Submitted to: International Union of Geodesy and Geophysics Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 7/30/1999
Publication Date: N/A
Technical Abstract: The Midwest is the largest and most intensive crop producing region of the USA. It accounts for 60% of the Nation's fertilizer use but comprises only 21% of the land surface. The intense use of agrichemicals has created concern about aquifer contamination. Since 1991, the U.S. Geological Survey and collaborators have been studying the regional occurrence of agrichemicals in shallow aquifers in the Midwest through 303 wells. To understand the relations of land use and soils on concentrations of nitrate in groundwater, detailed land use from aerial photographs and soil survey maps were recently obtained for a 2-km radius surrounding 100 wells penetrating shallow unconsolidated aquifers. The area of irrigated land surrounding a well was found to have a direct relation to nitrate concentrations in groundwater. Irrigation increases recharge accompanied by increased solute transport to shallow aquifers. The relative homogeneity in land use near all wells (by design all wells were surrounded by rowcrops) may have precluded the determination of significant relations with land use factors such as area in corn production. Soil properties significantly related to nitrate concentration include minimum water table depth and factors of water flux rates. Shallow water tables correlate with lower nitrate concentrations. Such areas have lower dissolved oxygen resulting in denitrification. The area of soil with slow water flux was inversely proportional to nitrate concentrations in groundwater, indicating these soils transport water and nitrate at a slower rate. Tight soils also tend to support denitrification and are more likely to be artificially drained, thus diverting nitrate to nearby streams.