Gastrointestinal (GI) illness risks associated with exposure to waters impacted by human and nonhuman fecal sources were estimated using quantitative microbial risk assessment (QMRA). Microbial source tracking (MST) results had identified Escherichia coli (E. coli) contributors to the waterbody as human and unidentified (10%), cattle and domestic animals (25%), and wildlife (65%) in a rural watershed. The illness risks associated with ingestion during recreation were calculated by assigning reference pathogens for each contributing source and using pathogen dose–response relationships. The risk of GI illness was calculated for a specific sampling site with a geometric mean of E. coli of 163 colony forming units (cfu) 100 mL⁻¹, and the recreational standard of E. coli, 126 cfu 100 mL⁻¹. While the most frequent sources of fecal indicator bacteria at the sampling site were nonhuman, the risk of illness from norovirus, the reference pathogen representing human waste, contributed the greatest risk to human health. This study serves as a preliminary review regarding the potential for incorporating results from library-dependent MST to inform a QMRA for recreational waters. The simulations indicated that identifying the sources contributing to the bacterial impairment is critical to estimate the human health risk associated with recreation in a waterbody.

Water quality standards are developed to protect and define when waterbodies support their designated uses including public water supply, recreational use, aquatic life use, and others. Recreational use categories include various activities that typically do not occur under similar hydrologic conditions making protection of all uses challenging. This paper presents a case study where Escherichia coli concentrations were grouped by flow rate to demonstrate potential effects of developing use-specific water quality standards for contact recreation. Adopting this approach requires a shift from current water quality policy which applies to all hydrologic conditions; however, it also requires additional data collection on actual usage types and occurrence before it can be implemented. This paper demonstrates that implementing an alternative water quality standards approach can still reasonably protect human health while minimizing taxpayer cost to restore impaired waterbodies.