Earthen Manure-Storage System Monitoring
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Iowa is the leading hog producing state in the United States. The total number of hogs marketed in Iowa has grown steadily from 20 million head in 1970 to 25.5 million head in 1992. While the total hogs marketed in Iowa has increased in recent years, the number of farmers raising hogs has decreased. Since 1970, the number of farms with hogs declined by over 60% while the average number of hogs per farm per year increased from 180 (1972) to 469 (1992). In 1965, 63% of the farms in Iowa reported raising hogs compared to 34% in 1992. This increased concentration of hogs per farm in Iowa signals a move to raising hogs in larger-scale confinement operations. There has been a growing concern over the expansion of large hog confinement operations in Iowa during the past several years. Concerns range from socio-economic to environmental, with the most often voiced concern being odor followed by potential surface water and groundwater contamination.
In response to potential groundwater contamination concerns from earthen manure storage facilities associated with confinement operations, the Geological Survey Bureau is monitoring shallow groundwater at three earthen lagoon sites across Iowa in Hancock, Linn, and Pottawattamie counties (location map). These locations were chosen to represent the different geologic regions of the state. If an earthen lagoon leaks, the geology of the area may either enhance or limit the potential movement of contaminants to shallow groundwater and possibly to nearby private water supplies.
The site in Hancock County is located on the Des Moines Lobe in north-central Iowa, the most recently glaciated area of the state (12,000-14,500 years ago). The area is blanketed by a relatively unweathered clay-rich glacial till ranging from 50 to 150 feet thick. In this area, poor drainage characteristics coupled with the presence of few, deep vertical fractures in the glacial till should limit the movement of contaminants to shallow groundwater. Near the margins of the Des Moines Lobe are hummocky areas, known as moraines. The deposits associated with hummocky topography are quite variable and chaotic; some knobs and ridges are sources of significant sand and gravel deposits while others are mantled by a sandy till with a core of clay-rich till. Also, many of the depressions linking this hummocky terrain are underlain with sand and gravel. In these areas, there is greater potential for leakage to shallow groundwater. Many private drinking water wells on the Des Moines Lobe are "protected" by a thick cover of glacial till over bedrock aquifers, however, others are in shallow sand and gravel lenses within the glacial till and may be susceptible to shallow groundwater contamination.
The Linn County site is located on the Iowan Erosion Surface. This landscape consists of weathered, deeply fractured older glacial tills (600,000 - 2 million years old) that are mantled by a variable thickness of loess (windblown silt). Drainage in this landscape ranges from poorly to well drained. Depth to bedrock is quite variable ranging from bedrock exposed at the surface to a depth of 400 feet. The majority of private drinking water wells in this area are drilled in fractured, carbonate bedrock to a depth in excess of 100 feet. Previous water-quality studies have identified regions in this area that have very little surficial cover over fractured rock. These karst (sinkhole) and shallow bedrock areas have a greater susceptibility to groundwater contamination.
The Pottawattamie County site is located in the Loess Hills region of western Iowa. The lagoon site is situated along a small stream valley underlain by silty alluvial (floodplain) deposits. The Loess Hills region is characterized by thick loess deposits blanketing older weathered and deeply fractured glacial till deposits. Depth to bedrock ranges from 150 to 200 feet. Many private water supplies in this region are commonly less than 50 feet deep and may utilize the "seepage well" design used in the Southern Iowa Drift Plain. Previous water-quality studies from western and southern Iowa have identified pervasive contamination of seepage ("water table") wells from agricultural contaminants.
Each lagoon site has been instrumented with three to seven small-diameter, shallow monitoring wells placed into saturated sediments to varying depths. Well depths range from ten to thirty feet and all wells are completed in unconsolidated geologic materials (i.e., windblown loess, glacial till, alluvium (floodplain) sediments). Wells are located uphill and downhill of the earthen manure lagoon. One is located in the berm of the lagoon to provide early detection of any leak that may occur. They are monitored monthly for a variety of chemical analyses, including nitrogen, chloride, and fecal coliform bacteria. Changes in the chemistry of the water from the wells from month to month will indicate whether animal waste is leaking from the lagoon into shallow groundwater. Monitoring will continue for three years.
Through this study we hope to better understand how effectively these earthen lagoons are containing manure in different geologic and hydrogeologic settings in Iowa. Results from this study will give an indication of the performance of lagoons in each geologic setting, however, these lagoons should not be considered representative of all lagoons located in each respective geologic region. A need still exists for collecting site specific information.