GEOLOGY OF MUSCATINE ISLAND, MUSCATINE COUNTY, IOWA
R.E. Hansen and W.L. Steinhilber
Iowa Department of Natural Resources, Geological Survey Bureau
Water-Supply Bulletin No. 11, 1977, 60 p.
Prepared cooperatively by the Iowa Geological
Survey, the United States Geological Survey,
and the Board of Water and Light Trustees of the City of Muscatine, Iowa
Muscatine Island is a wide segment of the west bank of the Mississippi River flood plain that covers about 50 square miles in Muscatine and Louisa Counties; the project area encompasses the 30 square miles in Muscatine County. The flood plain is underlain by thick, permeable alluvial deposits that comprise a water-table aquifer that is developed extensively for water supplies in the area. The aquifer consists principally of sand and gravel, interbedded with lenses of silt and clay. Its saturated thickness ranges from about 40 to 140 feet. The transmissivity and storage coefficients of the aquifer range from about 20,000 sq. ft./day and 0.15, respectively, in the western part of the Island to about 39,500 sq. ft./day and 0.24 in the eastern part. The amount of water stored in the aquifer, under normal conditions, is about 100 billion gallons.
Discharge from the aquifer is principally by pumpage, which has increased from about 1 mgd (million gallons per day) in 1906 to about 37 mgd in 1970. About 2.5 mgd is normally lost to seepage and evaotranspiration along a 9-mile reach of Muscatine Slough in Muscatine County. About 0.9 mgd is discharged by evaporation from gravel pits.
Recharge to the aquifer is by induced infiltration from the Mississippi River, seepage from the river during major flood events, precipitation, and seepage from the underlying limestone bedrock. Induced infiltration provides about 80 to 85 percent of the water withdrawn from the principal pumping centers along the river and also replaces about 70 to 80 percent of the water that is evaporated from the gravel pits; this amounted to about 30 mgd in 1971. Additional significant recharge from the river occurs during major floods, when prolonged high stages provide the head for considerable underflow to the aquifer. Recharge from precipitation on the Island was calculated to average about 6 inches per year or about 0.3 mgd per square mile. Seepage from bedrock is significant and is attributed to the increased head differential between the alluvial and bedrock aquifers in the areas of major pumping.
The chemical constituents of water from the aquifer are generally within the recommended limits established by the U.S. Public Health Service for drinking water.
Stresses on the hydrologic system have affected the position and configuration of the water table and the chemical quality of the ground water. The large-scale withdrawals, which began at the principal pumping centers in 1946, have caused the water table to decline about 1 foot in the interior of the Island to about 5 feet near the edges of the main pumping centers; the decline was more than 8 feet under the pumping centers. A slight increase in hardness of water from riverward wells in the pumping centers is attributed to the induced infiltration of slightly harder river water; a noticeable increase in hardness and iron content in water from landward wells is attributed to seepage of water from the bedrock. In the central irrigated area, which is underlain by very permeable, highly drained soils that are mulched with organic fertilizers, the nitrate content of the ground water is as high as 46 mg/1 (milligrams per liter). Land-use practices have had, and probably will continue to have, and impact on the quality and quantity of water available in the system.
The hydrologic system in 1971 was in dynamic equilibrium or in near-equilibrium with the stresses imposed on it to that date. This equilibrium would be disturbed by any additional stresses on the system and water levels would change until a new equilibrium was established. The effects of future stresses can be reasonably predicted by developing a digital model of the system. The data to develop such a model are available in this report. Continued and expanded monitoring of water levels would provide data for better model verification. Periodic monitoring of nitrate and other chemical constituents would permit early detection of changes in concentration before the concentrations reached excessive levels.