| Environmental Geochemistry and
Sediment Quality in Lake Pontchartrain
III. Data and Preliminary Interpretations (con't)
1. Delineation of contaminated vs. clean environmental conditions in bottom sediments
requires a detailed distribution of analyzed sediment samples. This process is greatly
enhanced by compilation of available historical samples from all sources. The present
cooperative research has increased samples available in one source from 100-200 to 1400.
These compilations have required data validation studies that found and flagged
questionable batches of data through use of discriminant functions and standard data sets.
Different analytical methods, deemed valid in their own terms, yield divergent results for
certain elements, especially Al, Fe, and Cr.
2. The bulk of Lake Pontchartrain sediments from the center of the water body have
metal concentrations that are below toxicity levels and largely fall within presumed
naturally-occurring elemental ranges. As one approaches point sources near urban coasts
contaminant distributions increase and become more erratic, and greater sample densities
are required. The highest contaminant levels occur in impacted inland waterways like Bayou
Trepagnier and Bayou Bonfouca.
3. Detailed chemical information is available for sediments in the vicinity of the
Bonnet Carré Spillway. These data do not show identifiable metallic contaminant trends in
Lake Pontchartrain sediments that can be attributed to influx of sediments from the
Mississippi River. However this conclusion cannot be extended to nutrients in the recent
floodwater releases, which have been confirmed to have caused large plankton blooms (see
discussions in Basics of the Basin Symposium, 1998).
4. Samplings from dated cores are needed to quantify the concentrations in
pre-anthropogenic contaminant levels and permit longer-term estimates of contaminant
fluxes to Lake Pontchartrain and surrounding estuaries. Samples taken in the current year
are expected to permit such analysis in the final year of the study (1999).
5. Recent releases of USEPA EMAP sediment data have been incorporated into the database
reported here, but have not been available long enough to permit an integrated
interpretive assessment of the results. Beside sediments, these data offer extensive data
on toxicity evaluation and organism analysis.