USING PRECIPITATION AND GROUNDWATER ISOTOPE DATA TO DELINEATE AND MAP HYDROGEOLOGIC UNITS, RECHARGE SOURCES AND GROUNDWATER FLOW, PLAINS, MONTANA

The Montana Bureau of Mines and Geology (MBMG) has collected groundwater, surface water and precipitation data in Lincoln and Sanders Counties as part of the Ground Water Assessment Program’s groundwater characterization effort (2018-2023). Monthly precipitation was collected in the Plains area between November 2019 and October 2023 (Montana Precipitation Isotope Network). The data include stable water isotope analyses (²H and ¹⁸O) for each month, two tritium analyses (³H) (May 2020-9.8TUs; October 2020 - 3.1TUs) and 2023 winter snowpack data. The snowpack data is a complete snowpack sample collected in March 2023 from a Cabinet Mountain SNOTEL. Four stable-water isotope samples were collected (seasonally) from the Clark Fork River at Plains. Periodic samples were collected at 3 wells and 1 spring. And another 22 wells were sampled once. Five wells were sampled and analyzed for ³H. The data are used to understand recharge sources to the aquifers, groundwater age and connection between aquifers.

Hydrogeologic units include a Precambrian fractured bedrock aquifer that surrounds and underlies a Quaternary alluvial aquifer that includes a shallow (0-35 ft), intermediate (40-180 ft), and deep layer (> 200 ft). The deep layer is exposed along the valley margins, directly above and adjacent to the fractured bedrock aquifer.

Isotope data indicate the bedrock aquifer and the deep alluvial layer are connected with respect to recharge source and age; the tritium data (<.8) indicate recharge older than 1950’s. Isotope composition for these two connected aquifers and the intermediate layer plot between relatively isotopically light winter snowpack (δ¹⁸O, -19‰; δ²H, -143‰) and the Clark Fork River’s heavier composition (δ¹⁸O, -16.5‰, δ²H, -126.5‰). The shallow layer is isotopically identical to the Clark Fork River and has a modern-day groundwater age (5.1 TU).

Groundwater-age differences between the intermediate and deep layers suggest a depositional contact between the deep and shallow-intermediate alluvium. This contact represents partial erosion of older Glacial Lake Missoula flood deposits that were subsequently covered with a younger set of flood deposits. The age of these inferred older flood deposits is unknown.