TRACING GROUND-WATER FLOW IN THE MISSOULA VALLEY AQUIFER, SOUTHWEST MONTANA

Ground water from the Missoula Valley aquifer in southwest Montana was analyzed for major ions and environmental tracers (chlorofluorocarbons [CFCs], oxygen-18, tritium and noble gases) to evaluate ground-water quality, sources, and ages. Samples were collected at 10 sites, including two nested shallow-deep well pairs, along two transects of ground-water flow between the Clark Fork (recharge source) and Bitterroot (discharge area) Rivers. The Clark Fork River was also sampled. The ground water has low total dissolved solids (less than 300 mg/L); however, concentrations of sodium, chloride, and nitrate generally increase along the flow path. CFC concentrations in most samples exceeded air-water solubility, rendering the samples unsuitable for age dating. Septic effluent appears to be the source of the excess CFCs; CFC-112 concentrations were 3 to 10 times greater in sewered than in unsewered areas. A seasonal d¹⁸O signal was detected in the Clark Fork River, the main source of aquifer recharge, with values relatively enriched (-16.5 to -17 ‰) during the late summer and fall, and depleted (-17 to -17.80 ‰) during the winter and spring. The seasonal signal in the river appears to be transmitted through the ground-water flow system; synoptic ground-water sampling revealed a 0.54 ‰ average enrichment of d¹⁸O values in the aquifer between June 1999 (runoff period) and March 2000 (base-flow period). All apparent tritium/helium-3 ages were less than 5 years, and most (7 of 12) were less than 2 years. In general, water age increased down flow path. Based on the age dates, ground-water flow velocities are between 7 and 18 feet/day. Bulk hydraulic conductivity values (900 to 2,300 feet per day) determined from the age dating are similar to values obtained from conventional aquifer tests. Helium-4 was present in surprisingly large concentrations given the young age of the water, and distributed in a pattern opposite of that expected based on flow path trends, with higher concentrations in the up-gradient part of the aquifer.