Michelle Soupir, Pramod Pandey 2016-02-26 23:56:39
In the U.S., 74,863 surface water quality impairments are on the EPA’s list of impaired waters, and the leading cause is high levels of pathogens or fecal indicator bacteria (FIB) in streams and lakes. To understand E. coli contamination in an agricultural stream, we monitored streambed sediment and the overlying water for E. coli concentrations over a range of flow conditions during storm events and characterized the impact of sediment E. coli on stream water quality. Data collected in the field were used to develop and test a model of E. coli resuspension that uses sediment transport theory to calculate the release of E. coli from the streambed sediment to the water column. A limitation of the existing criteria Typically, E. coli or enterococci are the two FIB used to detect the presence of pathogens originating from fecal contamination. More than 612,410 km of streams and 12 million acres of lakes and reservoirs are listed as pathogen impaired. High levels of pathogens or FIB in stream or lake water indicate a potential risk to human health. Ingestion of contaminated water may result in gastrointestinal illness in humans, including vomiting, diarrhea, fever, or rash. To protect public health, the ambient water quality criteria developed by the EPA in 1986 were recently revisited, and in 2012 the EPA established the new recreational water quality criteria (RWQC). The EPA’s current fresh water quality criterion for E. coli is a geometric mean not exceeding 126 colony-forming units (CFU) per 100 mL, which corresponds to an estimated illness rate of 36 per 1,000 recreators. These guidelines are important to protect public health, and improved modeling tools can help identify situations in which high levels of pathogens in streams can be expected. The levels of E. coli in streams are particularly driven by precipitation events. Our observations during high flow and normal conditions showed that the E. coli levels in streams change considerably within a short period. During precipitation events, fresh sources of FIB can enter a waterway through overland flow or subsurface pathways such as tile drainage. However, studies have also confirmed the presence of surviving “legacy” FIB in stream bottom sediment. These background organisms are a concern because the bottom sediment is resuspended, along with the FIB persisting in the sediment, during high flow conditions. The EPA water quality standards were established based on sampling of overlying water and do not consider the presence of these legacy organisms. The effect of legacy organisms To get additional insight into these legacy organisms, we collected sediment and water samples in the Squaw Creek watershed near Ames, Iowa, during the first significant precipitation event of the season in spring 2013. The E. coli concentrations in the stream water followed a pattern similar to that of the stream flow, with E. coli concentrations increasing and decreasing with flow. The E. coli concentration varied greatly during the precipitation event, ranging from 360 to 37,553 CFU per 100 mL. These values are about 3 to 300 times greater than the EPA recommended value (126 CFU per 100 mL) and were observed within 12 hours of the precipitation event. Because of the high flow caused by precipitation, the stream sediment characteristics changed dramatically during the study, with a decrease in the E. coli levels in the stream sediment corresponding to a decrease in the fraction of fine particles in the sediment. This observation is supported by previous studies, which found that E. coli tended to associate with cohesive particles and fine sediment. Flow patterns in the Squaw Creek watershed are flashy, so we performed a scenario analysis to assess the impacts of a wide range of sediment E. coli concentrations and stream flows on the E. coli concentration in the overlying water. Previous researchers have observed sediment-to-water ratios ranging from 1 to 10,000, so we used this range of values along with a resuspension model that we developed earlier. At sediment-to-water E. coli ratios of 1 and 100, when the flow was low, the water E. coli load was equal to the total E. coli load, indicating that the sediment E. coli contribution was negligible at low flows. However, as flow increased, the total E. coli load became considerably larger than the water E. coli load, indicating a significant contribution from the sediment E. coli. For example, for a sediment-to-water ratio of 100, the water E. coli load was less than 50% of the total E. coli load. The sediment E. coli contribution to the total E. coli load increased with flow; at high flows, resuspending E. coli dominated the total E. coli load. In addition, as the sediment-towater ratio increased, the resuspending E. coli load also exceeded the water E. coli load at lower flows. Future considerations While our study and other studies are advancing current understanding of legacy E. coli in stream sediment, many questions remain as to how this information should be incorporated into total maximum daily loads (TMDLs) and watershed management plans. Few watershed-scale water quality models include FIB resuspension as an in-stream process. Stakeholder groups are anxious to implement management practices to improve water quality, but few strategies exist for mitigation of sediment-associated FIB. ASABE member Michelle Soupir, Associate Professor, Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, USA, email@example.com, www.public.iastate.edu/~msoupir. ASABE member Pramod Pandey, Assistant Specialist/ Microbiologist, Department of Population Health and Reproduction, University of California-Davis, USA, firstname.lastname@example.org.
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