III.  Data and Preliminary Interpretations (con't)

C.  Inorganic Chemistry

Eight metals from among the many inorganic constituents reported in the major chemical table (Appendix 4), are plotted in figures 17 to 24. These are displayed in two ways. The size of the dots is proportional to their concentration, as shown in the legends. At the same time samples whose metal concentration exceeds screening guidelines for potential sediment toxicity to bottom organisms are plotted in yellow and red. It may be readily noted that the majority of yellow and red-indicated samples occur in coastal areas near the New Orleans urban area, or in inland waterways, especially Bayou Trepagnier. The Bayou is the locus of former waste discharge by a petrochemical complex (LDEQ, 1989). The inlet’s water now discharges into the Mississippi River rather than Lake Pontchartrain. Surficial samples near the inlet have not to date registered significant influence on Lake Pontchartrain sediments (Sarkar, 1995).

As explained, the specific analytical method affects the apparent distribution of certain constituents. Chromium (Fig. 21) in particular is present in relatively high concentrations in naturally-occurring (pristine) clays. The irregular sprinkling of low-level "toxicities" for this element are more likely the product of natural distributions than pollutant-related. The apparent "triggering" of warnings of low-level toxicity levels has been previously noted in scientific studies in other parts of the country, where Cr is analyzed by total dissolution methods. In like fashion, though, some of the anomalies occur logically at the mouth of canals and other waterways known to have discharged significant contaminated wastes. A few isolated datapoints will be reviewed in future studies and may not necessarily signify real enhanced contaminant deposits. In all cases isolated anomalies need to be viewed with caution.

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Beginning of Section Sediment Database and Preliminary Interpretations