146-9 Water Quality Impacts to Fort Belknap Indian Reservation Groundwater Resources from Landusky Mine in Little Rocky Mountains, North-Central Montana, 20 Years after Mine Closure

Session: Contaminants Near Groundwater-Surface Water Interfaces (Posters)

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Abstract:

The Landusky Mine operated in the Little Rocky Mountains of North-Central Montana from 1979-1988 utilizing cyanide heap-leach pads. Extensive water quality impacts are documented from mine activities. This project reports activities to identify a potable water source for tribal powwow grounds, in an adjacent mountain valley, on the Fort Belknap Indian Reservation. The research program includes surface water quality and flow monitoring, and the installation of two groundwater well clusters in the powwow grounds.  The ongoing study began in 2021 and demonstrates significant changes in water quality from data presented in a 1993 USGS reports on the area.  At King Spring, a former drinking water source for the community, a TDS from June 1990 was 313 mg/L and Calcium-Bicarbonate water type. Data from 3 recent samples show variable TDS near 1000 mg/L and Calcium-Sulfate water type.  Stream water quality in South Bighorn Creek flowing onto the reservation from the known area of mine impacted water, adjacent to King Spring, shows a similar Calcium-Sulfate water type with variable TDS up to 2830 mg/L with Fluoride up to 8.4 mg/L.  Stable stream temperatures around 8°C indicate these waters are from groundwater.  Variability in water quality is attributed to dilution of waters from snowmelt during spring/early summer when TDS is generally lower than late summer/fall dry conditions, and reflecting different annual conditions.  Piper/Stiff diagrams show these two waters have different water quality types. All of these waters infiltrate into the subsurface upgradient from the tribal powwow grounds.  The downgradient monitoring well clusters include shallow wells screened at the bedrock/alluvium interface, and deeper wells into the Paleozoic bedrock unit.  Data from June 2025 indicate that the shallow wells have Calcium-Sulfate water with TDS around 1000 mg/L, similar to King Spring.  The deep wells show different water types, with the upgradient well being Sodium-Sulfate with TDS at 1280 mg/L, and the downgradient well Calcium-Bicarbonate with TDS at 288 mg/L, similar to a sample from 1990.  The mine hydrology conceptual model infers two shear zones provide primary groundwater flow conduits through the area.  The current data indicate slower, more diffuse contaminated regional groundwater flow from the mining area onto tribal lands.  These impacts may prohibit developing a potable water source in this area. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10981

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