It is possible to calculate potential atmospheric removal by assuming full dissolution of the applied rocks. Realizable drawdown potential depends on mineral composition, particle size, kinetics, and applied volumes. It is also possible to directly analyze rock composition, which is critical not only for carbon accounting but also for tracking other ecosystem and agricultural impacts. Applied volume can be confirmed via an audit of operational records.

The effects of enhanced weathering should be understood as an overall shift in CO₂ fluxes as a result of introducing alkalinity to the system through rock weathering. While most applied minerals will ultimately weather, enhanced weathering approaches should only be credited when the impact on atmospheric CO₂ actually occurs. The rate of rock weathering, and therefore alkalinity introduction, depends on many factors including rock type, particle size, soil type, ambient temperature, precipitation, and irrigation. At a given point in time, it may be possible to measure how much rock has weathered via elemental analysis or by tracking changes in the concentration of tracer components that originate in the applied rocks and persist in the soil after rock weathering. Rock weathering may be facilitated by different sources of acid (carbonic or non-carbonic) and result in different weathering products, which can each have unique impacts on system CO₂ fluxes (see Terrestrial carbonate precipitation, Secondary mineral formation, and Neutralization of non-carbonic acid for more details.) If rock is weathered by carbonic acid, a potential product is dissolved inorganic carbon (DIC). To inform a lower bound on rock weathering and characterize how much additional DIC is leaving the site because of the weathering, effluent from the weathering site can be continuously monitored to detect changes in the carbonic acid system. These changes can be constrained by monitoring any two components of the carbonic acid system: pH, total alkalinity (TA), partial pressure of CO₂ (pCO₂), or dissolved inorganic carbon (DIC). Both approaches — monitoring of the rock in the weathering site and monitoring the effluent leaving the weathering site — are areas of active innovation.

If drawdown is being estimated on the basis of direct observation of mineral weathering rather than via estimation of alkalinity run-off (see Mineral Weathering for more details), it is important to characterize what fraction of the rock was weathered by non-carbonic acids. Non-carbonic sources of acidity in the soil may include protons associated with mineral and organic surfaces and other sources of acidity from organic acids, nitrogen transformations, or sulfide mineral oxidation. If applied rock is weathered by non-carbonic acid that would have otherwise caused outgassing downstream, it may be appropriate to count this avoided outgassing toward the net carbon drawdown of the enhanced weathering intervention depending on how confidently the counterfactual can be characterized and how quickly the outgassing would have otherwise occurred.

Secondary mineral formation — such as the formation of clays — can counteract the alkalinity addition associated with rock weathering. It therefore must be considered if drawdown is being estimated on the basis of direct observation of mineral weathering rather than via estimation of alkalinity run-off (see Mineral weathering for more details). Secondary mineral formation depends on many factors, including on the type of primary mineral dissolving as well as the local climate and biota present. It can be characterized via soil mineralogy techniques like x-ray diffraction.

The formation of solid carbonates decreases CO₂ drawdown relative to the aqueous reactions that result in alkalinity run-off. It therefore must be considered if drawdown is being estimated on the basis of direct observation of mineral weathering rather than via estimation of alkalinity run-off (see Mineral Weathering for more details). Terrestrial carbonate precipitation depends on the type of primary mineral dissolving, carbonate saturation state, and other local environmental conditions.

Groundwater can spend days to centuries in the ground before being discharged to streams. Once in surface waters, outgassing (evasion) might occur as groundwater equilibrates with the atmosphere or experiences pH shifts. Outgassing may also occur as surface waters interact with the surface ocean. Ultimately, the effects of enhanced weathering should be understood as an overall shift in CO₂ fluxes from the system as a result of the alkalinity introduced by rock weathering. Any changes in outgassing in the project scenario as compared to the counterfactual should be accounted for. The long timescale and large spatial scale of this process makes it difficult to study directly, but regional models could inform baseline- and project-scenario evasion estimates.

Eventually, the formation and burial of carbonate minerals will release CO₂ stored by mineral weathering. This process counteracts most or all of the carbon storage associated with weathering carbonate rock, and around half of the storage associated with silicate rock weathering. The carbonate precipitation process occurs slowly and is unlikely to have a significant impact on estimates of carbon storage on decadal to millennial timescales. However on geologic timescales, it is an important process to characterize.

In agricultural contexts, fine grained carbonate limestone (agricultural lime) is commonly applied to raise the pH of acidic soils. As the lime weathers, it may serve as a sink or source of carbon dioxide depending on the time frame considered, the extent of strong acid weathering, and the degree of secondary carbonate precipitation, among other factors (see corresponding pathway components). Weathering of a mole of carbonate can remove one mole of carbon dioxide, half of that amount removed through silicate weathering. Any change in lime use because of an enhanced weathering deployment, including replacing lime with another feedstock, requires accounting for the corresponding change in both life cycle emissions and carbon removal. We consider the uncertainty due to changing lime use to be medium (5-20%) because lime weathering efficiency may be reasonably well-known at sites that have applied lime for years, but tracking the fate of lime-derived carbon remains a challenge. If no liming occurs prior to enhanced weathering deployment, the uncertainty of the liming counterfactual is not applicable.

Adding minerals to soils has the potential to induce changes to soil carbon, including by affecting pH or impacting plant or microbial activity. Theoretically, these impacts could increase or decrease soil organic carbon (SOC) storage (i.e. the priming effect). There is relatively little research on these potential interactions to date, and they merit further exploration. Due to the high uncertainty of these potential effects, we recommend this component not be included in the accounting of net carbon removal. If mineral application does increase SOC storage, we recommend that these changes not be used to counterbalance project-related fossil CO₂ emissions nor counted as additional durable carbon removal due to the impermanence of soil carbon storage.

The embodied emissions of any materials consumed during operation, like mineral feedstocks, can be estimated based on a cradle-to-grave lifecycle assessment (LCA) of the material input. Emissions associated with any built infrastructure, including monitoring and maintenance equipment, should include both construction emissions and material embodied emissions. There are not yet consistent best practices around whether or how to account for the embodied emissions of equipment or infrastructure that is used but not owned by the project. Transparency around boundary assumptions, data sources, and uncertainties is critical for LCA consistency and comparability.

The emissions associated with energy use for the process should be estimated based on an assessment of lifecycle emissions for the specific electricity or energy sources consumed by the project. This should include the energy use associated with transporting and spreading mineral, combustion emissions associated with fuel use during the project, and energy use associated with monitoring activities.

Applied minerals may contain trace amounts of phosphorus and potassium, which could potentially reduce fertilizer requirements compared to conventional liming approaches. Application of alkaline silicates could also improve plant nitrogen uptake. Due to the potentially difficult nature of the counterfactual scenarios involved, quantifying these impacts may be challenging. If they are quantified, we recommend any associated avoided emissions be understood as a co-benefit and excluded from the calculation of net carbon removal.

There is some evidence that the effect of rock weathering on the pH and phosphorus levels of cultivated soils could reduce N₂O emissions. This is an area of active research, and we recommend this component not be included in the accounting of net carbon removal.

The formation of terrestrial carbonates depends on the type of primary mineral dissolving, carbonate saturation state, pH, and other local environmental conditions. In natural systems, terrestrial carbonates can be quite durable, persisting for many thousands of years as conditions permit. In enhanced weathering deployments, these minerals could be much less durable. If a deployment creates the conditions that allow terrestrial carbonates to form and persist, for example by raising the soil pH, then these minerals are likely to dissolve after the deployment activity stops. The uncertainty around terrestrial carbonate durability is not included in the calculation of this pathway's Verification Confidence Level (VCL) because it is also captured by the Terrestrial carbonate precipitation component.

The concept of ocean dissolved inorganic carbon (DIC) lifetime is a well-established theory and is accepted within the community to lie somewhere between 10,000 and 100,000 years. This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Marine carbonate precipitation component.

The precipitation and dissolution of carbonate minerals modifies ocean alkalinity (see DIC residence time). Carbonates preserved on the seafloor are likely to store carbon on geologic timescales. This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Marine carbonate precipitation component.