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 1970- 79. 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).
