Summary of Geological and Chemical Data (cont.)
Coastal and Inland Waterway Impacts and Sediment Toxicity Considerations
The highest contaminant concentrations in the Lake Pontchartrain Basin area are metal
concentrations in Bayou Trepagnier, a small inland water body adjacent to Bonnet Carré
Spillway (see opening illustration for location of
spillway; figures 17, 19, 21 for metal
concentrations). The bayou's water now discharges to the Mississippi River
rather than to Lake Pontchartrain. The central part of the bayou was the
site of long-continued metal discharge from a petrochemical plant (LDEQ,
1989). High values of chromium and lead have been observed in bank
sediments from the central part of this waterway. These metal
concentrations in sediments decrease markedly toward Lake Pontchartrain.
The few samples within the Bonnet Carré Spillway and the many located
near the mouth of the bayou in Lake Pontchartrain (fig. 31)
do not show significant contaminant
enrichment (Sarkar, 1995). Hence, the severely contaminated Bayou Trepagnier sediments
are probably not a hazard to the lake.
This conclusion and details about metal relationships in sediments and
waters of Bayou Trepagnier are given in a Ph.D. dissertation by Clymire
(1996) and in a summary by Flowers and others (1996).
The extent of organic contaminants dispersed into Lake Pontchartrain from the Bayou Bonfouca spill was discussed briefly in the section Organic Components. Although the spill was massive within the land area, there is little evidence of extensive residual contamination in the adjacent lake sediments, based on sparse sampling. Another spill at Madisonville (north-central shore of Lake Pontchartrain) in 1996 has not yet been assessed.
The breakdown of massive oil spills in Lake Maracaibo and in the Persian Gulf as a result of the Desert Storm campaign (Al-Lihaibi and Ghazi, 1997) demonstrates that in warm environments hydrocarbons can be broken down by microbial action over a span of decades or less. Thus, samples collected in the 1980's and 1990's would not necessarily show the full extent of creosote transport into the lake at the time of the action (1970).
White and Tittlebaum (1984) report additional sediment chemical data on a series of samples in minor waterways around New Orleans. The data are not precisely located and, hence, have not been included in the database. The maximum lead concentration found was 318 µg/g in the Violet Diversion, and the next highest was 131 µg/g in the Harvey Canal; the minimum for lead was 48 µg/g. Zinc levels as high as 198 µg/g and as low as 57 µg/g were measured. No chromium values higher than 72 µg/g were found.
The limited data of White and Tittlebaum (1984) and Landrum (1994) suggest that the Intracoastal Waterway sediments have elevated concentrations of metals that are occasionally potentially toxic (see table 8). Lead is the main (but not necessarily pervasive) hazard in the inland waterways excluding Bayou Trepagnier. Sampling is too sparse to detect possible small or patchy toxic metal concentrations. Bottom currents, especially those caused by ships' propellers, may cause transport and mixing of possible earlier hotspots.
In summary, contaminants discharged into these two important inland waterways, Bayou Trepagnier and Bayou Bonfouca, have not been transported on a significant scale into the lake itself. A qualification of this statement in the case of the hydrocarbons must be made because of the limited number of analyzed lake samples and the possibility that microbial action in the warm sediments would have broken down most of any creosote transported into the lake at the time of the Bayou Bonfouca spill (1970).