Biochar Storage in Subsurface Mines

This Module details the durability, reversal risks and requirements for storage of carbon as biochar in subsurface mine environments. A net decrease in atmospheric CO₂ will occur when biochar is emplaced and contained in a manner that substantially slows or arrests its chemical, microbial, and physical degradation, and prevents decay products from migrating to the atmosphere.

This Module was developed based on the current state of the art and publicly available science regarding biochar storage in subsurface mine environment. Biochar storage in subsurface mine environments is a novel biochar storage approach, so this Module incorporates requirements that may be more stringent than some current relevant regulations or other Protocols related to biochar for CDR. This Module will be reviewed when there is an update to scientific published literature which would affect net CO₂e removal quantification, durability claims or the monitoring guidelines outlined in this Module. Future versions of this Module may be altered, particularly regarding requirements for demonstrating durability of biochar, as the stability of CO₂ captured by biochar is better demonstrated and documented; quantification of biochar degradation is further improved and refined; and the overall body of knowledge and data regarding all processes, from feedstock supply to conversion and to permanent storage, is significantly increased.

Biochar is a carbon-rich material produced by the thermochemical conversion (pyrolysis) of biomass under oxygen-limited conditions¹. A significant fraction of the carbon in biochar is stable and durable over a time horizon of 1000 years and beyond (see Isometric Biochar Production and Storage Protocol). The proportions and characteristics of carbon fractions within biochar vary substantially with feedstock and pyrolysis conditions.

The labile fraction of biochar is subject to degradation over time through pathways that depend on the physical and chemical conditions of the storage environment. These pathways can include:

1. Physical: mechanical breakdown (dust loss, particle size reduction due to crushing or movement)^2,3.
2. Abiotic degradation: chemical reactions related to environmental factors including but not limited to: natural mineralization, photodegradation, leaching due to water flows or freeze thaw cycles ^3,4,5.
3. Microbial degradation: under aerobic conditions, including by saprotrophic fungi capable of degrading condensed aromatic structures⁶; under anaerobic conditions, biochar surfaces and pore networks can host methanogenic and sulfate-reducing microbial activity, with biochar's porous and electrically conductive structure capable of supporting direct interspecies electron transfer in methanogenic syntrophy⁷. The microbially accessible carbon fractions depend on biochar characterization: for biochars with H/Corg ≤ 0.4, the semi-persistent (SPC) fraction is the dominant microbially accessible pool over the credit horizon; for biochars with H/Corg > 0.4, both the distinct labile pool and the SPC fraction are accessible to microbial degradation.

Each of these pathways is directly influenced by the storage environment, with the relevant site characteristics determining which pathways may be most prevalent. The durability of biochar is dependent not only on storage site conditions but on biochar physical and chemical properties (see Section 4 for more information). Several storage options are available for biochar removal, including burial in low-oxygen environments. In the case of burial storage where low-oxygen conditions are achieved and maintained through engineered controls and specific site characteristics, abiotic oxidative degradation becomes negligible, leaving biotic degradation as the dominant pathway^8,9.

Subsurface mining environments, including operational underground mines (in mined-out areas), and closed underground mines in care and maintenance or in the process of closure, offer carbon-storage attributes that distinguish them from soil and near-surface burial. Once emplaced and sealed, these environments can provide functional anoxia through limited oxygen ingress, mechanical protection from surface disturbance, and physical isolation from active hydrological cycling. However, the chemical and geological diversity of mining environments is substantial: salt, hard-rock, coal, potash, and evaporite host rocks differ markedly in hydrogeological behavior, dissolution susceptibility, structural stability, and chemical reactivity with both biochar and any containment materials^10,11. Each host-rock setting presents a distinct reversal-risk profile that this Module addresses through host-rock-specific characterization requirements.

A defining feature of biochar storage in subsurface mining environments is the use of engineered containment systems, most commonly cement- or geopolymer-based grouts, backfills, plugs, or seals, to physically and chemically isolate the emplaced biochar from groundwater, oxygen, and microbial activity. Containment materials interact with biochar in non-trivial ways. Alkaline cement pore solutions (pH 12–13) suppress microbial activity in the short term, which is protective, but published studies of biochar exposed to cementitious media have also reported measurable structural and chemical changes to biochar, including declines in aromaticity, increases in matrix defectiveness, collapse of pore structure, reductions in mechanical properties, and formation of organometallic complexes between Ca²⁺ ions and oxygen-containing functional groups on biochar^11,12. The containment system has its own durability profile. Cement-based seals exposed to sulfate-bearing groundwater, common in evaporite host rocks and in environments where sulfide minerals oxidise to sulfate, are susceptible to sulfate attack, which proceeds through diffusion of sulfate ions, leaching of calcium hydroxide, ettringite and gypsum formation, decalcification of calcium-silicate-hydrate (C–S–H), and ultimately thaumasite formation, with progressive degradation of the cementitious matrix over decadal to centennial timescales^13,14. Thaumasite-form sulfate attack is particularly favoured by cold groundwater (≤15 °C, with optimal kinetics around 5–8 °C), conditions readily encountered in subsurface mine environments^14,15. Containment durability therefore directly constrains the durability of the biochar carbon sink, and this Module requires that containment design and host-rock chemistry be jointly assessed.

Long-term structural stability of the host mine is a further determinant of durability. Underground mining operations, whether active, in care and maintenance, or recently closed, can exhibit subsidence, pillar deterioration, and progressive degradation of mine workings over timescales relevant to credit horizons. Mechanisms include progressive rock-mass weathering, cyclic wetting-and-drying, swelling under humidity changes, micro-crack propagation, roof fracturing, bulk subsidence, and in dissolution-susceptible host rocks (e.g. evaporite, gypsum, potash), pillar narrowing and roof weakening through groundwater interaction¹⁶,^10,17,18. Post-closure groundwater rebound, the recovery of the water table following cessation of dewatering, is a near-universal feature of underground mine closure and typically completes on timescales of decades to over a century, depending on host-rock hydraulic properties¹⁹. The interaction between host-rock dissolution, water-bearing conditions, containment-material degradation, and structural stability of the host mine is the principal reversal-risk pathway addressed by this Module.

Quantification of the durability of biochar according to the current best available science must focus on rigorous characterization of the biochar to calculate the fraction of biochar that is stable beyond the desired crediting time horizon, coupled with conservative treatment of the uncertainty associated with that calculation (see Section 6.5 of the Biochar Production and Storage Protocol). In the case of subsurface mine storage, this characterization must be supplemented by site-specific evidence covering host-rock chemistry, hydrogeological setting, containment-system specification and durability, and verification and monitoring plan appropriate to the site's access conditions, including pre-seal verification of emplacement and containment, demonstration of containment design life, and any feasible non-intrusive monitoring, sufficient to support the durability claim over the Project lifetime.

This Module sets requirements on the storage of biochar in subsurface mining environments (see Section 2) for the purpose of carbon dioxide removal and describes how characterization of biochar, containment systems, host rock, and storage site conditions should be used to quantify the number of Credits that are issued for a Project storing biochar in those environments. It also includes details of the environmental conditions that must be met and documented in the Project Design Document (PDD) to ensure that biochar-C is stably sequestered in subsurface conditions for the credit horizon. This Module addresses storage site conditions and quantification of CO₂e stored for biochar emplaced in subsurface mining environments. For more information on biomass feedstock eligibility and accounting and pyrolysis conditions please refer to the Biomass Feedstock Accounting Module which must be followed as outlined in the Biochar Production and Storage Protocol and Section 9.0 of the Biochar Production and Storage Protocol.

The following requirements apply to all subsurface mining operations seeking eligibility under this Module:

• Subsurface mines must have existing excavations suitable for biochar storage.
• Subsurface mines must not be specifically opened, extended, or maintained for the purpose of carbon storage. Storage activities must be incidental to, not the driver of, mine operation or post-closure management decisions¹¹.
• The host rock and surrounding geological setting must be characterized to demonstrate suitability for long-term carbon storage, including assessment of structural stability, hydrogeology, and host-rock chemistry.

The following mining operation types may be considered as applicable biochar storage Project locations under this Module:

Operational Subsurface Mines. Active underground mining operations where biochar storage can be integrated into mined-out areas without interfering with ongoing mineral extraction, subject to the following conditions:

• Storage activities must not extend the economic life-of-mine (LOM) or incentivize continued or increased mining operations beyond the trajectory of the underlying mineral extraction business case.
• Stored biochar must be located in areas permanently undisturbed by future mining activities, with physical and operational separation from active workings documented in the Project Design Document.
• The host rock and mine workings hosting the storage area must be of a type and condition compatible with the durability requirements specified in this Module.

The following mining operation types are presumptively eligible based on typical host-rock and structural characteristics, subject to site-specific characterization:

• Hard-rock metal mines (e.g., gold, copper, nickel, zinc, lead): typically hosted in low-permeability crystalline or sedimentary rock with limited dissolution susceptibility.
• Coal mines: hosted in sedimentary sequences whose long-term stability is well-characterized in the mining engineering literature^17_,20.
• Halite (rock salt) mines: hosted in evaporite sequences whose mechanical behaviour is dominated by viscoplastic creep and damage self-healing properties internationally recognized as favourable for long-term underground storage of hazardous and radioactive waste^21_,7,22,23.
• Potash mines: hosted in evaporite sequences sharing viscoplastic creep behaviour broadly analogous to halite, subject to site-specific characterization^24,22.
• Diamond mines: typically hosted in kimberlite pipes and surrounding hard-rock country rock.

Dissolution-susceptible evaporite and carbonate mines (e.g., gypsum, anhydrite, soluble carbonate sequences) are eligible subject to site-specific characterization, but are not presumptively eligible. Unlike halite and potash, gypsum and anhydrite do not exhibit self-healing viscoplastic creep and are soluble in circulating groundwater, giving them an elevated dissolution and sulfate-attack risk profile. Projects hosted in dissolution-susceptible rock must additionally demonstrate:

• the dissolution susceptibility of the host rock, and of any dissolution-susceptible backfill or fill material in contact with the storage zone, together with the extent and duration of groundwater contact over the credit horizon;
• long-term pillar, roof, and floor stability accounting for dissolution-driven degradation over the credit horizon (see Section 7.3.3.1)
• sulfate-attack resistance and design life of cementitious or other containment materials under the site-specific sulfate exposure, including sulfate generated by dissolution of the host rock or of the backfill itself (see Section 8);
• a groundwater monitoring approach able to detect active dissolution (sulfate concentration, calcium concentration, saturation indices) where applicable (see Section 7.3.2.3).

Other mining operation types may be considered on a Project-by-Project basis, subject to documented evidence of comparable long-term host-rock stability, hydrogeological suitability, and compatibility with the durability and reversal-risk framework of this Module.

Closed or Closing Subsurface Mines. Closed underground mining operations may be considered eligible under the following conditions:

• The mine is currently in care and maintenance status, or in the process of completing closure plans but not yet fully remediated.
• Infrastructure required for biochar emplacement and verification remains accessible and in serviceable condition for the duration of emplacement and verification activities. Access to storage areas is expected to be lost once emplacement and sealing (e.g., backfilling with debris and concrete) are complete; ongoing access to sealed storage areas after that point is not required (see Section 10.2).
• The host rock and mine workings have been characterized and demonstrated to satisfy the durability requirements of this Module, including post-closure groundwater rebound considerations²⁵.
• Storage activities must not extend the period of care-and-maintenance status beyond the duration that would otherwise have applied in the absence of The Project^26,27.

The following mining operation types are not eligible biochar storage Project locations under this Module:

Mines Developed or Maintained for Carbon Storage. Subsurface mining operations that have been opened, extended, or are maintained primarily for the purpose of carbon storage. This includes mines kept in care and maintenance status, or whose closure schedule has been delayed, primarily to accommodate carbon storage activities^26,27.

Mines with Life-of-Mine Extensions Driven by Carbon Storage Activities. Operational subsurface mining operations that have extended, or are likely to extend, the LOM as a direct or indirect result of carbon removal activities²⁶.

Note: Where a mining operator has extended the LOM for reasons not related to carbon removal activities, a signed affidavit by the operator may be provided as evidence. Such affidavits must be supported by documented business-case evidence (e.g., updated reserve estimates, commodity price assumptions, capital expenditure decisions) demonstrating that the LOM extension would have occurred independently of The Project.

Fully Closed and Remediated Mines. Closed mining operations, including both surface and subsurface, that have completed closure plans and have been remediated in accordance with the applicable jurisdictional requirements and the operator's closure plan. Such mines are excluded both because reopening would compromise the closure rehabilitation and because verification of emplacement is typically not feasible.

Surface Mining Operations. Open-pit surface mining operations of any type. Surface mines do not provide the functional anoxia, mechanical protection, and isolation from hydrological cycling that are foundational to the durability framework of this Module (see Section 1.1).

Hydrogeologically Unsuitable Operations. Subsurface mining operations with active groundwater flow through proposed storage areas that cannot be diverted or adequately managed for the duration of the credit horizon. Active groundwater flow drives multiple reversal-risk pathways, including host-rock dissolution in soluble lithologies, sulfate attack on cementitious containment materials, and biochar leaching^27,16,28 (see Section 3).

The Project must document in The Project Design Document the following:

• The contaminant levels, geochemical parameters, and physical conditions that will be monitored to detect potential impacts on surrounding soil, surface water, groundwater, and air quality, together with the frequency and methods of testing.
• The environmental baseline against which Project-related impacts will be assessed, including pre-emplacement characterization of host rock chemistry, groundwater chemistry, and any pre-existing contamination.
• The site management practices in place, in collaboration with the mine operator and any applicable land managers, regulators, or rights holders, to maintain or enhance environmental quality during emplacement and over the post-emplacement controlling period.

Where soil quality, surface water quality, or groundwater quality is demonstrated or anticipated to be adversely affected as a result of Project activities, including but not limited to leachate generation, host-rock interaction with stored biochar, sulfate release from host-rock dissolution, or compromise of engineered containment systems, The Project Proponent must:

• Implement site storage management practices that prevent further adverse impact and remediate existing impact. For subsurface mine storage, such practices may include containment system reinforcement, groundwater management (diversion, pumping, treatment), modified emplacement procedures, and adjustments to The Project closure plan.

Guidelines on appropriate emplacement procedures, containment system specification, groundwater management, and site closure are provided in Sections 7.1, Section 10, and Section 11. These are designed to minimize adverse impacts on soil, air, surface water, groundwater, host rock, mine workers, and surrounding human and ecological communities.

Where applicable jurisdictional regulation governs any of the above (e.g., national mining safety regulations, national or regional groundwater protection standards, EU Water Framework Directive²⁹, waste storage and waste classification regulations), Projects must demonstrate compliance with the more stringent of the applicable regulation or this Module.

• Confined-space and ventilation procedures during biochar emplacement, particularly where host rock or mine atmosphere may contain residual mine gases (notably methane in coal-mine settings) or where sulfate-reducing conditions could generate hydrogen sulfide (H2S) in saturated, sulfate-rich environments³⁰.
• Dust management during handling, transport, and emplacement of biochar, with reference to inhalation exposure standards for fine carbonaceous particulates^2,31.
• Fire and self-heating risk management during biochar handling and emplacement, including consideration of co-storage with any residual coal or sulfide minerals in the host rock.
• Compliance with the applicable jurisdictional mine safety regulations of the storage location.

For Projects involving cross-border transport of biochar between the production site and the storage site, The Project Proponent must document compliance with the relevant transport, classification, and waste-shipment regulations of all applicable jurisdictions.

Biochar storage in subsurface mine environments differs from biochar application in agricultural soils or near-surface burial in several important respects: the biochar is physically isolated from biological systems by host rock and engineered containment, and is not in functional contact with soil microbial communities, plant root systems, or surface ecosystems. In these circumstances, subsurface mine storage of biochar may contribute to environmental co-benefits:

• Beneficial reuse of subsurface void space that would otherwise require filling with primary or secondary fill materials, potentially displacing higher-emissions fill alternatives. Where The Project Proponent claims emissions displacement of this nature, baseline assumptions must be documented in the PDD with reference to the counterfactual fill material that would have been used in the absence of The Project.
• Contribution to mine closure and rehabilitation where biochar emplacement is incorporated into the operator's approved closure plan. Such contributions must not extend The Project's underlying mining or care-and-maintenance activity.

Project Proponents may report in the PDD any specific environmental co-benefits or contributions to mine closure objectives associated with The Project. Such co-benefits are not credited under this Module but should be reported separately for Project transparency.

This Section provides the requirements for the characterization of biochar for durable storage in mine environments, to determine if the material is eligible for Crediting under the Biochar Production and Storage Protocol. Durability refers to the length of time for which CO₂ is removed from the Earth's atmosphere. Biochar physical and chemical characteristics will be highly influenced by the biomass feedstock type and pyrolysis conditions. This Section will not set requirements or guidelines for biomass feedstock eligibility or pyrolysis conditions. Please refer to the Biomass Feedstock Accounting Module and Section 9 of the Biochar Production and Storage Protocol for guidance and discussion on these two topics.

Some of the required measurements in this Section have minimum or maximum thresholds that determine eligibility for Crediting by Isometric. Others may be required but have no associated eligibility threshold. Additionally, certain measurements are not mandatory; however, Project Proponents are strongly encouraged to measure and report them to support scientific progress in understanding biochar durability in subsurface mine environment. Analytical methods provided are examples of eligible methodologies, but they are not the only ones permitted.

For each parameter, the selected methodology or analytical technique, along with an appropriate standard reference (e.g., ISO, ASTM, DIN), where applicable, must be specified in the PDD.

All laboratories used for analysis must conform to ISO 17025 or equivalent. Alternatively, laboratories may be eligible in consultation with Isometric if they can provide adequate QA/QC data. Sample preparation should be performed in adherence to ISO 13909-4:2025.

Biochar must be characterized prior to mine storage to ensure environmental safety and suitability for CO₂ removal, of particular relevance to subsurface mine storage:

• Residual heavy metals derived from feedstock or pyrolysis equipment, which under aqueous mobilization (Risk Category B, in Section 10.3) may leach into surrounding groundwater.
• Polycyclic aromatic hydrocarbons (PAHs) and other organic contaminants formed during pyrolysis under suboptimal conditions³².
• Other persistent organic pollutants (POPs) including PCBs, dioxins, and furans (PCDD/F) at trace levels.

Project Proponents must outline in the PDD the legal and regulatory requirements with which The Project complies in respect of biochar composition, both in the jurisdiction of biochar production and in the jurisdiction of storage.

In the absence of binding regulation, or where regulation does not address the specific contaminant, Projects must adhere to the upper bounds set by the World Biochar Certificate (WBC)³³ for the following parameters:

Some of these measurements will be used in the quantification of CO₂e_stored, as outlined in Section 6.1.1. The required and recommended measurements listed below investigate multiple mechanisms of reactivity (or prevention of), including aromaticity and aromatic condensation, functional groups, and volatility. The redundancy of characterizing reactivity potential via different mechanisms serves to reduce the uncertainty surrounding the durability of biochar, and provides multiple indicators of confidence that durability will exceed the crediting time horizon. All analyses documented in the table below meet or exceed the WBC standards.

All results pertaining to the calculation of carbon removal must be reported on a dry basis. Reporting on a dry basis provides a standardized, stable reference point for comparing material properties, making the data repeatable and reliable regardless of the sample's water content at the time of testing.

Table 1: Recommended and required measurements of biochar chemical properties

*A note on PAH requirement - PAH is required as outlined in Table 1, unless it can be demonstrated that stringent risk mitigation has been carried out, this is applicable to high tech, continuous production processes only. This would include pre-agreeing the risk mitigation with Isometric and detailing this in the PDD.
Risk mitigation may include the following:

• Demonstrating a sufficiently high pyrolysis temperature to ensure thermal cracking of PAHs.
• Reactor design choices such as:
  • Increased reactor residence time
  • Testing to show a thermal destruct unit removes PAHs to negligible levels during pyrolysis, coupled with evidence that those operating conditions for pyrolysis are maintained to remove the need for testing of PAHs.
• Evidence of how post-pyrolysis treatment of biochar removes PAHs.
• Previous evidence of the same production process producing acceptably low PAH concentrations.

Table 2: Additional chemical characterization required for 1000 year durability

For the required measurements in Table 1, samples should be taken using the same sampling regimes outlined in Section 8.3.1 of the Biochar Production and Storage Protocol for measuring carbon content.

A batch associated with any one project may have a unique history or set of characteristics that could require individual consideration for recommended measurements. Feedstock characteristics and pyrolysis conditions will influence biochar homogeneity. These include, but are not limited to; the biomass feedstock type and particle size distribution, pyrolysis temperature and reactor type. The sampling plan specified in Section 8.3.1 of the Biochar Production and Storage Protocol takes a conservative approach to sampling with enough frequency to capture the impacts of any heterogeneity in biochar. These considerations include, but are not limited to, biomass feedstock type and particle size distribution, pyrolysis temperature, reactor type, etc.

Project Proponents should utilize accredited analytical services such as UKAS, MCERTS, DWTS, and ISO whenever feasible. Where a Project Proponent utilizes laboratory facilities within an academic institution, or a non-accredited commercial laboratory, periodic external validation must be undertaken with an accredited facility. The frequency of these external checks will vary by project and analytical procedure, and will be agreed with Isometric on a case-by-case basis.

Laboratories must complete standard quality assurance procedures on a schedule in accordance with their quality management plans and accreditation requirements to include:

• Instrumentation calibrations and analysis of calibration standards or certified reference materials;
• Analysis of technical replicates, and;
• Analysis of blanks (where possible and appropriate);

Project Proponents must maintain data records for a minimum of five years following the date of data collection. It is also recommended that a similar approach is taken towards sample archiving, with a representative subsample (e.g. 100 g) dried and archived for a minimum of five years, to allow re-analysis of these, or additional parameters.

Project Proponents must report data such that the data analysis methods used are easily identified, verified and replicated. This Module requires that any data reports include the raw data from which any data analysis and or processing was performed, including reference standards and replicate measurements and any other data associated with quality control and assurance. A summary of the proposed sampling regime must be included in the PDD. Analytical uncertainty, number of samples taken and analyzed, standards used and number of standard runs, standard deviation and percentage error on the standards must also be included in the data report for the VVB.

For example, this may take the form of a spreadsheet containing four dataframes, in all cases appropriate identifiers should be used to allow samples to be easily identified:

1. Summary sheet detailing metadata:
   1. Number of samples run;
   2. Analytical uncertainty;
   3. Standards used;
   4. Number of standards run;
   5. Standard deviation;
   6. Percentage error on standards.
2. Reduced data sheet (data summary);
3. Data processing sheet (if applicable; e.g. processing of ICP-MS data);
4. Raw data;

The formula to calculate $CO_2e_{stored}$ is:

$$CO_2e_{stored} = C_{biochar} \times m_{biochar} \times F_{durable} \times \frac{44.01}{12.01}$$

(Equation 1)

Where

• $C_{biochar}$ is the carbon content of the biochar (empirical).
• $m_{biochar}$ is the dry mass of biochar applied.
• $F_{durable}$ is the fraction of durable biochar that remains in the mine for the full duration of the crediting timeline (i.e. 1,000 years), and can be credited under this Module.
• $\frac{44.01}{12.01}$ is the ratio of molar masses of carbon dioxide and elemental carbon.

This Module only Credits for the durably stored $C_{org}$ fraction in biochar, which is used to calculate $CO_2e_{stored}$. While biochar associated inorganic carbon generally makes up a small fraction of total biochar C. However, the fate of biochar-associated $C_{inorg}$ is much less predictable in the environment ^36,37. Thus, $C_{biochar}$ is calculated using the following equation:

$$C_{biochar} = Total\:Carbon\:Content-C_{inorg}$$

(Equation 2)

Where:

• $Total\:Carbon\:Content$ is the Total Carbon Content of the biochar as analyzed using the methods described in Table 1.
• $C_{inorg}$ is the inorganic carbon content of the biochar as analyzed using the methods described in Table 1.

Please refer to Section 8.3.1 of the Biochar Production and Storage Protocol for full guidelines on number of samples required for the measurement of biochar carbon content, $C_{biochar}$.

Please refer to Section 8.3.1.1 of the Biochar Production and Storage Protocol for full guidelines on measurement of mass of biochar applied, $m_{biochar}$.

The quantification framework for determining the CO₂e_stored for 1000-year durability is based on the quantification approach set out by ³³,³⁴. This approach quantifies biochar on the random reflectance value of the biochar, compared to inertinite as a proxy for geologically stable carbon. Using petrographic analysis, ³³,³⁴ identified that biochars with a mean random reflectance (R₀ ≥ 2%) exhibit structural characteristics equivalent to inertinite macerals in fossil coals and chars, which are known to persist over geological timescales. While biochar meeting the benchmark of R₀ ≥ 2% can be considered permanent, additional peer-reviewed research^34,38,39_,35 has been published that further validates the experimental work of ³³,³⁴.

As outlined in Section 4 of this Module, Project Proponents must report a set of at least 500 measurements of R₀, calculated at the maceral-level, for at least three replicate samples of their biochar. Batches that adopt this measurement approach can be credited for the percentage of their biochar which passes the 2% R₀ benchmark, as outlined in ³³,³⁴. The histogram of the R₀ values must be submitted at the point of project verification for this Crediting option. This method was further updated in ³³,³⁵ to refine the methodology to only account for the recalcitrant fraction of biochar (discounting the reactive fraction, determined by thermogravimetric analysis).

To ensure a conservative approach when Crediting biochar durability, we account for uncertainty in both the fraction of biochar passing the 2% R₀ benchmark and the proportion of carbon that is non-reactive. Specifically, for each replicate sample we determine the fraction of R₀ measurements that meet or exceed the 2% benchmark. The credited inertinite fraction is the mean of these fractions across all samples analyzed, reduced by one standard deviation of the fractions (Equation 3). This is applied to the non-reactive carbon fraction, itself reduced by one standard deviation (Equation 4), to give the credited durable fraction (Equation 5). This ensures the durability estimate reflects a lower-bound confidence level, mitigating the risk of overestimating long-term carbon storage.

As such, $F_{durable}$ is calculated through:

Calculating the sample standard deviation of $R_0$ quantifying the typical deviation of individual $R_0$ measurements from their mean, which is used to account for uncertainty and conservatively adjust the estimated durable fraction of biochar carbon, as:

$$s_{R_0} = \sqrt{\frac{1}{n_{R_0}-1} \sum_{i=1}^{n_{R_0}} (R_{0,i} - \bar{R_0})^2}$$

(Equation 3)

Where:

• $s_{R_0}$ is the standard deviation of the fractions passing the 2% benchmark.
• ${n_{R_0}}$ is the number of samples analyzed (i.e. > 3).
• $R_{0,i}$ is the fraction of R₀ measurements ≥ 2% in the $i$-th sample.
• $\bar{R_0}$ is the mean of the fractions (≥ 2%) across all samples.

Calculating the sample standard deviation of $C_{non-reactive}$ quantifying the typical deviation of individual $C_{non-reactive}$ measurements from their mean, which is used to account for uncertainty and conservatively adjust the estimated durable fraction of biochar carbon, as:

$$s_{C_{non-reactive}} = \sqrt{\frac{1}{n_{C_{non-reactive}}-1} \sum_{i=1}^{n_{C_{non-reactive}}} \left(C_{\text{non-reactive},i} - \bar{C}_{\text{non-reactive}}\right)^2}$$

(Equation 4)

Where:

• $s_{C_{non-reactive}}$ is the standard deviation of $C_{non-reactive}$ measurements
• $n_{C_{non-reactive}}$ is the number of samples analyzed (i.e. > 3).
• ${C_{non-reactive,i}}$ is the individual measurement for the $i$-th sample.
• $\bar{C}_{\text{non-reactive}}$ is the mean of all $C_{non-reactive}$ measurements

Then:

$$F_{durable,1000} = min(0.95 , max( 0 , (\bar{R_0} - s_{R_0}) (\bar{C}_{\text{non-reactive}} − s_{C_{non-reactive}}) ) ) )$$

(Equation 5)

Where:

• $F_{durable,\:1000}$ is the fraction of durable (inert) carbon in the biochar after 1000 years, adjusted conservatively for uncertainty.

• $\bar{R_0}$ is the mean of the fractions (≥ 2%) across all samples.

• $s_{R_0}$ is the standard deviation of the $R_0$ passing the 2% benchmark.

• $\bar{C}_{\text{non-reactive}}$ is the mean of all non-reactive carbon measurements.

• $s_{C_{non-reactive}}$ is the standard deviation of $C_{non-reactive}$.

The maximum and minimum functions are applied to ensure that the fractions are bounded.

The permit must identify biochar as acceptable for storage at the site.

Projects must adhere to regulations regarding the responsible operation of mines, including mines which are operational, closed, or undergoing remediation.

In addition, The Project must comply with all applicable local environmental, ecological, and social requirements as well as those set out in the relevant Protocol and the relevant section of the Isometric Standard.

At present, Projects with storage sites located in jurisdictions governed by the United States, Canada, United Kingdom, the European Union, Switzerland, Norway, Australia, or New Zealand are eligible under this Module. Projects in other locations may be eligible for crediting if the Project Proponent can demonstrate adherence to an equally rigorous set of requirements for permitting, mine safety, hydrogeological protection, and environmental protection as would be required for a similar Project in one of the above jurisdictions. Such exceptions must be approved by Isometric.

Note: For the purposes of this Module, 'location' refers to the location of the storage site. Upstream biochar production must comply with the jurisdictional requirements of the relevant Biochar Production and Storage Protocol.

This section details the potential risks and leak pathways applicable to subsurface mine storage Projects. Leak pathways represent routes through which carbon stored as biochar could be lost from the storage reservoir.

Projects must conduct a site-specific assessment of all potential pathways for leaks when characterizing the storage environment, and design appropriate engineering controls and MRV systems for the monitoring of stored biochar (see Section 10).

The following three risk categories are recognized for potential risks:

The magnitude of gas-phase reversal risk depends strongly on biochar characterization. Independent of biochar characterization, the properties of biochar that make it suitable for long-term carbon storage, high aromaticity and resistance to microbial degradation, substantially reduce the magnitude of gas-phase reversal risk relative to storage of fresh biomass.

Nevertheless, gas-phase reversal risk is not zero and remains a material consideration under the following conditions:

• A fraction of stored biochar carbon may undergo degradation over the credit timeline. The relative production of CO₂ versus CH₄ depends on the available terminal electron acceptors (oxygen, sulfate, nitrate, ferric iron), microbial community composition, temperature, and host-rock chemistry^8,7.

Gas migration out of the storage area may occur via two distinct pathways

1. Migration through anthropogenic infrastructure. Storage areas in subsurface mines are typically sealed using engineered barrier systems designed to prevent gas migration through mine infrastructure and boreholes. Barriers may include cement-slurry seals, concrete bulkheads, mud slurries, rock fills, plugs, and combinations thereof. The integrity of these barriers over the credit horizon is the dominant control on gas-migration risk through anthropogenic features. Barrier integrity may be compromised by:
   • Sulfate attack on cementitious barriers where host-rock chemistry, groundwater chemistry, or sulfide-mineral oxidation generates sulfate-bearing solutions in contact with the seal.
   • Insufficient bonding between barrier and host rock, creating preferential migration pathways along the barrier-rock interface that bypass the bulk seal material.
   • Consolidation, shrinkage, and cracking of mud slurries or low-grade cement fills, with potential gas channelling along developed cracks.
   • Structural failure of barriers under loading or ground movement, including failure due to host-rock subsidence
2. Migration along geological features. Stored carbon released to the gas phase may migrate vertically through the overlying rock mass via natural or mining-induced geological discontinuities. These pathways include:
   • Geological faults, where rock displacement has created zones of crustal weakness. Active or recently active faults may create high-permeability conduits for gas migration. The potential for migration depends on fault activity, orientation, fault zone characteristics, and hydraulic connectivity to the surface⁴⁰.
   • Fracture zones and joint networks, where smaller-scale discontinuities (joints, bedding planes, cleavage) collectively provide migration pathways. Connectivity and permeability are influenced by rock type, burial depth, stress history, and the degree of mineralization or cementation.
   • Soluble bedrocks in the overlying stratigraphy, including limestone, dolomite, and evaporite sequences, which are susceptible to dissolution-driven creation of voids and preferential flow paths. These features must be evaluated where carbonate or evaporite units are present in the geological column above the storage area.
   • Mining-induced fracture zones developing above mined-out areas, including caved zones, fractured zones, and continuous deformation zones whose vertical extent depends on the geometry of mining and the time elapsed since extraction¹⁷.

CO₂ stored as biochar in subsurface mines may be mobilized through aqueous pathways as DOC, DIC, POC, or dissolved CH₄, and subsequently migrate out of the storage area. Although biochar exhibits substantially lower aqueous mobility than fresh biomass, owing to its hydrophobic aromatic structure and low solubility of the recalcitrant carbon fraction, empirical evidence demonstrates measurable release of dissolved black carbon (DBC) under specific conditions, including freeze–thaw cycling⁵, prolonged contact with alkaline cement pore solutions^5,11, and physical disintegration over time⁴. Aqueous-phase reversal risk is typically the dominant reversal pathway for subsurface mine storage projects, and is materially elevated for Projects located in dissolution-susceptible host rocks.

Aqueous mobilization of stored carbon may occur through the following pathways:

1. Groundwater infiltration: Where water contacts biochar, host rock, and containment materials, soluble organic and inorganic species can be leached and transported, while contaminating species can be introduced. Substantial groundwater flow within the storage area represents a significant long-term reversal-risk mechanism.
2. Surface water intrusion through mine infrastructure. Surface water intrusion during flood, precipitation, or snowmelt events can introduce oxygenated water, suspended sediment, and nutrients to the storage area, creating conditions conducive to enhanced decomposition, and can physically mobilize and transport biochar particles. Engineered containment, mine depth, and surface-drainage management collectively determine the likelihood of surface water intrusion. Projects must demonstrate in the PDD that storage depth and engineered seals are sufficient to preclude surface water ingress under extreme precipitation or flood scenarios applicable to the site.
3. Leachate generation and water–rock–biochar geochemical interactions. Biochar contains a labile fraction of soluble organic compounds that can be mobilized upon contact with water, generating leachate with elevated DOC concentrations and modified pH^3,4. The chemistry of this leachate depends on biochar properties, pH and ionic composition of the contacting water, contact time, temperature, and the presence of co-located materials. Specific concerns for subsurface mine storage include:
   • Interactions between biochar and alkaline cement pore solutions, where prolonged exposure has been documented to reduce biochar aromaticity, increase matrix defectiveness, collapse pore structure, and form organometallic complexes between Ca²⁺ ions and oxygen-containing functional groups on biochar^5,11,12. These interactions may alter the long-term stability of biochar carbon and increase its aqueous mobility relative to biochar emplaced in non-cementitious matrices.
   • Acidic leachate driving carbonate dissolution in mine host rocks where carbonate mineralogy is present (calcite, dolomite, siderite). Dissolution releases Ca²⁺, Mg²⁺, and DIC into solution, altering local pH and carbonate equilibria, and potentially triggering CO₂ degassing where solutions become oversaturated.
   • Sulfate generation from host-rock dissolution in evaporite sequences (gypsum, anhydrite, sulfate-rich sediments), which feeds back into Risk Category A through accelerated sulfate attack on cementitious containment materials⁴¹. This is the principal coupled reversal-risk pathway for Projects in dissolution-susceptible sulfate host rocks.
   • Active oxic flow due to groundwater flow in the vadose zone, that raises the loss rate. Shallow, oxygenated, actively flushing flow of biochar stored in the vadose zone with groundwater flow could accelerates degradation and DOC/POC export at the margin.
   • Complexation of dissolved metals (Fe, Mn, Al) by organic ligands in leachate, potentially altering metal mobility and forming secondary phases that may either protect or compromise pore structure.

Physical disturbance of the storage area, through seismic activity, mine subsidence, structural failure of host rock, or failure of engineered containment, can compromise containment integrity, generate new gas or aqueous migration pathways, or directly expose biochar to the atmosphere.

Seismic activity and ground displacement. The likelihood and severity of seismic impact depends on regional seismicity, site-specific ground motion amplification, and the design robustness of engineered containment. All Projects must include a seismic hazard assessment in Site Characterization; containment systems must be designed to withstand the design-basis ground motion appropriate to the storage area's seismic hazard classification and the credit horizon.

Mine subsidence and structural failure. Subsurface mines can experience long-term subsidence as rock pillars yield, roof spans collapse, or host-rock strength degrades. Relevant subsidence mechanisms include:

• Gradual rock-mass deterioration through weathering, cyclic wetting-and-drying, micro-crack propagation, and humidity-driven swelling and shrinkage^17,42.
• Dissolution-driven pillar narrowing in dissolution-susceptible host rocks, particularly evaporite (gypsum, anhydrite, halite, potash) and soluble carbonate sequences, where groundwater flow progressively reduces pillar cross-section and strength. Documented multi-stage collapse models include roof fracturing, bulk subsidence of rock mass along discontinuities, and central depression formation with peripheral cracking¹⁸. The timescale of dissolution-driven failure depends on dissolution rate, which scales with groundwater flow rate and host-rock solubility.
• Tensile fracturing extending from mine depth toward the surface, creating gas and water migration pathways through the overburden (feeding back into Risk Categories A and B).
• Compression or shearing of storage chambers under overburden loading, which may breach seals or create new openings.
• Alteration of surface drainage and groundwater flow as a consequence of subsidence, potentially directing water toward storage areas.

In addition, biochar emplaced in storage chambers may undergo gradual volume change over the credit horizon as the labile and semi-persistent fractions degrade, contributing to localized settlement, altered void distribution, modified mechanical loading on roof spans and pillars, and creation of preferential flow paths for gas or water. While the magnitude of this volume change is small for stable biochars relative to fresh biomass, it must be accounted for in long-term containment-integrity assessment.

The rationale is based on (a) isolation from the zone of active surface weathering and soil formation, which typically extends 10–50m below surface but can reach 60m or more in deeply weathered lithologies^43,44, requiring the storage horizon to sit demonstrably beneath the locally established base of weathering rather than at a fixed nominal depth; (b) isolation from seasonal temperature and moisture fluctuations, which damp out at approximately 10–20m below surface depending on host-rock thermal diffusivity^45,46; and (c) reducing the risks to durability of stored carbon from surface disturbance, land use change, and atmospheric exchange [A, B, C]. Tying the requirement to the site-specific base of weathering, rather than a single nominal depth, ensures the storage horizon clears the active weathering zone in both shallowly and deeply weathered settings [A, B]⁴⁷.

The depth requirement is also established to ensure:

• Geological Isolation: sufficient overburden beneath the base of weathering to provide multiple barriers between stored biochar and surface disturbances, such as surface water and / or groundwater infiltration through mine infrastructure, and a buffer from surface land use changes. Isolation provides additional protection against potential migration out of the targeted storage reservoir [A, B]⁴⁷.
• Atmospheric Stability: below the seasonal thermal and moisture zone, natural ventilation exchange with the surface is minimized, temperature and humidity conditions are more stable, and the risk of oxygen ingress from the surface is reduced.
• Physical Protection: depth beneath the weathering zone provides enhanced protection from surface erosion and mass wasting, agricultural or construction activities, wildfire or other surface disturbances, and human excavation, unauthorized access, or disturbance [C].

Project Proponents are required to provide accurate depth measurements for all storage locations. This must include the depth measured as true vertical depth from ground surface to the shallowest point of the biochar storage area; survey measurements from mine establishment may be used. Project Proponents must also establish the local base of the active weathering zone at the storage location and demonstrate that the storage horizon sits beneath it with the required clearance. Depth and weathering-base verification documentation must be available upon request.

Evidence required to demonstrate the storage area depth and weathering-base determination must include at least one of the following:

• Mine survey plans;
• 3D modelling (e.g., leapfrog modelling) showing storage location;
• Cross-sections showing depth profile and weathering base from surface to storage areas;
• Geological or geotechnical logs characterizing the weathering profile;
• Topographic maps showing surface elevation.

In addition, Project Proponents must account for any variation in storage area depth, and monitoring must distinguish between different depth zones if any variability is identified. Biochar must not be stored within an actively circulating, oxygenated groundwater flow zone at the Project storage location. Storage within a saturated horizon is permitted where groundwater is stagnant or slow-moving and prevailing conditions are anoxic/reducing.

Depth beneath ground surface does not by itself preclude groundwater flow: the water table commonly sits within tens of metres of surface, and active meteoric circulation routinely extends well below the depths required above. The Storage Depth criterion isolates the storage horizon from vertical, top-down oxygen and water ingress through the weathering zone and mine infrastructure; it does not constrain lateral or ambient groundwater flow through the storage horizon itself, which this criterion addresses. What governs durability is not the presence of groundwater but its character: an actively circulating, oxygenated flow regime supplies oxygen and exports dissolved and particulate organic carbon, accelerating degradation and loss, whereas a saturated, slow-moving, anoxic horizon suppresses oxidation and is protective of stored carbon [A, B].

Project Proponents are required to provide a hydrological condition assessment for each storage location, see Section 7.3.2.3 and Section 10.

Project Proponents must conduct comprehensive characterization of the mine through 2D mapping of all storage areas, entries, and infrastructure, including calculations of available void space suitable for biochar storage based on environmental criteria such as stability and accessibility.

Void space calculations must account for biochar packing efficiency, anticipated settling and compaction over time, and the presence of any existing infrastructure within chambers (e.g., rails, supports, or equipment) that may reduce usable storage volume. 3D mapping of the Project mine is recommended for all Projects.

The geological evaluation must characterize the host rock lithology, stratigraphy, structural features (e.g., faults, folds, and fractures), and overburden depth, while employing recognized rock mass classification systems (e.g., RMR, Q-system) to assess rock quality through strength testing and discontinuity analysis⁴⁸.

For room-and-pillar configurations, pillar stability analysis is critical, requiring width-to-height ratio calculations, stress assessments, and safety factors, along with evaluation of pillar performance history⁴⁸,⁴⁹.

Additionally, roof and floor stability must be assessed through span analysis, competency evaluation, ground support requirements, and documentation of any fall or heave history to ensure the mine's structural integrity for safe, phased biochar storage operations [A].

Project Proponents must provide geological survey and structural mapping documents demonstrating the presence or absence of faults, fracture zones, and other vertical discontinuities or geological features within the area of the storage site and a buffer zone of approximately 50m radius [A, C].

If active faults, defined by the USGS Earthquake Hazards Program as faults that have moved one or more times in the last 10,000 years, are identified, the Project Proponent must ensure the chambers and sealing barriers are designed to withstand seismic loads. If any fault (active or inactive) is found to intersect with the storage chambers, the Project Proponent must:

• Characterize fault orientation, geometry, activity status, and hydraulic properties.
• Assess whether faults provide a hydraulic connection between storage chamber(s) and overlying aquifers or the surface.
• Assess whether host-rock dissolution may enhance fault zone permeability over the credit horizon, particularly in evaporite and soluble carbonate host rocks⁴¹.
• Model potential gas migration routes, rates, and transit times to the surface.
• Demonstrate the geological confining system is free of transmissive faults and fractures and of sufficient extent and thickness to ensure the containment of biochar and potential gas migration.
• Undertake a baseline assessment of subsurface structures including any faults or artificial penetrations (e.g., abandoned wells).
• Determine the ground displacement risk.
• Prepare a monitoring and emergency response plan identifying appropriate assessments and remediation actions.

In addition, Project Proponents must ensure mine ventilation conditions enable the effective monitoring of atmospheric mine conditions, as specified in the monitoring plan (fugitive gases, oxygen, etc.) over the full Crediting Period.

Where natural ventilation is insufficient, engineered control systems must be implemented to ensure monitoring is representative and any compromised storage is identified [A].

Project Proponents must submit all characterization information within the PDD. Such evidence must demonstrate compliance with all local and national regulations applicable to the Project operation, including those governing mine structural safety, subsurface operations, occupational health and atmospheric monitoring, and environmental protection.

Project Proponents must conduct infrastructure assessment evaluating the condition and stability of all access routes including portals, adits, and shafts to ensure the capability for biochar transportation to storage locations and sustained monitoring access throughout the period of biochar emplacement and the subsequent Project Reporting Period.

Existing infrastructure must be inventoried for condition and utility, including availability of power for monitoring systems, communication systems for remote monitoring capabilities, and storage facilities for equipment and materials necessary to support long-term biochar storage operations [A].

Documentation and reviews of the mining history for the site must be submitted, with records of the type of operations, materials extracted, timelines of mining and closure activities, any hazardous materials used or stored, and post-closure uses of mine workings. Such information will be used to assess the suitability of the chosen mine operation for storage of biochar.

The mine's current regulatory closure status must be documented, including existing closure plans and integration strategies for biochar storage with closure requirements, and the identification of responsible parties for long-term mine stewardship to ensure regulatory compliance and environmental protection throughout the storage duration [A, B, C].

If water management of the storage site is planned to finish upon site closure, hydrological flow through the storage chambers must be slow enough to prevent the physical mobilization and loss of emplaced biochar, and to limit aqueous transport of stored carbon out of the storage area [B]. Groundwater contact with, or full saturation of, the storage area is permitted, provided the Project Proponent demonstrates that flow velocities remain below the threshold at which biochar particles are entrained or dissolved carbon is exported. Saturated, oxygen-limited storage conditions may enhance biochar durability by suppressing aerobic oxidation and microbial decomposition of stored carbon.

As part of site characterization, the Project Proponent must characterize the groundwater regime of the storage area and demonstrate one of the following:

• the storage area is not in contact with groundwater; or
• groundwater flow through the storage area is below a site-specific maximum seepage velocity, defined and justified by the Project Proponent for the emplaced biochar's particle size distribution, below which neither particulate biochar loss nor aqueous carbon export out of the storage area occurs over the credit horizon.

Evidence must characterize the groundwater flow regime of the storage area and include visual inspection of mine walls and surrounding tunnels, supported by at least two of the following methods:

• Historical Mine Data, for example historical mining reports classifying the Project mine or target storage chamber as "dry", or documenting historical inflow and pumping rates.
• Literature Data, examining the hydrogeology of the mine.
• Field Measurements, such as piezometers in the mine/chamber walls, to quantify hydraulic pressure and groundwater flow within the mine.

The emplaced biochar, any associated containment fill or backfill material, and the host rock immediately surrounding the storage area must collectively exhibit sufficiently low permeability to preclude aqueous transport of stored carbon out of the storage area over the credit horizon.

In addition, a comprehensive groundwater evaluation must be undertaken to identify nearby aquifers, determine groundwater flow directions, velocities, and hydraulic gradients, measure hydraulic conductivity of host rock and overburden, assess surface water infiltration potential, and define the zone of influence of mine workings on local and regional groundwater systems⁵⁰,⁵¹. The evaluation must additionally include:

• Projected post-closure groundwater rebound trajectory for closed or closing mines, including expected timing, magnitude, steady-state water table depth relative to the storage area, and the resulting flow regime the storage area will experience once rebound is complete²⁵,¹⁹.

The evaluation of suitability for the Project site, and any groundwater monitoring systems, must be certified by a qualified groundwater scientist (e.g., certified hydrogeologist or equivalent in the jurisdiction of the Project) and must comply with the sampling and analytical procedures outlined in the site permit or by applicable regulations [B].

The design of the groundwater monitoring plan (e.g., sampling locations, spacing, depth) must be determined on a site-specific basis by the Project Proponent and must be included in the PDD. The plan must be designed to track groundwater flow velocity and indicators of biochar mobilization or carbon export (e.g., dissolved organic carbon, turbidity), and should reflect applicable regional and local regulations, aquifer thickness, depth, groundwater flow rate, direction, and other relevant geological and hydrogeological characteristics of the site.

While surface water intrusion through mine infrastructure as a result of flood events is unlikely, Project Proponents must undertake a site characterization assessment to identify the likelihood of flood events capable of driving transient high-velocity flow through the storage area.

Such assessments should include:

• 1,000-year storm modelling for the Project location.
• An assessment of potential surface-to-subsurface hydrologic connections through fractures, old workings, or subsidence features.
• A monitoring plan for flow through the storage chambers, with drainage or pumping triggered if flow velocity exceeds the defined biochar mobilization threshold.

Additional environmental baseline assessment must be conducted including air and water quality measurements, identification of any contamination from previous mining activities, and an evaluation of any materials incompatible with biochar storage.

Geomicrobiological Characterization

All Projects must undertake a literature-based screening of the potential for microbially-mediated decomposition of stored biochar in the storage environment⁵²,⁵³. For biochar storage specifically, the principal microbial considerations are (a) potential degradation of biochar's labile and semi-persistent fractions over decadal-to-centennial timescales, and (b) the role of biochar's electrically conductive surface in supporting direct interspecies electron transfer (DIET) where co-located organic substrates are present⁸,⁷.

Full baseline geomicrobiological characterization is required only where the environmental baseline assessment identifies co-located labile organic carbon or an elevated in-situ thermal regime, either of which could support microbially-mediated carbon loss. Where triggered, the characterization must include enumeration and characterization of indigenous microbial communities present within the mine, with particular attention to functional guilds relevant to organic matter degradation, including aerobic heterotrophs, fermenters, sulfate-reducing bacteria, iron-reducing bacteria, and methanogenic archaea⁵², and must be conducted prior to biochar emplacement to establish a pre-emplacement baseline against which post-emplacement microbial changes can be assessed [A, B].

Thermal Environment Characterization

Project Proponents should document the thermal regime of the storage environment, including in-situ temperature profiles at the proposed storage depth. Temperature exerts a primary control on microbial metabolic rates and the kinetics of geochemical reactions relevant to carbon stability⁵⁴; elevated temperatures accelerate both aerobic and anaerobic decomposition processes and increase rates of mineral dissolution and organic compound leaching^54,55. Thermal characterization should include measurement of the ambient geothermal gradient at the site and assessment of any localized heat sources (e.g., exothermic oxidation reactions, residual mining equipment) that could elevate temperatures within storage chambers. This information should be submitted in the PDD and used to inform predictions of long-term biochar stability [A, B].

Project Proponents must conduct geotechnical hazard evaluation including a comprehensive subsidence assessment incorporating historical monitoring data, predictive modelling, analysis of surface effects from past or potential future subsidence, and a risk assessment of subsidence impacts on storage containment integrity [C].

Project Proponents must conduct and provide a geotechnical evaluation of long-term mine stability, including:

• Analysis of pillar stress and factor of safety against yielding.
• Overlying layer stability assessments for storage chambers.
• Prediction of potential subsidence magnitude and extent over 1,000+ year timescales.
• Evaluation of whether predicted subsidence could induce surface fracturing or compromise containment.

Where Project Proponents can provide evidence from a partnered mining operator that satisfies the above requirements, the Project Proponent will not be required to undertake additional geotechnical evaluations. In such instances, the Project Proponent shall provide evidence and documentation that the mining activities and storage chambers are in compliance and certified by the relevant engineering methodologies and regulations.

For mines with historical subsidence or marginally stable conditions, enhanced safeguards must be outlined in the PDD, which must at minimum include:

• Ground support and stabilization measures to reduce subsidence risks.
• Placement of storage chambers in geotechnically superior locations (e.g., solid coal or rock pillars).
• Surface monitoring for subsidence indicators.
• Increased buffer pool contributions to account for elevated reversal risk.

Seismic hazard characterization must be undertaken, encompassing regional assessments of both natural and induced seismicity [C]. Seismic hazard is treated as a continuum and assessed on the basis of site-specific ground motion, not on a Project's location within any designated zone. The characterization must cover the 1,000-year durability horizon, incorporating data from historical seismicity records within the mining district, evaluation of mine-induced seismicity potential from past or ongoing operations, and an analysis of mine working structural response to seismic loading to ensure long-term stability and safety of biochar storage areas under potential geotechnical hazard scenarios^47,56.

The site-specific peak ground acceleration (PGA) for the storage area must be calculated, through a probabilistic seismic hazard assessment, and reported. This PGA screening determines the Project's seismic obligations:

• Where the base PGA is less than 0.1 g at 10% probability of exceedance within 250 years, the storage area is classified as low seismic hazard. The Project Proponent must document the assessment and its result in the PDD; no seismic monitoring program is required on the basis of seismicity alone, although the containment system must still tolerate the design-basis ground motion (see Section 8).
• Where the base PGA is 0.1 g or greater at 10% probability of exceedance within 250 years, the storage area is classified as elevated seismic hazard. All systems must be designed to withstand the maximum horizontal acceleration and to prevent permanent deformation of the emplaced materials, and a seismic monitoring program may be required (see Section 10.1.2).

Each containment material must be specified by reference to a recognised national or international standard (e.g., ASTM, EN, ISO, BS) and accompanied by the relevant mix design, technical datasheet, and supplier certification in the PDD.

Cementitious materials must conform to the relevant cement standard for the jurisdiction (e.g., EN 197-1 in the EU; ASTM C150 / C595 / C1157 in the US) and, where the host rock or groundwater is sulfate-bearing, must additionally conform to a sulfate-resistant classification appropriate to the in-situ sulfate concentration (e.g., EN 197-1 SR cements, ASTM C1157 Type HS, or equivalent).

Geopolymer and alkali-activated containment materials are permitted provided the Project Proponent submits an equivalent specification covering composition, activator chemistry, and performance testing under in-situ conditions.

Naturally derived backfill materials (e.g., mud, slurry, mine tailings, native rock fill) are permitted provided their geotechnical and geochemical properties are characterised, and they are not the sole containment for chambers where gas or aqueous reversal pathways are identified in site characterisation.

Recycled or waste-derived containment materials are permitted provided their composition, contaminant profile, and long-term stability are documented to the same level as virgin materials.

Where a Project Proponent utilizes an engineered backfill material for the purpose of storage and sealing [A], a full description of the backfill composition must be provided within the PDD upon submission to Isometric and the Project VVB. Projects may utilize a backfill or containment material that consists of biochar alone (e.g., in discrete packaging units), biochar mixed with geological materials (such as mud or slurry), or biochar emplaced separately and surrounded by cementitious or geopolymer containment.

Where biochar is mixed into the Project backfill material, or where biochar replaces a baseline backfill material that would have been used in the absence of the Project, Project Proponents are required to demonstrate the composition and geotechnical performance of the backfill compared to materials that would have been utilized in the absence of the Project. Where the Project Proponent claims emissions displacement based on substitution of a higher-emissions baseline backfill material, baseline assumptions and material specifications must be documented in the PDD with reference to the counterfactual fill that would have been used.

If the Project requires additional excavation of land, in excess of excavation that would have occurred in the absence of the Project activities, the baseline carbon stocks must be established and monitored after disturbance, with reductions in stocks accounted for as foregone counterfactual storage. Such excavations may be for the purpose of providing additional geological material, such as mud, as a binder or slurry feedstock that is mixed with biochar prior to emplacement and storage.

Where the project activities necessitate excavations that would not have occurred in the counterfactual scenario, Project Proponents must account for any reductions in soil organic carbon due to these excavations. This must be accounted for by measuring the drop in concentration of soil organic carbon compared with a representative control site. Increases in soil organic carbon are not considered creditable removals under this Module.

For each containment material and for the containment system as a whole, the Project Proponent must specify and justify the following performance parameters with reference to laboratory or field testing under conditions representative of in-situ exposure:

¹ Where the host rock provides natural hydraulic isolation, the bulk seal permeability requirement may be relaxed subject to justification in the PDD and demonstration that the host rock independently meets equivalent performance. ² The reversal-risk budget is set by the project's quantification framework and must be documented in the PDD.

The containment system must be designed for a service life consistent with the credit horizon. For projects claiming 1000-year durability, the Project Proponent must provide:

• A documented service-life prediction for each containment material under in-situ chemical, mechanical, hydrological and thermal conditions, using an established degradation model (e.g., DuraCrete, fib Model Code for Service Life Design, or an equivalent published model for non-cementitious materials).
• Sensitivity analysis covering principal degradation pathways relevant to the host rock and groundwater chemistry, including sulfate attack, carbonation, alkali-silica reaction, leaching, and biologically mediated attack.
• Where service-life predictions extend beyond the validated range of the underlying model, justification supported by analogue evidence (e.g., archaeological cementitious materials, natural cement deposits, long-duration laboratory studies).
• A contingency plan describing remediation or compensating measures if monitoring indicates degradation ahead of the predicted trajectory.

Where the predicted service life of any individual containment component is shorter than the credit horizon, the containment system must be designed as a multi-barrier system such that no single component failure compromises overall containment integrity over the credit horizon.

Each unit of biochar emplaced under this Module must be identifiable to a single Production Batch (as defined in the Biochar Production and Storage Protocol) and to a single Storage Batch corresponding to a defined emplacement event in a defined mine chamber. Batch identifiers must be unique and persistent, must accompany the biochar at every stage of handling, and must be cross-referable to the analytical data used to characterize the batch under Biochar Characterization.

The Project Proponent must maintain transfer records at each of the following points. Where biochar is handled in sealed packaging units whose dry mass is weighed and recorded at packaging, the packaged dry mass may be carried through the subsequent points by verifying the integrity and count of the sealed units rather than by re-weighing, provided any damaged, opened, or lost units are identified and accounted for:

• Dispatch from the production facility, including weighed mass, moisture content at dispatch, and batch identifier.
• Receipt at any intermediate storage or transshipment facility, including weighed mass, or verification of sealed-unit integrity and count; condition assessment; and identification of any losses.
• Arrival at the mine surface, including weighed mass, or verification of sealed-unit integrity and count; and visual inspection.
• Final emplacement, including weighed mass, surveyed mass, or verified count of sealed units emplaced; and the location identifier of the emplacement chamber.

Cumulative mass losses across the chain of custody must be reconciled and documented. Where reconciled mass at emplacement deviates from dispatched mass by more than 2% on a dry basis, the deviation must be investigated and explained in the PDD, and only the reconciled emplaced mass may be credited.

All weighing equipment used in the chain of custody must be calibrated to a recognised national or international standard, with calibration certificates retained per Record Keeping.

For each transport leg, the Project Proponent must retain:

• Shipping or consignment documents identifying the carrier, origin, destination, batch identifier, and dispatched mass.
• Customs and waste-shipment documentation where transport crosses a jurisdictional boundary.
• Evidence that transport conditions were maintained throughout transit, specifically, that the biochar was kept at an appropriate moisture level to limit dust generation, that fine particles were contained (e.g., in sealed or covered bags or containers), and that the load was protected from contamination by other materials. Such evidence may take the form of the transport method and packaging used, together with the condition assessment recorded on receipt.

Custody of biochar transfers between parties (e.g., producer to transporter, transporter to mine operator, mine operator to emplacement contractor) must be documented by signed transfer records identifying the parties, batch identifier, mass transferred, and condition at transfer. The Project Proponent retains overall responsibility for the integrity of the chain of custody irrespective of subcontracting arrangements.

Where custody transfer steps occur under the control of a partner (e.g., the mine operator or an emplacement contractor), the corresponding transfer records may be generated and held by that partner under the project agreement, provided they remain available for VVB review and support reconciliation of emplaced mass.

Storage records must be able to trace the dry mass of biochar from production to the section of the mine that it is ultimately stored in. Emplacement is the irreversible step at which biochar enters the storage reservoir. The Project Proponent must implement a documented QA/QC program, with sufficient evidence to confirm to the VVB that the specified mass of biochar has been emplaced in the specified location under the specified conditions.

Descriptions must outline how geological permanence and any leaks will be assessed and monitored, and how implemented storage strategies will ensure durability of stored carbon. These must be detailed within the PDD.

Note: Requirements that align with the specific risk categories (Risk Categories A, B, and C) outlined within Section 7 can be identified within squared brackets.

Subsurface biochar storage is a nascent CDR approach, therefore addressing potential risks to durability is important for ensuring robust quantification and monitoring of CO₂ removals.

The biochar emplaced under this Module must be characterized in accordance with the requirements of Biochar Characterization (Section 4) and the sampling requirements of the Biochar Production and Storage Protocol.

Where the Project's emplacement method involves mixing biochar with a slurry, mud, or other geological binder material, the following parameters must additionally be characterized for the biochar-slurry mixture:

• Plasticity of the slurry materials, including the liquid limit and plasticity index values for all materials.

• Sorption capacity of the slurry materials, including the mineral composition, especially the abundance of expandable phyllosilicates.

• Permeability of the biochar-slurry mix. Where the Project's emplacement method involves placing biochar in discrete packaging units (e.g., big bags, sealed containers) and isolating these from the host rock and mine atmosphere via cementitious or geopolymer seals, backfill, or other engineered barriers, the following parameters must additionally be characterized for the containment materials:

• Composition and mix design of the sealing or backfill material, including identification as virgin or waste/secondary material, and cement type where cementitious materials are used.

• Sulfate-attack resistance of the sealing material where the host rock, groundwater or backfill material is sulfate-bearing, with reference to the relevant national or international standard (e.g., ASTM C1012, EN 196-1) [A, B].

• Permeability of the cured sealing material under representative in-situ conditions.

• Projected service life of the sealing material under the in-situ chemical, mechanical, and thermal conditions over the credit horizon.

As specified in the Section 7, all Projects must perform a site-specific seismic hazard assessment. Where that assessment classifies the storage area as elevated seismic hazard (base PGA ≥ 0.1 g at 10% probability of exceedance within 250 years), the Project Proponent must establish a baseline of seismic hazards [C] ⁸, and a seismic monitoring program may be required. Where the storage area is classified as low seismic hazard, a seismic monitoring program is not required on the basis of seismicity, and the Project Proponent need only document the hazard assessment result in the PDD.

A seismic monitoring program may be required at the discretion of the relevant regulatory authority (e.g., UIC Director or equivalent) in areas of increased seismic risk, or where it is demonstrated that seismicity may impact the durability of the carbon storage. The monitoring program should adhere to the following specifications:

• The monitoring system should include deeper wireline or cemented subsurface geophones for microseismic monitoring. This should ideally be combined with at/near ground level stations as part of an integrated detection strategy.
• The objective of the monitoring is to determine the presence or absence of: Induced micro-seismic activity associated with facility wells (if applicable); Activity near subsurface discontinuities, faults, or fractures; Any seismic activity within the Area of Review (AOR) of the facility and the storage reservoir of Magnitude 2.7 or greater.

If groundwater infiltration into the storage chambers does occur, the Project Proponent must [B]:

• Calculate groundwater flux rates through storage chambers.
• Measure or calculate dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) production from stored biochar under relevant conditions.
• Calculate potential DOC and DIC export rates and cumulative carbon loss over crediting timescales, and demonstrate how these will be deducted from net removal calculations.
• Implement groundwater monitoring systems to detect intrusion and measure DOC and DIC concentrations downstream of storage areas.
• Where the host rock is dissolution-susceptible (evaporite sulfate sequences, soluble carbonate sequences), additionally monitor sulfate concentration, calcium concentration, and saturation indices for gypsum, anhydrite, and any relevant carbonate minerals, as indicators of active host-rock dissolution and potential sulfate-attack feedback on cementitious containment²⁸.

Note: Where groundwater infiltration does occur, the Project Proponent is required to consult with Isometric on potential remediation steps. The suitability of remediation steps to restrict groundwater infiltration will be assessed on a Project-by-Project basis.

Given the potential for aqueous mobilization of stored carbon and contaminants under conditions of groundwater contact (Risk Category B), Projects must establish a groundwater monitoring program around the storage area. At minimum, this program must include:

• Baseline characterization of groundwater chemistry prior to emplacement, including major ions (Ca, Mg, Na, K, sulfate, chloride, carbonate, bicarbonate), pH, electrical conductivity, dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC).
• Sentinel monitoring locations placed in down-gradient flow paths from the storage area, selected on the basis of the site-specific hydrogeological assessment.
• Ongoing monitoring at a frequency proportionate to groundwater flow rate, host-rock dissolution susceptibility, and the duration of emplacement activity, with monitoring continuing through the post-emplacement controlling period.
• Trigger thresholds for changes in dissolved organic/inorganic carbon, sulfate, pH, and any contaminants of concern that initiate further investigation and reporting to Isometric.

Where the host rock is susceptible to dissolution (e.g., evaporite sulfate sequences including gypsum and anhydrite, soluble carbonate sequences), groundwater monitoring must additionally track sulfate concentration, calcium concentration, and saturation indices for gypsum, anhydrite, and any relevant carbonate minerals, as indicators of active host-rock dissolution and potential containment-material attack.

Where soluble carbonate or evaporite sequences are present in the overlying stratigraphy and could provide preferential migration pathways (see Risk Category A), monitoring must additionally include surface or near-surface gas flux measurement at locations determined by the hydrogeological and geological assessment.

Geotechnical and/or structural seals are required between each storage chamber and remaining mine infrastructure [A, B, C].

Seals may include:

• Geological sealing of stored biochar by overburden strata.
• Engineered sealing with reinforced concrete or cement-slurry barriers.
• Engineered backfilling of chambers with geological materials.

These barriers must ensure there are no leaks through the barriers over relevant timescales (1,000+ years) under site representative conditions and characteristics (e.g., expected pressure gradients, hydrostatic head, geochemical exposure including sulfate attack where applicable, and mechanical stress).

The Project Proponent must provide a detailed plan for seal integrity testing of sealing barriers between storage chambers, in conditions representative of the storage site.

The plan must specify the test methods and inspection approach appropriate to the identified transport pathways, the acceptance criteria, the minimum testing frequency, and the procedures for addressing non-conformities. Any seal that does not meet acceptance criteria must be remediated and retested prior to commencing storage operations in the relevant chamber, and material deviations and corrective actions must be documented in the PDD.

Post-emplacement monitoring must be proportionate to the reversal risk of the storage configuration and to what is physically and legally achievable at the site. For sealed biochar storage in subsurface mine environments, durability assurance rests primarily on the conservative durability quantification, the demonstrated design life of the containment system (see Section 8), and verification carried out before each chamber is sealed (see Section 5). Post-emplacement monitoring supplements these controls; it is not the primary basis on which durability is established.

Consistent with the Risk of Reversal and Buffer Pool section, reversals in sealed, low-oxygen biochar storage are not expected to be directly observable or attributable at the project level. Post-emplacement monitoring is therefore directed at confirming the continued integrity of the storage configuration and detecting the aqueous and physical conditions that could compromise it, rather than at direct quantification of gas-phase carbon loss.

• Surface or near-surface monitoring, where feasible and permitted, monitoring appropriate to the reversal pathways identified in site characterization.
• Physical conditions of the site, where storage chambers, seals, or adjacent workings remain accessible, regular inspection of confining materials (e.g., cementitious seals, geopolymer seals, backfill, synthetic liners) for signs of degradation.
• Other site-specific parameters, mutually agreed upon by the Project Proponent and Isometric, such as:
  • Temperature and humidity.
  • Gaseous control measurements (e.g., background CH4 production) using representative control sites or chambers.
  • Crack/strain gauges.
• Operator and regulatory monitoring, where the mine operator or the responsible authority already conducts monitoring relevant to storage integrity (e.g., ground-stability and subsidence monitoring, regulatory groundwater monitoring), the Project Proponent may rely on and document this monitoring rather than duplicating it.
• Groundwater chemistry parameters where the site is subject to a groundwater monitoring programme (see Section 7), continued monitoring of the parameters specified there at down-gradient locations, addressing the aqueous reversal pathway. This does not require access to the sealed chamber.

The post-closure plan is intended to demonstrate:

• that containment integrity was verified before sealing (see Section 8);
• that the containment system's design life covers the credit horizon (see Section 8.3); and
• how monitoring conducted by the operator or regulator, together with any feasible surface, near-surface, or groundwater monitoring, will be used to identify conditions relevant to storage integrity.

Where post-closure monitoring of accessible parameters is undertaken, the minimum monitoring period is 20 years after site closure, unless a shorter period is justified by documented site-access constraints and agreed with Isometric and the VVB. Monitoring may be conducted through continuous or discrete sampling; the sampling approach and frequency must be justified in the PDD and agreed with Isometric and the VVB.

• A description of how final closure of the facility will be achieved.
• An estimate of the maximum amount of possible hazardous additives (if used) kept on site during the facility's operating life.
• A detailed description of closure methods.
• A description of any other required steps, such as groundwater monitoring and leachate management.
• A schedule of closure activities, including closure dates for each unit and the entire facility.
• A description of how the management of each hazardous additive (if used) will be performed.

The Project Proponent must provide a post-closure care plan that includes:

• A monitoring plan (see Section 10).
• A description of planned maintenance activities for carbon/biochar storage (e.g., liners, final covers and sealing systems, leachate management systems, monitoring infrastructure).
• Contact information for the responsible party during the required post-closure care period.

In addition, the Project Proponent must provide any supplementary information which is required of the Project by the local permitting and regulatory authorities.

To limit long-term environmental impacts from storage facilities, Project partner operators (where relevant) must formalize closure plans in accordance with local regulations and standards.

Project closure plans must be integrated with the mine operator's existing or updated mine closure and remediation plans. This requirement is particularly relevant for Projects utilizing naturally derived materials for biochar containment in subsurface mine storage chambers.

Project Proponents must provide the Project closure plan, as well as the mining operation's closure plan, within the PDD upon submission to Isometric and the Project VVB.

Note: Where the mining operator's mine closure plan cannot be provided due to confidentiality or access issues, the Project Proponent must provide a signed affidavit from the mining operator outlining the impact of the Project on the existing mine closure plans, as well as any changes to plans as a result of the Project activities.

• Future mining operations (coal, metals, evaporite, or other minerals) could intercept storage chambers, either at the same mining operation or from adjacent properties.
• Drilling for water, oil/gas, geothermal resources, or site investigation could inadvertently penetrate storage zones.
• Construction or development of infrastructure such as foundations, tunnels, or underground facilities could disturb storage chambers.
• Future land use changes might involve excavation or other activities that disturb buried biochar.
• Future entities could deliberately mine or remove biochar for other purposes (e.g., energy recovery) if economic or regulatory incentives change.

Where Project Proponents can provide a signed affidavit from a partner operator that demonstrates safeguards against potential leaks that may be a result of intentional or inadvertent human disturbance of storage sites, additional assessment may not be required.

Note: The suitability of an operator affidavit in lieu of a distinct assessment will be assessed on a Project-by-Project basis by Isometric and the Project VVB.

Project Proponents must assess the potential for reversals as a result of human activities, both during the Project Crediting Period and post closure. Mitigation plans, or actions, must be outlined within the PDD upon submission to Isometric and the VVB.

Mitigating future human activity risks may require legal and institutional measures rather than purely technical controls. Such measures may include:

• Establish legally binding restrictions on future land and mineral use; this may include conservation easements, mineral rights restrictions, or long-term stewardship agreements.
• Ensure storage site locations are permanently documented in geological surveys, mining cadastres, and land use databases accessible to future users.
• Where feasible, maintain buffer zones around storage areas where surface activities are restricted.

There is no single credible mechanism that can ensure, without uncertainty, that biochar buried in the subsurface will remain undisturbed in perpetuity given the relative nascency of such legal mechanisms relative to the time horizons required in the Isometric Standard. Land durability claims are subject to social and political factors and are thus different in nature from claims regarding physical or geologic durability. Isometric has developed a set of land security eligibility criteria that align with current best practices for legal strategies to restrict future uses of land. The Protocol and this Module also considers the risk associated with land ownership as a risk factor in determining the Risk of Reversal and corresponding buffer pool.

Where the Project Proponent has partnered with a mining operator, for the purpose of biochar storage, evidence is required to demonstrate intended land use following Project cessation and subsequent mine closure. Such evidence may take the form of the operations closure plan or any relevant permitting documents. Where closure plans and remediation schemes have been updated, or amended, to take into consideration the storage of biochar within the mines subsurface, such documents must be submitted to Isometric and the Project VVB for review.

Projects using this Storage Module are typically deemed to have No Observable Risk of reversal, according to the Isometric Standard Risk Assessment Questionnaire (also found in the Biochar Production and Storage Protocol), as all Projects Crediting against this Protocol are credited conservatively to account for degradation of labile pools of biochar within the relevant crediting time horizon. Based on present understanding, reversals in biochar storage in low oxygen burial environments will not be directly observable with measurements, nor attributable to a particular project. Following the Risk of Reversal Section of the Isometric Standard, storage uncertainty for open systems is primarily accounted for within the removal quantification framework. This results in a 0% buffer pool for Projects using this Storage Module. For more details, refer to the Reversals and Buffer Pools Section of the Isometric Standard. This reversal risk will be reassessed at the renewal of the Crediting Period, or when new scientific research and knowledge are produced.

All records associated with the characterization, design, construction, burial operations, monitoring, site closure, and site maintenance must be developed and submitted to proper authorities as required by any applicable permitting authority.

Records of laboratory analyses and relevant permit limitations to demonstrate compliance must be maintained in accordance with the permit and available for review at any point during the Crediting Period or post closure. Where not required by the permit, records of all analyses and storage events must be maintained by the storage facility or Project Proponent and provided for verification purposes for a minimum of five years after the end of the monitoring period.

All closure and post-closure monitoring records must be maintained by the Project Proponent for a minimum of 10 years after closure. These records must be available to be consulted by interested parties for future clarifications if needed.

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