Mass-Balance Chain of Custody: How Greentruth Tracks Fungible Gas and RNG

For MRV teams, pathway holders, and ESG teams trying to articulate to an auditor exactly how a buyer ends up with a Scope 1 reduction claim from a unit of biomethane that physically commingled with fossil gas in a shared pipeline, mass-balance chain of custody is the accounting model that answers the question. Used across the major biofuel certification schemes, recognized by RED III, accepted under California's LCFS, and built into the EU Methane Regulation's downstream documentation expectations, mass balance is the standard chain-of-custody discipline for fungible commodities — and it's the model Greentruth uses for natural gas and RNG. This page explains what it is, how it differs from book-and-claim, and how the Greentruth implementation works from physical batch to retired QET.

Mass-balance chain of custody, in one paragraph. Mass-balance chain of custody is an accounting model in which physical inputs (verified RNG or low-CI gas injected into a pipeline system) and certificate outputs (QETs issued, transferred, and retired) are tracked at the system level so that total certificates can never exceed total verified physical volume. It is the standard model for fungible commodities — gas, RNG, ethanol, biomethane — that commingle in shared infrastructure and cannot be individually traced at the molecular level. Greentruth implements mass balance under ISO 14064-3 reasonable assurance, with volumetric equivalence enforced at the registry layer and full audit trails preserved on-chain from physical batch through to retired token.

What a QET is in the first place

The chain-of-custody question — can a buyer credibly claim the environmental attributes of a unit of fuel they cannot physically identify? — has four standard answers in commodity accounting. Identity-preserved tracks a specific physical batch end to end. Segregation keeps the certified material in physically separate streams. Mass balance allows commingling but enforces a strict volume-in / volume-out discipline at the system boundary. Book-and-claim fully decouples the certificate from the physical flow.

The third model — mass-balance accounting for natural gas, RNG, and similar fungible streams — sits in the middle of that spectrum, and it's the one that fits how gas networks actually operate. The principle is mechanical:

• Verified physical input. A specific volume of qualifying material — RNG from an anaerobic digester, low-methane-intensity gas from a producer with primary MRV data, biomethane from a landfill-gas project — is injected into a shared system. The volume is measured, verified, and recorded.
• System boundary. The physical mixing happens inside the boundary (a pipeline network, a refinery, a fuel-storage facility). Inside the boundary, the commingled molecules are physically indistinguishable.
• Certificate output. Certificates are issued representing the environmental attributes of the verified physical input. The total volume of certificates ever issued, transferred, and outstanding never exceeds the total verified physical volume that entered the system.
• Audit-grade ledger. A continuous record reconciles inputs to outputs. An independent verifier confirms that the books balance. The ledger is what makes the claim defensible against an auditor or regulator.

Put differently: the model does not assert that the buyer received the specific molecules the producer injected. It asserts that the producer injected enough physical material to back the certificates the buyer eventually retires, and that the accounting at the system boundary is auditable.

How double counting is structurally prevented

Natural gas, RNG, biomethane, ethanol, and biodiesel are fungible commodities — once they enter a shared pipeline, blending tank, or storage facility, individual molecules become physically indistinguishable from the surrounding stream. There is no chemical marker that survives commingling; no operator can point to a single methane molecule downstream and say "this one came from the digester in Iowa, that one came from the well in West Texas."

For fungible commodities, identity-preserved chain of custody and physical segregation are operationally impractical at scale. You'd need parallel infrastructure for every certified pathway, which the network economics do not support. So the practical choice is mass balance — which is exactly why the major biofuel and biomaterial certification schemes built it into their rules decades ago, and why the regulatory frameworks (RED III, LCFS, EU Methane Regulation) all recognize it for renewable gases and renewable fuels.

For RNG specifically, the physical reality is captured in the QET-RNG Compliance Passport: once biomethane is injected into the common-carrier pipeline grid, it commingles entirely with conventional fossil gas. Yet major reporting frameworks (GHG Protocol, SBTi, CDP, TCR) require robust proof that the buyer is not double-counting attributes and is not claiming fuel they cannot physically access. The model reconciles those two requirements: commingling is permitted, but the volumetric accounting is rigorous.

For the QET-RNG product in full

For the biogenic accounting treatment

The distinction between the two models is the single most important concept on this page, because it's what determines whether a claim will or will not survive an auditor's review.

Book-and-claim fully decouples the certificate from the physical flow. The certificate represents an environmental attribute generated somewhere in the system, and it can be sold to a buyer who has no physical connection to where the attribute was produced. The classic example is voluntary RECs — a corporate buyer in New York can retire a REC backed by a wind farm in West Texas without any physical-delivery linkage. Book-and-claim is useful for early-market scaling and for certain electricity-market structures, but the integrity threshold is lower: the buyer cannot defend a claim that the underlying physical commodity actually reached their facility.

Mass balance retains the physical-delivery linkage. The certificate corresponds to a verified physical input that entered the same interconnected system the buyer draws from. The buyer cannot claim the specific molecule — physical fungibility makes that impossible — but they can claim that the physical material flowed into the system they consume from, in a volume that supports the certificate's environmental attributes.

For Scope 1 fossil-CO₂ reduction claims under GHG Protocol, SBTi, CDP, and TCR, the higher bar is what the frameworks expect. Book-and-claim is increasingly disallowed for direct emissions claims; the physical-delivery-anchored model, executed with auditable rigor, remains valid.

This is why the QET-RNG Compliance Passport's five-point verification gateway explicitly tests for pipeline connectivity confirmed (both injection and delivery on the same interconnected system) and volumetric equivalence (total certificates issued never exceed total physical RNG injected). The first is the physical-delivery linkage that book-and-claim lacks; the second is the volumetric discipline at the heart of mass-balance accounting.

How RNG delivery is certified

The model is not a Greentruth invention — it's the accepted chain-of-custody approach across the major regulatory frameworks the platform serves.

RED III (EU Renewable Energy Directive III). RED III's chain-of-custody requirements for renewable gases and renewable fuels recognize mass balance as a valid accounting model, subject to the audit and verification requirements built into the directive's implementing acts. For biomethane injected into European gas grids, it is the practical discipline that supports cross-border attribute trading.

California LCFS. The Low Carbon Fuel Standard's pathway integrity rules accept it for renewable fuels — including biomethane, ethanol, and biodiesel — that commingle in shared transport or distribution infrastructure. LCFS-compliant pathways must demonstrate the volumetric reconciliation the model enforces.

EU Methane Regulation. Article 28's chain-of-custody requirements for imported natural gas operate alongside the same principles: the operator chain along a pathway is auditable, the volumes injected and delivered must reconcile, and certificates cannot exceed verified physical throughput.

The practical implication: this is the chain-of-custody approach that produces claims that survive in regulated environments. It's also the approach that interoperates with the major industry certification schemes for adjacent commodities (biofuels, biomaterials), which means a customer building a procurement program across multiple feedstocks can apply a consistent accounting discipline across all of them.

How to comply under LCFS

How to comply under EU Methane

Readers familiar with biofuel and biomaterial certification will recognize ISCC (International Sustainability and Carbon Certification) and RSB (Roundtable on Sustainable Biomaterials) as the major international certification schemes that have used this model as their primary chain-of-custody approach for years.

Both schemes are useful industry-context references for understanding how it operates in practice across fungible-commodity supply chains. ISCC EU has been the dominant chain-of-custody scheme for biofuels entering the EU market under previous renewable-energy directives, and its methodology is well-documented and widely audited. RSB applies analogous principles across a broader set of biomaterials.

Greentruth's implementation is not a certified ISCC or RSB pathway — Greentruth operates under its own ISO 14064-3 reasonable-assurance verification methodology and its own Governance Framework Section 6.1.6 cross-registry rules. But the underlying accounting principles are the same family of disciplines that ISCC and RSB use: verified physical input, volumetric equivalence, system-level reconciliation, and independent third-party assurance.

For customers operating across both ISCC/RSB-certified biofuel programs and Greentruth's gas and RNG markets, the conceptual model is consistent: the chain-of-custody discipline travels across commodities even where the specific certification body and the specific methodology differ.

ISCC EU mass balance methodology

RSB chain of custody

The Greentruth implementation maps directly to the platform's QET lifecycle. The integrity controls sit at four points:

At the producer (mint). Primary MRV data — site-specific measurement, continuous monitoring, segment-level reporting where applicable — feeds the methodology that produces the verified attribute value. An ISO 14065:2020 accredited verifier reviews the producer data under ISO 14064-3 reasonable assurance and confirms the physical volume. The token is minted on-chain with the verified volume attached as an attribute.

At the system boundary (volumetric equivalence enforcement). The registry layer enforces single-mint rules: a specific physical unit produces exactly one token. The Hedera-anchored registry structurally rejects any attempt to issue a second token for the same MMBtu, kWh, or tonne. This is the volumetric equivalence discipline at the infrastructure level — total certificates outstanding for any verified batch cannot exceed the verified physical input.

At the retirement (irrevocable claim). When a buyer retires a QET against a reporting claim, the retirement is irrevocable on-chain. The retired token cannot be re-retired, re-used, or rolled forward into a subsequent period. The retirement record carries the volumetric reconciliation that ties the buyer's claim back to the producer's verified physical input.

At the verification layer (independent third-party assurance). Across the full lifecycle, the verifier's ISO 14064-3 reasonable-assurance opinion sits on-chain alongside the token. The opinion confirms that the producer's MRV stack supports the attribute claim, that the methodology was applied correctly, and that the materiality threshold was met. Any auditor downstream can re-trace the chain from a Greentruth retirement back to the producer's primary data via the on-chain references.

For RNG specifically, the Greentruth implementation also runs through the Compliance Passport's five-point gateway: injection point identified, delivery point identified, pipeline connectivity confirmed, volumetric equivalence verified, temporal matching confirmed. The combination produces the Fuel Attribute Documentation Certificate that supports Scope 1 fossil-CO₂ reduction claims under the major reporting frameworks. Where one or more criteria fail, the certificate is downgraded to a Production Documentation Certificate — valid for voluntary support, not for Scope 1 reduction.

How ISO 14064-3 underpins this

For natural gas and RNG, the volumetric ledger is anchored to the physical pipeline corridors the commodities actually flow through. The proprietary Lower-48 pipeline network behind the Greentruth Map of Pipelines provides the spatial structure: every QET-NG and QET-RNG retirement record carries the operator chain along the pathway from injection point to delivery point, and the volumetric equivalence is reconciled at the system level the pathway defines.

The ledger answers three questions an auditor or regulator is likely to ask:

• Was the physical material actually injected? Verified through the producer's MRV stack and the third-party verifier's opinion.
• Did the injection point and the delivery point sit on the same interconnected pipeline system? Confirmed through the network model that backs the Map of Pipelines (criterion 3 of the QET-RNG five-point gateway).
• Did total certificates outstanding ever exceed the verified physical volume? Reconciled through the registry-layer single-mint enforcement and the irrevocable retirement records.

The audit trail from physical batch to retired token is end-to-end traceable on-chain. A reviewer in 2028 examining a retirement record made in 2026 can follow the references back through the retirement event, the acquisition event, the mint event, the verification opinion, and the producer's primary data — every step preserved as an immutable on-chain record per the Governance Framework's seven-year retention requirement.

For the pipeline network behind the ledger

For the downstream Scope 3 product that consumes these claims

A few important boundaries — these matter because readers often arrive at this page from adjacent chain-of-custody concepts.

It is not identity-preserved tracking. It does not claim that the buyer receives the specific molecules the producer injected. Physical fungibility makes molecular identity-preservation impossible for gas and RNG; the model does not pretend otherwise.

It is not book-and-claim. Certificates are not fully decoupled from physical flow. The injection point and the delivery point must sit on the same interconnected system (for RNG, this is criterion 3 of the QET-RNG verification gateway); total certificate volume must never exceed verified physical input.

It is not a Greentruth-only concept. The accounting discipline is recognized across RED III, LCFS, the EU Methane Regulation, ISCC EU, RSB, and most other major fungible-commodity certification schemes. Greentruth implements the discipline under its own ISO 14064-3 verified methodology, but the underlying principles are industry-standard.

It is not a substitute for primary MRV data. This is an accounting layer that sits on top of primary measurement. The producer's MRV stack still has to generate the verified attribute values; the volumetric reconciliation governs how those values flow through the certificate system. Without primary MRV, there is nothing for the ledger to track.

It is not a Scope 1 transfer mechanism. Retiring a QET tracked under this model does not transfer the producer's direct emissions to the buyer. Scope 1 stays with the entity that physically combusts the fuel; the certificate conveys the verified environmental attribute (carbon intensity, biogenic content, pathway provenance), not the GHG Protocol scope itself.

For MRV teams, pathway holders, and ESG teams working through the documentation an auditor or regulator will eventually open, the mass-balance ledger is the part of the system where claims either survive or fail. The walkthrough traces a specific batch from physical injection through verification, mint, transfer, and retirement — every step preserved on-chain, every volume reconciled.

For the QET-RNG product

For the downstream Scope 3 Category 3 product

Read the full Governance Framework