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
Individuals experiencing unsheltered homelessness face significant barriers to accessing water, sanitation, and hygiene services, but the risks associated with this lack of access and barriers to service provision have been largely understudied. We analyzed water samples upstream and downstream of three homeless encampments in the San Diego River watershed and interviewed service providers from public and nonprofit sectors to assess local perceptions about challenges and potential solutions for water and sanitation service provision in this context. Water upstream from encampments contained detectable levels of caffeine and sucralose.
Escherichia coli
concentrations downstream of the encampments were significantly greater than concentrations upstream, but there was no significant change in the concentrations of other pollutants, including caffeine and sucralose. The HF183 marker of
Bacteroides
was only detected in one sample upstream of an encampment and was not detected downstream. Overall, there was insufficient evidence to suggest that the encampments studied here were responsible for contributing pollution to the river. Nevertheless, the presence of caffeine, sucralose, and HF183 indicated that there are anthropogenic sources of contamination in the river during dry weather and potential risks associated with the use of this water by encampment residents. Interviews with service providers revealed perceptions that the provision of water and sanitation services for this population would be prohibitively expensive. Interviewees also reported perceptions that most riverbank residents avoided contact with service providers, which may present challenges for the provision of water and sanitation service unless trust is first built between service providers and residents of riverine encampments.
Water quality benchmarks
for fecal indicator bacteria (FIB) are
often exceeded in many urban streams in southern California. Possible
sources of elevated stream FIB concentrations within urban areas include
sanitary sewer exfiltration, sanitary sewer overflows (SSOs), illegal
discharges, and human or animal fecal material on the ground surface.
Teasing apart the different sources remains a challenge, especially
when untreated wastewater and runoff from open defecation sites both
contain human fecal material. To distinguish the various sources of
microbial contamination in an urban stream, temporal trends in biological
and chemical markers of anthropogenic contamination were evaluated
in the San Diego River and its tributaries during storm events in
two consecutive hydrologic years. Temporal trends in FIB, HF183, pepper
mild mottle virus (PMMoV), caffeine, sucralose, chloride, bromide,
specific ultraviolet absorbance, and fluorescence index indicated
that untreated wastewater flushed from the vadose zone was the main
source of microbial pollution to the San Diego River, while open defecation
near homeless encampments in the river margins was not a major source.
We demonstrated that the combined use of caffeine/sucralose ratios
and HF183 and PMMoV holds promise for identifying sewage inputs to
surface waters. These findings highlight the need for maintenance
and repair of aging sewer infrastructure.
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