In 1981 arsenic was discovered in ground water beneath Milltown, Montana (Woessner et al., 1982). Subsequent researchers have attempted to determine the source, delineate the extent, and predict the future configuration and concentration of the ground water contamination in the valley. Of particular interest to the residents of Missoula, Montana, is the degree of hydraulic connectedness of the Hellgate Valley Aquifer/Milltown Reservoir Sediments Superfund Site with the Missoula Valley Aquifer and the probability that ground water contamination from the Milltown Reservoir Sediments Superfund Site may migrate into the Missoula Valley Aquifer.

I collected 397 gravity stations distributed about the valley in an attempt to build a model of the 3-D configuration of bedrock in the Hellgate Canyon and Bandmann Flats area to use as the lower boundary of the Hellgate Valley Aquifer in a future ground water flow model. The final 3-D model is most likely accurate to ± 5 meters. Where we can accurately determine the correlation, the standard error of the depth estimate from the final model is 5 meters.

My final result, a 3-D model of the bedrock clearly shows a bedrock shelf of protuberance toward the north end of valley where the Gestring's (1994) model shows a smooth U-shaped bedrock profile. Gestring's (1994) depth to bedrock model has a larger cross sectional area across the northern portion of the basin than my new 3-D model shows. A reduction in the aquifer cross-section in this area may cause ground water gradients to increase locally to accommodate ground water through-flow. The new bedrock model should reduce errors in any future ground water model by providing a better 3-D characterization of the lower aquifer boundary and improving estimates of aquifer transmissivities.