GROUNDWATER QUALITY, HYDROGEOLOGY,
AND AGRICULTURAL DRAINAGE WELLS: FLOYD AND MITCHELL COUNTIES, IOWA

R.D. Libra, D.J. Quade, G.R. Hallberg and J.P. Littke


Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 29, 1994, 64 p.

ABSTRACT


Studies in Iowa, based on geological criteria, have defined areas where the underlying aquifers are susceptible to agriculturally-related contamination. In Floyd and Mitchell counties, stratigraphic relationships within the underlying Devonian System aquifers also influence the potential for groundwater contamination. Part of Floyd County contains a concentration of agricultural drainage wells (ADWs), which are potential contaminant delivery mechanisms. ADWs deliver tile drainage and/or surface runoff into the underlying Devonian carbonate strata. An estimated 92 ADWs are present in Floyd County, draining about 11,500 acres, or 4% of the county. Approximately 60% of the ADWs are less than 100 feet deep, while 4% appear to exceed 250 feet in depth. To evaluate the impacts of ADWs and stratigraphic relationships on groundwater quality, four complete core penetrations of the Devonian aquifers were drilled in Floyd and Mitchell counties. Three of these corehole sites were completed as four-piezometer nests to allow for hydrologic and chemical monitoring of specific aquifer intervals. Private wells located near ADWs were also monitored for ADW-related effects. Other private wells, located in analogous geologic settings but distant from ADWs were monitored as a control. Results of the coring and hydrologic analysis confirmed the Devonian strata form a three-part aquifer system in these counties. Results of hydrologic and water-quality monitoring suggest that ADWs do deliver agricultural contaminants--specifically nitrate and commonly-used corn and soybean herbicides-- to groundwater. Monitoring at a well nest located 500 feet from a 300 foot-deep ADW showed significant ADW-related contamination at some depths, and negligible contamination at others. Some private wells located within 1 to 2 miles of concentrations of ADWs are impacted by the drainage wells, while others are not. The interplay between private well depth and construction, ADW depth, and stratigraphy is the likely cause of this variability. Discernible effects of ADWs are further limited to areas that have a low natural susceptibility to ag-contamination, including "deep bedrock areas" where the upper Devonian aquifer is overlain by greater than 50 feet of relatively low-permeability glacial deposits and/or shales. ADWs that penetrate the middle or lower Devonian aquifers also cause discernible water-quality effects within these units. Tritium age-dating of upper aquifer groundwater from deep bedrock areas, and groundwater from the middle and lower aquifers, indicates pre-1953 groundwater is present in these units in the absence of ADWs. In susceptible areas, the delivery of ag-contaminants by natural processes masks ADW effects. Tritium and nitrate data suggest denitrification may occur within the Devonian aquifers in the Floyd County ADW area.