USGS - Pontchartrain Geochemistry

Environmental Geochemistry and Sediment Quality in Lake Pontchartrain

III. Data and Preliminary Interpretations (con't)

D. Organic Components

Organic components are presented in five subtables, corresponding to major organic constituent categories as follows:

General (total) organics
Polychlorinated biphenyls (PCB) and pesticides
Polyaromatic hydrocarbons (PAH)
Aliphatic hydrocarbons (USEPA EMAP)
Miscellaneous organic components

The frequency of occurrence of the organic constituents is recorded in Table 7. Although organic constituents were analyzed in the extensive Louisiana Department of Environmental Quality resource studies of 1983-84 (Overton, 1984, Schurtz and St. Pé, 1984), they were mostly below detection limits. The EMAP studies of 1991-94, utilizing more sensitive methods, have yielded more quantitative data on the many related congeners or organic species. The EMAP studies also provide chemical data on biota (e.g. fish), water column and toxicity studies. The sediment data have been entered into Appendix C and D just prior to publication, but the data have not yet been reviewed or interpreted for this publication.

Organic constituents in generic groups, like PAH's, often show systematic relationships when plotted on log/log scales (see Figure 25). Closer correlations are expected between congeners having closer chemical relationships based on molecular weight, specific gravity, melting point, and solubility. Thus, phenanthrene shows closer correlationship with pyrene (pink squares) than with the low-molecular-weight naphthalene (yellow triangles). Essentially, such plots measure the consistency in ratios over wide ranges of absolute concentrations.

Fig. 25. Phenanthrene vs. PAHs

The earlier data indicated detection of organic contaminants mainly in close proximity to the New Orleans shoreline, especially a few hundred meters distance from canals (Overton and others, 1986). Other comments on toxic organics are presented in section F. (Coastal and Inland Waterway Impacts).

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Beginning of Section Sediment Database and Preliminary Interpretations