GROUNDWATER RESOURCE ASSESSMENT OF THE SOUTHERN FORT BELKNAP RESERVATION, NORTH-CENTRAL MONTANA

The Nic-Mni Water Center at Aaniiih Nakoda College implemented a water quality assessment program in 2019, focused on alluvial aquifers as a potable water source for the local communities. Potable water is also obtained from bedrock wells. The study area comprises north flowing drainages from the Little Rocky Mountains, an “island” range located in the southern part of the Fort Belknap Reservation in North Central Montana. Precipitation in the mountains, especially winter snowpack, provides recharge to surface and groundwater in the semi-arid plains adjacent to the mountains. The Zortman/Landusky gold mines, utilizing extensive cyanide heap-leach pads, operated from 1979-1998 along the watershed divide in the mountains. Acid mine drainage and cyanide spills contaminated surface and groundwater adjacent to the mines.

Monitoring location comprise wells installed in the alluvial aquifers by the USGS during 1987-1988. Data from the USGS studies provides baseline conditions for comparison with recent results. Water level data was collected with a combination of dataloggers and hand measurements. The water level data shows the connection to stream recharge near the mountain front, with stable water levels in the alluvial aquifers downgradient. Water quality samples were collected in 2019, with wells resampled in 2020. Samples were analyzed for major ions, nutrients and trace metals. Data results show Calcium-Bicarbonate groundwater near the mountain front, with increases in Sodium and Potassium, and Sulfate downgradient from the two primary communities. Water quality continues to decrease downstream, transitioning to a Sodium-Sulfate water type with an increase in dissolved solids. Downgradient water levels show little variation, with the alluvial aquifers disconnected from the streams by a layer of glacial till confining the system in this area. The downgradient water quality changes appear related to bedrock geology in Cretaceous shales bounding the alluvial aquifer. No significant changes to trace element chemistry were detected in the water samples. The results show that alluvial groundwater near the mountains provides a good potable water source. The impacts from mining to local water resources are limited to near the mine; the long-term disposition of the impacted waters are not well understood.