INVESTIGATING GROUNDWATER AND SURFACE-WATER INTERACTION USING STABLE WATER ISOTOPES IN THE BITTERRROOT RIVER VALLEY, MONTANA

Evaluation of the stable isotopes of water (oxygen-18 and deuterium) is a valuable tool for hydrogeologic investigations and provides an established method to characterize source waters, determine their relative contributions to recharge, and track groundwater/surface-water interaction. Stable water isotope samples were collected in the Bitterroot River Valley, near Victor, Montana to evaluate groundwater/surface-water interactions.

Results of the analyses of δ¹⁸O and δD were compared to regional and global meteoric water lines to estimate local evaporation lines (LELs) and to identify spatial and temporal variations in the isotopic composition of waters in the study area.

In the intermountain basins of the Western U.S., variations in the isotopic composition of surface waters generally follow a trend of isotopically light snowmelt during spring runoff followed by isotopically heavier baseflow in late summer and early fall. Precipitation events (common in late fall/early winter) can further enrich the isotopic signal in surface waters.

Comparing the isotopic results of this study to the global meteoric water line show the waters in this area are isotopically heavier indicating the effects of continentality and latitude. The analytical results of our local sampling are similar to regional meteoric water lines.

Sampling results from tributary streams to the Bitterroot River indicate more enriched water coming from the east side of the study area than the west. Groundwater isotopic composition suggests that the groundwater is influenced by surface water recharge from eastern and western tributaries. The isotopic composition of groundwater in the floodplain indicate that groundwater is mostly influenced by recharge from the Bitterroot River with groundwaters in the lower elevations of the east valley margin having a slight influence from the Bitterroot River.

The isotopic composition of the irrigation ditches that flow within the valley floodplain reflect a groundwater recharge component as they flow downgradient, whereas, the ditches on the east valley margin above the floodplain, do not. The ditches on the east valley margin have the same isotopic seasonal variations as their Bitterroot River source water.