Carbonated Material Storage and Monitoring

This Module details the determination of reversal risk for carbonated materials stored in closed or open systems. Within this Module, durability refers to the length of time for which carbon is removed from the Earth’s atmosphere and cannot contribute to further climate change.

The stability of carbon within the carbonated materials, and thus its durability, depends on their interactions with the surrounding environment. Potential risks to the expected durability of carbonate minerals include dissolution by strong acids (e.g. H₂SO₄, HNO₃, etc) fluids, changes in pressure and temperature and changes in geochemical and environmental parameters. These risks will likely be greater if storage occurs within mining operations, and very low within most other environments. Note that, where carbonic acid is the acidity source, dissolution of carbonate minerals will lead to storage of CO₂ as dissolved bicarbonate (HCO₃^-).

Within this Module, reversal risk is determined based on storage environment, described further in Section 4.0. This assessment is conducted via either a geochemical model or direct monitoring, incorporating risk factors specific to each storage type. For low-risk environments, reversal risk is accounted for by application of a conservative uncertainty discount based on this risk assessment. Where possible, additional site characterization and monitoring can be used to reduce the uncertainty discount. This is described further in Appendix 1. For high-risk environments, such as mine sites, direct monitoring for reversals at the storage site is required. Site characterization and monitoring requirements for high-risk storage sites are given in Appendix 1. Additional uncertainty discounts may also be applied in the event that reprocessing of carbonated mine tailings is described as a possibility in the mine closure plan.

This Module is applicable to Projects storing carbonated materials, made via open- or closed-system mineralization reactions. Storage can take place in:

• Closed systems (e.g. lined and capped landfills)
• Open systems (e.g. open pits)
• High-risk environments, such as mine sites, which may constitute open or closed system storage

This Module does not include storage in the built environment, as engineering fill or as an agricultural amendment.

The Project Proponent must demonstrate that all relevant permitting requirements are met. The permit must specifically include the storage location as well as identify the feedstock being stored. This permit must be shared with the Validation and Verification Body (VVB) and Isometric prior to project crediting.

This Module requires characterization of all carbonated materials in accordance with Isometric's Rock and Mineral Feedstock Characterization Module:

The two primary pathways by which CO₂ can be lost from carbonated materials are exposure to high temperatures (e.g., >300 °C)¹ and contact with low pH fluids. These conditions, though not necessarily common globally, can occur in some instances; for example, mine tailing are sometimes heated to high temperatures during reprocessing,² and the pH of groundwater that is impacted by industrial pollution (e.g. acid mine drainage, AMD) can be below 3.³

Carbon loss resulting from high temperatures is referred to as calcination and occurs by the reaction:

$CaCO_3(s) \to CaO(s) + CO_2(g)$

Equation 1

The kinetics of this reaction are strongly dependent on both temperature and crystal structure.⁴ Multiple polymorphs of CaCO₃ exist in nature, the most common of which is calcite. Studies have shown that, depending on the reaction conditions, calcination of calcite can occur between 680–900°C.⁵ Other possible CaCO₃ polymorphs include aragonite and vaterite, both of which decompose at lower temperatures than calcite (450–725°C).⁴ Depending on the feedstock mineralogy, magnesium carbonates may also be present, which can decompose at temperatures as low as 300 °C.¹ Thus, conservative estimates of carbon loss due to calcination require either detailed carbonate mineralogy data or the assumption that at least some of the CaCO₃ present is in non-calcite carbonate phases.

Carbon loss resulting from contact with low pH fluids is dependent on a) pH, b) the acid source, c) ambient temperature, d) the material's initial composition and porosity, and e) length of exposure. In particular, interaction of carbonate minerals with strong acids, such as sulfuric, nitric or phosphoric acid, lead to release of CO₂ following the reactions:

$CaCO_3 (s) + H_2SO_4(aq) \to CaSO_4 + CO_2 (g) + H_2O(l)$

Equation 2

$CaCO_3 (s) + 2HNO_3 (aq) \to Ca(NO_3)_2 + CO_2 (g) + H_2O(l)$

Equation 3

$CaCO_3 (s) + H_3PO_4 (aq) \to CaHPO_4 + CO_2 (g) + H_2O(l)$

Equation 4

Conversely, where the acid source is carbonic acid, interaction with calcium carbonates can result in either carbon loss or carbon storage depending on the pH of the solution. Dissolution of CaCO₃ by carbonic acid (here represented as CO₂ + H₂O) occurs following the reaction:

$CaCO_3+CO_2+H_2O \to Ca^{2+}+2HCO_3^-$

Equation 5

In this reaction, dissolution of 1 mole of CaCO₃ results in the capture of 1 mole of CO₂ as bicarbonate (HCO₃^-). Bicarbonate is part of the carbonic acid system, the speciation of which is highly pH-dependent ⁶. As pH decreases, the relative concentration of dissolved CO₂ increases. This dissolved CO₂ can exchange with the atmosphere, resulting in a release of CO₂.

This Module considers three broad categories of storage environment: open system, closed system and high-risk. The reversal risk for carbonated materials is determined separately for each type of storage according to the likelihood of exposure to conditions resulting in CO₂ loss from carbonate minerals.

Storage of carbonated materials in a location that does not fall under these categories may be allowable in consultation with Isometric. Project Proponents are required to provide details on the storage location, including storage type, in the PDD. The storage type designation must be justified in the site description.

Carbonated materials stored in closed systems are isolated from ambient environmental conditions. An example of closed system storage is a purpose-built, dedicated use facility that is constructed for storage of carbonated materials associated with project activities. Under most circumstances, this means that CO₂ should remain durably stored for a 1,000 year period. Projects storing carbonated materials in closed systems are not subject to an additional uncertainty discount, though they must maintain a buffer pool as described in Section 4.4. Project Proponents must provide evidence that the system is reasonably isolated from external conditions, including ambient environmental conditions and interactions with groundwater through the presence of one or more impeding layers. If the closed system storage site has additional risk factors that may impact the durability of the carbonated materials, it will be treated as a high-risk storage environment (see Section 4.2.3).

Where a dedicated facility is constructed, the Project Proponent should ensure that the storage site is designed and constructed based on a conceptual site model and in compliance with the relevant regulatory authority's permit or equivalent. Documentation and records of well construction should be maintained and available for review for the duration of the Crediting Period, as well as 10 years post closure.

Carbonated materials stored in open systems are exposed to ambient environmental conditions, including precipitation, temperature fluctuations and wind. Examples of open system storage include open pit storage, storage in unlined and/or uncapped landfills and storage in non-dedicated use facilities, such as construction and demolition waste landfills. Depending on the environmental conditions, this exposure may lead to loss of CO₂ from the carbonated materials (Section 4.1) or additional carbon removal, such as by additional mineralization or dissolution of carbonate minerals by carbonic acid (Equation 5). This leads to uncertainty in the amount of CO₂ that remains stored over a 1,000 year period unless additional measurements are performed at the storage site. As such, Projects storing carbonated materials in open systems are subject to an uncertainty discount as described in Section 4.3.1.

Within this Module, a high-risk environment is defined as a storage location where there is significant risk that carbonated materials may be exposed to either high temperatures or acidic solutions (see Section 4.1). This includes storage at mine sites, where there may be contamination by acid mine drainage (AMD) or where carbonated tailings may be reprocessed in the future. Projects storing carbonated materials in high-risk environments are required to conduct additional site characterization and ongoing monitoring in accordance with Appendix 1. In addition to required measurements, Projects storing carbonated materials in mining environments may be subject to an additional uncertainty discount associated with high-temperature reprocessing of carbonated tailings (Section 4.3.2).

Within this Module, reversal risk is determined based on the storage type. As stated in Section 4.2.1, Projects storing carbonated materials in closed systems with no additional risk factors are not required to assume an uncertainty discount, but are required to maintain a buffer pool as described in Section 4.4.

To ensure that uncertainty discounts are conservative, the reversal risk approach described here assumes the worst-case scenario for environmental conditions if no additional information can be provided. For example, the reversal risk for Projects operating in open systems outside of high-risk areas will be determined based on the lowest groundwater pH within the operational region. Project Proponents can provide additional information for model inputs that may result in a lower uncertainty discount. This may include (but is not limited to):

• Average pH of groundwater at the storage location
• Average pH of precipitation at the storage location
• Average annual temperature at the storage location
• Where applicable, description of any tailings reprocessing procedures (or lack thereof), signed by the mining operator

As described in Section 4.1, interaction with surrounding fluids is the most direct reversal risk for carbonated materials stored in open systems or high-risk environments. This can be determined using geochemical models, such as PHREEQC or Geochemist's Workbench. Geochemical modeling must consider the following inputs:

• Mineralogy of the materials stored through direct measurement, including:
  • Carbonate content and mineralogy
  • Any other minerals present
• Climatic conditions of the storage site, determined by direct measurement or weather station data, including:
  • Average annual precipitation volume
  • Average precipitation pH
  • Average annual temperature
• Groundwater chemistry, determined by direct measurement or open source data, including:
  • pH
  • Alkalinity
  • Carbonate saturation

Of these parameters, mineralogy of the stored materials must be directly measured. It is recommended to directly measure climatic conditions and groundwater chemistry where possible, but where direct measurement is not possible and open source data is unavailable, conservative assumptions must be applied as described above. Project Proponents may run their own models based on the parameters given above and must provide the details of the model, including the software used, input parameters and any additional code to Isometric. Project Proponents may also request that Isometric runs a PHREEQC model, and in this case must provide data on all required parameters to Isometric.

Though less likely than reversal through interactions with surrounding fluids, there are some environments where reversal from exposure to high temperatures may occur. This includes high-temperature reprocessing of materials stored within mine sites and high-temperature landfill fires. Note that, due to lack of data on the incidence of high-temperature landfill fires, the reversal risk approach described here does not currently account for this reversal pathway. However, in the event that a fire occurs at the storage facility, Project Proponents must report this to Isometric to determine if a reversal is likely to have occurred.

Reprocessing of mine tailings is not currently widely implemented, due to factors such as difficulties in handling fine-grained materials, heterogeneity of tailings piles and economic considerations.² However, declining ore grades, advances in reprocessing technology and increased focus on sustainable mining practices may increase demand for reprocessing in the future.² As such Project Proponents storing carbonated materials in mine sites are required to report any current reprocessing practices utilized by the operator or that may be implemented as part of the closure plan. If that information is unavailable, Project Proponents must report any reprocessing techniques that are used for the tailings types relevant to project activities. Where these practices include high-temperature reprocessing, an audit will be conducted, in consultation with Isometric, to determine a conservative discount.

Based on present levels of scientific knowledge, Projects applicable to this Module are categorized as having a Very Low Risk Level of Reversal according to the Isometric Standard Risk Assessment Questionnaire. This storage pathway does not have a documented history of reversals; however, certain environmental conditions (described above in Section 4.1) can lead to reversal. As a result, a 2% buffer pool will be set aside as a precaution. This reversal risk will be reassessed every 5 years, aligning with the Crediting Period, or when new scientific research and knowledge are produced.

Table A1. Site Characterization

Table A2. Site Monitoring

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2. Abbadi, A., and Mucsi, G. (2024). A review on complex utilization of mine tailings: Recovery of rare earth elements and residue valorization. Journal of Environmental Chemical Engineering, 12, 113118. https://doi.org/10.1016/j.jece.2024.113118 ↩ ↩² ↩³

3. Nordstrom, D. K., Blowes, D. W., and Ptacek, C. J. (2015). Hydrogeochemistry and microbiology of mine drainage: An update. Applied Geochemistry, 57, 3–16. https://doi.org/10.1016/j.apgeochem.2015.02.008 ↩

4. Zhang, D., and Shao, Y. (2016). Effect of early carbonation curing on chloride penetration and weathering carbonation in concrete. Construction and Building Materials, 123, 516–526. https://doi.org/10.1016/j.conbuildmat.2016.07.041 ↩ ↩²

5. Vogler, N., Drabetzki, P., Lindemann, M., and Kühne, H.-C. (2022). Description of the concrete carbonation process with adjusted depth-resolved thermogravimetric analysis. Journal of Thermal Analysis and Calorimetry, 147, 6167–6180. https://doi.org/10.1007/s10973-021-10966-1 ↩

6. Zeebe, Richard E., and Dieter Wolf-Gladrow. CO₂ in seawater: equilibrium, kinetics, isotopes. Vol. 65. Gulf Professional Publishing, 2001. https://shop.elsevier.com/books/co2-in-seawater-equilibrium-kinetics-isotopes/zeebe/978-0-444-50946-8#full-description ↩