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Hydrogeology and ground-water flow of the drift and Platteville aquifer system, St Louis Park, Minnesota

Water-Resources Investigations Report 94-4204

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Abstract

Three aquifers and two confining units have been delineated within the drift underlying the area near the site of a former coal-tar distillation and wood-preserving plant in St. Louis Park, Minnesota. The hydrogeologic units of the drift, in descending order, are the upper drift aquifer, the upper drift confining unit, the middle drift aquifer, the lower drift confining unit. and the lower drift aquifer. A contamination plume consisting of coal-tar derivatives exists in the drift aquifers and in the Platteville aquifer underlying the southern part of the plant site and areas to the south and east of the plant site. The upper drift aquifer has a maximum saturated thickness of about 25 feet. Horizontal hydraulic conductivities of the upper drift aquifer range from less than 1 to about 25 feet per day in peat areas and from about 50 to 400 feet per day in sand and gravel areas. The upper drift confining unit generally is less than 20 feet thick, with a maximum thickness of 62 feet. The saturated thickness of the middle drift aquifer generally is 20 to 30 feet in areas where the aquifer is both overlain and underlain by a confining unit. The horizontal hydraulic conductivity of the middle drift aquifer ranges from about 50 to 500 feet per day. The lower drift confining unit is as much as 50 feet thick. Model-computed vertical hydraulic conductivities for the upper and lower drift confining units ranged from 0.0002 to 5 feet per day. The lower drift aquifer consists of discontinuous sand and gravel deposits overlying Platteville Formation bedrock and has a maximum thickness of 20 feet where it is overlain by the lower drift confining unit. Water in the drift aquifers and in the Platteville aquifer generally flows from the northwest to the southeast under a hydraulic gradient of about 10 feet per mile. The drift confining units and the Glenwood confining unit. when present, control the vertical movement of water through the aquifers. Discontinuities in these confining units greatly influence patterns of ground-water flow. A numerical cross-sectional ground-water-flow model was used to test concepts of flow of ground water through the drift aquifers and the Platteville aquifer. particularly the effects of confining units and bedrock valleys on vertical flow. The model has eight layers representing, in descending order: ( 1) the upper drift aquifer. (2) the upper drift confining unit, (3) the middle drift aquifer, (4) the upper part of the lower drift confining unit, (5) the lower part of the lower drift confining unit and lower drift aquifer, (6) the Platteville aquifer and bedrock valley deposits, (7) the St. Peter aquifer, and (8) the Prairie du Chien-Jordan aquifer. A sensitivity analysis indicated that model-calculated hydraulic heads in the drift aquifers and in the Platteville aquifer were most sensitive to variations in: (1) the horizontal hydraulic conductivities of the middle drift aquifer, (2) the transmissivities of the Platteville and St. Peter aquifers, (3) the vertical hydraulic conductivities of the lower drift confining unit and the drift material filling the bedrock valley, and (4) the vertical hydraulic conductivity of the basal St. Peter confining unit. The model-calculated water budget indicated that recharge from infiltration of precipitation to the upper and middle drift aquifers and the upper drift confining unit accounts for about 41 percent of the total sources of water. The remaining 59 percent is from subsurface inflow from the west (through specified-head cells). About 70 percent of the outflow from the eastern model boundary was simulated as discharge from the model layers representing the Platteville aquifer and bedrock valley deposits and the St. Peter aquifer. The calibrated simulation indicated that about 99 percent of the total leakage of water from the drift aquifers and from the Platteville aquifer to the underlying St. Peter aquifer occurs through areas where the Glenwood confining unit is abs

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Hydrogeology and ground-water flow of the drift and Platteville aquifer system, St Louis Park, Minnesota
Series title:
Water-Resources Investigations Report
Series number:
94-4204
Edition:
-
Year Published:
1995
Language:
ENGLISH
Publisher:
U.S. Geological Survey ; U.S.G.S. Earth Science Information Center, Open-File Reports Section [distributor],
Description:
vi, 79 p. :ill., maps ;28 cm.