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A three-dimensional, ground-water-flow model was developed to gain an improved understanding of the ground-water-flow system and its response to withdrawals near the Minneapolis Water Works in Fridley, Minnesota. Eight hydrogeologic units are represented in the ground-water-flow model. Aquifers represented are the unconfined-drift, confined-drift, St. Peter, and Prairie du Chien-Jordan. Confining units represented are the upper drift, basal-drift, Decorah-Platteville-Glenwood, and basal St. Peter confining units. The ground-water-flow model was calibrated for steady-state conditions fbr a period before substantial ground-water development (1885-1930) and for a period of significant pumping stress (winter conditions, 1970-79). The principle of superposition was used in the steady-state simulation for 197079. Transient conditions were simulated for an aquifer test conducted at the Minneapolis Water Works site and for seasonal variations in ground-water withdrawals resulting in seasonal fluctuations of hydraulic heads of as much as about 45 ft. Sensitivity analysis indicated that hydraulic heads in the confined-drift and St. Peter aquifers and Prairie du Chien-Jordan aquifer were most affected by varying the vertical hydraulic conductivity of the upper drift confining unit and recharge to the confined-drift and St. Peter aquifers. Spatially variable leakage to the confined-drift and St. Peter aquifers in the steady-state simulation for 1885-1930 ranged from 1.0 to 2.3 inches per year. Leakage to the confined-drift and St. Peter aquifers in the steady-state simulation for 1970-79 increased 0 to 3.0 inches per year above the initial steady-state results. This increase represents additional leakage caused by the lowering of hydraulic heads due to ground-water withdrawals. Simulated leakage to the confined-drift and St. Peter aquifers for the transient simulation for 1987 varied both seasonally (0.4 to 2.1 inches per stress period) and spatially (2.6 to 5.7 inches per year). The calibrated transient simulation for 1987 was used to determine the effects of hypothetical ground-water withdrawals near the Minneapolis Water Works under transient conditions. A simulation assuming total additional ground-water withdrawals of 27.5 million gallons per day, during late summer (July, August, and September), from the Prairie du Chien-Jordan aquifer in four model cells resulted in a maximum increased drawdown of about 80 feet in the Prairie du Chien-Jordan aquifer and about 60 feet in the confined-drift and St. Peter aquifers at the end of September. Hydraulic heads rebounded following the cessation of the hypothetical withdrawals and at the end of the 1-year simulation period, the heads were only about 2.5 feet lower than they were at the beginning of the simulation. The water supplying the additional ground-water withdrawals was derived mostly from (1) changes in leakage from hypothetical between the Mississippi River and the unconfiried percent), (2) increased ground-water inflow boundaries (about 34 percent), and (3) water aquifers (about 29 percent). A 5-year was done to determine if a cumulative drawdown occur. The seasonal recharge and ground-water the 1-year hypothetical simulation were cycled confined-drift and St. Peter aquifers and the at the end of late summer (September) and the ber) were about the same during the fifth year after only 1 year. A hypothetical transient tional ground-water withdrawals of 55 million summer from the Prairie du Chien-Jordan aquif in a maximum increase in drawdown of about 13C Jordan aquifer and about 105 feet in the confi at the end of September. The calibrated simulation for 1970-79, which is based on the principle of* superposition, was used to determine possible ground-water withdrawals near the Minneapolis -drift aquifer (about 34 areas beyond the model released from storage in the transient simulation L in hydraulic heads would withdrawal rates used in five times. Drawdowns in the Prairie du Chien-Jordan aquifer end of the annual cycle (Decemof simulation as they were imulation, assuming total addigallons per day during late in eight model cells, resulted feet in the Prairie du Chienned-drift and St. Peter aquifers er long-term effects of increased Water Works assuming steady-state conditions. A hypothetical steady-state simulation with additional ground-water withdrawals totaling 14.0 million gallons per day from the Prairie du ChienJordan aquifer in four model cells resulted in a maximum increase in drawdown in the aquifer of about 70 feet. The water supplying the additional groundwater withdrawals was derived from (1) leakage of water from the Mississippi River to the aquifer system and (or) reduction in the discharge of water from the aquifer system to the river (about 81 percent), and (2) increased groundwater inflow from areas beyond the model boundaries (about 19 percent). Contaminated water from areas of known depressions in the potentiometrie surfaces of aquifers and Prairie du Chien-Jordan aquifer withdrawn near the Minneapolis Water Works, beneath the Minneapolis Water Works and confining unit create the potential for the from the surficial sand and gravel deposits to if contamination could move toward the confined-drift and St. Peter additional ground water were presence of a bedrock valley in the upper drift downward movement of contaminants the underlying aquifers. discontinuities

Simulation of ground-water flow in the Prairie du Chien-Jordan and overlying aquifers near the Mississippi River, Fridley, Minnesota