WATER RESOURCES OF THE OCHEYEDAN-LITTLE SIOUX ALLUVIAL AQUIFER
C. A. Thompson
Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 86-3, December 1986, 90 p.
A study of the alluvial aquifers of the Ocheyedan and Little Sioux River valleys from the Minnesota border south to the Woodbury-Monona County line was conducted to provide information on water availability and water quality.
The geologic history of the alluvial valleys is complex. A major change in the drainage area and course direction of the Ocheyedan and Little Sioux rivers occurred as a result of disruption by Wisconsin glaciations.
Seismic refraction surveys were used to define the thickness and extent of the alluvial deposits within these valleys. Nineteen traverses covering a total of 25 miles were done during the summers of 1983 and 1984. The alluvial valley floors are commonly wide and flat. The thickness of the alluvial deposits ranges from less than 10 feet (3.0 meters) to 90 feet (27 meters) and averages about 25 feet (7.5 meters). Characteristically, the alluvial materials consist of highly stratified, fine- to coarse-gravel units with occasional fine-sand lenses. Cut-and-fill sequences are common. On late Wisconsinan terraces the alluvial sequence is often capped by a coarse gravel unit. Older terraces along the Little Sioux are loess-mantled and the fluvial deposits are fine textured. Often these terraces are not hydraulically connected to the present alluvial aquifer.
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 streams, supplying as much as 70% 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 low flows are expected to recur frequently on the Ocheyedan and Little Sioux rivers, particularly in the upper reaches.
Transmissivities calculated from pumping test data range from 30,000 to 800,000 gpd/ft. In most of the Ocheyedan Valley, transmissivities of 200,000 gpd/ft are not uncommon. Only small drawdowns occur because of the highly transmissive nature of the aquifer. Water in storage in the Ocheyedan alluvial system is estimated to be at least 2.4 billion gallons.
Water levels were measured monthly and ranged from one foot above ground level to 18 feet below ground level. Most wells varied an average of five feet during the course of this study. Water table gradients are low, varying between 0.001 (5 ft/mi) to 0.005 (26 ft/mi). Vertical gradients are generally low, being within measurement error of zero. Two wells, however, show strong upward gradients of 0.01 to 0.14. Gradients in the aquifer appear to control the migration of contaminants.
A total of 31 observation wells were installed in the Ocheyedan-Little Sioux alluvial system. These were sampled monthly for nitrate and bacteria with a few wells being analyzed for pesticides. The groundwater can be classified as slightly alkaline freshwater with calcium and magnesium the dominant cations and bicarbonate the major anion. The results of the nitrate monitoring have shown that although nitrate levels are not excessively high, extensive areal contamination has occurred. Nitrate levels vary temporally and generally increase in response to increased infiltration. Pronounced vertical stratification of nitrate has been found, with nitrate levels decreasing with depth. Highest concentrations are generally found in the upper 10 feet (3.0 meters) of the saturated zone. Tritium dating has shown that this stratification may be age-related with older water in the lower zones. However, even in these lower zones tritium is present, indicating at least some of the water must be less than 30 years old.
There are some locations where nitrate is consistently not detected. Denitrification is suspected as the mechanism for nitrate losses in these areas. Denitrification is the transformation of nitrate to nitrogen gas which is then lost to the atmosphere. The greatest potential for denitrification occurs in oxygen deficient, water-saturated soil with an available supply of biodegradable carbon. Conditions favorable for denitrification exist at some places in the alluvium.
High bacteria levels were seen in almost all wells sampled. Much of this bacterial contamination may result from leakage or contamination introduced during sampling. Fecal coliform contamination at one well suggests that some migration of bacteria through the aquifer can occur.
Five different pesticide compounds were detected. Highest concentrations were found in surface waters, although all concentrations detected were below acute toxicity levels. Most pesticides were detected in the early summer months, after field application. Atrazine was the only compound detected during fall sampling.
The largest amount of water presently allocated is for irrigation followed by municipal, rural water system, livestock, and rural-domestic use. Adequate water is available during most seasons to meet current needs and to support projected future increases. Further degradation in water quality could limit certain uses of this water resource.