Biomass Storage in Subsurface Mines

This Module outlines the requirements for the storage of biomass in subsurface mining operations for the purpose of carbon dioxide removal. A net decrease in atmospheric CO₂ will occur when biomass is processed and stored in a way to substantially slow or arrest microbial degradation and prevent decay products migrating to the atmosphere. When these carbon cycle interventions are combined with engineered controls and favorable site characteristics to limit disturbance, this stored carbon may persist for long periods of time.

While this Module provides specific requirements on how biomass can be stored to significantly delay the degradation of biomass and contain decay products, it is noted that adherence to the storage provisions outlined in this Module does not guarantee durability of carbon storage. Given the significant diversity in the chemical characteristics of biomass, biomass processing prior to storage, storage environments, and other site-specific considerations, the durability claims associated with removals utilizing this Module must be evidenced with direct, time-series evidence and a detailed, quantitative description of how those storage conditions will be maintained throughout the Project lifetime. Additionally, this Module describes the requirements of monitoring and reporting of fugitive gasses. Any biomass processing and carbon quantification must be conducted in accordance with other Isometric Protocols and Modules.

Biomass is inherently labile with respect to degradation by fungi, microorganisms and other biogeochemical redox reactions. While there are many biomass processing technologies (e.g., desiccation, biochar, bio-oil) and storage options (e.g., burial, injection, sinking) present in the market today, scientific literature has not yet reached consensus on the carbon storage durability of different processing and storage techniques.

Given this current state of understanding of subsurface biomass burial for carbon dioxide removal, Isometric takes a restrictive approach to Project eligibility. Specifically, Isometric will assess the current state of scientific understanding and consensus on subsurface biomass burial technologies on an individual Project basis. Once such technologies have demonstrably met a reasonable burden of scientific evidence, they will be added to the Eligible Biomass Processing and Storage Options listed below as the Module is updated. Isometric will update this list as frequently as necessary, guided by scientific research and market learnings.

All Projects must have biomass feedstocks that meet all the applicable eligibility requirements outlined in the Isometric Biomass Feedstock Accounting Module including sustainability, market leakage, and counterfactual storage criteria and quantification.

Eligible subsurface mining operations for subsurface biomass storage will be assessed by Isometric on a Project by Project basis.

Eligibility requirements for all mine types:

• Subsurface mines must have existing excavations suitable for biomass storage.
• Subsurface mines must not be specifically opened or extended for the purpose of carbon storage.

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

• Operational subsurface mining operations
  • Metal, coal, potash and diamond mines are considered applicable under this Module. Other mining types may be considered on a Project by Project basis.
• Closed subsurface metal or coal mining operations that are currently in either; care and maintenance or in the process of completing closure plans.

Operational Subsurface Mines

• Active underground mining operations where biomass storage can be integrated into mined-out areas without interfering with ongoing mineral extraction.
• Operations must demonstrate that biomass storage does not extend the economic life of mine (LOM) or incentivize continued/increased mining operations.
• Storage of feedstocks must be in areas permanently undisturbed by future mining activities.

Closed or Closing Subsurface Mines

• Closed underground mines that are currently in care and maintenance status.
• Mines in the process of completing closure plans but not yet fully remediated.
• A closed mining operation where infrastructure remains accessible and in serviceable condition.

The following mining operation types will not be considered as applicable biomass storage Project locations under this Module:

Mining Operations Developed for Carbon Storage

• Operational or planned subsurface mining operations that have been opened or planned specifically for the purpose of carbon storage.

Mining Operations with Life-of-Mine Extensions

• Operational subsurface mining operations that have extended or are likely to extend the LOM as a direct result of carbon removal activities.

Note: Where a mining operator has extended the LOM for reasons not related to carbon removal activities, a signed affidavit may be provided by the operator as evidence.

Fully Closed and Remediated Mines

• Closed mining operations, including surface and subsurface, that have completed closure plans and have been remediated (in line with The Project closure plan).

Surface Mining Operations

• Open pit surface mining operations of any type.
• Operational mines with active groundwater flow through storage areas that cannot be diverted or adequately managed.

At present, Projects in locations governed by the US, Canada, United Kingdom and European Union are eligible under this Protocol. 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 and environmental protection as would be required for a similar Project in one of the above jurisdictions. Such exceptions must be approved by Isometric.

The permanence of carbon storage through biomass burial in subsurface mines depends on preventing or minimizing potential leaks out of the storage area which could lead to stored carbon could be re-emitted to the atmosphere or mobilized to other subsurface environments. Within this Module ‘storage area’ is defined as the chamber or sealed area within which biomass is placed for the purpose of permanent burial, as well as the surrounding area of expected diffusion. In this context, leaks refers to any release of GHGs that originate from the stored biomass, including CO₂ and methane generated by anaerobic decomposition of that biomass after burial and storage.

This section details the potential risks and leak pathways in subsurface biomass storage Projects. Leak pathways represent routes through which CO₂ stored in biomass could be lost from storage reservoirs, through gaseous emissions (e.g., CO₂, CH₄), aqueous transport (dissolved organic/inorganic carbon (DOC/DIC) or particulate organic carbon (POC) and/or physical displacement (subsidence, seismic activities) ¹. Projects must conduct a site-specific assessment of all potential pathways for leaks when characterising the storage environment and design appropriate engineering controls and MRV systems for the monitoring of stored biomass (see Section 7).

Even under anoxic conditions designed to minimize decomposition, some fraction of stored biomass may undergo anaerobic degradation, producing GHGs. The gas phase formed by biomass degradation may leak from the storage area and provide the most direct pathway for GHG release to the atmosphere. This is especially important for coal bearing mines that have natural sources of CH₄ emissions. Such GHG migration may occur via:

• Migration through anthropogenic infrastructure (such as mine shafts or boreholes).
• Migration along geological features.

Storage chambers in subsurface mines are typically sealed using engineered barrier systems designed to prevent gas migration through mine infrastructure and boreholes. These barriers may include: mud slurries, bulkheads (concrete walls), rock seals, and combinations of the three. However, horizontal gas migration may still occur depending on design and material quality ².

For example, the effectiveness of mud slurries as a barrier can be comprised by consolidation, shrinkage, thickness, gaps and cracks, exposure to mine ventilation, erosion and displacement by groundwater flow, as well as heterogeneous emplacement of the material. For concrete walls leaks may occur when there is insufficient bonding to host rocks, degradation from sulfate attack (particularly in coal mines) or other geochemical processes, as well as structural failure if inadequately designed.

Abandoned boreholes from exploration, water wells, or mining operations may provide direct vertical pathways if not properly sealed. These anthropogenic features are not always fully documented in historical records, requiring careful investigation during site characterization.

Gases may also migrate upward through the overlying rock mass via natural geological discontinuities. These pathways include geological faults, fracture zones, soluble bedrocks (karst topography).

Geological faults represent zones of crustal weakness where rock displacement has occurred. Active or recently active faults may create high-permeability conduits for gas migration. The potential for migration through faults depends on factors such as fault activity, orientation, zone characteristics, and hydrological connectivity. Fracture zones are rock masses containing networks of smaller-scale discontinuities (joints, bedding planes, cleavage) that can collectively provide migration pathways. The connectivity and permeability of these networks are influenced by rock type, burial depth, stress history, and the degree of mineralization or cementation.

Soluble bedrocks (such as karst topography found in limestone, dolomite, or evaporites) are susceptible to dissolution, creating voids, caves, and preferential flow paths. While less common in typical mining lithologies, these features must be evaluated when carbonate or evaporite units are present in the overlying stratigraphy.

The Project Proponent must ensure that no water infiltrates the storage area. However, if water intrusion into storage chambers does occur it can accelerate decomposition, transport dissolved carbon species, and physically erode or displace biomass. Stored carbon can be mobilized through aqueous pathways in the form of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), or particulate organic carbon (POC) and dissolved gases (CH₄aq), this can then migrate out of the storage area. Water may intrude into the storage chamber either by:

• Groundwater infiltration.
• Surface water intrusion through mine infrastructure during flood events.

Changes in subsurface conditions could result in groundwater flow through or around storage chambers which could mobilize carbon, where water contacts with biomass, rocks and minerals can leach soluble organic and inorganic compounds, transporting carbon. If substantial groundwater flow occurs through storage chambers, this could represent a significant long-term leak mechanism. Additionally, groundwater flow could transport dissolved oxygen, electron acceptors (e.g., sulfate, nitrate, ferric iron), and nutrients into storage chambers, accelerating aerobic or anaerobic decomposition processes. Finally, if there is high groundwater fluxes, it could physically erode, suspend, or transport particulate organic matter, further promoting leaks.

Surface water intrusion through mine infrastructure could introduce large volumes of oxygenated water, suspended sediment, and nutrients, creating conditions conducive to rapid aerobic decomposition. Additionally, surface water can physically mobilize and transport biomass particles.

Projects utilizing this Module are unlikely to be impacted by surface water intrusion, due to the implementation of leak/seepage safeguards and minimum depth requirements. It is the expectation that engineered sealing barriers employed between storage chambers will be designed to restrict/block surface water intrusion.

Physical disturbances, such as structural failures and ground movement within the storage area, could result in leaks and the release of the stored carbon.

In areas that are seismically active, earthquakes and associated ground displacement can compromise storage integrity through:

• Seismic shaking can crack or displace engineered barriers (e.g., concrete bulkheads), creating new pathways for gas and/or water migration.
• Fault activation, creating new fracture permeability and vertical migration pathways.
• Tunnel/chamber collapse or wall failures in mine chambers, potentially disrupting containment systems or exposing biomass.
• Groundwater regime changes as a result of seismic events, alter fracture connectivity and groundwater flow patterns, potentially increasing water infiltration to storage chambers.
• In extreme cases, surface-rupturing earthquakes can create direct connections between storage depth and the surface.

The likelihood and severity of seismic impacts depends on regional seismicity, site-specific ground motion amplification, and the design robustness of engineered systems.

Subsurface mines can experience long-term subsidence as rock pillars yield or roof spans collapse. Relevant subsidence impacts that could result in leaks include:

• Subsidence can induce tensile fractures extending from mine depth to the surface, creating gas migration pathways.
• Compression or shearing of storage chambers that can breach seals or create openings.
• Subsidence can alter surface drainage patterns and groundwater flow, potentially directing water toward storage areas.

Project Proponents must assess potential leaks that may be a result of intentional or inadvertent human disturbance of storage sites. Such potential scenarios may include, but not be limited to:

• Future mining operations (coal, metals, 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/development of Infrastructures such as foundations, tunnels, or underground facilities could disturb storage chambers.
• Future land use changes might involve excavation or other activities that disturb buried biomass.
• Future entities could deliberately mine or remove biomass 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.

Project Proponents are required to describe and provide evidence to demonstrate how biomass will be stored within subsurface systems at the Project mining operation. Descriptions should 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: Links to the associated risk section numbers are given in square brackets.

To be considered for crediting under this Module, biomass must be stored at a minimum depth of 150m below the ground surface at the Project storage location. The rationale for this requirement is based on considering the zone of active surface weathering and soil formation (approximately <10m) ^{3 4}, seasonal temperature and moisture fluctuations (approximately <50m)⁵⁶, reducing risks to durability [Section 4.1 and Section 4.4]. Below this 150m there is also minimal risk of impact from potential surface disturbances and land use activities and potential surface disturbances and land use activities. The minimum depth requirement is established to ensure:

• Geological Isolation: Where there is sufficient overburden to provide multiple barriers between stored biomass and surface disturbances, such as surface water infiltration through mine infrastructure, isolation from seasonal temperature fluctuations and buffer from surface land use changes. In addition, isolation provides an additional protection against potential migration out of the targeted storage reservoir ⁷.
• Atmospheric Stability: At >150m depth, natural ventilation exchange with surface is minimized, temperature and humidity conditions are more stable, less susceptible to seasonal atmospheric variations and reduced the risk of oxygen ingress from surface.
• Physical Protection: Deep storage 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.

Isometric may accept Projects with a minimum depth of <150m, where a Project Proponent provides justification for a reduction in minimum depth. Reductions in minimum depth requirements will be assessed on a case by case basis by Isometric, in consultation with The Project VVB.

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 biomass storage area, survey measurements from mine establishment may be used. Depth verification documentations must also be available.

Evidence required to demonstrate the storage area depth 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 from surface to storage areas, topographic maps showing surface elevation. In addition, Project Proponents must also account for any variation in storage area depth, monitoring must distinguish between different depth zones if any variability is identified.

Storage chambers must be shown to not be impacted by groundwater, and no groundwater infiltration must occur [Section 4.2]. This must include the mine showing no signs of leakage from the walls into the chambers or surrounding tunnels during visual inspections and further verification from at least two of the following methods:

• Historical Mine Data, for example, historical mining reports which explicitly classify The Project mine or target storage chamber as "dry".
• Literature Data, examining the hydrogeology of the mine.
• Field measurements, such as piezometers in the mine/chamber walls, to demonstrate that there is no increase in hydraulic pressure from groundwater within the mine.

In addition, it must be shown that the biomass-geologic material mix must have a low permeability, this could be done via permeability testing prior to biomass emplacement.

As mine chambers will likely not be completely sealed to the surrounding strata, The Project Proponents must ensure that there is limited diffusion of any gas generated from biodegradation [Section 4.1] and determine the expected diffusion of any gases produced in order to determine the extent of the storage area.

Gas migration out of the biomass must be limited to minimize the risk of leaks and ensure the durability of the stored carbon. Therefore biomass must be mixed with a mud, clay or other geological media/matrix within the storage chambers to ensure a sufficiently low diffusivity. Diffusivity of the biomass and expected migration distances should be calculated in line with Section 5.3.3 of this Module.

Mines must be chosen that have a sufficiently low permeability and porosity to ensure minimal diffusion of any gases produced into the surrounding strata. Diffusivity of the surrounding strata and expected migration distances should be calculated in line with Section 5.3.3 and used to define the Project storage area.

For Projects relying on minimal diffusive transport to ensure permanence, leak assessments should include robust diffusion modeling that estimates the migration rate and distance of GHGs under varying site-specific conditions ⁸. These models must be fully documented and included within the PDD upon submission to Isometric and the Project VVB. The selection and applicability of specific gas diffusion models will be assessed on a case-by-case basis.

Diffusion within geological media follows Fick’s laws of diffusion, proceeding at a rate determined by the material's effective diffusion coefficient (D_eff). To allow such quantification, Project Proponents must:

• Measure or obtain literature values for effective diffusion coefficients of the containment layer material properties.
• Calculate expected diffusion distances over relevant timescales (minimum 1,000 years).
• Consider uncertainty in diffusion parameters and account for spatial heterogeneity.

If appropriate, include biogeochemical consumption/mineralization terms that may further limit migration. The relationship between time and distance in a diffusive system is described by the characteristic diffusion timescale:

$$t = \frac{L^2}{D_{\mathrm{eff}}}$$

(Equation 1)

Where:

• L = The length of the diffusion path (meters). In the context of containment, this represents the thickness of the confining layer.
• ${D_{\mathrm{eff}}}$ = The effective diffusivity of the material (m²/s)
• t = The characteristic time (seconds) required for gas to migrate across distance (L).

To determine the potential migration distance of gas over a specific Project lifespan, this formula can be rearranged to solve for the length of the diffusion path (i.e., confining layer thickness) (L):

$$L = \sqrt{t \cdot D_{\mathrm{eff}}}$$

(Equation 2)

Using the relationship in Equation 2, the time (t) can be set to 1000 years (converted to seconds), and Deff varies based on the grain size and material properties. This allows the calculation of the distance GHGs produced from biomass will diffuse during the lifespan of The Project, and determine whether GHGs are expected to reach the surface ⁹.

To calculate amount of instantaneous diffusive flux, Fick’s laws of diffusion can be used:

$$J = -D_{\mathrm{eff}} \cdot \frac{dC}{dx}$$

(Equation 3)

Where:

• J = flux (mol m^-2 s^-1).
• ${D_{\mathrm{eff}}}$ is the effective diffusion coefficient in the medium m² s^-1.
  • Note: Where there are multiple mediums, such as slurry and rock, the calculation must be carried out for each medium.
• $\frac{dC}{dx}$ is the concentration gradient mol (m^-3 · m^-1).
  • Note: The minus sign means diffusion goes from high concentration to low concentration.

This allows the estimation of diffusive leakage flux per unit area, either as a conservative maximum (J_max) or as a time dependent flux (J(t)). By further multiplying this value with the disposal footprint area A (i.e., vertical area of the containment layer) (m²), the calculation allows the estimation of the mass transfer rate (mol s^-1) across the containment layer. As part of the modelling exercise, the transient diffusion should be modeled with appropriate boundary conditions and reporting time dependent flux J(t), including the peak flux J_max reporting and cumulative mass transferred over the assessment period ¹⁰.

Geological and/or structural seals are required between each storage chamber and remaining mine infrastructure [Sections 4.1, 4.2 & 4.3]. This could include:

• Geological sealing of stored biomass by overburden strata.
  • Engineered sealing with reinforced concrete 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, 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.

Projects utilizing subsurface biomass burial have diverse characteristics, using a range of feedstocks, processing technologies, and storage solutions. Additionally, the environmental context will differ from operation to operation. Given this diversity in Project level characteristics and the potential diversity in true durability of the associated removal, suppliers utilizing this Module must provide evidence of their durability claims with the following requirements. These requirements are in place to assess durability of biomass, as well as microbial degradation potential [Section 4.1].

• Direct, time-series observation of GHG evolution from representative biomass that has been processed in the same manner as the feedstock, and incubated in a context analogous to the storage conditions.

  • A detailed description and quantitative justification as to how the storage conditions will be maintained for a minimum of the claimed durability period is required. In most cases, this will include a direct calculation of time integrated permeation on chemical species that may compromise biomass (e.g., if biomass durability is predicated of low water activity, time integrated diffusion of water into stored biomass must be determined).
  • Time-series evidence of biomass stability must be included in the PDD with a minimum of six months of observation demonstrating biomass stability compared to control observations (longer observation windows are recommended).
  • Suitable tests could include standard ASTM tests for aging of polymers in landfill conditions in both anaerobic and aerobic conditions (ASTM D5526-18: accelerated anaerobic degradation in landfills; and ASTM D7475-20: accelerated aerobic degradation in landfills).
  • This must include a microbial inoculum representative of the storage site. Including a microbial inoculum during testing provides a conservative scenario forecast of expected durability.
  • The Project Proponent must demonstrate that the observed GHG production rate, when extrapolated to the time horizon of the durability claim, is consistent with the durability requirements of Section 2.5.8 of the Isometric Standard. It may be the case in some instances that no GHG evolution is observed over the period of time-series analysis. In such instances, the Project Proponent must include either a theoretical or empirical description of the minimum detection limit of the methods and analyses used, and extrapolate that minimum detection limit to the claimed durability period, with appropriate compounding error.
  • The evidence used to support a durability claim must be specific to a feedstock and a processing method. Evidence of durability may be reused for multiple Projects that utilize a demonstrable similar feedstock as well as processing and storage methods.

• The Project Proponent must also include a detailed description of how the burial conditions facilitating biomass preservation will be maintained for the claimed durability period, including details of natural and engineered controls that will be considered. This must include how low oxygen concentrations and minimal microbial degradation will be maintained. Wherever engineered controls are used to maintain storage conditions (e.g., low permeability/high plasticity clay and synthetic liner to impede liquid water), the Project Proponent must provide quantitative characterization of the durability and physical/chemical properties underpinning these controls (e.g., permeability of water and oxygen through synthetic wrapping material). The Project Proponent must provide calculations extrapolating the storage conditions of the buried biomass to the claimed durability period.

Project Proponents must submit the required evidence of biomass durability to the Isometric Science Team and the VVB for review and approval prior to crediting.

When Project Proponents choose a mining operation for the purpose of subsurface biomass storage, several operational attributes need to be assessed for their suitability as a storage site according to the criteria listed below. Project Proponents must provide detailed information on the following site characteristics and Project documentation and demonstrate that they pose limited risk to the Project. Links to the risks catagory sections addressed are within square brackets.

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 biomass storage based on environmental criteria such as stability and accessibility. 3D mapping of the Project mine is recommended for all Projects.

Where the partner operator has existing 2D or 3D mapping models of the mine, Project Proponents may submit these within the PDD, in lieu of, or as well as, characterization mapping carried out by the Project Proponent. Mapping of the Project mining location must include, at a minimum, the storage chambers that will be utilised for Biomass emplacement and storage (See Section 6.9).

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 analysis. 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 history ¹¹. 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 biomass storage operations.

Project Proponents must submit this information within the PDD. Such evidence must demonstrate compliance with all local and national regulations applicable to the Project operation.

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 biomass transportation to storage locations and sustained monitoring access throughout the period of biomass emplacement and 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 biomass storage operations.

Documentation and reviews of the mining history for the site must be undertaken, 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 biomass.

Environmental baseline assessment must include air and water quality measurements, identification of any contamination from previous mining activities, and an evaluation of any materials incompatible with biomass storage. The mine's current regulatory closure status must be documented, including existing closure plans and integration strategies for biomass 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.

There must be no groundwater infiltration into the storage chambers [Section 4.2]. The Project Proponent must provide evidence of this when characterizing the site in alignment with Section 5.2 of this Module. In addition, the Project Proponent must determine the hydraulic conductivity of host rock and mine floor/walls. Where natural isolation is insufficient, engineered controls must be implemented, proven effective and agreed with Isometric to ensure stable conditions for biomass storage for the lifetime of the storage site.

In addition, a comprehensive groundwater evaluation must be undertaken to identify nearby aquifers, determine groundwater flow directions 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 systems ^{12 13}. Project sites must be located outside of the zone of influence of local groundwater and aquifers, both laterally and vertically. The evaluation of suitability for Project site, additionally all groundwater monitoring systems must be certified by a qualified groundwater scientist (e.g., certified hydrogeologist or similar in the jurisdiction of the Project) and must comply with the sampling and analytical procedures outlined in the site permit or by applicable regulations.

The design of groundwater monitoring plan (e.g., sampling location sites, spacing, depth etc.) must be determined on a site-specific basis by the Project Proponent and must be included in the PDD. The plan should be designed based on applicable regional, 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, are unlikely, Project Proponents must undertake a site characterization assessment to identify the likelihood of flood events. Such assessments should include:

• 1,000-year storm modeling for the Project location.
• An assessment of potential surface-to-subsurface hydrologic connections through fractures, old workings, or subsidence features.
• A monitoring plan for water accumulation in storage chambers and implement immediate pumping/drainage if water exceeds certain time residence criteria.

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.

Project Proponents must conduct geotechnical hazard evaluation including a comprehensive subsidence assessment incorporating historical monitoring data, predictive modeling, analysis of surface effects from past or potential future subsidence, and a risk assessment of subsidence impacts on storage containment integrity [Section 4.3].

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 [Section 4.3]. The modeling must cover 1,000-year durability, 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 biomass storage areas under potential geotechnical hazard scenarios ¹⁴.

The peak acceleration for the storage area must be calculated and reported. Base peak acceleration should be less than 0.1 g with >10% probability of exceedance within 250 years. However, if sequestration is determined to be subject to >0.1 g base acceleration then all systems must be designed to withstand maximum horizontal acceleration and prevent permanent deformation of the sequestered materials.

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 [4.1 & 4.3].

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 Proponents 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.
• 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 biomass 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.

If faults are activated after the Project has started, the Project Proponent is required to consult with Isometric to identify appropriate assessments and remediation actions.

Project Proponents must provide detailed documentation of all engineered barrier systems and the documentation must include (addresses leakage pathway 4.1& 4.2]):

• Barrier design specifications and installation procedures.
• Material properties (permeability, strength and durability) supported by testing data.
  • Material property testing should be carried out in accordance with local and national standards that are relevant to the Project location.
  • In the absence of local or national standards for geotechnical characterization, Project Proponents should defer to International Organization for Standardization (ISO) standards
  • Recommended geotechnical characterization standards can be found in section 3.3.1 of the Rock and Mineral characterization Module (v.1.1).
  • The application of materials within barrier designs may vary by Project, as such material characterization programs should be agreed in consultation with Isometric.
• Analysis of potential failure modes and associated probabilities.
• For Projects relying on horizontal diffusion barriers, quantitative modeling of gas migration through barrier systems over relevant timescales (1,000+ years).

Project Proponents must outline the process by which biomass will be placed and stored in subsurface mine chambers. Such outlines must include the following information:

• A flow diagram demonstrating the placement and storage process for the Project.
• The frequency and mass of biomass placements.
• The composition of the biomass feedstock to be used, together with any details of any carrier, stabilization, binder or backfill matrix, such as clay/silt slurry, cemented paste or membranes.

Subsurface biomass burial is a nascent storage technology for carbon dioxide removal, therefore addressing potential risks to durability is important for ensuring robust quantification and monitoring of CO₂ removals.

Monitoring frequency must be defined based on monitoring phases, monitoring frequency must be the highest during the post-closure phase where chambers are sealed and anoxic environments are established. These establishments are gradually reduced as systems stabilize.

The Project Proponent must describe all the methods, equipment, detection limits and any applicable standards that will be used for monitoring. The Project Proponent must also provide either a theoretical or empirical justification that reversals of CO₂, as well as emissions of CH₄ and N₂O (if relevant) corresponding to the decay of 1% of the stored biomass by mass over the 20 year period of the stored biomass will be detectable.

All equipment used for sampling must be properly adjusted for atmospheric temperature and pressure (ATP) and calibrated per manufacturer requirements, with documentation available upon request. All meters must be calibrated by the manufacturer or a certified third-party calibration service as per the manufacturer’s guidance. Calibration certificates must be maintained in accordance with Section 11 below.

Direct monitoring of stored biomass is a core component of ensuring durability. Prior to emplacement of biomass into target mine chambers, Project Proponents must periodically sample the biomass and slurry mixture in line with the requirements outlined in the relevant Protocol. In addition the following parameters must be characterized:

• Plasticity of materials mixed into biomass.
  • Including the liquid limit and plasticity index values for all materials.
• Sorption capacity of materials mixed into biomass.
  • Including of the mineral composition, especially abundance of expandable phyllosilicates.
• Permeability of the biomass-slurry mix.

Projects must monitor gas concentrations and airflow outside mine chambers utilised for storage of biomass. Gas concentrations within the storage chambers must be monitored, until such a point that the storage chambers have been filled. Project Proponents must, at a minimum, undertake ongoing monitoring of CO₂, CH₄ and airflow, over the Project's monitoring period. It is recommended that Project Proponents measure temperature and relative humidity (RH) outside of the filled storage chamber over the Project Reporting Period. Baseline measurements that must be collected prior to the injection and storage of biomass within mine chambers.

Where Projects employ sealing barriers between differing mine chambers, Project Proponents must undertake ongoing monitoring within every individual chamber utilized for biomass storage. In situations where ongoing monitoring within every individual chamber is not feasible, due to engineering or access restrictions that are out of the control of the Project Proponent, alternative monitoring approaches may be considered. In such instances the Project Proponent should engage with Isometric to assess the suitability of alternative approaches, which will be determined on a Project by Project basis.

Gas monitoring outside chamber seals is required to detect potential gas breakthroughs. This should include:

• Placement of gas probes in mine airways between storage chambers and mine entries.
• Monitoring of adjacent chambers or pre-existing boreholes for elevated CO₂/CH4.

Where chambers, and sealing barriers, are no longer accessible for monitoring, due to the progression or closure of the mining operation, the Project Proponent must report this to Isometric. Such situations will be assessed on a case by case basis in consultation with the Project Proponent and the Project VVB.

Where a Project Proponent utilizes an engineered backfill material for the purpose of storage and sealing, 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 material that is composed of Biomass and geological materials (such as mud or slurry). Where biomass is mixed into the Project backfill material, Project Proponents are required to demonstrate the composition and geotechnical efficiency of the backfill compared to materials that would have been utilised in the absence of rthe Project, the baseline scenario.

If the Project requires the excavation of land, 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 geological material, such as mud, as a binder/ slurry feedstock that is mixed with biomass prior to injection 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.

As specified in the Site Characterization section, for Projects that take place within active seismic zones, Project Proponents must perform a site-specific study of regional seismicity to establish a baseline of seismic hazards ¹⁵. 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 ¹⁶. 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 Proponents must:

• Calculate groundwater flux rates through storage chambers.
• Measure or calculate dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) production from stored biomass 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 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.

Project Proponents are required to conduct monitoring at the storage site for a minimum of 20 years after site closure. A post-closure monitoring plan must be included in the PDD and must include the following monitoring at the wall of the last accessible storage chamber:

• Detection and measurement of greenhouse gasses, including CO₂, CH₄, N₂O, in any open chambers or at the sealing layer/barrier of the last accessible chamber.
  • Note: Where the monitoring of N₂O is not applicable suitable for a specific feedstock, the Project Proponent must provide justification of why the monitoring of N₂O is not relevant. Such omissions will be assessed on a Project by Project basis by Isometric.
  • Projects may utilize tracing techniques (e.g., isotope tracers) in order to quantify how much of the gas sampled is the result of biomass decomposition or mining activities. If no such methods are used, all increases in greenhouse gas will be considered reversals. Detection of reversals must be immediately reported to Isometric and will be handled according to Isometric’s reversal policy (See the Isometric Standard for more details).
• Pressure and airflow outside of storage chambers.
• Physical conditions of the site, including regular inspections of confining materials (e.g., synthetic liners).
• 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/chambers.
  • Crack/strain gauges.
• Oxygen concentration.

Incremental changes of CO₂, CH₄, and/or pressure may indicate that there is a decay of feedstock within the storage site. Monitoring of gasses, pressure and other parameters as applicable, may be conducted through continuous or discrete sampling. If discrete sampling is used, weekly measurements are required over a three-year period to confirm functional stability, with ongoing monitoring for reversals required for 20 years. Where the Project Proponent conducts discrete sampling, sampling must be conducted weekly until conditions at the site are shown to be stable and less frequent monitoring is warranted in consultation with Isometric and the VVB.

Project Proponents utilizing this storage Module must provide a closure plan that describes the details of how the site/facility will be closed and storage chambers maintained after biomass burial activities have concluded. The site closure must include:

• 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 (described below).
• A description of planned maintenance activities for carbon storage (e.g., liners, final covers/ sealing systems, leachate management systems).
• Contact information 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 operators must formalize closure plans in accordance with local regulations and standards. Project closure plans must be integrated with the mine operators' existing, or updated, mine closure and remediation plans. This requirement is relevant for Projects utilizing naturally derived materials for feedstock processing and containment in subsurface mine storage chambers. Project Proponents must provide the Project closure plan, as well as the mining operations closure plan within the PDD, upon submission to Isometric and The Project VVB. 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 crediting Project on the existing mine closure plans, as well as any changes to plans as a result of the Project activities.

Project Proponents are required to identify how the implementation of the Project may impact mine permitting, operation, and closure, specifically where the implementation of a Project may impact waste production volumes, waste management and the mine operators net emissions.

Where a Project is undertaken within an active mining operation, the Project Proponent must engage with the operator and Engineer of Record (where appropriate), to ensure compliance with relevant environmental permitting and regulations. A Project Proponent must engage with the Engineer of Record prior to undertaking CDR activities to assess the potential impacts and suitability of undertaking CDR activities at the Project location.

The Project Proponent is responsible for collecting and submitting Project data, including data generated by a partner operator, where a Project is undertaken within an active extractive operation or quarry. Such data may be used for both environmental monitoring, site characterization and carbon removal quantification, as long as the data is judged as suitable for such purposes by Isometric and the Project VVB. Where data submitted has been generated, or collected, by a partner operator, the Project Proponent must submit information related to uncertainty analysis, as well as any standard operating procedures (SOP) used by the partner operator.

Project Proponents must provide documentation that demonstrates the VVB entrusted with validating removals are able to visit the project site to undertake inspections at agreed intervals, at a minimum of every 2 years. Where access is limited due to safety concerns of the active mine operator, the Project Proponent must consult with Isometric to resolve validation issues. Such resolutions may include engaging the Project’s or operation’s Engineer of Record, where applicable, or a qualified third-party, to undertake measurements required by the VVB.

Note: It is the sole responsibility of the Project Proponent to retrieve, manage and submit all data and information required to meet the requirements of this Module and relevant Protocols, even in instances where data has been generated by a partner operator.

Project Proponents must incorporate land-use restrictions to ensure that stored biomass remains undisturbed. Project Proponents must incorporate a legally binding mechanism on the storage site, such as a conservation easement, covenant, or other similarly restrictive agreement relevant to the jurisdiction which transfers between land owners. The Project Proponent must demonstrate that the restrictive agreement provides legal protection against biomass disturbance for either 1,000 years or a restriction that is enforceable in perpetuity. The purpose of such a legal mechanism is to prevent excavation of and/or the interference with the storage facility for the entirety of the Project lifetime, at a minimum. All of the following eligibility criteria must be met for a Project to be considered sufficiently legally protected for the purposes of carbon Credit generation:

Table 2. Land Durability Requirements

Projects utilizing this storage Module must assess the Risk of Reversal according to the Isometric Standard Risk Assessment Questionnaire in consultation with Isometric. The Risk of Reversal may vary due to Project specific considerations. The Risk of Reversal will be reassessed every 5 years, aligning with the Crediting Period, or when new scientific research and knowledge are produced that highlights significant risks to the durability of the stored carbon.

When quantifying reversals, the Project Proponent must use a 100-year time horizon when selecting GWP. For this Module, GWP100 values provided in the IPCC 6th Assessment Report ¹⁷ (for CH₄, N₂O, and other relevant non-CO₂ gasses must be used, or most recent updates. For methane, biogenic and non-biogenic GWP differentiation can be applied, where appropriate.

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.

All records must be maintained for a minimum of 10 years. All post-closure monitoring records must be maintained by the Project Proponent for a minimum of 10 years after collection.

There is no single credible mechanism that can ensure, without uncertainty, that biomass 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. 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 biomass 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 biomass within the mines subsurface, such documents must be submitted to Isometric and the Project VVB for review.

Isometric would like to thank following contributors to this Module:

• Rebecca Tyne (The University of Manchester)
• Josh Steinberg (Rewind Earth)
• Nitai Amiel (Rewind Earth)
• Eitan Brettler (Rewind Earth)

Isometric would like to thank the following reviewers of this Module:

• Hannah Murnen (Graphyte)

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