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Earthen Waste Storage Structures (EWSS) that store waste from animal feeding operations have raised serious concerns about ground-water and surface-water contamination risks. Thirty-four of 639 permitted EWSS in Iowa were investigated to characterize their hydrogeologic setting. Sites were selected to proportionally represent five aquifer vulnerability regions of Iowa. Data used in the analysis included digital-soils data from the Natural Resources Conservation Service, topographic data from the U.S. Geological Survey, and oblique aerial photographs taken at 1,000 feet (ft) altitude.
Nearly 18 percent of the 34 selected sites were constructed over alluvial aquifers. Contaminants reaching these aquifers could affect many water supplies in the State. Sites located on alluvial aquifers also lie in flood plains with a continual risk of flooding and contamination of surface water from manure application and structure failure. High and fluctuating water tables associated with floods may compromise EWSS liners increasing the risk of failure. Large areas within 2 miles of most sites have soils with a saturated permeability of > 1 inch per hour (in/hr). These areas also include substantial well-drained soils or moderately- to well-drained soils. The dominance of EWSS depths exceeding 10 ft and areas with water tables less than 5 ft deep, suggests that most sites are below the water table. The frequency of sites with a combination of these indicators of contaminant movement indicates EWSS expose ground water to a substantial risk of contamination. Ephemeral streams were found within 500 ft at 21 percent of the sites, and perennial streams were found within 500 ft at 12 percent of the sites. One site had been built by impounding the valley of a small ephemeral stream, and one was immediately upstream from a major aquatic recreation area. Many sites had unmapped drainageways that led from the EWSS to ephemeral or perennial streams.
Reduction of risks to ground-water and surface-water resources by EWSS may be attained by using siting criteria that incorporate geologic, hydrogeologic, and soils data as outlined in this paper. EWSS sites built on alluvial aquifers should not be permitted unless measures are taken to ensure that the aquifer is not being contaminated. Controlling the timing of manure application and avoiding manure application on frequently flooded soils, such as those on flood plains, may reduce the risk of contamination of ground water and surface water. Application of well established, scientifically defensible ground-water-monitoring techniques should be used to locate the position of the water table during construction and throughout the life of the EWSS. These techniques may help identify whether the recommended hydraulic separation between the EWSS and the water table will be maintained. In many instances, a shallow water table should preclude siting of an EWSS. Setback distances from surface-water courses should be based on local hydrogeologic and topographic conditions. These considerations, used with appropriate construction designs, would reduce the potential for contamination of surface water resulting from seepage, overflow, or failure of EWSS. Uniform setback distances may not be appropriate for all topographic, hydrogeologic, and ecologic settings.
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1U.S. Department of Agriculture, Agricultural
Research Service,
National Soil Tilth Laboratory, 2150 Pamuel Drive, Ames, IA 50011
(burkart@nstl.gov
2Iowa State University, Department of Geological and Atmospheric
Sciences (bsimp@iastate.edu)
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