WATER RESOURCES OF THE ROCK RIVER ALLUVIAL AQUIFER C. A. Thompson Iowa Department of Natural Resources, Geological Survey Bureau, Open File Report 87-1, 1987, 109 p. ABSTRACT A study of the alluvial aquifer of the Rock River valley from the Minnesota border to its intersection with the Big Sioux valley was conducted to provide information on water availability and water quality. The river valley was formed by meltwater from the late Wisconsinan glaciation. The alluvial valley is wide and flat, and flanked by non-continuous terraces. The thickness of alluvial deposits ranges from seven to 87 feet, but averages only about 20 feet. The alluvial materials consist of coarse sands and gravel. Occasional finer sand underlies the coarser material. Recharge to the alluvial system occurs primarily from infiltration of precipitation. Most recharge occurs during the early spring and fall. In summer, evapotranspiration losses exceed precipitation, and groundwater levels usually decline. During most of the year, alluvial groundwater discharges to the stream, supplying as much as 70 percent of annual stream flow. As groundwater levels decline, flow to the stream diminishes and stream levels fall. Flow-duration and low-flow data show that moderately low flows are expected to recur frequently on the Rock River. Transmissivities in the aquifer range from 95,000 to 400,000 gallons per day per foot. Water in storage in the Rock River alluvial system is estimated to be at least 3.2 billion gallons. Water levels were measured monthly and ranged from 0.5 feet above ground level to 13 feet below ground level. Water levels varied an average of five feet during the course of the study. Water table gradients are low ranging from .001 (5 ft/mi) to .0018 (9.5 ft/mi). Both strong downward (.01-.07) and upward (.006-.38) gradients were observed at the nested well sets. A total of 12 observation wells were installed at eight sites in the Rock River alluvial system. These were sampled monthly for nitrate and bacteria with a few wells being analyzed for pesticides. The groundwater can be classed as slightly alkaline freshwater with calcium and magnesium the dominant cations and bicarbonate the dominant anion. Nitrate concentrations are high and extensive areal contamination has occurred. Nitrate levels vary temporally and generally increase in response to increased infiltration. Higher nitrate levels are found in the Rock River system than in other alluvial systems studied in northwest Iowa. Land use, geology, and chemical processes all combine to effectively increase nitrate inputs to the Rock River alluvial system. High bacteria levels were seen in almost all wells sampled. Much of this bacterial contamination may result from leakage along the casing or contamination introduced during sampling. Limited pesticide sampling was done in the Rock River alluvial system. Atrazine was the only compound detected in groundwater. Four pesticides were detected in surfacewater. All concentrations detected are below acute toxicity levels. The largest allocation of water at present is for irrigation, followed by municipal, rural-water system, livestock, and rural domestic uses. Adequate water is available during most seasons to meet current needs and to support projected future increases. Further degradation in water quality could limit use of this water resource.