Siting of Animal Confinement Operations and Manure Application Areas: A GIS Analysis of Geology and Soils
D.J. Quade, E.A. Bettis III, B.E.
Hoyer and R.D. Libra
|Figure 1. Location of the Des Moines Lobe in the Upper Midwest.|
The DML resulted from a lobate extension of the last great continental glacier that advanced into Iowa approximately 14,500 years ago. The DML was selected for mapping because it occupies roughly one-fifth of the states area, is the most productive agricultural region in Iowa, and contains the main concentration of Iowas animal confinement facilities.
|Figure 2. Permitted hog confinement operations by landform region, 1987 through 1997.|
As of December 1997, there were 670 permitted hog confinement operations (HCOs) in Iowa. This number represents less than 4% of hog farms in the state. Approximately 308 of the 670 permitted sites (46%) are located on the DML; 94 of these units (31%) contain more than 4,166 hogs. An estimated 1.5 million hogs reside in permitted sites on the DML, and one-sixth of these (250,000) reside in HCOs with more than 13,333 hogs.
|Figure 3. A cross-section through the stagnant Des Moines Lobe glacier illustrates a glacial karst system. A network of drainage tunnels was present within and at the base of the stagnant glacier. A few of the larger tunnels eventually became major river valleys.|
|Figure 4. A model for the development of present-day Des Moines Lobe topography. Top schematic: The glacial karst system with low-order (smaller) tunnels connecting to high-order (larger) tunnels within and at the base of the stagnant glacier. Bottom schematic: The modern landscape following melting of the glacial ice.|
|Figure 5. Aerial photo showing the
subsurface hydrologic connections between potholes on the
Des Moines Lobe, Story County, Iowa.
Photo by Gary Hightshoe, Iowa State University.
Iowans are concerned about the quality of drinking-water supplies and protection of the states water resources. A better understanding of DML surficial sediments will aid environmentally sound management and informed land-use planning decisions. By identifying the types, properties, and distribution of glacial landforms and their associated sediments, we can better predict contaminant fate and transport in shallow groundwater in this region of the state.
Recent geologic studies on the DML have identified a complex sequence of sediments that is the product of a stagnant wasting glacier. As the ice slowly melted, tunnels within the stagnant glacier functioned as an internal drainage network. Eventually, these tunnels became clogged with stratified deposits of silt, sand and gravel, and were then covered with poorly sorted sediments either melted out from the overlying ice or transported by mudflows and streams on the unstable surface of the disintegrating glacier. Shallow, porous sand and gravel bodies now occupy the former glacial tunnels and function as links between modern-day, semi-closed depressions and successively larger surface drainage routes on the DML. We recognize these subsurface drainage routes as linked drainage areas on the DML.
|Figure 6. Stratigraphic, particle-size and bulk-density data from a core site on the Des Moines Lobe. Properties of two types of glacial till are shown.|
Deposits that accumulated in direct contact with the DML glacier compose the Dows Formation, which consists of two different kinds of glacial till. Sediment deposited beneath or at the base of the glacier is a uniform, massive, dense loam textured, slowly permeable, subglacial till called the Alden Member. This deposit is overlain by the Morgan Member, a less dense, variably textured till formed by mudflows, debris flows, and flowing water at the surface of the ice. The Morgan Member also contains continuous to discontinuous sand, gravel and silt deposits that accumulated in the former glacial karst system draining the stagnant glacier. These porous sand and gravel deposits may function as preferred pathways along which shallow groundwater can move faster than through the adjacent finer-grained tills.
|Figure 7. Illustration of a typical earthen manure-storage structure on the Des Moines Lobe. Environmental concerns include a regionally high water table and the presence of porous sand and gravel bodies within the Morgan Member till that may function as preferential pathways linking shallow groundwater with surface waters.|
Expansion of large-scale HCOs and the concentration of large numbers of hogs and manure in several DML counties have prompted water quality concerns. Reasons for this concern include the regionally high water table, often within 5 feet of the land surface. Current Iowa rules state that the floor of a basin or lagoon must be at least 2 feet above the water table or construction of a slurry wall is necessary. The landscape position of manure-storage structures is also an important factor. A common site for lagoons and basins is downgradient of the confinement buildings. It is not uncommon on the DML to encounter discontinuous subsurface sand and gravel lenses in the upland areas, and more extensive sand and gravel bodies downslope, along former and present drainage channels. These factors must be considered in order to properly site and construct earthen storage units and to assure appropriate application of manure on the land.
|Figure 8. Location of Hancock County with respect to former ice margins on the Des Moines Lobe in Iowa.|
Hancock County was selected for a surficial geology and susceptibility mapping project because it contains most of the diverse landform types and complex sediment packages associated with the DML. In addition, the Iowa Department of Natural Resources, Geological Survey Bureau (GSB) had existing geologic data from previous studies, digitized soil survey information, and high-altitude photo imagery. GSB staff also had worked closely with the soil mappers during field work for the most recent county soil survey.
|Figure 9. Digitized soil survey for a portion of T. 94N., R. 24W. (Twin Lake Township) in Hancock County. Color coding is by soil series.|
|Figure 10. Preliminary susceptibility assessment of the digitized soil survey for a portion of T. 94N., R. 24W. (Twin Lake Township) in Hancock County using existing geologic information and GIS analysis. Map shows a highly susceptible area with pervasive linked drainages in a moderate-relief landform area (Altamont Moraine).|
|Figure 11. Preliminary susceptibility assessment of the digitized soil survey for a portion of T. 94N., R. 25W. (Amsterdam Township) in Hancock County using existing geologic information and GIS analysis. Map shows a moderately susceptible area with many linked drainages in a low-relief landform area.|
The early stages of map construction involve application of existing landscape models to identify regional landscape patterns. This landscape analysis uses comprehensive data sources such as high-altitude imagery, USGS topographic maps, and county-wide digitized soil surveys. Data sources available in digital form enable the implementation of GIS analysis.
|Figure 12. Preliminary surficial geology map of Hancock County derived from existing geologic information and GIS analysis.|
|Figure 13. Preliminary susceptibility map of Hancock County derived from existing geologic information and GIS analysis.|
The latter stages of map construction involve the difficult process of applying the information gained from limited, site-specific, subsurface borings to larger areas of the county for a reasonable map of regional surficial geology. GIS tools are useful for combining data layers and for analyzing regional information into one comprehensive layer and numerous derivative layers. These maps can be powerful management tools for making informed land-use decisions.
Presented as a poster at Managing Manure in Harmony with the Environment and Society, February 10-12, 1998, Iowa State University, Ames, IA.