Abstract
The City of Wilmington, Delaware, is in the downstream
part of the Brandywine Creek Basin, on the main stem of
Brandywine Creek. Wilmington uses this stream, which drains
a mixed-land-use area upstream, for its main drinking-water
supply. Because the stream is used for drinking water, Wilmington
is in need of information about the occurrence and
distribution of specific fecally derived pathogenic bacteria
(disease-causing bacteria) and their relations to commonly
measured fecal-indicator bacteria (FIB), as well as information
regarding the potential sources of the fecal pollution and
pathogens in the basin.
This study focused on five routinely sampled sites within
the basin, one each on the West Branch and the East Branch
of Brandywine Creek and at three on the main stem below the
confluence of the West and East Branches. These sites were
sampled monthly for 1 year. Targeted event samples were collected
on two occasions during high flow and two occasions
during normal flow.
On the basis of this study, high flows in the Brandywine
Creek Basin were related to increases in FIB densities, and
in the frequency of selected pathogen and source markers, in
the West Branch and main stem of Brandywine Creek, but
not in the East Branch. Water exceeding the moderate fullbody-contact single-sample recreational water-quality criteria
(RWQC) for Escherichia coli (E. coli) was more likely to contain
selected markers for pathogenic E. coli (eaeA,
stx1, and rfbO157 gene markers)
and bovine fecal sources (E. hirae and LTIIa gene markers),
whereas samples exceeding the enterococci RWQC were more likely to contain
the same pathogenic markers but also were more likely to carry a marker
indicative of human source (esp gene marker).
On four sample dates, during high flow between October
and March, the West Branch was the only observed potential
contributor of selected pathogen and bovine source markers to
the main stem of Brandywine Creek. Indeed, the stx2 marker,
which indicates a highly virulent type of pathogenic E. coli,
was found only in the West Branch and main stem at high flow
but was not found in the East Branch under similar conditions.
However, it must be noted that throughout the entire year of
sampling there were occasions, during both high and normal
flows, when both the East and West Branches were potential
contributors of pathogen and microbial-source tracking markers
to the main stem. Therefore, this study indicates that under
selected conditions (high flow, October through March), West
Branch Brandywine Creek Basin was the most likely source of
elevated FIB densities in the main stem. These elevated densities
are associated with more frequent detection of selected
pathogenic E. coli markers (rfbO157,
stx1) and are associated
with MST markers of bovine source. However, during other
times of the year, both the West Branch and East Branch
Basins are acting as potential sources of FIB and fecally
derived pathogens.