Quantitative microbial risk assessment with microbial source tracking for mixed fecal sources contaminating recreational river waters, Iowa, USA

Authors: Burch, Tucker; STOKDYK, JOEL - Us Geological Survey (USGS); FIRNSTAHL, AARON - Us Geological Survey (USGS); Opelt, Sarah; Cook, Rachel; Heffron, Joseph; BROWN, AMANDA - Polk County Conservation; HRUBY, CLAIRE - Drake University; Borchardt, Mark

Submitted to: ES&T Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2024
Publication Date: 6/27/2024
Citation: Burch, T.R., Stokdyk, J.P., Firnstahl, A.D., Opelt, S.A., Cook, R.M., Heffron, J.A., Brown, A., Hruby, C., Borchardt, M.A. 2024. Quantitative microbial risk assessment with microbial source tracking for mixed fecal sources contaminating recreational river waters, Iowa, USA. ES&T Water. https://doi.org/10.1021/acsestwater.3c00652.
DOI: https://doi.org/10.1021/acsestwater.3c00652

Interpretive Summary: Fecal contamination of surface water poses a risk of acute gastrointestinal illness (AGI) to recreators from waterborne pathogens like Cryptosporidium and Salmonella. Human, livestock, and wildlife fecal sources present different levels of risk compared to one another. However, the prevalence of these sources is unknown for most recreational water sites. ARS researchers in Marshfield, Wisconsin, estimated risk of AGI for six recreational sites near Des Moines, Iowa, using quantitative microbial risk assessment (QMRA). Risk was estimated for swimming and limited contact activities like kayaking and fishing based on water samples collected over two years and analyzed for a variety of bacterial, viral, and protozoan pathogens along with microbes specific to humans, bird, cows, and pigs. Risk varied by site and activity type, but no single fecal source dominated risk. Risk was generally highest when multiple fecal sources were present at the same time. The risk of AGI was compared to USEPA’s monitoring threshold for E. coli, and high E. coli concentrations were generally more frequent than periods of elevated risk. These results inform broader understanding of recreational water risks in the context of multiple mixed fecal sources by illustrating that identification of a single dominant source of risk is less important than recognizing the number of different sources that might impact a particular site.

Technical Abstract: Fecal contamination of surface water can cause acute gastrointestinal illness (AGI) among recreators. AGI risk varies among human, livestock, and wildlife fecal sources, but the prevalence of individual sources is unknown for most recreational sites. We estimated AGI risk for six sites near Des Moines, Iowa, using quantitative microbial risk assessment combined with microbial source-tracking (MST). We collected 147 water samples over two years and tested them for 9 waterborne pathogens and MST markers specific to avian, bovine, human, and porcine fecal sources. Average swimming risk across all sites was 5 (95% CI: 0.0030 – 142) to 67 (16 – 215) AGI cases per 1,000 recreators. Individual fecal sources were rarely associated with swimming exposures where risk was > 36 AGI cases per 1,000 recreators; most high-risk exposures were associated with simultaneous occurrence of multiple fecal sources. Iowa’s beach action value for Escherichia coli (235 MPN per 100mL) identified 66 – 100% of high-risk exposures across sites, so was generally protective of risk in this setting. For sites influenced by mixed fecal sources, results illustrate that identifying a single dominant source of risk is less important than recognizing the number of unique fecal sources that impact AGI risk.