Lake Pontchartrain
Basin: Bottom Sediments and Related Environmental Resources |
Summary of Geological and Chemical Data (cont.)
Organic Components
Organic components are presented in five subtables of the database (appendix
D), corresponding to major organic
constituent categories as follows:
- General (total) organics
- Polychlorinated biphenyls (PCB) and pesticides
- Polyaromatic hydrocarbons (PAH)
- Aliphatic hydrocarbons
- Miscellaneous organic components
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![freq.gif (3033 bytes)](../images/freq_sm.gif)
Table 10
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The frequency of occurrence of the organic constituents is recorded in table
10. Although organic constituents were analyzed in the
extensive Louisiana Department of Environmental Quality resource studies of 1983-84
(Overton and others, 1984; Schurtz and St. Pé, 1984; Chew and Swilley, 1987), they were mostly below detection limits. The
EMAP studies of 1991-94
(Macauley and Summers, 1998) have yielded more quantitative
data on the many related congeners (organic chemical groups) utilizing more sensitive
methods. The EMAP studies also provide
data on biota, water column, and organism toxicity tests. These data are not included in the
appendix D database but are available from
USEPA at http://www.epa.gov/emap/html/dataI/estuary/data/.
Organic constituents in generic groups, like
polyaromatic hydrocarbons (PAH), often show systematic
relationships when plotted on log/log scales (fig. 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 correlation 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. |
![PAHplot2.gif (15746 bytes)](../images/PAHplot2.gif)
Figure 25. Phenanthrene versus PAHs
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The largest and most prominent group of organic constituents detected in Lake
Pontchartrain is the polyaromatic hydrocarbons and polychlorinated biphenyls (PCBs).
Unlike PCBs, which are exclusively manmade compounds that have wide industrial uses, PAHs
are of both natural and anthropogenic origin. Perylene is an example of a hydrocarbon that
is a common breakdown product of natural organic matter. It is not listed in leading
registers of toxic organic components. It is, however, included in the analytical schemes
designed to recover the principal PAH congeners (structurally related organic compounds).
Figure 26 shows that perylene is found in highest
concentrations (up to 541 parts per billion, ppb) in Lake Maurepas, which is mainly surrounded by
swamps and marshes. Next in concentration are sediments from the western half of Lake
Pontchartrain, which likewise has many sources of natural organic input from marshes or
trees (see fig. 14). Perylene is enriched in the
finer grained sediments in the center part of the lake. Still lower concentrations are
found in the eastern half of Lake Pontchartrain, including areas adjoining the New Orleans
shoreline south of the lake and Slidell, LA, on the northeastern shore.
Figure 26
![perylene.GIF (14475 bytes)](../images/peryl_sm.GIF)
Perylene concentrations in sediments from Lakes Pontchartrain and
Maurepas |
Figure 27
![pyrene.GIF (14720 bytes)](../images/pyren_sm.GIF)
Pyrene concentrations in sediments from Lakes Pontchartrain and Maurepas |
Pyrene is a high-molecular- weight PAH compound. It is unsubstituted and is commonly
found as an advanced and refractory (resistant) breakdown product of weathering and
transport of both petroleum products and coal tars. In contrast to perylene, the largest
concentrations of pyrene were found along the New Orleans shorefront close to abundant
sources of urban waste products (fig. 27).
The same pattern shown by pyrene is reflected in other PAHs of anthropogenic
origin (phenanthrene, benzo a pyrene, etc.). Special interest was focused on the northeastern
part of the lake, where a major creosote spill took place in Bayou Bonfouca in the
1970s. Did the spill enter the lake? The few samples near the bayou show low
concentrations of all PAHs. The pyrene/perylene ratio was examined; the sample
adjacent to the bayou (Unique ID 88) shows a greatly enhanced value (~6). Ratios over 2
are generally restricted to areas where urban or industrial wastes can be documented. This
suggests influence of anthropogenic hydrocarbons on this sample, although further sampling
would be needed to better define the possible presence and extent of creosote dispersal
into the lake. The low absolute values in ID 22 and other peripheral samples suggest that
the impact on the lake itself is likely to be small. Enrichment of pyrene in the Lake
Maurepas area is limited to sites within the Amite River (northwest of the
lake).
Figure 28
![PCB44.GIF (20044 bytes)](../images/PCB44_sm.GIF)
PCB 44 concentrations in sediments from
southeastern Louisiana waterways |
The distribution of PCB 44 (a congener on the low end of the PCB molecular
weight spectrum) is shown in figure 28 for the entire
sample distribution in the database. All of the values within the lakes and in the
Intracoastal Waterway were less than 1 ppb in sediments. The highest PCB values (below 1
ppb) were generally outside the lake, with the exception of a few samples near the New
Orleans shoreline. Similar observations hold for pesticides (DDT and its breakdown
products, chlordane, dieldrin, and aldrin). |
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