Water
quality impairments at urban beaches can be challenging to
manage due to a wide range of possible pollutant sources, including
both regional and local sources. In this study, we used high-resolution
time series data as the input into a 3D hydrodynamic model to predict Escherichia coli (E. coli) and human Bacteroides marker concentrations at
nearby beaches. Model results predicted that both organisms reach
beaches following select wet weather events and during combined sewer
overflows (CSOs), but in 50% of these events, the E.
coli model predictions were <235 CFU/100 mL when
human sources were present. Local pollutant sources were also evaluated
during a variety of conditions using fecal source markers, including
human, gull, and dog bacterial markers. Gull waste was frequently
detected at beaches. In contrast, human markers were rarely detected,
but when present, detection generally coincided with CSOs and low
levels of E. coli, suggesting dilute
regional contamination. Together, the results suggest that regional
fecal pollution containing human sources does not coincide with elevated E. coli levels, and localized contamination likely
drives E. coli levels at beaches. This
work highlights the need to consider risk-based metrics rather than
relying on a single easily measured indicator at beaches.