State hydrogeologist summarizes Park County groundwater: most wells in basin-fill aquifers; irrigation contributes recharge

Sarah Edinburg, a hydrogeologist with the Montana Bureau of Mines and Geology, told the Park County Planning Board on Feb. 20 that most local water users rely on shallow basin-fill aquifers and that snowpack and irrigation strongly influence groundwater levels.

Edinburg summarized countywide hydrogeology, well counts and water-quality measurements and demonstrated the bureau's Groundwater Information Center database. She emphasized that most wells in Park County are completed in basin-fill aquifers and described differences between unconfined (water-table) and confined (artesian) systems.

The presentation gave the board a technical foundation for later questions about proposed subdivisions and river-related planning. Edinburg said the bureau's county-by-county groundwater characterization work is intended to support local planning and project-level investigations and that her team makes its data publicly available online.

Edinburg said Park County has roughly 5,600 recorded wells and that domestic wells make up the largest share by count. "Eighty-nine percent of the wells in Park County are for domestic purposes," she said, noting that domestic wells account for a small fraction of total daily groundwater withdrawals. She told the board the county's groundwater withdrawals total about 3.6 million gallons per day and that groundwater accounts for only a small share of overall water withdrawals in the county when surface-water diversions are included.

On seasonal behavior, Edinburg said Paradise Valley aquifers receive most recharge from mountain snowpack and from small mountain streams that lose water to groundwater as they descend into valley-bottom alluvium. She used hydrographs to show that shallow wells near streams respond quickly to spring runoff while deeper wells in glacial deposits show delayed, broader peaks later in the year.

On water quality she told the board that basin-fill aquifers across most of the county generally have low total dissolved solids (TDS) and are suitable for drinking; median TDS values in many sampled basin-fill wells were well under 500 parts per million. She warned that Gardiner Basin and geothermal areas near Lava Hot Springs show higher TDS and localized arsenic and uranium concentrations tied to geothermal sources. "A lot of those (elevated values) are connected to geothermal activity," she said.

Edinburg demonstrated the Groundwater Information Center (GWIC) mapping and data portal and showed how to pull well logs, water-level hydrographs and project datasets. She also described the bureau's Groundwater Investigation Program, which accepts project proposals and can perform more detailed, multi-year studies and groundwater-flow modeling for communities or watershed groups that seek it.

Board members and attendees asked technical and policy questions about irrigation recharge, modelable scenarios for increased residential pumping, and where to get help if the county needs a formal study. Edinburg and Alan English (who identified himself as manager of the bureau program during Q&A) advised that the bureau's Groundwater Investigation Program can run scenario models but projects are selected through a periodic proposal cycle and typically take one to a few years from kickoff to final report.

The presentation was delivered for information and discussion only; no formal board action was taken on the subject at the Feb. 20 meeting.

Edinburg and bureau staff left printed materials and contact information for board members and county staff; she encouraged planners and landowners to use GWIC for well logs, water-quality samples and published characterization reports.

Ending: County staff and several board members thanked the bureau for the overview and noted the material will be useful when reviewing future subdivisions and water-resource questions.