Application of Quantitative Microbial Risk Assessment and Bacterial Source Tracking to Assess the Associated Human Health Risks from Multiple Fecal Sources During Recreational Exposure in the Leon River Watershed

Abstract

Applying a risk assessment framework, such as quantitative microbial risk assessment (QMRA), can be used to estimate the human health risk associated with recreation in a waterbody impaired for elevated levels of fecal indicator bacteria (FIB). Recent efforts to identify the sources contributing to a waterbody’s bacterial impairment have been facilitated by bacterial source tracking (BST) analysis for several watersheds in Texas, including the Leon River Watershed. A QMRA was conducted to calculate the human health risk for a recreational waterbody impacted by both human and non-human sources of fecal contamination. Waterborne reference pathogens were used to represent the different fecal contamination sources and the risk of a GI infection and illness. The GI illness risk for contact exposure to recreational waters within the Walnut Creek tributary of the Leon River Watershed were calculated for site LEO 2, with a geometric mean of 163 cfu 100 mL^-1, and the U.S. recreational standard of 126 cfu 100 mL^-1 for Escherichia coli (E. coli). Three different scenarios were modeled to estimate the potential risks of a GI illness in recreational waters impacted by different proportions of human and non-human sources of fecal contamination. The analysis found that: a) the dominant fecal source in a waterbody may not be the greatest contributor to the human health risk; b) risks associated with wildlife fecal contamination were significantly lower than that of the cattle/domestic animals and human fecal contamination; and c) while considering norovirus as a representative pathogen for human fecal contamination, the estimated risk was much higher. The results indicate that identifying the sources contributing to a bacterial impairment and conducting a QMRA for the recreational waterbody can greatly assist in developing site-specific standards, especially if the site is not predominantly impacted by human fecal contamination.