CONCLUSIONS

1.  The present study has demonstrated the feasibility of compiling chemical and other sediment data from heterogeneous sources in the Lake Pontchartrain Basin area. Data on bulk samples from highly valued water bodies have been increased from around 100 samples in any given electronic data source prior to this investigation (1996) to about 1,600 samples in the database described here. This database is designed to be dually available on a CD-ROM and a web site (https://pubs.usgs.gov/prof/p1634j/). The enhanced data set greatly increases capability for assessing contaminant distribution in sediments from Lake Pontchartrain and surrounding water bodies.

2.  The bulk of Lake Pontchartrain sediments from the more central regions of the water body have metal concentrations below toxicity-screening levels. Although the concentrations largely fall within elemental ranges characteristic of uncontaminated, fine-grained estuarine sediments, from 5 to 72% of metal concentrations in surficial sediments may be attributable to anthropogenic sources. Contaminant levels related to point sources near urban coasts increase sharply and become more erratic, especially in the vicinity of New Orleans canals. Similar shoreward increases in organic contaminants like pesticides and PCBs occur at levels below screening-level values for toxic effects on organisms. Unlike metals, the concentration of nonchlorinated organic compounds may have been reduced by biodegradation in the warm waters.

3.  Very high contaminant levels, especially of lead and chromium, occur in Bayou Trepagnier sediments in sites close to a petrochemical complex. Extensive sampling in adjoining Lake Pontchartrain sediments shows little evidence that the contaminants have migrated into the lake in significant quantities up to the present. Outflow from this water body has been redirected to the Mississippi River. Fewer data are available for lake sediments in the vicinity of another impacted waterway, Bayou Bonfouca, where a major creosote spill occurred in 1970.

4.  Discriminant analysis techniques and comparisons with "standard" data sets have been used to assess data quality and comparability for the different data sets. All acquired data have been retained in the database, but certain data batches show unexplained deviations beyond the range of relationships expected or found in well-controlled data. These have been flagged in the database and have not been included among environmental evaluations in this report. It is suggested that the labeled data be rechecked before utilization in scientific evaluation. Systematically lower values are obtained from leachate-based extractions versus "total dissolution" extractions. Both types of analyses are represented in the database. Each may be valid for the original purpose for which the analyses were performed. However, precautions (for example, normalization) should be undertaken when combining such data.

5.  The relationships between Lake Pontchartrain sediments and sediment influxes from the Mississippi River via the Bonnet Carré Spillway are complex. No statistical differences in metallic contaminant trends can be discerned between sediments near the spillway mouth and more distant parts of the lake, when factors like texture are taken into account. An emerging explanation for this relationship is the likelihood that the bulk of surficial sediments in Lake Pontchartrain are, in fact, derived from the Mississippi River through periodic overflows. Particulate influxes from the Mississippi River have had variable contaminant concentrations with time, reaching maxima in the late 1970's. A more immediate impact on the lake was caused by incoming nutrients and freshwater in the 1997 floodwater release. These triggered a massive plankton bloom that created temporary anoxic bottom conditions in much of the lake.

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Table of Contents: Sediment Database and Geochemical Assessment