Research · published 2026-07-07 · download the county-level CSV

1 in 10 Montana well records sits within a quarter mile of a documented fuel leak

Montana does not require anyone to test a private well after it is drilled — sampled once at construction, then never again unless the owner pays for it. The state also carries one of the country’s largest backlogs of underground fuel-tank leaks it has documented but not finished cleaning up. We mapped where those two facts meet.

Comparing 295,562 Montana well records (GWIC) against the U.S. EPA’s UST Finder database of underground-storage-tank releases — documented leaks of gasoline, diesel, and heating oil at 4,660 Montana sites:

• 28,729 well records (9.7%) lie within 1,320 ft (a quarter mile) of at least one documented petroleum release.
• 11,862 (4.0%) lie within a quarter mile of a release the state still lists as open — reported but not yet cleaned up (EPA snapshot, 2026-06-12).

None of these households is required to test their water. Public water systems near the same leaks are monitored on a federal schedule; private wells fall entirely outside that law.

Counties with the most records near an open leak

Download the full county table (CSV)

What this does and does not say

• Proximity is not contamination. Distance from a documented release is a screening signal, not evidence any specific well is affected. Whether a plume reaches a well depends on hydrogeology, depth, direction, and remediation history. Testing is the only way to know — which is the point: no one is required to.
• What we counted. Montana well records — drilling events, which include some replacement and abandoned wells.
• “Open” status lags. The EPA release snapshot dates to 2026-06-12; states close releases every year, so the current open count is somewhat lower. The “any documented release” figure — cumulative history — is not affected.

Methodology

• Wells: 295,562 Montana well records with coordinates from GWIC (refreshed weekly; see our general methodology).
• Releases: 4,660 Montana release sites with coordinates from EPA UST Finder (990 listed open in the snapshot).
• Join: great-circle (haversine) distance, well-to-release, threshold 1,320 ft. Each well is counted once no matter how many releases are nearby.
• Why a quarter mile: peer-reviewed work behind UST Finder used 1,500 ft as the outer bound of observed petroleum plume lengths (Weaver et al. 2017); we use a tighter 1,320 ft, so our threshold is the more conservative of the two.
• Checks: results were spot-verified at the address level (flagged wells and their nearest release share streets and ZIP codes) and screened for coordinate artifacts (no bulk co-located records).

Sources

• Montana well records: GWIC
• EPA UST Finder (release locations and status)
• EPA, Semiannual Report of UST Performance Measures, FY2025 (state cleanup backlogs)
• Weaver, Murray & Kremer (2017), Science of the Total Environment 609:1589–1599 (plume-length basis for the distance threshold)
• Murray, Hall, Weaver & Kremer (2021), JAWRA 57(5):828–843 (EPA’s domestic-well proximity estimates)
• ASTSWMO/EPA UST Finder briefing (state-by-state wells-near-releases counts)