Lake Pontchartrain Basin:  Bottom Sediments and Related Environmental Resources

Summary of Geological and Chemical Data (cont.)

Coastal and Inland Waterway Impacts and Sediment Toxicity Considerations

Inland Waterways

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).

Another severe contamination event affected Bayou Bonfouca (Slidell) in northeastern Lake Pontchartrain in 1970. Tanks belonging to a 100- year-old firm spilled creosote over a 55- square-mile area as a result of fire. The site became a USEPA Superfund site in 1982, and the site was listed as having achieved final construction (remediation) in August 1997 (

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).


Toxicity guidelines

     Figure 34
histogram thumbnail Recent guidelines relating chemical composition of bulk sediment to effects-based toxicity criteria provide a screening-level assessment of the environmental status of sediments with respect to specific contaminants. These are illustrated in histograms for three of the heavy metals in figure 34.  The dashed lines refer to effects-based animal toxicity criteria of Long and others (1995). The data on which the threshold values are based were gained from a nationwide set of data that include both laboratory toxicity tests on sensitive organisms and contaminant analyses on bulk sediment samples (dry weight basis). The "ERM" values are defined at concentration ranges associated with a 50-percentile level of acute toxicity symptoms in test organisms. The "ERL" level corresponds to the 10-percentile level (see table 8). Since this type of toxicity criterion is statistically based on national samplings, it can only suggest toxicity potential in relative terms to specific local samples or areas. It cannot assess bioaccumulation and other more specific issues for given species. However, the ERL and ERM guidelines have gained wide use as a conservative screening tool that is particularly useful in identifying areas that should be subjected to more detailed analysis ("higher tier" toxicity testing). This type of criterion is the only one applicable to historical sediments. Alternative approaches are summarized in Baker and Kravitz (1992).

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Sediment Database and Geochemical Assessment
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