Ross
D. Miller
University of Montana
Geology Department
A
computer flow model of the Missoula aquifer was constructed as part of the
initial phase of the EPA Wellhead Protection Program. The goal of the modeling
effort was to develop a regional flow model that would; 1) provide detailed
information on ground water flow directions and velocities, 2) simulate the
effects of recharge to, and discharge from, the aquifer over time, 3) assist in
defining key hydrologic parameters of the aquifer, and 4) provide a tool for
future miu1ilgement decisions regarding the aquifer.
As part of the project, all available water level
data from February 1986 to October, 1989 which were useful to the project were
combined in one database. Monthly water level surface maps were then constructed
for a one-year period (February 1986 to January1987) based on observed water
levels at about 100 1ocations.
Five pump tests were conducted on municipal supply wells
using nearby observation wells and precise water level measuring techniques to
obtain time verses draw down data. Values of transmissivity and hydraulic
conductivity were obtained which are believed to be accurate within plus or
minus 20%.
A computer flow model of the aquifer was constructed which simulated the water
level surfaces observed for the period February 1986 to January 1987. The model
was successfully calibrated using hydraulic conductivity values obtained from
the project pump tests. The model simulated leakage from the Clark Fork River
into the aquifer in the eastern portion of the study area, and return flow from
the aquifer to the river in the western portion of the study area. Outflow from
the aquifer to the Bitterroot River was also simulated. Based on model results,
all approximate ground water budget of inflows to the aquifer and outflows from
the aquifer was developed.