PFAS CONTAMINATION
PFAS in groundwater: EPA drinking water limits, sources and how PFAS plumes behave
PFAS “forever chemicals” in groundwater: EPA’s 2024 drinking water limits expressed in mg/L, the 2025 changes, common sources, and why PFAS plumes travel so far.
By Argos Tellus · Updated
KEY TAKEAWAYS
- In April 2024, EPA set the first national drinking water limits for PFAS: 0.000004 mg/L each for PFOA and PFOS, 0.00001 mg/L each for PFHxS, PFNA and HFPO-DA (GenX), and a hazard index for certain mixtures.
- In May 2025, EPA said it will keep the PFOA and PFOS limits, intends to extend the compliance deadline from 2029 to 2031, and will rescind and reconsider the limits for PFHxS, PFNA, HFPO-DA and the hazard index. Follow-on rulemaking and litigation were still pending at this update.
- A USGS study detected at least one PFAS in about 45% of U.S. tap water samples, and USGS estimates 71–95 million people in the lower 48 states may rely on groundwater with detectable PFAS.
- PFAS barely degrade and move readily with groundwater, so plumes from fire-training areas, airports, industrial sites and landfills can extend for miles.
What are PFAS and why are they in groundwater?
Per- and polyfluoroalkyl substances (PFAS) are a large family of synthetic chemicals used since the 1940s in non-stick coatings, stain- and water-resistant treatments, industrial processes and aqueous film-forming foam (AFFF) for fighting fuel fires. The carbon–fluorine bond that makes them useful also makes them extraordinarily persistent — hence “forever chemicals.”
PFAS reach groundwater when they are released to soil or surface water and infiltrate downward. Once in an aquifer they are not meaningfully broken down by natural processes, so groundwater can carry them long distances to drinking water wells.
EPA PFAS drinking water limits (in mg/L)
EPA finalized the PFAS National Primary Drinking Water Regulation on April 10, 2024. Concentrations below are expressed in mg/L.
| Compound | Maximum contaminant level (MCL) | Status after May 2025 announcement |
|---|---|---|
| PFOA | 0.000004 mg/L | Retained; compliance date proposed to move to 2031 |
| PFOS | 0.000004 mg/L | Retained; compliance date proposed to move to 2031 |
| PFHxS | 0.00001 mg/L | EPA intends to rescind and reconsider |
| PFNA | 0.00001 mg/L | EPA intends to rescind and reconsider |
| HFPO-DA (GenX) | 0.00001 mg/L | EPA intends to rescind and reconsider |
| Mixtures of PFHxS, PFNA, HFPO-DA and PFBS | Hazard index of 1 (unitless) | EPA intends to rescind and reconsider |
What changed in 2025 — and what is still uncertain
On May 14, 2025, EPA announced it would keep the MCLs for PFOA and PFOS, extend the compliance deadline for those two compounds from 2029 to 2031, and rescind and reconsider the regulatory determinations for PFHxS, PFNA, HFPO-DA and the hazard index mixture. Those changes require new rulemaking, and the 2024 rule is also being challenged in federal court, so the final requirements for the four other compounds remain unsettled.
For site owners and consultants the practical takeaway is stable: PFOA and PFOS limits at 0.000004 mg/L are the near-certain federal benchmark, and several states set their own PFAS standards for drinking water or groundwater. Check the applicable state rules for any site.
Common sources of PFAS in groundwater
- Firefighting foam (AFFF) — military bases, commercial airports and fire-training areas are among the most common sources of high-concentration plumes.
- Industrial facilities — PFAS manufacturing and users such as metal plating, textile, paper and carpet treatment.
- Landfills — leachate from consumer products and industrial waste.
- Wastewater and biosolids — treatment plants do not remove most PFAS; land-applied biosolids can carry PFAS into soil and groundwater.
How PFAS plumes behave in groundwater
Petroleum plumes such as benzene usually shrink over time because microbes degrade them (natural attenuation). PFAS do not. Their plumes tend to grow longer and more dilute as groundwater moves, and they can persist for decades after the source is removed.
Transport varies by compound. Longer-chain PFAS such as PFOS sorb more strongly to organic carbon and accumulate at air–water interfaces in the unsaturated zone, which creates a long-lived source in soil above the water table. Shorter-chain compounds move faster and farther. Some precursor compounds transform into terminal PFAS such as PFOA in the subsurface, so concentrations can rise down-gradient.
That combination — persistence, mobility, ongoing release from the vadose zone and very low regulatory limits — is why PFAS characterization needs dense sampling data, careful non-detect handling and forecasts of where concentrations will exceed limits over time.
How PFAS are treated
EPA identifies granular activated carbon (GAC), ion exchange resins and high-pressure membranes (reverse osmosis and nanofiltration) as effective treatment technologies for drinking water. In groundwater remediation, pump-and-treat with these technologies is common for plume containment; destructive technologies are still emerging.
Frequently asked questions
What is the EPA limit for PFAS in drinking water?
EPA’s April 2024 rule set maximum contaminant levels of 0.000004 mg/L each for PFOA and PFOS and 0.00001 mg/L each for PFHxS, PFNA and HFPO-DA, plus a hazard index for mixtures. In May 2025 EPA said it will keep the PFOA and PFOS limits and reconsider the others.
When do water systems have to comply with the PFAS rule?
The 2024 rule required MCL compliance by 2029. In May 2025 EPA announced it intends to extend the PFOA and PFOS compliance deadline to 2031 through new rulemaking.
How common is PFAS in U.S. groundwater and tap water?
A USGS study detected at least one PFAS in about 45% of tap water samples nationwide, and USGS estimates 71–95 million people in the lower 48 states may rely on groundwater with detectable PFAS for drinking water.
Why are PFAS called forever chemicals?
Their carbon–fluorine bonds are among the strongest in chemistry, so PFAS resist breakdown by heat, water, sunlight and microbes. In groundwater they persist for decades and travel with the flow.
Do PFAS plumes shrink over time like petroleum plumes?
Generally no. Benzene and other petroleum compounds biodegrade, so their plumes often stabilize or shrink. PFAS do not biodegrade meaningfully, so plumes tend to lengthen and persist unless contained.
HOW ARGOS HELPS
SOURCES
- U.S. EPA — Final PFAS National Primary Drinking Water Regulation (April 2024)
- U.S. EPA — EPA announces it will keep maximum contaminant levels for PFOA, PFOS (May 2025)
- USGS — Tap water study detects PFAS “forever chemicals” across the U.S. (2023)
- USGS — Millions in U.S. may rely on groundwater contaminated with PFAS for drinking water (2024)
Figures are taken from the primary sources above and dated as of the update shown. Concentrations are expressed in mg/L. Spot an error? Email support@argostellus.com.
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