Subsurface Biomass Storage

Plants are photosynthetic, autotrophic organisms that use energy from sunlight and reducing equivalents from water to convert atmospheric CO₂ into biomass. Plant biomass represents a standing stock of carbon on the order of several hundred gigatonnes ¹. Net primary productivity, or the net amount of new carbon fixed into biomass by autotrophic organisms annually, is a flux of approximately 100 gigatonnes of carbon per year, with about half of this flux occurring on land ². Every year, approximately the same mass of carbon is returned to atmospheric CO₂ (and other GHGs such as methane) through microbial degradation and other biogeochemical reactions. At steady state, this suggests that the typical residence time of carbon in terrestrial biomass is about 10 years (assuming 500 Gt standing stock and 50 Gt/year flux). Biomass storage for carbon removal aims to decrease the amount of carbon dioxide in the atmosphere through projects that act to prolong the residence time of carbon in stored biomass far beyond this typical residence time.

This Module provides guidance on the storage of biomass in the shallow subsurface 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. 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, it is noted that adherence to the storage provisions outlined in this Module does not guarantee durability of carbon storage. The first version of this Module primarily focuses on highly controlled subsurface storage settings, such as a dedicated burial site, where external influences like groundwater and soil biogeochemical cycling will have limited interference to the ability to monitor for and detect reversals. Given the significant diversity of biomass chemical characteristics, processing prior to storage, storage environments, and other site-specific considerations, it would be inappropriate to claim a minimum, maximum, or average durability. Instead, durability claims associated with removals utilizing this Module must provide direct, time-series evidence of the underlying claims 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 as well as buffer pool requirements. 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 and microorganisms and other biogeochemical redox reactions. While there are many biomass processing technologies and storage options 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 shallow 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 shallow subsurface biomass burial technologies on an individual basis. Once such technologies have demonstrably met a reasonable burden of scientific evidence, they will be added to the Eligible Biomass Processing and Storage Option list below as the Module is updated in line with the Public Consultation process. Isometric will update this list as frequently as necessary, guided by scientific research and market learnings.

When biomass is dried to sufficiently low water activity, biological activity is severely inhibited. It has been shown that production of carbon dioxide from labile organic matter will be severely inhibited below 12% water content (corresponding to a water activity of approximately 0.6) ^3,4. While 12% water content is considered a minimum threshold in this Module, it is recommended that the Project Proponent include a safety factor of an additional 2% (or 10% water content in biomass) to mitigate the risk of water uptake during processing or storage. Project Proponents conducting biomass drying must sample and measure biomass water content at the same frequency and cadence of carbon content sampling.

Dried biomass is eligible for storage in a dedicated burial site (i.e., dried biomass and associated storage materials are the only constituents of the burial site. The burial site must consist of multiple impermeable layers, including, at a minimum, a synthetic liner consistent with durability claims and compacted earthen material with the hydraulic conductivity specifications outlined in Section 5.1. The Project must also include measures for maintaining biomass water activity within the site. Examples of measures that may be used to maintain biomass water activity include biomass salting (20% salt or higher) or wrapping in low permeability materials ⁵.

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

• 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 direct calculation of time integrated permeation of chemical species that may compromise biomass (e.g., if biomass durability is predicated on 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 should ideally include microbial inocolum representative of the storage site. Where biomass is being wrapped, there will be minimal exposure to microbes and moisture once sites are closed. 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 of 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 the same feedstock and processing 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. Wherever engineered controls are used to maintain storage conditions (e.g., low permeability/high plasticity clay and synthetic liner to impeded 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.

• The Project Proponent must provide a detailed plan for seal integrity testing of wrappers chosen for the Project, in conditions representative of the storage site. A suitable pressure test should be chosen (e.g. Haug test) and choices on how that test will be carried out to be representative of the storage site conditions must be justified in full in the PDD. The passing rate for quality testing must be >99%.

• Testing must also be carried out to determine the shear force that causes slope collapse, a condition which would expose wrapped biomass to tearing risk. The storage site should be designed to avoid tearing risk as identified in this aspect of testing.

Project Proponents must submit the required evidence of biomass durability and burial setting stability set out in this section to the Isometric Science Team for review and approval prior to crediting.

Hypothetical example of a Project Proponent meeting the above requirements

A Project Proponent supplies removals that involve drying biomass to a water activity that is sufficiently low to inhibit all biological activity. The Project Proponent incubates the dried biomass under controlled laboratory conditions for six months, and finds that no measurable greenhouse gasses have been produced. The Project Proponent uses the extrapolation of the minimum detection limit to evidence that < 1% of the biomass is expected to decay in a 1,000 year time horizon. The Project Proponent seals the biomass in synthetic wrapping before aggregating the wrapped biomass in a landfill that includes a sealed synthetic liner and a clay layer. The Project Proponent in this example must include in the PDD the water and gas permeability and other relevant material properties of the wrapping, landfill liner, and clay layer. The Project Proponent must provide calculations using these material properties to demonstrate that the dry storage conditions can be maintained over the claimed durability period. The Project Proponent calculates that under a worst case scenario, the water activity will increase by 2% in the vicinity of the biomass over a 1000-year period. The Project Proponent adjusts their biomass processing operations to ensure a 2% safety factor in water activity (i.e., stored biomass has a water activity 2% below that which has been shown to fully inhibit biological activity).

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.

For underground biomass storage site selection, several site attributes need to be assessed for their suitability as a storage site according to the criteria listed below ⁶. These categories include surface water, groundwater, rock and soil, and regional seismicity. Project Proponents must provide detailed information on the following site characteristics and demonstrate that they pose limited risk to the Project.

Hydrology - surface water
Storage sites must be located outside of the flood plain of all major waterways and arterial streams based on the governmental flood maps with longest available time duration. Where natural permeability of soils is impacted by the development of storage sites, adequate drainage must be installed to transfer all precipitation and meltwater away from the site. No pooling of water is permitted within the burial chamber which remains for more than 48 hours following a precipitation event. If any sedentary water is found within the burial chamber during a course of regular inspection, it must be immediately reported to Isometric and addressed with a permanent solution within 90 days. Where floodplain maps are drawn for 1,000-year storms based on historical considerations and multi-decadal records, or where future climate change is likely to considerably affect the distribution of suitable locations, modeling or conservative additional buffer zones must be applied. For all sites located near existing water bodies, such as the ocean, ponds, or lakes, modeling must confirm that the proposed site will not be impacted over a 1,000-year period by changes in sea level or shifting precipitation patterns.

Hydrology - groundwater
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 sites, as well 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 ⁷.

The required number of groundwater sampling location sites, spacing, and depth is determined on a site-specific basis, based upon applicable regulations, which may depend on aquifer thickness, groundwater flow rate and direction, and the other geologic and hydro-geologic characteristics of the site. The Project Proponent must also assess the general direction and magnitude of groundwater level change that may result from climate change.

Local Geology and Soil Properties
While most eligible projects will necessarily utilize impermeable membranes or other means of separation between the stored biomass and the underlying soils, rocks, and minerals of the storage site, lithology must be considered by Project Proponents with no barrier in place. For such projects, the Proponent must describe the following properties, including how each property will be evaluated and addressed to ensure permanence of the stored biomass.

• Permeability
  • Including soil composition and texture, plus bedrock composition and texture
• Soil properties
  • Including pH and clay content
• Hydraulic conductivity
  • Consisting of hydraulic conductivity values for all materials
• Plasticity
  • Consisting of liquid limit and plasticity index values for all materials
• Sorption capacity
  • Consisting of mineral composition, especially abundance of expandable phyllosilicates
• Seismicity
  • The Project Proponent must characterize any material seismic risk in the vicinity of the storage site. This must include the following.
    • Peak Ground Acceleration (PGA) - PGA from shock waves, modeled for 50-year or longer timescale and wave-amplification potential implied by local lithology
    • Peak Ground Displacement (PGD) - mechanical response model for site materials and geologic evidence for absence of land surface disturbance by earthquake activity

Permeability, soil properties, hydraulic conductivity, plasticity, and sorption capacity are included to assess the risk of biomass disturbance by infiltration of surface water and/or groundwater. Peak ground acceleration and peak ground displacement are included to assess the risk of biomass disturbance from seismicity.

Table 1 contains target values against which a potential biomass storage site should be assessed and compared. Any deviations from the ranges specified here must be justified in the PDD. The use of sufficiently non-permeable liners may be a justification for relaxing hydraulic conductivity target values.

Table 1. Design Guidelines and Target Values

These target values were chosen based on conventions and requirements used in municiple solid waste landfills ^8,9, as well as parameters that would quantitatively restrict passive water intrusion on sufficently long time scales (e.g., 1000+ years).

If the Project requires the excavation of land, baseline soil carbon stocks must be established and monitored after disturbance, with reductions in stocks counted as foregone counterfactual storage. If excavation has already occurred, the Project Proponent must identify an analogous plot of land nearby that has the same or similar soil type and site characteristics. The concentration of soil organic carbon in the nearby undisturbed site may be used as an estimate for the baseline soil organic carbon at the project site. Increases in soil organic carbon are not considered creditable removals under this Module.

Project Proponents must incorporate land-use restrictions to ensure a high likelihood that stored biomass remains undisturbed. Project Proponents are required to 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 or 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

When developing on agricultural land, Project Proponents must account for the land use impacts by calculating Replacement Emissions for the quantity of agricultural production that would counterfactually have been conducted on the site used. Developing wetlands lands for the purposes of biomass burial is not allowed. Deforesting old-growth forests is also prohibited. Regardless of Project location, Project Proponents should use the United States Environmental Protection Agency definition of wetlands when evaluating the suitability of a site ^10,8. The Project Proponent must consider counterfactual CO₂ sequestration associated with vegetation at the project site(s) that was present before the site was cleared for the project purposes. This must include all project sites that were cleared as a result of the CDR project including sites for project facilities and storage sites. The quantification of CO₂e Counterfactual must consider counterfactual CO₂ sequestration using annual CO₂sequestration rates and consider sequestration over a period of 15 years.

Project Proponents utilizing this storage Module must provide a closure plan that describes the details of how the site will be closed and 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, 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.

The closure standards for municipal solid waste facilities in the United States (MSWLFs) ⁷ require owner/operators to install a final cover system to minimize infiltration of liquids and soil erosion. While biomass storage facilities may or may not be landfills in a legal sense, biomass storage facilities and their contents have many of the same characteristics and will experience the same influences from precipitation, heat and pressure and as such must actively reduce the impact of these influences throughout the project's lifespan. Unlike a landfill for municipal waste disposal, where leachate, odor and methane gas from chemical and biological activities are of major environmental concern, these are not major concerns as long as storage conditions are maintained.

To limit long term environmental impacts from storage facilities, project operators must formalize closure plans in accordance with local regulations or, in absence of local regulations, with the US EPA standards ⁷. This requirement is relevant for projects utilizing naturally derived materials for feedstock processing and containment in storage chambers.

It is required that sites are reclaimed to support habitat and biodiversity enhancement or another use as decided through local community engagement within 3 years of covering sites and prior to the last issuance of Credits at the end of the initial Crediting Period, that could be confirmed during a site visit and visual inspection. Carbon removal projects, including shallow subsurface biomass storage projects should not, for any reason, encourage or incentivize industrial wastelands due to project activities. The reclamation of project sites and their surroundings, while potentially required by the legal mechanism for land preservation (eg conservation easement), will be supportive of positive stewardship of lands and demonstrating respect for local stakeholders.

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. Direct monitoring of stored biomass is a core component of ensuring durability. Project Proponents are required to conduct monitoring at the storage site for a minimum of 50 years after site closure. A post-closure monitoring plan must be included in the PDD and must include the following:

• Temperature and moisture content in the vicinity of the biomass for early detection of stored biomass decomposition
• Greenhouse gasses produced in the vicinity of the stored biomass including CO₂, CH₄, N₂O
• Pressure
• Physical condition of the site, including regular inspection of confining materials (e.g., synthetic liners)
• Other site-specific parameters mutually agreed upon by the Project Proponent and Isometric

Incremental changes of CO₂, CH₄, water vapor, temperature and/or pressure may indicate that there is a decay of feedstock within the storage site. If these variables indicate the possibility of decay, additional testing for N₂O must be conducted. Monitoring of gasses, temperature, moisture, and pressure may be conducted through continuous or discrete sampling. If discrete sampling is used, frequent measurements are required over a three-year period to confirm functional stability, with ongoing monitoring for reversals required for 50 years. It is recommended that, if the Project Proponent conducts discrete sampling, sampling be conducted weekly until conditions at the site are shown to be stable and less frequent monitoring is warranted.

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 on the order of 1% over the 50 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 per manufacturer’s guidance or more frequent, if needed. Calibration certificates must be maintained in accordance with Section 11 below.

Projects may utilize tracing techniques (e.g., isotope tracers) in order to quantify how much of the gas sampled is the result of ambient air or soil gas inflow that occurs during the sampling process. 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.

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 be higher 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.

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 ¹¹ 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 shall be developed and submitted to proper authorities as required by any applicable permitting authority.

All records shall be maintained for a minimum of 10 years. All post-closure monitoring records shall 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 shallow 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.

Isometric would like to thank following contributors to this Module:

• National Renewable Energy Laboratory (NREL)
• Lawrence Livermore National Lab (LLNL)
• EcoEngineers
• Ning Zeng, Ph.D., University of Maryland

NREL and LLNL were supported by the DOE Office of Technology Transitions in collaboration with the Office of Clean Energy Demonstrations (OCED), Office of Fossil Energy and Carbon Management (FECM), Office of Energy Efficiency and Renewable Energy (EERE), and the Bioenergy Technology Office (BETO).

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