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State hydrogeologists brief committee on Fox Hills–Hell Creek aquifer study: early findings and next steps
Summary
The Montana Bureau of Mines and Geology reported early findings from a legislatively mandated study: the Fox Hills–Hell Creek aquifer covers much of eastern Montana, has roughly 6,500 completed wells (mostly domestic/livestock), shows localized declines in some areas, and will produce a final report and preliminary groundwater model for the legislature by June 30, 2027.
Sarah Edinberg, hydrogeologist with the Montana Bureau of Mines and Geology, told the Water Policy Interim Committee that HB 935 authorized a bureau study of the Fox Hills–Hell Creek aquifer after landowner concerns about declining artesian flow. The bureau’s work to date includes compiling historical data, prioritizing wells with long records, installing pressure transducers and building preliminary structure maps to support groundwater modeling.
Key points from MBMG’s presentation:
- The Fox Hills–Hell Creek formation underlies much of eastern Montana; MBMG estimates roughly 6,500 completed wells in the unit, most used for domestic or livestock purposes and fewer than 4% used for irrigation or public supply. Irrigation is under 1% because the aquifer’s high chloride content limits agricultural use.
- Long‑term monitoring wells show heterogeneous behavior: some wells show decades‑long declines (one site near Terry exhibited ~35 feet net decline of water level), while other wells, e.g., near Jordan, have seen modest increases since the early 1980s. That heterogeneity argues against a single, statewide conclusion and requires localized analysis.
- MBMG has instrumented nine sites with hourly data loggers and visited about 120 sites (76 with historic data). The bureau will complete structure maps (top of aquifer elevation and thickness) using petroleum and water well logs and will publish a bureau report and web app (expected mid‑to‑late June) with interactive well records and hydrographs.
- A preliminary groundwater model and water‑budget will be completed as part of the final report due to the legislature on 2027‑06‑30.
Committee members asked about recharge rates, check‑valve retrofit costs for free‑flowing wells and program uptake. MBMG said recharge is slow where outcrops are limited, retrofit costs vary by well condition and program uptake depends on landowner interest. The bureau said its interim work is high level because HB 935 provided limited funding; more focused studies would require additional funds.
MBMG’s data collection and the web app will provide a public resource for landowners and managers and should inform future policy and local mitigation or retrofit programs.
(Reported from the HB 935 presentation and Q&A.)
