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USGS1996

Movement of agricultural chemicals between surface water and ground water, lower Cedar River basin, Iowa

Movement of agricultural chemicals alachlor, atrazine, cyanazine, deethylatrazine, deisopropylatrazine, and metolachlor between surface water and ground water is documented by data collected from May 1989 through July 1991 at an unfarmed study site adjacent to the Cedar River in Iowa. During periods of runoff, these chemicals moved from the Cedar River into the alluvial aquifer with bank-storage water. Results of simulation of ground-water flow conditions during March-April 1990 indicated that bank-storage water moving through the river bottom accounted for 70 percent of the total bank-storage water, whereas the remaining 30 percent moved through the riverbank. The largest concentrations of the chemicals in bank-storage water during 1990 were: alachlor, 2.1 micrograms per liter ( μ g/L); atrazine, 4.7 μ g/L; cyanazine, 3.2 μ g/L; deethylatrazine, 0.54 μ g/L; deisopropylatrazine, 0.33 μ g/L; and metolachlor, 2.2 μ g/L. Larger concentrations of some herbicides and their metabolites were detected in the ground water after the study site was inundated by floodwater between June and August 1990. The concentrations in a water sample from one well after this flooding on February 5, 1991, were: alachlor, 0.06 μ g/L; atrazine, 18 μ g/L; cyanazine, 1.3 μ g/L; deethylatrazine, 1.4 μ g/L; deisopropylatrazine, 0.40 μ g/L; and metolachlor, 7.0 μ g/L. During base-flow conditions, the movement of agricultural chemicals from ground water to surface water was quantified for two periods of time in 1989 and 1990 along a 117-kilometer reach of the Cedar River. The principal source of atrazine, deethylatrazine, deisopropylatrazine, and metolachlor in the Cedar River during base flow was ground water discharged directly from the alluvial aquifer adjacent to the Cedar River. This discharge exceeded the combined tributary inflow of the chemicals along the entire reach. Bank storage is probably an important source of agricultural chemicals discharged from the alluvial aquifer but becomes depleted with time after surface runoff. Herbicides discharged from the alluvial aquifer during periods of extended base flow entered the alluvial aquifer with ground-water recharge at some distance from the river. The movement of nitrate between surface water and ground water is minor, when compared to the herbicides, even though nitrite was detected in the Cedar River during runoff.

Iowa, Minnesota↗

USGS1995

Hydrogeology and water quality of the Mississippi River alluvium near Muscatine, Iowa, June 1992 through June 1994

A study of the Mississippi River alluvium near Muscatine, Iowa, was conducted to evaluate ground-water flow and water quality using data collected from June 1992 through June 1994. The study area included approximately 80 square miles in parts of Muscatine and Louisa Counties in Iowa and Rock Island and Mercer Counties in Illinois. A steady-state, ground-water flow model was constructed using February 1993 hydrologic conditions. Model results indicate that drawdown in the lower alluvium caused by the pumping centers in Iowa extends beneath the Muscatine Slough in the northwest part of the study area and beneath the Mississippi River in the central and northern parts of the area. The primary sources of ground water in the alluvium are recharge from precipitation, leakage from the Mississippi River, and infiltration of upland runoff. The bedrock is not a major contributor of ground water to the alluvium. The areal distribution of selected water-quality properties and constituents in ground water results from several factors. Localized large chloride and nitrite-plus-nitrate nitrogen concentrations could be indicative of contamination from human activity. Specific conductance and calcium, magnesium, and sulfate concentrations are larger in ground water near the boundary between the river valley and upland area and could result from infiltration of upland runoff or lithologic differences in the alluvium. Large iron or manganese concentrations occur in the ground water near the Mississippi River and Muscatine Slough that result from microbial processes and the presence of dissolved organic carbon. Temporal variations of concentrations for selected water-quality constituents in groundwater samples attest to the dynamic nature of the ground-water system as it responds to natural and human-induced changes in water quality. Leakage from the Mississippi River affects ground-water quality in the alluvium adjacent to the river. Temporal variations in water quality of the Mississippi River can be caused by seasonally, amount of discharge, or upstream human activities. The quality of ground water induced from discrete zones of the alluvium by the pumping centers in Iowa has implications for the entire ground-water resource. The ground-water flow model calculated that 10 percent of the water that enters the zone of active pumping on the Iowa side of the Mississippi River originates from the zone west and north of Muscatine Slough, and that 5.2 percent originates from the zone on the Illinois side of the Mississippi River east of the center of the river channel.

Illinois, Iowa↗