- Mark A. Borchardt, US Department of Agriculture
- Maureen A. Muldon, Department of Geology, University of Wisconsin-Oshkosh
- Randall J. Hunt, US Geological Survey
- Davina E. Bonness, Kewaunee County Department of Land and Water Conservation
- Aaron D. Firnstahl, US Geological Survey
- Burney A. Kieke, Marshfield Clinic Research Institute
- David W. Owens, US Geological Survey
- Susan K. Spencer, US Department of Agriculture
The Silurian dolomite aquifer is an important water source in northeastern Wisconsin. Previous studies have shown that the aquifer is vulnerable to contamination because the dolomite is extensively fractured and many regions have thin soils that do not attenuate contaminants moving from the land surface to the water table. Wastewater from septic systems and livestock manure from farming operations are possible contaminant sources, while environmental conditions and geology (e.g., precipitation, groundwater recharge, and depth to bedrock) can affect the timing and extent of private well contamination.
We address these aspects of groundwater contamination in six main objectives that produced nine key findings. Private domestic wells completed in the Silurian dolomite aquifer and located in Kewaunee County, Wisconsin were sampled and tested in three ways: 1) Wells were randomly selected based on depth to bedrock and tested for coliform bacteria and nitrate during two synoptic events, the fall of 2015 and summer of 2016. 2) Samples were collected on a seasonal basis during five sampling events and analyzed by quantitative polymerase chain reaction (qPCR) for microbes that originate in fecal material, including those specific to human fecal wastes and manure from ruminant animals and cattle. 3) Automated sampling devices collected time-series samples and continuously monitored water-quality parameters.
Overall, we collected 980 samples from 624 wells. Depth-to-bedrock data were abstracted from well construction reports, and sentinel wells monitored groundwater levels and a variety of water-quality parameters that were assessed for their utility as indicators of recharge. Key findings are as follows: 1) The Kewaunee county-wide contamination rate for wells positive for coliform bacteria or with nitrate-N concentrations greater than 10 mg/L was 26% in the fall of 2015 and 28% in the summer of 2016. 2) In regions with shallow depths to bedrock the contamination rate can be much higher than the county-wide average. 3) Both human wastewater and cattle manure were identified as sources of fecal contamination among those wells positive for coliforms or with nitrate-N concentrations greater than 10 mg/L; 60% of tested wells had evidence of fecal contamination. 4) Pathogens that cause illness in people were detected in private wells using qPCR. 5) Analysis of existing nitrate data for the Silurian dolomite aquifer in northeast Wisconsin suggests increased nitrate-N concentrations correlate with nutrient application rather than periods of groundwater recharge. 6) Water-quality data collected at non-pumping sentinel wells suggest that sharp inflections in specific conductance and dissolved organic matter are good recharge indicators (in addition to rising water levels). 7) Water-quality data collected by automated samplers placed in households indicate that private well water quality is highly variable over time. 8) Microbial data collected by the automated samplers indicate the presence of microbes in private wells depends on both recharge events and the strength of the contamination source. 9) Forecasts of runoff predicted from the Runoff Risk Advisory Tool appear to be associated with bovine contamination of private wells.
Other research activities related to this report but not funded by the Wisconsin DNR include multivariable statistical models of risk factors related to private well contamination, DNA sequencing confirmation of well water samples positive for human Bacteroides and ruminant Bacteroides, genetic subtyping analyses of samples positive for rotavirus group A to determine the fecal source of the virus, and estimates of illness risk in Kewaunee County caused by private well contamination. These additional findings are presented in the scientific papers resulting from this work. Considered in toto, findings from the present study and from previous hydrogeological and water-quality investigations of the Silurian dolomite aquifer provide many opportunities for ensuring clean groundwater for the residents of northeastern Wisconsin.