Tracing Evidence of Nitrates and Bacteria in Groundwater

Onsite system design and maintenance investigation may yield clues about human fecal contamination of water wells.

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As reported previously in Pumper, an ongoing study is being conducted in southwestern Wisconsin to test private wells for evidence of contamination. The Southwest Wisconsin Groundwater and Geology study began in 2018 with two objectives: (1) Evaluate private well contamination using indicator bacteria (total coliform and E. coli) and nitrate based on randomized synoptic sampling events, and (2) identify the source of contamination in a subset of total coliform- and nitrate-positive wells once per season using microbial tests that distinguish between human, bovine and swine fecal sources.

Two phases of the study have been completed, and the results are concerning. During a first sampling, 42% of the wells showed either bacteria contamination or elevated nitrate levels above the 10 mg/L health standard. A second sampling showed 27% of the wells were contaminated. Higher percentages of wells showed contamination with bacteria as opposed to nitrate, which was surprising to me, as nitrate (nitrogen) moves more easily through soil and is not captured by adsorption and filtering as bacteria are when septic tank effluent moves by unsaturated flow through soil.

In the second study phase, 34 contaminated wells were evaluated for bacteria associated with fecal contamination from either animals or humans. Animal fecal contamination was found in 25 wells and human fecal material in 14 wells.


This led one local sanitarian to make a statement and some recommendations as the researchers move into the next phases of the study. The sanitarian stated, “Seeing the human fecal contamination being the majority of the last two testings, we are looking at our maintenance program and making sure everyone is following the rules. Even though we’re waiting for all four of the rounds to be complete, there are some steps that are being taken to look and see if there are things we can do right now.”

The sanitarian recommended homeowners have their septic systems checked thoroughly every three years and keep up on cleaning filters, fixing soil compaction issues and addressing any ponding in the drainfield.

None of us should have any problem with these recommendations, and we should be recommending them daily to our customers. Cleaning filters and checking systems is always good practice. By themselves, they will not eliminate the possibility of well contamination by fecal bacteria. As indicated, the systems should be checked on a regular basis and any problems corrected.

Determining the reason for well contamination requires a close look at individual residences, including the soils and geology at the site, septic system location in relation to the topography, condition of each system component, any evidence of surfacing and runoff, along with condition of the well, the depth where the well is finished and other conditions occurring in the area or region.

The next phase of the research is to carry out geologic studies and analyze well construction practices in the three-county region. The goal is to determine correlations between water quality, geology and well construction. The geology in the area consists of shallow soils over limestone bedrock, and there are three separate aquifers at different depths in the bedrock. The hope is to understand where the water in the wells is being drawn and whether the wells are properly cased and finished to avoid contamination from the surface. Contamination may be coming from sources other than septic systems.

In areas of geology with creviced bedrock, the actual source of contamination can be quite a distance from the site of the well. From our perspective in worrying about treatment of septic tank effluent, this is important because the system causing the problem may not be the one serving that residence.


There are several ways septic systems could contribute to or be the cause of the fecal contamination. If the drainfield is hydraulically overloaded and surfacing, there is the potential for the effluent at the surface to move as runoff to the well. And if the well is not properly constructed, the effluent can move directly into the well and aquifer. So as the sanitarian noted, identifying potential ponding and correcting the problem is important to ensure there is not a direct connection.

Condition of the septic tank is key; it needs to be watertight with no leaking into the surrounding soil. If the septic tank is installed at the top of or into the bedrock and it is not watertight, it can be a source of contamination. If the tank is leaking effluent, it may move directly into the creviced bedrock and to the aquifer to contaminate not only the nearest well, but also others finished in the same aquifer. To evaluate whether the tank is structurally sound and watertight, the tank should be opened and pumped to examine the inside. It would not be enough to simply observe whether the tank is operating at the correct level. As the sanitarian indicated, when the tank is pumped, the filter should be cleaned.

In addition to looking for surfacing in the drainfield area, an evaluation should determine that the bottom of the sewage trenches are at least 3 feet above the creviced bedrock. Has biomat developed to slow flow through the soil and ensure the flow is under unsaturated conditions? As I stated earlier, if the system is designed and installed so unsaturated flow is ensured, soil is an excellent media to capture bacteria and virus preventing movement into the aquifer.

In sensitive geologic areas like in southwestern Wisconsin, identifying and fixing problems as soon as possible is important to prevent groundwater contamination and associated health problems. Stay tuned for updates as the next phase of this study is completed. 


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