HYDROGEOLOGY AND WATER QUALITY OF THE UPPER DES MOINES RIVER ALLUVIAL AQUIFER
C. A. Thompson
Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 84-5, 1984, 170 p.
A study of the Des Moines River alluvial aquifer from Boone County north to the Minnesota border was conducted to provide information on water availability and water quality. The study was partially funded through a contract with the U.S. Environmental Protection Agency. The data collection techniques included a seismic refraction survey of the alluvial valley to determine alluvial thicknesses, well-installation and measurement of water-levels, and collection of water samples for chemical analysis.
The alluvial materials which comprise the system were deposited by outwash streams during the melting of the late Wisconsinan glaciers. The outwash materials along the west fork are very coarse, anisotropic, and stratified. They range from matrix-supported pebbly layers to clast-supported cobble layers with occasional sand lenses. Textures in the east fork are finer with more sand being present. The terraces along the lower river show a three-fold stratigraphy of fine and coarse layers described.
Seismic refraction surveys were done during the summer of 1982. Twenty-five miles of seismic lines were run at 38 different locations. The sands and gravels comprising the alluvial aquifer were found to range from less then ten to greater than fifty feet. The thickest parts of the aquifer occurred along the west fork north of Bradgate and along the east fork, south of Irvington. The alluvial valley below the junction of the two forks is characterized by numerous terrace and bench deposits. These are variable in thickness, but are often no more than a thin veneer. The alluvial sands and gravels are, for the most part, underlain by Des Moines Lobe till, but also can be underlain by Mississippian limestone and Pennsylvanian shales, sandstones and limestones.
Recharge to the alluvial system is from precipitation. Interactions between the stream and the aquifer will also affect the water table. Normally the water table slopes from the aquifer to the stream, however, at high stream stage conditions, the stream can contribute water to the aquifer. Water in storage in the alluvium along the west fork north of Bradgate was computed to range from 3 to 74 billion gallons.
Streamflow in the Des Moines varies from year to year and season to season. The drainage basin of the Des Moines is not well-integrated. The hydrographs are characterized by diffuse peaks. Flow-duration data indicate that in the upper ends of the basin streamflow is highly variable and groundwater inflow is negligible and is not capable of maintaining streamflow. Variability decreases downstream as groundwater recharge becomes more significant.
Water levels in wells ranged from two feet above ground level to 24 feet below ground level and averaged about six feet below ground. Water levels in any one well varied by a maximum of 8.5 feet. Horizontal gradients in the alluvial system range from .0007 parallel to the river to .001 perpendicular to the river. The water table generally slopes to the river, but many other configurations were also seen. Vertical gradients were downward at one location and not well-defined at others.
A pumping test was conducted four miles north of Emmetsburg. The well was pumped at 1,000 gallons per minute for 16 hours. Maximum drawdown at the observation well located 60 feet away was 2.5 feet. Transmissivities were calculated to be between 30,000-550,000 gpd/ft with storage coefficients from .005 to 0.2.
Well yields will be highest along the west fork from Bradgate north to Graettinger. Yields suitable for irrigation can be obtained in this area. Yields adequate for rural-domestic and livestock use will be generally obtainable anywhere in the alluvial sections. Exceptions are on high terraces which are not hydraulically connected to the river and areas of extremely thin alluvium. Higher yields may be obtainable from areas where the alluvium is thin or storage is negligible, however, there will be a greater impact on the stream.
Groundwater in the study area is a calcium magnesium bicarbonate type. Total dissolved solids are generally less than 1,000 mg/L. Information from either past data or from the water-quality inventory showed that 20-30% of the wells have nitrate concentrations greater than the recommended limit of 45 mg/L. Monthly sampling of the monitoring network showed that nitrate does not appear to be infiltrating the groundwater system to any depth. In general, wells open between the water table and 10 feet below the water table show nitrate. There are exceptions where no nitrate is occurring even at the water table. These appear to be regulated, in part, by soil type which may hinder infiltration and promote denitrification. Coliform bacteria also appears prevalent in the groundwater, occurring in over 50% of the samples.
The largest amount of water presently allocated is for irrigation, followed by municipal, rural-domestic and livestock. Adequate water will be available during most seasons to meet current needs and to support some increases. During drought conditions, irrigation water may have more of an impact on stream levels due to more frequent usage and lower storage capability in the aquifer.