USGS
Releases Bacteria Data on Lake Pontchartrain
November 22, 2005
In response to public concern for the water
quality of Lake Pontchartrain following Hurricane Katrina, the
U.S. Geological Survey (USGS), in collaboration with the
Louisiana Department of Environmental Quality (DEQ), U.S.
Environmental Protection Agency (EPA), National Oceanic and
Atmospheric Administration (NOAA) and the Food and Drug
Administration (FDA), is conducting intensive studies of water,
sediment, and seafood quality of Lake Pontchartrain.
Findings suggest that, despite expectations
that hurricane-related flooding in New Orleans could cause
uniformly high concentrations of fecal bacteria in Lake
Pontchartrain, water samples from sites in and around the lake
commonly were within limits acceptable for recreational waters.
These results represent a first round of
testing following Hurricanes Katrina and Rita. The Louisiana DEQ
is continuing to monitor bayous along the north side of Lake
Pontchartrain, areas found to be ‘hot spots’ in samples
collected immediately following Hurricane Rita.
USGS scientists measured fecal-indicator
bacteria Escherichia coli (E. coli), enterococci and fecal
coliforms over a three week period. These indicator bacteria are
not themselves pathogens, but scientists monitor for them
because they are useful indicators of fecal contamination and
the possible presence of pathogens. Concentrations in samples
they collected in the third and fourth week after passage of
Hurricane Katrina commonly were less than EPA criteria for E.
coli and enterococci in fresh or marine waters and also met the
Louisiana DEQ standard for fecal coliform bacteria. A week
later, following the passage of Hurricane Rita, concentrations
at several tributary sites were well above the criteria and
standard, while concentrations in the lake remained generally
below or near those limits.
Scientists collected water samples at 22
sites—including most inflows to the lake, sites within the lake
and the major outflows to the Gulf of Mexico by way of Lake
Borgne and the Mississippi Sound. Nineteen of the sites are
routinely sampled as part of the Louisiana DEQ ambient
monitoring network. Corresponding data on water temperature,
specific conductance (a measure of salinity), pH and dissolved
oxygen also were collected at each site at the time of sampling.
The scientists were especially interested to
find the highest concentrations of fecal contamination in north
shore tributaries rather than in the south shore canals that
carried floodwater from New Orleans into Lake Pontchartrain.
Dennis Demcheck, the USGS hydrologist at Baton Rouge, La. who
led the sampling effort, attributed this to a "settling pond"
effect in New Orleans, which held floodwaters during the weeks
prior to pumping them back into Lake Pontchartrain.
The study included an extensive
quality-control data set, and that data set largely indicates
satisfactory analytical performance, even though scientists were
working out of a mobile lab in less than ideal field conditions.
These results are intended to help in completing the regional
Interagency Environmental Assessment underway by USGS, U.S. EPA,
NOAA and FDA.
© 2005 Scranton Gillette Communications, Inc. All rights reserved.
Source: USGS November 22, 2005
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