D. R. Bruner and G.R. Hallberg

Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 87-3, 1987, 36 p.


Geologic and groundwater quality information were reviewed by the Iowa Department of Natural Resources-Geological Survey Bureau (IDNR-GSB) in a cooperative agreement with the United States Department of Agriculture-Soil Conservation Service (USDA-SCS) as part of an effort by the USDA-SCS to develop an integrated soil and water conservation plan for the Skunk River basin. The basin covers all or part of 21 counties, from north-central Iowa to the Mississippi River in the southeast corner of Iowa. The bedrock and Quaternary geology are variable across the basin. Unlike other portions of Iowa, there is not a continuous bedrock aquifer through the region that produces adequate quantities of good quality groundwater. Hence, adequate quantities of good quality groundwater are deficient in much of the basin. Deeper aquifer sources in the basin are often highly mineralized and some exhibit natural quality problems with radionucleides.

The population, in much of the Skunk River basin, relies on shallow groundwater sources for their drinking water supplies; 50% of all private wells analyzed in the basin were less than 50 feet deep. Deeper sources are generally unavailable to many people in the basin. Many public water supplies utilize surface water for all or part of their drinking water supplies. This dependence on shallow groundwater and/or surface water makes the southern portions of the basin, in particular, vulnerable to increasing water contamination problems.

Nitrate analyses from private water supplies were reviewed to determine if contamination of the groundwater supplies has occurred or could occur. These data showed a general trend of decreasing nitrate concentration in the groundwater with increasing well depth, as has been shown in other areas of Iowa. Wells in the Skunk River basin that were less than 100 feet deep had significantly higher nitrate concentrations than deeper wells. 'Shallow Bedrock' areas (areas with less than 50 feet of glacial materials covering bedrock) and 'Deep Bedrock' areas both showed the general trend of decreasing nitrate concentrations with depth. However, the data from Shallow Bedrock areas show (statistically) significantly greater nitrate concentrations than Deep Bedrock areas, especially in the shallower wells. While data are limited, pesticide residues are also present in shallow wells, coexisting with high nitrate concentrations.

While the median nitrate concentrations on the shallow groundwater are lower than in northeast Iowa, the percentage of wells over the recommended drinking water standard are about the same; 17%. Water quality is much more variable than in northeast Iowa, likely because of the differences in the geology, soils, and agricultural land-use between the two areas.