Manure Management to Protect Water Resources

Methods of raising livestock in Iowa have changed dramatically in recent years. Today, increasing numbers of livestock are raised in concentrated populations within confined environments (see maps above). These operations produce large volumes of manure that are temporarily stored in nearby earthen lagoons or basins (aerial image, below) until applied to farmland as a nutrient source. As a result of this trend in livestock production, animal confinement operators are now required to submit a Manure Management Plan to the Iowa Department of Natural Resources (DNR) for approval. Each plan estimates the volume of manure generated at a particular facility, and proposes how and where the manure will be applied to agricultural land.

Storage of manure in lagoons adjoining large, confined-animal feeding operations, and subsequent application of manure to the land, are environmental concerns in geologically vulnerable areas of the state. (Simulated photo image)

The concentrated populations of livestock, the large volumes of stored manure, and the application of manure to farmland have increased concerns for safeguarding Iowa's water resources from the chemical nutrients and pathogens found in animal manure. These include nitrate, ammonia, and fecal bacteria. Environmental concerns involve the potential for excessive seepage of these contaminants from the lagoons and basins in geologically vulnerable areas. The same concerns apply to excessive concentrations of manure being applied to the land. Geologic vulnerability occurs where porous earth materials allow fluids and water-borne contaminants to move into an aquifer, potentially affecting private and public water wells in the area. Contaminants can also drain into a nearby stream or river. To address these potential contamination problems, site assessments are conducted by the DNR. Geologists review the known hydrologic and geologic conditions at each permitted manure storage structure, as well as the geologic deposits underlying the areas identified for manure application.

A vital tool used for these assessments is Geographic Information System (GIS) technology. State-of-the-art computer applications utilize a wide range of geologic and cultural data layered together in map form, which allows for comparison and analysis at any location. Bringing together accurate, up-to-date information from numerous data sources is an efficient and effective method for evaluating a site’s potential for groundwater contamination.

To conduct a site assessment for a proposed (or existing) manure storage structure, or for land designated for manure application, requires compilation of the site’s GIS "coverages." This is done by combining layers of data such as rivers, state highways, and urban area outlines. Information is initially compiled at a county-wide scale, and later the viewer can "zoom in" on the smaller geographic area of interest.

Once common geographic reference features are compiled into a base map, other relevant data layers, such as sinkholes and public and private wells, are added until the final site coverage is constructed (see map below). Using this site coverage, an assessment can be made on groundwater vulnerability, and on whether other environmentally sensitive areas are located near the manure storage structure or the land application area in question.

GIS MAP OF THE DECORAH AREA (Winneshiek County)
Used to Evaluate Groundwater Protection

Horizontal map distance = 24 miles

Groundwater vulnerability is determined by several factors that can be evaluated using GIS technology. One important variable is the type and depth of groundwater aquifers that underlie a site. There are several types of aquifers in Iowa, and one or more could be present beneath a proposed manure storage structure or manure application area. Two of the most vulnerable geologic settings in Iowa are alluvial and shallow bedrock aquifers. Alluvial aquifers are composed of porous sand and gravel deposits that occur at or near the land’s surface along streams and rivers. They are particularly vulnerable to seepage or spills from manure storage structures, or from land-applied manure because they have little if any natural protection.

Other aquifers that are particularly vulnerable are shallow bedrock aquifers, which are found in several counties in northeast Iowa and at scattered locations elsewhere in the state. These aquifers are composed of fractured limestone and dolomite, carbonate rocks that are subject to dissolution by infiltrating groundwater. Associated sinkholes and springs seen at the land surface are vivid expressions of their shallow, fractured, and easily infiltrated nature. Like alluvial aquifers, shallow bedrock aquifers have little or no protection from surface contaminant sources.

The vulnerability of Iowa’s deeper bedrock or buried alluvial aquifers is dependent on the type and thickness of glacial age materials that overlie them. These glacial materials may be sand, gravel, silt, or pebbly clay, and their horizontal and vertical distribution varies greatly across the state. Understanding the porosity and thickness of these materials is essential to assessing how well protected an underlying aquifer might be.

Another significant factor in evaluating a site involves the location, type, and depth of any nearby drinking water wells that tap a specific aquifer. Physical characteristics of these wells will determine the potential risk to human health posed by a proposed manure management site. Ecologically sensitive areas can also be identified on a site coverage. These may include communities of threatened, rare, or endangered plant or animal species, as well as state and county parks and preserves.

Based on the information found during the site assessment process, recommendations are made regarding needed modifications to the Manure Management Plan, the general suitability of a site for placement of a manure storage structure, or use of the land for manure application. his process is designed to protect Iowa’s valuable water resources through the application of knowledge derived from on-going geologic research in the state.