Overview
On 19 March 2026, the United States Court of Appeals for the D.C. Circuit denied a motion by the US Environmental Protection Agency to sever and indefinitely stay legal challenges against the Maximum Contaminant Levels for four PFAS compounds regulated under the Hazard Index approach. The ruling forces the EPA to defend the scientific and procedural basis of its 2024 cumulative risk assessment framework for PFHxS, PFNA, HFPO-DA (GenX), and mixtures containing PFBS in open court.
The EPA had sought to pause litigation on these specific compounds while it actively works to rescind the regulations, citing procedural errors in the original determination process. The court’s refusal to grant a stay means the legal challenge will proceed, potentially testing the validity of the Hazard Index methodology that underpins how regulators worldwide assess the combined toxicological risk of PFAS mixtures.
Key details
The 2024 US EPA National Primary Drinking Water Regulations introduced two distinct regulatory mechanisms for PFAS in drinking water. Individual Maximum Contaminant Levels were established for PFOA at 4 parts per trillion and PFOS at 4 parts per trillion. For four additional compounds — PFHxS, PFNA, HFPO-DA, and PFBS — the EPA applied a cumulative Hazard Index approach that accounts for the combined exposure risks from multiple PFAS compounds present simultaneously in a water supply.
The Hazard Index methodology represents a significant departure from traditional single-compound regulation. Rather than setting individual concentration limits, it calculates a combined risk score by dividing the measured concentration of each compound by its respective health-based reference value, then summing the results. A Hazard Index exceeding 1.0 triggers a regulatory exceedance, even if no individual compound exceeds its own reference value in isolation.
The current EPA administration has signalled its intention to abandon support for the Index PFAS regulations while continuing to defend the stricter individual MCLs for PFOA and PFOS. The motion to sever the Index PFAS litigation from the broader PFOA and PFOS challenges was designed to allow the EPA to rescind the Hazard Index rules without the complication of an active court challenge. The D.C. Circuit’s denial of this motion means both sets of regulations remain subject to simultaneous judicial scrutiny.
Australian context
This US court decision has direct implications for Australian environmental practitioners. The US EPA’s toxicological assessments and risk methodologies serve as primary reference points for Australian regulators when developing and updating domestic screening criteria for PFAS compounds.
The PFAS National Environmental Management Plan (NEMP) Version 3.0, which came into effect in March 2025, already incorporates cumulative risk assessment principles for PFAS mixtures. The NEMP screening criteria assess the sum of PFOS and PFHxS concentrations against combined investigation and screening levels, reflecting the same underlying toxicological rationale as the US EPA’s Hazard Index approach. If the US court proceedings discredit or weaken the scientific basis of cumulative PFAS risk assessment, it could create uncertainty around the Australian framework’s regulatory foundations.
The Australian Drinking Water Guidelines are also undergoing updates that reference US EPA toxicological data. Any judicial finding that the EPA’s Hazard Index methodology was procedurally flawed or scientifically unsupported could prompt a reassessment of how Australian water utilities and environmental consultants apply cumulative risk screening to PFAS-contaminated water sources.
For contaminated site practitioners, the practical concern is regulatory volatility. Site conceptual models, risk assessments, and remediation targets that rely on cumulative PFAS screening criteria may need to be revised if the underlying methodology is successfully challenged. Consultants should ensure their assessment frameworks are sufficiently flexible to accommodate potential changes in how PFAS mixtures are regulated.
Practical implications
Environmental consultants advising clients on PFAS-contaminated sites should document which regulatory framework and version of screening criteria their assessments rely upon. If the US Hazard Index methodology is weakened through litigation, Australian regulators may respond by updating the NEPM or ADWG guidance, potentially changing the compliance status of sites that were previously assessed as meeting criteria.
Site management plans for properties with multiple PFAS compounds present should include contingency provisions for changes in cumulative risk assessment methodology. This is particularly relevant for sites where individual PFAS concentrations fall below standalone screening levels but the combined Hazard Index approaches or exceeds 1.0. These are the sites most vulnerable to changes in how cumulative risk is calculated.
For property transactions involving PFAS-contaminated land, due diligence assessments should explicitly address the regulatory uncertainty created by this litigation. Purchasers and their advisers should understand that current screening criteria for PFAS mixtures may not represent the final regulatory position, and remediation cost estimates should account for the possibility of either tightening or relaxation of cumulative risk thresholds.
Laboratories providing PFAS analytical services should ensure their reporting suites include all compounds relevant to the Hazard Index calculation, not just PFOA and PFOS. As regulatory attention shifts toward mixture effects, incomplete analytical coverage will leave gaps in site characterisation that may require costly additional sampling rounds.
References and related sources
- JD Supra — Court Denies EPA’s Motion to Sever and Stay Challenges to MCLs for Four Index PFAS
- Australian Government DCCEEW — PFAS National Environmental Management Plan
- NHMRC — Australian Drinking Water Guidelines
- Association of Metropolitan Water Agencies
How iEnvi can help
iEnvi provides specialist environmental consulting services for PFAS site characterisation, cumulative risk assessment, and remediation strategy development. Our CEnvP Site Contamination Specialists assist property developers, industrial operators, and government agencies with navigating the evolving PFAS regulatory landscape.
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This is an iEnvi Machete news summary. Prepared by iEnvi to summarise the source article for contaminated land, groundwater, remediation, approvals and site risk professionals.
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