Association of runoff risk with private well contamination in a fractured dolostone aquifer

In the United States, private well owners are responsible for monitoring and maintaining their drinking water but have little control over neighboring contamination sources and limited information about daily water quality. Potential for rapid contamination in karst settings exacerbates the challenge. The Runoff Risk Advisory Forecast (RRAF) is a real-time online tool available in five US states that helps nutrient applicators avoid times and locations of high runoff risk. We wanted to determine if RRAF could be repurposed in areas susceptible to groundwater contamination to inform well owners of conditions when contamination is likely. Private wells were repeatedly sampled on seven dates (32-36 wells per date) and analyzed for nitrate-nitrogen, total coliforms, and bovine manure markers. Three-day windows of RRAF risk prior to sampling were selected post hoc, ranging from 1 to 3 days prior for manure markers to 10-12 days prior for nitrate. Contaminants were associated with RRAF values of ≥75% within these windows based on observed weather. A hydrologic association between RRAF risk and groundwater depth was also apparent. Overall, results suggest the RRAF model anticipates conditions of elevated contamination risk for susceptible wells. For nitrate, between-well variation and delayed peak concentration suggested that well location and construction factors may be more informative than runoff events. Microbial contamination was rapid and distributed among wells, with one or more detections of coliforms in 31% of wells and bovine markers in 61% of wells. Thus, a warning system based on the RRAF model could help identify conditions of acute pathogen exposure.