PRELIMINARY ISOTOPE HYDROLOGY OF WATER RESOURCES OF THE LITTLE ROCKY MOUNTAINS AND FORT BELKNAP RESERVATION, MONTANA.

The Nic?-Mni Water Center at Aaniiih Nakoda College began research on characterizing the hydrology of water resources on the Fort Belknap Indian Reservation. The primary field area comprises north flowing drainages from the Little Rocky Mountains on the southern part of the Reservation. 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. Precipitation in the mountains, especially winter snowpack, provides recharge to surface and groundwater in the semi-arid plains adjacent to the mountains, with winter/spring recharge and more recent summer drought conditions. The research program was designed to collect data of current conditions, used to assess long-term impacts to local water resources from mining while providing baseline data for water security programs for future water supply development. The water isotope program began in 2022 to provide additional data to characterize the local hydrologic system. The sampling program for water isotopes includes composite precipitation samplers designed specifically for this type of sampling. A station was established at the Landusky mine near the top of the Little Rocky Mountains, with another station at Aaniiih Nakoda College in the northern part of the reservation. Data from the precipitation samplers are used to develop local meteoric water lines (LMWL) for the area, for comparison with surface and groundwater data results. Surface and groundwater data generally follow the LMWL for an area, with deviations resulting from processes such as evaporated waters, or mixing of water from different recharge sources. The sampling program includes water isotope samples with surface and groundwater samples collected with Nic-Mni Water Center monitoring programs. Data from the 2022 and 2023 field seasons, considered preliminary with respect to the full study, show LMWL consistent with other studies in Montana. The results support conclusions on surface and ground water connections based on water quality, water levels and streamflow monitoring results, with streams recharging alluvial groundwater systems near the mountain front. The presentation will incorporate date results from the first part of the 2024 field season.