| Environmental Geochemistry and
Sediment Quality in Lake Pontchartrain
Sediment Database and Preliminary Interpretations
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
Both organic and inorganic constituents have nutrient functions (carbon, nitrogen,
phosphorus, copper, iron, zinc, and others) 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, pre-settlement levels, they
may upset environmental balance. Contaminants can reach levels that are toxic to bottom
organisms like clams or worms, which in turn are fed upon by fish and other swimming
organisms. Contaminants from sediments may be resuspended to the water column and ingested
Besides their potential for toxic effects, the distribution of chemical constituents
may prove helpful to better knowledge of the overall role and transport of toxic materials
to the ecosystem. They provide a complement to short-term measurements of discharges
through water measurements. The studies in this report are mainly limited to surface
sediments. The final report for the five-year Pontchartrain program will include data on
cores. These are expected to provide a more definitive delineation of pre-settlement
background levels of trace metals and organic matter with which surficial sediment
concentrations can be compared.
The first objective of the work was to delineate the status of sediments in Lake
Pontchartrain Basin by inclusion of all available chemical, physical and other data on
estuarine and coastal sediments. This effort has increased the number of samples yielding
information and available in one source from 100 to 200 at the beginning of the work in
1996, to 1400 currently. These data have been placed in a database from which users can
readily retrieve data using standard desktop computers and a variety of commercial
software. The data is prepared in formats that make it suitable for mapping and display in
many ways, including incorporation in geographic information systems (GIS), to be compared
or used with other kinds of data. The combination of basic data and interpretations are
designed to assist in management decisions as well as scientific and public information
regarding the environmental status of the water bodies.
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 to
also 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 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 be made available electronically on the Internet, and on
CD-ROM. As discussed in the section on CD-ROM and electronic data,
the present data 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.
This presentation is a preliminary summary of available information. Additional types
of data (e.g. radiometric dating, supplemental samples and analyses especially for core
data) are planned for inclusion in the final report of the Lake Pontchartrain Basin
Project to be completed in 1999.
Problems associated with compilations of heterogeneous data
Many of the basic techniques and approaches for compiling heterogeneous data on
sediment chemistry were adapted from chemical sediment database assessment systems
described in Manheim and Hathaway, 1991, Manheim and others, 1998, and Buchholtz ten Brink
and others (in preparation). The data are obtained from documents that have come from
different organizations and reflect varying objectives and methodologies. Therefore,
reliance on standard quality control (QA/QC) protocols utilized for more recent federal
agency work were either not feasible or only partly applicable for many of the data sets.
Yet, potentially valuable data exist in material from the diverse sources. Consequently,
procedures developed earlier (Manheim et al, 1998) were employed to minimize problems with
data comparability. Assessment and interpretation took place at intervals throughout data
entry and processing, supplemented by contacts with persons knowledgeable about data
sources, wherever possible.