Iowa State University team finds few problems in a lengthy groundbreaking study of earthen livestock storage systems.
A groundbreaking study of earthen livestock manure storage structures has found few have leakage problems and none pose serious contamination concerns.
In an 18-month study commissioned by the Iowa legislature, a research team of Iowa State University (ISU) agricultural engineers and geologists conducted a statewide study of manure structures. Thirty-four sites were included in the geologic evaluation - 33 in the management and maintenance study, 31 sites in the soil sampling study and 28 sites in the seepage measurement and soil analyses study.
The legislature charged the scientists with studying point and non-point sources of contamination.
Structures Not So Bad In their survey of the earthen structures (14 lagoons and 19 storage basins), the team concluded that seepage is within acceptable ranges. The structures pose little impact on the major surface waters of the state.
The structures studied were built between 1987 and 1994 under the state Department of Natural Resources (DNR) 1987 standards for seepage loss rates. The standards allowed a seepage rate of no more than 1/16-in./day when structures are filled to a depth of 6 ft.
Based on that rule, the team found that most structures meet the standards to which they were built, says Stewart Melvin, ISU agricultural engineer and research team leader. Analyses showed:
* Twelve of 28 sites had seepage rates below the DNR standard;
* One lagoon had a higher rate than the standard; and
* Fifteen sites were about equivalent to the DNR standard.
The major soil sampling effort yielded more than 11,000 soil core samples, says Tom Glanville, ISU associate professor of agricultural engineer. Each 8-ft.-deep sample from outside the lagoon's berm was broken up into 1-ft. samples, looking for concentrations of ammonium nitrogen attached to the soil, ammonium nitrogen in the soil runoff, nitrate nitrogen, chloride and sulfate. The results were compared with an average sample obtained from soil cores adjacent to each site.
"The majority of soil cores indicated no significant increase in contamination levels compared with background levels," points out Melvin. "There was some localized contamination but no evidence of general contamination."
High levels of ammonium and chloride showed up in some soil core samples (1-2/site), usually in cases where the site was previously used as open-front livestock lots or frequently used for manure loading and unloading where frequent manure spills occurred, says Melvin. Almost all sites had at least one soil core containing high levels of all five chemicals.
However, five sites had high ratios for nitrogen that weren't explained by previous manure handling problems. Analysis points to a migration of nitrogen from inside the lagoon to the outside berm, a distance of 30 to 50 ft. But none of those samples saturated the soil. The saturation point is 1,000 ppm, says Melvin.
Contamination Causes Seepage from lagoons or storage basins isn't the only source of chemical contamination.
High nitrate nitrogen levels have been detected in shallow groundwater under corn and soybean fields for several years in the Corn Belt. The major cause is thought to be cropping combined with excessive application of nitrogen fertilizer, stresses Melvin.
Management lapses also enter into the picture. ISU researchers did on-site interviews. They observed management practices that could lead to higher water quality risks at 76% of the sites.
These practices included minor spills during unloading (55%), erosion of compacted clay liners or berms (27%), plugging or freezing of flow inlet pipes (12%), tree growth on berms (6%) and poor freeboard upkeep (6%), according to the ISU study. Major manure spills had occurred at three sites.
New Standards Public controversy has pressured pork producers into storing manure in deep pits inside hog buildings, says Glanville.
Point in fact, only a handful of construction permits were approved last year to build livestock lagoons/ manure storage units in Iowa.
Older storage structures like those cited in this report only have to meet standards of the 1987 DNR code. But Glanville says those structures are fairly sound even by today's standards.
The new DNR standards call for no more than 1/16-in.-seepage/day in all earthen manure storage structures at full depth.