Temporal and spatial variation of nitrate, fecal indicator bacteria, and microbial source tracking markers in private well water from a karst aquifer

Authors: Cook, Rachel; Opelt, Sarah; FIRNSTAHL, AARON - Us Geological Survey (USGS); Sherman, Jessica; Burch, Tucker; Heffron, Joseph; Young, Eric; WAYNE, RICK - University Of Wisconsin; GOOD, LAURA - University Of Wisconsin; STOKDYK, JOEL - Us Geological Survey (USGS); Borchardt, Mark

Submitted to: American Society for Microbiology Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 2/27/2024
Publication Date: 6/16/2024
Citation: Cook, R.M., Opelt, S.A., Firnstahl, A.D., Sherman, J.F., Burch, T.R., Heffron, J.A., Young, E.O., Wayne, R., Good, L., Stokdyk, J.P., Borchardt, M.A. 2024. Temporal and spatial variation of nitrate, fecal indicator bacteria, and microbial source tracking markers in private well water from a karst aquifer. American Society for Microbiology Annual Meeting. June 13-17, 2024.

Interpretive Summary:

Technical Abstract: Rural residents use private wells for drinking water in northeastern Wisconsin, USA, where a large percentage of the land is used for agriculture and private septic systems are common. Cattle manure and human wastewater are sources of enteric pathogens and nitrate that can contaminate private wells. Groundwater quality can change rapidly in the region’s fractured bedrock aquifer, so predicting the timing and distribution of private well water contamination is difficult, which presents a challenge for protecting public health for residents that rely on private wells for drinking water. We aimed to assess water quality variation in a vulnerable aquifer over time and space by comparing nitrate and microbial contamination across a spectrum of hydrological conditions. Private wells (n = 36) were sampled repeatedly (up to 7 times) over 13 months using Wisconsin’s Runoff Risk Advisory Forecast (RRAF) tool to determine a range of sampling conditions. The RRAF was developed by the United States National Weather Service for predicting risk of cropland runoff in the Great Lakes region using soil and weather conditions. Water samples (n = 235) were tested for fecal indicator bacteria (FIB: total coliforms, E. coli, and Enterococci) by Colilert and Enterolert trays, 6 human and bovine microbial source tracking (MST) markers by qPCR, and nitrate. Overall, 19 wells were positive for at least one FIB or MST marker, and microbial detections were intermittent; only 11 of 36 wells were positive for FIB or MST markers in multiple sampling events. FIB detections were most common in spring and fall, while MST detections were most common in February. Nitrate concentrations averaged 2.3 mg NO3-N/L (median 0.01 mg NO3-N/L), and nitrate concentrations varied by well more than within wells over time (a = 0.05). FIB detections varied by well and sampling event (a = 0.05). By repeatedly sampling the same wells over time, these results demonstrate the transience of groundwater fecal contamination in the karst aquifer of northeast Wisconsin. Results also suggest that spatial variability may be more significant in groundwater contamination by nitrate, while microbial contamination may be more driven by temporal changes within individual wells.