An Illinois State Geological Survey study reveals that deep manure pits under hog barns have little effect on nearby groundwater quality.

Two-thirds of newly constructed hog barns in Illinois use either a deep pit below the barn or an above ground slurry storage system to contain manure.

An on-going, two-year study by the Illinois State Geological Survey (ISGS) and the University of Illinois, Champaign-Urbana, investigates whether deep pits affect groundwater quality. The research was funded by the Illinois Council for Food and Agricultural Research and the Illinois DNR Environmental Trust Fund.

Based on nine months of data collected from 18 wells at two hog finishing sites, the quick answer is that deep pits have little or no impact on groundwater.

Ivan Krapac, ISGS geochemist, outlines locations of the research:

Site D is a 2,400-head finishing facility located on 19.8 ft. of silt, loess, loam and sandy soil. Under the soil is a thick layer of shale and limestone. Water wells in the area are deeper than 230 ft.

Site E is a 2,300-head facility located on 16.5 ft. of silty clay diamicton and sandstone. Most water wells pump from the sandstone layer and are less than 99 ft. deep.

“These facilities are representative of what is being built today,” Krapac says. “The presence of a sandstone aquifer less than 19 ft. below ground surface at Site E suggests a greater potential vulnerability for groundwater contamination than at Site D.”

The researchers inspected the pits under each barn during construction and found that cracks in the pit walls were most prevalent near pump-out stations. Thus, monitoring wells were located near the pump-outs at each facility.

Six wells were drilled at Site D, and 12 wells were drilled at Site E. Approximately 150 groundwater and pit manure samples were collected during the first nine months of the study. They were analyzed for inorganic and bacteriological constituents.

Results and Conclusions

Researchers conclude that Site D, after two years of operation, did not have significant pit leakage.

Although nitrate concentrations near Site D were greater than background tests (samples collected up-gradient of the facility), tests indicated the nitrates were likely not from manure.

Site E, after 18 months of operation, has a limited impact on local groundwater quality, Krapac says.

Elevated nitrate and chloride concentrations were detected in samples taken within 13 ft. of the pit, but testing was inconclusive in determining the nitrate source.

“If it is leaking, the seepage has a very limited and localized effect on groundwater,” he says.

These results are based on the relatively short-term operation of these facilities, and continued monitoring is required to determine the long-term impacts of these facilities on the environment, Krapac reminds.

Fecal bacteria were detected in many of the groundwater samples. The use of manure on nearby fields and the presence of other indigenous animals near the facilities makes it hard to tell the source of fecal bacteria, he says.

Monitoring will continue at each site, and further research is planned to evaluate and identify the potential sources of bacteria and to determine the occurrence of viruses and antibiotics in groundwater.

The researchers also point out that trace and heavy metal concentrations in groundwater samples collected from both sites were less than drinking water standards. Although some evidence suggests that pit leakage may be occurring, the presence of inorganic constituents in the groundwater would have little impact on human health, they report. A potential concern may be the presence of fecal bacteria in the shallow groundwater, although this water is not a drinking water source.