Sewage loading and microbial risk in urban waters of the Great Lakes

Authors: MCLELLAN, SANDRA - UNIVERSITY OF WISCONSIN; SAUER, ELIZABETH - UNIVERSITY OF WISCONSIN; BOOTSMA, MELINDA - UNIVERSITY OF WISCONSIN; CORSI, STEVE - U.S. GEOLOGICAL SURVEY (USGS); BOEHM, ALEXANDRIA - STANFORD UNIVERSITY; SPENCER, SUSAN; BORCHARDT, MARK

Submitted to: Elementa: Science of the Anthropocene
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2018
Publication Date: 6/20/2019
Citation: McLellan, S.L., Sauer, E.P., Bootsma, M.J., Corsi, S.R., Boehm, A.B., Spencer, S.K., Borchardt, M.A. 2019. Sewage loading and microbial risk in urban waters of the Great Lakes. Elementa: Science of the Anthropocene. 6:46. https://doi.org/10.1525/elementa.301.
DOI: https://doi.org/10.1525/elementa.301

Interpretive Summary: Water quality managers can test for specific bacteria to indicate whether rivers and lakes are polluted with fecal wastewater from people, agricultural livestock, or wildlife. One limitation of the current approach is that while the fecal pollution can be detected, it is difficult to translate the measurement to human health risk, that is, the probability of becoming ill from exposure to the contaminated water. We related two indicator bacteria, human Bacteroides and human Lachnospiraceae, to the concentration of the human pathogen norovirus in the Milwaukee River Basin entering Lake Michigan in Milwaukee, Wisconsin. Using these data in a risk assessment framework we determined for the two indicator bacteria the concentrations in recreational water that if exceeded, the health risks were greater than the current acceptable level in the United States of 30 illnesses per 1000 swimmers per swimming event. With this information in hand, water quality managers can go beyond simply stating the water is polluted to where they can now begin to use indicator bacteria concentrations to estimate the human health risks.

Technical Abstract: Despite modern sewer system infrastructure in urban areas, sewage released from deteriorating pipes and sewer overflows is a major water pollution problem in US cities, particularly in coastal watersheds that are highly developed and have large human populations. We quantified fecal pollution sources and loads entering Lake Michigan from a large, mixed land use watershed using host-associated indicators. Wastewater treatment plant influent had stable concentrations of human Bacteroides (HB) and human Lachnospiraceae (Lachno2) with geometric mean concentrations of 2.77x107 and 5.94x107 copy number (CN)/100 ml, respectively. These human associated indicator levels were four orders of magnitude higher than virus concentrations, suggesting they could be sensitive indicators of pathogen risk. Norovirus concentrations in these same samples were used in quantitative microbial risk assessment calculations. Assuming a typical recreational exposure to untreated sewage in water, concentrations of 7,800 CN/100 ml HB or 14,000 CN/100 ml Lachno2 corresponded to an illness risk of 0.03. These levels were exceeded in estuary water during storm events with greater than 2 inches of rainfall. Following combined sewer overflows, concentrations were 10 fold higher than rainfall with 2.98x105 CN/100 ml HB. High frequency automated sampling allowed for loads of human associated markers to be determined, which could then be related back to equivalent volumes of untreated sewage that was released. Evidence of sewage contamination decreased as ruminant associated indicators increased approximately one day post-storm, demonstrating the delayed impact of upstream agricultural sources on the estuary. These results demonstrate urban areas are a diffuse source of sewage contamination to urban waters and storm driven release of sewage, particularly when there are sewage overflows, creates a serious but transient human health risk.