacts N/A Notes Precipitation data provides context to assess water inputs available for weathering. Determining a water balance for the field is important for converting measurements of weathering products or carbon, which are expressed as concentrations (mass / volume), into weathering or carbon removal fluxes (mass / time). This data may be obtained from a local weather monitoring station or site records, if available. Comments Regional precipitation data products may provide some useful context, but can vary in quality and may not be appropriate to use as input data to sophisticated models. References Maher and Chamberlain, 2014
Project emissions	LCA	Essential	N/A	N/A	N/A	N/A
Rock application Essential Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage N/A Transient N/A Type Other Category Other Impacts N/A Notes Project emissions must be accounted for when estimating net carbon removal from an enhanced weathering deployment. Key components to consider include transport and application of rock material. Comments At present, few LCAs of enhanced weathering exist in the scientific literature. References Lefebvre et al., 2019 Calabrese et al., 2022 Tan and Aviso, 2021
River outgassing	Default discount factor	N/A	N/A	N/A	Primary	N/A
Rock application N/A Initial weathering N/A Field processes N/A Watershed transport Primary Ocean storage N/A Transient N/A Type Other Category N/A Impacts N/A Notes Default factors may be used to account for leakage from watershed processes that result in CO₂ outgassing (e.g., carbonate precipitation or secondary mineral formation). These default factors may be established, for example, by estimating a worst case scenario given a certain set of eligibility criteria and applying it across the board. Comments If large enough, default factors may result in adequate discounting for leakage from river systems. However, applying default factors rather than explicitly quantifying leakage does not support learning nor geographically-tailored understanding of enhanced weathering deployment.
River outgassing	Process or empirical models	N/A	N/A	N/A	Primary	N/A
Rock application N/A Initial weathering N/A Field processes N/A Watershed transport Primary Ocean storage N/A Transient N/A Type Model Category N/A Impacts N/A Notes Process or empirical models of river outgassing may be used to characterize watershed processes that affect net carbon removal, such as carbonate precipitation or evasion, and their impact on carbon removal uncertainty. Empirical models are based on statistical analysis of observed river data. If enhanced weathering deployments scale and signals become detectable at the watershed-to-regional level, process models such as river inversion models could be useful tools for identifying enhanced weathering contributions to river carbon transport. Comments It is unlikely that smaller-scale projects will be able to estimate river outgassing based on direct measurement given the potentially large spatial scale and long temporal scale of these processes. However, few river models exist today that are tailored to a consideration of river outgassing that happens as a result of enhanced weathering. Developing and validating regional models of these river processes is an area of active research. References Marx et al, 2017 Saccardi and Winnick, 2021
Rock application	Record keeping, weighing, monitoring	Essential	N/A	N/A	N/A	N/A
Rock application Essential Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage N/A Transient No Type Measurement, Records Category Rock, Management Impacts Silicate inhalation Notes Details about the application of a weathering material — such as records of application rates, dates, and rock moisture content at the time of application — are essential for correctly determining the mass of applied material and its weathering potential. Comments Dust monitoring may be required to understand local environmental hazards of rock application, particularly if the material is especially fine, is drier, or is not being incorporated into the soil. Appropriate health & safety protections should be taken for workers involved in application. A system for record keeping should be established up front, as commerical farm record keeping priorities may differ from enhanced weathering needs. References Beerling et al., 2018
Rock elemental composition	XRF, complete digestion plus ICP-MS or ICP-OES	Essential	N/A	N/A	N/A	N/A
Rock application Essential Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Rock Impacts Heavy metals, Crop health Notes Elemental composition identifies the abundance and distribution of chemical elements within the rock, which is useful for determining weathering potential and understanding the aggregated rock composition, including silica content, cation ratios, trace metals, and macro- or micro-nutrients. Concentrations of trace metals should be assesed with attention to appropriate regulations. Elemental data is typically measured via lithium borate fusion coupled with X-ray fluorescence (XRF). It may also be measured using various whole-rock digestions followed by inductively coupled plasma atomic emission spectroscopy or mass spectroscopy (ICP-MS or ICP-OES). Comments This measurement should be performed prior to applying the weathering material to determine its suitability for enhanced weathering. Care should be taken to construct a sampling regime that appropriately accounts for the potential variability of the weathering material. References Campbell et al., 2023 Almaraz et al., 2022
Rock mineralogy	QEMSCAN, XRD	Essential	N/A	N/A	N/A	N/A
Rock application Essential Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Rock Impacts N/A Notes Mineralogy identifies the relative proportions of minerals within a rock. Since the maximum theoretical potential for enhanced weathering is determined by the mineral composition of the applied rock, this places an upper bound on theoretical carbon removal and helps reveal what fraction of the rock is likely to weather quickly, or if there is a portion (e.g., quartz) that should be considered "unweatherable" given the relevant time scales for enhanced weathering. The mineralogy of rock amendment samples can be characterized using X-ray diffraction (XRD), quantitative evaluation of materials by scanning electron microscopy (QEMSCAN), or similar tools. Comments This measurement should be performed prior to applying the weathering material to determine its suitability for enhanced weathering. Care should be taken to construct a sampling regime that appropriately accounts for the potential variability of the weathering material. References Campbell et al., 2023 Silva et al., 2021
Rock particle size distribution	Particle size analyzer or sieve method	Essential	N/A	N/A	N/A	N/A
Rock application Essential Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Rock Impacts Silicate inhalation Notes Particle size distribution is valuable for estimating weathering potential and suitability of rock material, as coarser material will weather more slowly. Various methods exist to characterize particle size distribution, but particle size analyzers (PSA) using laser diffraction are typical. Comments This measurement should be performed prior to applying the weathering material and requires a small rock sample. Care should be taken to construct a sampling regime that appropriately accounts for the potential variability of the weathering material. Particle size distribution may also be used to consider potential impacts on soil physical properties, for example if the texture is signficantly coarser or finer than the native soil texture. Mass fraction sieving or texture-by-hydrometer may be used to provide more general insight into particle size if a particle size analyzer is not available. References Campbell et al., 2023 Almaraz et al., 2022
Rock surface area	BET	Secondary	N/A	N/A	N/A	N/A
Rock application Secondary Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Rock Impacts N/A Notes In addition to particle size distribution, the specific surface area of the applied weathering material can provide additional information to model weathering potential. The Brunauer, Emmett, and Teller (BET) method uses gas adsorption to mineral surfaces to determine the specific surface area of a small rock sample. Alternative gases and specific measurement approaches may be used. Comments This measurement should be performed prior to applying the weathering material and requires a small rock sample. Care should be taken to construct a sampling regime that appropriately accounts for the potential variability of the weathering material. References Campbell et al., 2023 Almaraz et al., 2022
Secondary mineral formation	Various (e.g., SEM-EDS, TEM, XANES, NanoSIMS, XRD)	N/A	N/A	Primary	N/A	N/A
Rock application N/A Initial weathering N/A Field processes Primary Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts N/A Notes A variety of approaches may be used to characterize secondary mineral formation, which can bind up cations released during initial rock weathering and therefore reduce net carbon removal. Some methods examine changes to soil mineralogy as a whole, for example using x-ray diffraction (XRD). Others examine secondary mineral formation processes at a small scale, for example on the surface of a weathering mineral, with approaches like transmission electron microscopy (TEM), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), nanoscale secondary ion mass spectrometry (NanoSIMS), or synchrotron X-ray absorption spectroscopy. Comments If the approach to measuring initial weathering is focused on observing direct evidence of rock dissolution (see, e.g., "Isotopic tracers" and "Mobile / immobile element ratios"), it is important to characterize the secondary mineral formation that may take place in the field. However, if initial weathering is characterized by observing weathering product run-off (see, e.g., "Cations / anions," "Total alkalinity," or "Dissolved inorganic carbon"), the impacts of secondary mineral formation may be captured in those measurements. Any soil-sample based measurement should be accompanied by bulk density measurements in order to rigorously extrapolate to field-scale stock estimates (see "Bulk density"). The possibility of secondary mineral formation in estuaries and oceans must also be accounted for — often called "reverse weathering" in scientific literature — and will likely be characterized by models (see "River outgassing"). References Campbell et al., 2023 Haque et al., 2020 Bonneville et al., 2009 Nieva et al., 2019
Soil and air temperature	Sensors, local weather station, record keeping	N/A	Secondary	Secondary	N/A	N/A
Rock application N/A Initial weathering Secondary Field processes Secondary Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Soil, Environment Impacts N/A Notes Soil and air temperature measurements could potentially be used to constrain weathering rates and equilibria dynamics. Temperatures may be measured using in situ sensors that are installed permanently for continuous monitoring, or inserted only at time of measurement. These measurements could be critical for the interpretation of other primary measurements, such as GHG fluxes (see "greenhouse gas fluxes"). Comments Regional temperature data products may provide some useful context, but can vary in quality and may not be appropriate to use as input data to sophisticated models.
Soil biology	Various (e.g., PLFA, DNA sequencing)	N/A	Extra	Extra	N/A	N/A
Rock application N/A Initial weathering Extra Field processes Extra Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts Soil health Notes Enhanced weathering could have impacts on soil biology, including microbial diversity and functioning, for example by changing pH or micronutrient availability (see "pH" and "Rock elemental composition"). Changes to soil biology, such as to the microbial community, might impact weathering rates or other in-field processes, like carbonate precipitation. The interaction between enhanced weathering and soil biology is an area of active research, and could be informed by project-level monitoring. The soil microbial community can be characterized by various techniques, including analysis of phospholipid fatty acids (PLFA) or DNA sequencing. Comments Characterizing soil microbes can be intensive from a sampling perspective, require access to specific facilities (e.g., may require -80 C sample storage), and should be guided by an appropriate microbial sampling protocol.
Soil inorganic carbon	Various (e.g., TGA, pressure calcimeter, acid dissolution)	N/A	N/A	Primary	N/A	N/A
Rock application N/A Initial weathering N/A Field processes Primary Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts N/A Notes Measuring the inorganic carbon content of soil solids can be used to characterize in-field carbonate precipitation. This is a critical process to understand, as carbonate precipitation affects the net carbon removal. This process may be characterized in the field by analyzing soil samples using tools such as thermogravimetric analysis (TGA), pressure calcimetry, or acid dissolution. Comments Soil inorganic carbon measurements are less commonly processed by commerical or academic labs. Additionally, it may be difficult to detect small changes as a result of carbonate precipitation if absolute concentrations are very low or if carbonates are heterogeneously distributed. Significant changes to soil carbonates should also be reflected in soil total carbon measurements (see "Soil total carbon"). Any soil-sample based measurement should be accompanied by measurements of bulk density in order to rigorously extrapolate soil samples to field-scale stock estimates (see "Bulk density"). The possibility of carbonate precipitation in rivers and oceans must also be accounted for, and will likely be characterized by models. References Khalidy et al., 2021 Haque et al., 2020
Soil moisture	Sensors, soil samples	N/A	Secondary	Secondary	N/A	N/A
Rock application N/A Initial weathering Secondary Field processes Secondary Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Soil Impacts N/A Notes Soil moisture measurements could potentially be used to inform the water balance and moisture impacts on weathering rates. Soil moisture may be measured using in situ sensors that are installed permanently for continuous monitoring, or based on moisture content of soil samples at specific timepoints. Comments Pre-existing soil moisture data products may provide some useful context, but can vary in quality and may not be appropriate to use as input data to sophisticated models. Soil moisture may also be affected by the addition of rock dust.
Soil organic carbon	Various (e.g., EA, LOI; can be combined with methods for isolating SOC fractions)	N/A	N/A	Extra	N/A	N/A
Rock application N/A Initial weathering N/A Field processes Extra Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts N/A Notes Adding minerals to soil has the potential to induce changes to soil organic carbon (SOC), for example by affecting pH, impacting plant or microbial activity, or changing secondary mineral content. Theoretically, these impacts could increase or decrease SOC storage. There is some limited evidence that addition of silicates may increase mineralization of organic carbon (yielding increased CO₂ efflux) which could decrease net carbon removal, though other preliminary evidence suggests neutral to positive effects on SOC. Comments There is relatively little research on enhanced weathering's impact on soil organic carbon (SOC), and these potential interactions merit further exploration. Although SOC changes are not the direct focus of enhanced weathering, it's important to monitor SOC for possible changes. If a clear understanding of enhanced weathering's impacts on SOC across weathering material, soil types, and agricultural systems emerges, this monitoring may become less necessary. Changes to SOC would also be reflected in soil total carbon measurements (see "Soil total carbon"). Any measurement of SOC should be accompanied by measurements of bulk density in order to rigorously extrapolate soil samples to field-scale stock estimates (see "Bulk density"). If soil organic carbon is a focus, analysis may be deepened with measurements of soil organic matter. References Yan et al., 2023
Soil physical properties (water infiltration, aggregate stability)	Various (e.g., infiltrometers, wet sieving, ultrasonic dispersion, immersion, SLAKES)	N/A	Secondary	N/A	N/A	N/A
Rock application N/A Initial weathering Secondary Field processes N/A Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts Soil health Notes Characterizing soil physical properties, like water infiltration or aggregate stability, is likely not essential, but could inform an understanding of initial weathering processes as well as changes to the soil following rock application. It is likely to take time for any changes to soil physical properties to appear. Changes may be more likely if the rock amendment particle size is substantially different from native soil texture (see "Rock particle size distribution") or if rock is applied repeatedly (see "Mineral application"). The appropriate measurement approach may vary depending on soil type, and care must be taken with measurement timing and frequency. Comments Pre-existing data products characterizing soil physical properties may provide some useful context, but can vary in quality and may not be appropriate to use as input data to sophisticated models. References Fajardo et al., 2016 Flynn et al., 2020
Soil texture	Particle size analyzer, texture by hydrometer	N/A	Secondary	Secondary	N/A	N/A
Rock application N/A Initial weathering Secondary Field processes Secondary Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts Soil health Notes Soil texture measurements are important for characterizing the soil system, including its suitability for enhanced weathering, field processes that may influence net carbon removal, and likely impacts of enhanced weathering for soil health. For example, enhanced weathering could impact soil texture via the rock dust incorparation or secondary mineral formation, but these potential impacts are likely to differ depending on the contrast between the native soil texture and the rock amendment. Soil texture measurements are likely not critical, but may be a supporting or explanatory measurement for other indicators. Soil texture may be determined by a particle size analyzer (PSA) or via the hydrometer method. Comments Pre-existing data products characterizing soil texture may provide some useful context, but can vary in quality and may not be appropriate to use as input data to sophisticated models.
Soil total carbon	Elemental analyzer (EA)	N/A	N/A	Primary	N/A	N/A
Rock application N/A Initial weathering N/A Field processes Primary Watershed transport N/A Ocean storage N/A Transient No Type Measurement Category Soil Impacts N/A Notes Soil total carbon measurements characterize the combined organic and inorganic carbon content of soil samples. These measurements could reflect changes in inorganic carbon pools due to carbonate precipitation, or changes in organic carbon pools, for example due to enhanced weathering's impacts on impacting plant or microbial activity. Comments Soil total carbon is a very standard measurement, but further analysis is needed to differentiate changes to inorganic carbon, for example via carbonate precipitation (see "Soil inorganic carbon"), versus organic carbon (see "Soil organic carbon"). Any measurement of soil total carbon should be accompanied by measurements of bulk density in order to rigorously extrapolate soil samples to field-scale stock estimates (see "Bulk density").
Total alkalinity (TA)	Titrations	N/A	Primary	Primary	N/A	N/A
Rock application N/A Initial weathering Primary Field processes Primary Watershed transport N/A Ocean storage N/A Transient Yes Type Measurement Category Soil, Water Impacts N/A Notes Changes in total alkalinity (TA) can provide evidence of weathering, as bicarbonate ions are generally the dominant contributor to alkalinity. Alkalinity determinations via acidemetric titration may be made on soil extracts (e.g., saturated paste), soil pore water samples (e.g., from a lysimeter), or in downstream water samples. Comments Bicarbonate alkalinity is subject to fluctuations as pH and other system parameters change, altering carbonate speciation. Therefore, alkalinity measurements are context-dependent and do not necessarily represent lasting outcomes. Continuous monitoring of total alkalinity (TA) would be required to capture the total flux of additional bicarbonate as a result of enhanced weathering. However, this poses a challenge given realistic sampling constraints and equipment interference with farming operations. Care must be taken that other field management changes are not responsible for observed shifts in alkalinity (e.g., changing irrigation source or rate), which emphasizes the importance of establishing a dynamic baseline rather than solely monitoring alkalinity prior to rock application. Alkalinity may also be estimated by measuring cations (see "Cations / anions") or potentially by monitoring electrical conductivity as a proxy (see "Electrical conductivity"). Note that there may be a delay between initial weathering and a total alkalinity signal as a result of interactions between weathered cations and the soil exchange complex (see "Cation exchange complex (CEC) and exchange complex cations"). References Campbell et al., 2023 Olin Neal, 2001 Weihermüller, et al., 2007
Trace metals	ICP-AES, ICP-MS, AAS	N/A	Primary	N/A	N/A	N/A
Rock application N/A Initial weathering Primary Field processes N/A Watershed transport N/A Ocean storage N/A Transient Maybe Type Measurement Category Soil, Water, Crops Impacts Heavy metals Notes Monitoring changes in trace metal concentrations may inform estimates of initial rock weathering. Measurements of trace metals in the environment should be accompanied by a clear characterization of the elemental composition and trace metal content of the applied weathering material (see "Elemental composition"). Various forms of spectroscopy can be applied to liquid or solid samples to determine the trace metal content, such as inductively coupled plasma atomic emission spectroscopy or mass spectroscopy (ICP-AES or ICP-MS) or atomic absorption spectroscopy (AAS). Comments It is important to monitor trace metal concentrations like nickel (Ni) and chromium (Cr) in soil, water, and crops to ensure safe levels of heavy metals are maintained. Many jurisdictions regulate allowable concentrations of trace metals, and relevant regulations should thus inform rock application, measurement approaches, and threshold concentrations. If a field is subject to repeated applications of weathering material, cumulative levels of trace metals should be monitored. References Campbell et al., 2023 Haque et al., 2020 Dupla et al., 2023
Upstream emissions	LCA	Essential	N/A	N/A	N/A	N/A
Rock application Essential Initial weathering N/A Field processes N/A Watershed transport N/A Ocean storage N/A Transient N/A Type Other Category Other Impacts Industrial development Notes Upstream emissions must be accounted for when estimating net carbon removal from an enhanced weathering deployment. Key components to consider include the mining, grinding, and transportation of rock. Comments At present, few LCAs of enhanced weathering exist in the scientific literature. An LCA may also be used to inform the consideration of non-carbon impacts associated with mining, grinding, and transporting the rock. References Lefebvre et al., 2019 Calabrese et al., 2022 Tan and Aviso, 2021
Weathering rate and field outgassing	Process or empirical models	N/A	Secondary	Primary	N/A	N/A
Rock application N/A Initial weathering Secondary Field processes Primary Watershed transport N/A Ocean storage N/A Transient N/A Type Model Category N/A Impacts N/A Notes Process models like reactive transport models can be used to characterize initial rock dissolution and subsequent reactions as weathering products move within the soil column. Reactive transport models can be designed at various levels of spatial complexity (1D, 2D, or 3D), and explicitly represent different combinations of biogeochemical processes and deployment practices that influence net carbon removal. In contrast, empirical models are based on statistical analysis of observed data, and do not contain a mechanistic representation of initial weathering or field processes. Comments There is broad consensus that models are not sufficiently rigorous to characterize initial weathering or field processes on their own. Any models used should be validated by in-field measurement. Empirical models based on mesocosm or greenhouse studies are unlikely to generalize well to field deployments. References Kanzaki et al. 2022 Lewis et al. 2021 Kelland et al., 2020 Maher and Chamberlain, 2014

About

The goal of this tool is to support broad community engagement around enhanced weathering monitoring, reporting, and verification (MRV). To do so, we catalog quantitative methods that may be used to estimate net carbon removal from enhanced weathering. In the context of this tool, we focus on enhanced weathering that is performed by spreading ground rock on agricultural soils.

CarbonPlan collaborated with Iris Holzer (University of California, Santa Barbara), Noah Sokol (Lawrence Livermore National Lab), and Eric Slessarev (Yale University) to build this tool. Contributions from all authors were made in their personal capacity under contract with CarbonPlan. CarbonPlan received no specific financial support for this work.

Please cite the tool as:

I Holzer, E Slessarev, N Sokol, K Martin, F Chay (2023) “Quantifying Enhanced Weathering” CarbonPlan

Content

Each entry in the tool connects two pieces of information: a variable that may influence or respond to enhanced weathering, and a set of methods for generating data on that variable.

We map each combination of variable and methods to one or more phases of the enhanced weathering process which they could inform: rock application, initial weathering, field processes, watershed transport, and ocean storage. We indicate how relevant the variable is for characterizing a given enhanced weathering phase with qualitative tags.

In the table above, you can click to expand several elements to access more detailed information. By clicking the plus sign by a column header, you can read more about the enhanced weathering phases and the relevance mapping described above. By clicking on any entry within the table, you can see additional information on the entry and read our notes and comments. Finally, by clicking on the information icon throughout the tool, you can access definitions and extra context.

Note that this tool does not present an exhaustive catalog of quantitative methods and should not serve as a substitute for a more comprehensive inventory when making MRV implementation decisions. You can download the content in this tool as a CSV or JSON file. These files include all the same data as rendered on this website. Read the accompanying explainer for more details.