PRELIMINARY RESULTS OF A GROUNDWATER RESOURCE INVESTIGATION IN A FRACTURED ROCK AQUIFER, WESTERN MONTANA

The Blue Mountain region, located southwest of the city of Missoula, Montana, is underlain by a fractured bedrock aquifer. The lithology of the aquifer includes argillites, quartzites, and siltites of the Missoula Group of the Precambrian Belt Supergroup. The area is developed with domestic wells and on site sewage disposal sites. This study investigates the hydrologic system of the fractured rock aquifer, including the occurrence, transport, and fate of nitrate and bacteria in the Blue Mountain aquifer. Preliminary work indicates a portion of the aquifer is vulnerable to septic effluent. Combined nitrate and nitrite concentrations collected from monitoring wells in the area range from less than .04mg/L to 15.52mg/L. Our work includes establishing a monitoring well network, creating potentiometric surface maps, obtaining general aqueous chemistry, generating a water balance, and developing a structural map of the region. Final results from this study will be used by Blue Mountain residents and the Missoula City/County Health Department to evaluate future growth and anticipate water needs.