Evaluation of drawdown and sources of water in the Mississippi River alluvium caused by hypothetical pumping, Muscatine, Iowa

A study was conducted to evaluate drawdown and volumetric changes in sources of water in the Mississippi River alluvium caused by hypothetical pumping. A steady-state, ground-water flow model was constructed for a previous study to simulate February 1993 hydrologic conditions, which were assumed to be an acceptable estimate of the ground-water system at equilibrium. The flow model was modified for this study to simulate six hypothetical pumping scenarios: five pumping scenarios to simulate hypothetical pumping at five pumping scenario sites, and a total pumping scenario to simulate cumulative hypothetical pumping from the five pumping scenario sites.

The evaluation of drawdown for the six hypothetical pumping scenarios indicates that hypothetical pumping causes simulated drawdown that varies from about 10 ft to greater than 50 ft relative to February 1993 conditions at the five hypothetical pumping scenario sites. The simulated drawdown is less than half of the estimated saturated thickness of the alluvium during February 1993 at these sites.

The primary sources of water (inflows) to the alluvium needed to balance the increased ground-water withdrawals (outflows) caused by hypothetical pumping are a combination of increased river leakage and decreased leakage to Muscatine Slough. Compared to February 1993 conditions, larger inflow rates occur as river leakage from the Mississippi River for the six hypothetical pumping scenarios. However, smaller outflow rates to Muscatine Slough compared to February 1993 conditions indicate that an important source of water for hypothetical pumping is ground-water discharge that would have become streamflow in the slough.

Increased pumping at the hypothetical pumping scenario sites could affect long-term water quality and hydrology in the study area. The greater amounts of river leakage might affect overall ground-water quality in the alluvium. The lesser amounts of ground water being discharged to streamflow could have a long-term impact on the hydrology of the slough and adjacent wetland areas.

The simplified steady-state flow model does not account for dynamic (transient) conditions (natural or development-related). The steady-state model does not indicate time needed to reach new equilibrium conditions. Attaining equilibrium might take many years and is complicated by varying climatic and hydrologic conditions; noncontinuous pumping and pumping that is cycled among well fields; and changing and seasonally varying irrigation pumpage.