Fecal bacteria contamination of floodwaters and a coastal waterway from tidally-driven stormwater network inundation

Inundation of coastal stormwater networks by tides is widespread due to
sea-level rise (SLR). The water quality risks posed by tidal water rising
up through stormwater infrastructure (pipes and catch basins), out onto
roadways, and back out to receiving water bodies are poorly understood but
may be substantial given that stormwater networks are a known source of
fecal contamination. In this study, we (1) documented temporal variation
in concentrations of Enterococcus spp. (ENT), the fecal indicator bacteria
standard for marine waters, in a coastal waterway over a two-month period
and more intensively during two perigean spring tide periods, (2) measured
ENT concentrations in roadway floodwaters during tidal floods, and (3)
explained variation in ENT concentrations as a function of tidal
inundation, antecedent rainfall, and stormwater infrastructure using a
pipe network inundation model and robust linear mixed effect models. We
find that ENT concentrations in the receiving water body vary as a
function of tidal stage and antecedent rainfall, but also site-specific
characteristics of the stormwater network that drains to the waterbody.
Tidal variables significantly explain measured ENT variance in the
waterway, however, runoff drove higher ENT concentrations in the receiving
waterway. Samples of floodwaters on roadways during both perigean spring
tide events were limited, but all samples exceed thresholds for safe
public use of recreational water. These results indicate that inundation
of stormwater networks by tides could pose public health hazards in
receiving water bodies and on roadways, which will likely be exacerbated
in the future due to continued SLR.