Abstract
Sewer overflows and exfiltration can potentially contaminate water bodies with pathogens from wastewater. Microbial source tracking (MST) methods such as the detection of the HF183 gene target of Bacteroides have been proposed to monitor human fecal pollution inputs to surface waters; however, the persistence of HF183 and other MST markers in water flushed from soils after contamination events is not well understood. In this study, the persistence and decay of two culture-based fecal indicators, Escherichia coli and enterococci, and two molecular MST markers, HF183 and pepper mild mottle virus (PMMoV), were evaluated in riverbank soils spiked with untreated sewage, which were left idle for 1, 14, 28, 60, and 121 days under dark conditions and then flushed with synthetic rainwater. All four microbial indicators were still detected in flush water 4 months after the soil was contaminated. PMMoV persisted much longer and had a slower decay rate than the other microbial indicators, and E. coli degraded most rapidly. In consecutive flushing experiments with fresh (1 day) sewage-spiked soils, HF183, E. coli, and enterococci were all detected after 20 consecutive flushes with rainwater, but PMMoV was not detected after the fifth flush. Our findings indicate that water (e.g., stormwater interflow) flushing through riverbank soils that have previously been contaminated by sewer overflows or sewer exfiltration can potentially be a source of microbial pollution to surface waters, even for several months after the contamination occurs. Results from this study also demonstrate the benefits of using multiple human-associated fecal indicators to distinguish pollution from different microbial groups in water bodies.Graphical abstract
Highlights
Human-associated fecal contaminants and pathogens have been detected in rivers during storm events (Schiff et al, 2016), and fecal contamination to ocean water from river outfalls after rain events has been associated with increased rates of gastrointestinal illness and multiple infections amongst surfers (Arnold et al, 2017)
After the 5th flush, dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations changed very little, with < 10% of the initial concentration remaining. The decrease in both DOC and TDN as a function of flushing volume followed a decrease that appeared to be consistent with secondorder rate processes (Supplemental Figure S1 shows that relationships of 1/concentration vs. flush volumes had r2 values ranging from 0.98 to 0.999)
It is worth noting that controls did experience decreases in DOC and TDN concentration, which are naturally found in soils, but the starting concentrations were lower than in spiked soils, with 55 mg C/L and 3.7 mg N/L measured in the first flush; the decrease in concentrations during the first 5 flushes was not as pronounced as in the spiked soil flushing experiments
Summary
Human-associated fecal contaminants and pathogens have been detected in rivers during storm events (Schiff et al, 2016), and fecal contamination to ocean water from river outfalls after rain events has been associated with increased rates of gastrointestinal illness and multiple infections amongst surfers (Arnold et al, 2017). Microbial source tracking (MST) methods such as the detection of the HF183 gene target of Bacteroides are increasingly being used to monitor human fecal pollution in surface waters Traditional fecal indicator bacteria, such as E. coli and enterococci, do not strictly indicate human fecal pollution, while MST markers such as HF183 and pepper mild mottle virus (PMMoV) are more specific to human fecal pollution (Symonds et al, 2018). Molecular methods such as qPCR and RT-qPCR, which are used to detect HF183 and PMMoV, respectively, do not provide information about viability and may pick up DNA and RNA from nonviable microorganisms.
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