Title

A Computer Flow Model of the Missoula Aquifer

Abstract

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 January 1987) 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.

Start Date

20-4-1990 9:20 AM

End Date

20-4-1990 9:40 AM

Document Type

Presentation

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Apr 20th, 9:20 AM Apr 20th, 9:40 AM

A Computer Flow Model of the Missoula Aquifer

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 January 1987) 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.