Lake Pontchartrain Basin:  Bottom Sediments and Related Environmental Resources




This report completes an assessment of the environmental chemistry of bottom sediments from Lake Pontchartrain and adjoining estuaries and water bodies. The work was begun in 1996 by the USGS Center for Coastal and Marine Geology (Woods Hole, MA) in cooperation with many scientists and organizations cited in Participants and Cooperators. The first step in this effort was to compile available data from all sources into a comprehensive database and apply quality-control standards to the results. The efforts have increased the number of electronically accessible samples from about 100 at the outset of the work to about 1,600 in appendix D. These data are prepared in formats that are suitable for mapping and display in many ways, including incorporation into geographic information systems (GIS). This document extends earlier progress reports (Manheim and others, 1997; Manheim, 1998). It offers interpretations of the status and origin of both inorganic and organic contaminants in these estuarine sediments.

The report reviews data sources, methodological and quality-control issues, regional background information, distribution of contaminants in surficial sediments, the role of urban and industrial areas, and the influx of Mississippi River sediments. It discusses relationships between sediment composition and other data provided by project participants. Implications of the 1997 opening of the Bonnet Carré Spillway on sediment accumulation and composition in Lake Pontchartrain are briefly reviewed.

The data are documented as fully as possible so as to reduce need to consult original sources -- some of which are unpublished. The data and interpretations are designed to assist in management decisions as well as to provide scientific and public information regarding the environmental status of the water bodies.


What are bottom sediments and why are they important?

Bottom sediments in waterways accumulate from mineral and organic particles washed out of wetlands and soils and transported through rivers, streams and canals or by erosion from shoreline deposits. Additional contributions come from atmospheric dust particles and waste materials from vessels or discharge pipes. Sediments in waterways sensitively record human activities surrounding them: waste discharge and disposal, automotive emissions, farming and fertilization, water and air pollution, dredging, flood control, and other activities.

Both organic and inorganic constituents (carbon, nitrogen, phosphorus, copper, iron, zinc, and other elements) have nutrient functions for bottom organisms and nutrient supplies to plankton growth in the water column. When trace metals and nutrient concentrations substantially exceed levels that are typical for the natural, presettlement levels, they may upset environmental balance. Contaminants can reach levels that are toxic to bottom organisms like clams or worms, which are fed upon by fish and other swimming organisms. Contaminants from sediments may be resuspended to the water column and ingested or redeposited.

Knowledge of the distribution of chemical constituents will also contribute to an understanding of the overall role and transport of toxic materials to the ecosystem. The studies in this report are mainly restricted to surface sediments, but limited studies of core data are in preparation (Manheim and others, 1999). These are expected to provide a better delineation of presettlement background levels of trace metals and organic matter with which surficial sediment concentrations can be compared.


Not just "how dirty is dirty" but "how clean is clean"?

Much concern has developed in recent years about environmental degradation. It is important not only to document where contaminants occur and in what quantities, but also to confirm where conditions approach natural pristine or healthy levels and all gradations in between. This allows concerns to be better and more efficiently focused, to expand scientific and public understanding of sources and transport pathways for contaminants, and to make predictions of contaminant dispersion and accumulation.

The evolution of computer technology and information transfer in recent years now allows data of the present kind to be made available electronically on the Internet and on CD-ROM. As discussed in the chapter on >data visualization (Hayes and others, this volume), the present data set is made available in a form such that it can be combined with other data through Geographic Information Systems (GIS) software.


The work plan

The development of the current database involved four phases: 1) Discussions and interactions with knowledgeable individuals and agencies to identify and retrieve pertinent data from many sources, including unpublished data, 2) Selection and augmentation of a data dictionary to provide consistent field/parameter definitions and background documentation, 3) Encoding and integration of the data tables in a master flat-file database system involving eight basic tables linked by unique sample identification codes, 4) Selective querying, mapping, and interpretation of the data with the help of standard data sets to identify potential outliers and quality problems and to identify key geochemical and environmental relationships and processes.

Environmental issues include linking the chemical data to bulk sediment toxicity criteria, searching for possible contaminant sources, and identifying transport and geochemical processes. The in-depth inventory and assessment was only possible with the help of many partners and cooperators.


Problems associated with compilations of heterogeneous data

Until the 1990's, combining chemical data on sediments from diverse sources was not favored because of the possibility for systematic error or incompatibility. Even with the establishment of analytical protocols, Federal agencies may use significantly different analytical methods due to varying objectives (Macauley and Summers, 1998; NS&T, 1996). However, assessment methods described in Manheim and Hathaway (1991) and Manheim, Buchholtz ten Brink, and Mecray (1998) have shown the feasibility of data compilation from heterogeneous sources, including data from older sources. As described in the following sections, analytical review and interpretation took place iteratively throughout data entry and processing phases, supplemented by contacts with people knowledgeable about data sources and analytical methodologies, wherever possible.


Table of Contents: Sediment Database

Forward to Database Structure

[an error occurred while processing this directive]