| Environmental Geochemistry and
Sediment Quality in Lake Pontchartrain |
Table 9. Other interpretive notes
A. Inorganics
| Metal |
Related
Notes |
| Ag |
Ag/Zn ratio refers to Ag/Zn x 1000. Note that
data of Schurtz & St. Pe, Demcheck, and Byrne (LL) are leachate analyses, which yield
lower recovery than total dissolution methods |
| Al |
Leachate analyses for Al are expected to yield
significantly lower concentrations than total dissolution analyses (e.g. Flowers, 1990,
EMAP, 1998). |
| As |
As/Zn ratios have particularly wide scatter for
this element. Some data, analyzed by leachate techniques, showed low but consistent ratio
patterns. These are attributed to low recovery of noncontaminant As. Data sets with
anomalously high discriminant levels that could not be validated are marked with
"W". |
| Cd |
A number of EPA-EMAP data fall below the low
alert level for Cd (.04). These values may represent local background levels for natural,
uncontaminated sediments. Two data sources reported many samples with anomalous Cd values
and discriminant ratios (Cd/Zn) that were not corroborated by other investigators’
data in comparable areas. The entire data sets were marked with "W". |
| Cr |
Extreme, high-alert values of Cr are found only
in Bayou Trepagnier, where major metal concentrations were discharged into the waterway
from a petrochemical complex (see also Mo, Pb, and Zn). Normal aluminosilicate minerals
(clays) have relatively high Cr values, which may be only partly extracted by leachate
methods. |
| Mo |
Elevated Mo values are exclusively found in
Bayou Trepagnier, where this metal may have been leached from molybdenum-containing
stainless steels in a petrochemical plant. |
| Pb |
Extreme Pb values are found mainly in Bayou
Trepagnier. |
| Zn |
Extreme Zn values are found mainly in Bayou
Trepagnier |
B. Source-specific Notes
| Dry vs Wet weight |
The data of Byrne and others, and Voyksner are
reported on a wet weight basis. All others are on a dry weight basis. This means that all
their data will be lower by the proportion of water in the samples. |
| Organic constituents and toxicities |
There is no low alert range for organic
compounds, because many are not found naturally in sediments. Toxic ranges begin at
roughly 0.18 ug/g for the most toxic PCB ("ERM" levels. Long and others (1995)
report toxicity ranges for PAH congeners in the .5 to 9.6 ug/g range, with Total PAH
having much higher values. Total PCBs have a range of about .027, and pesticides
.005-.025. Organics are not quality controlled in this report. The analyses reported in
Schurtz and St. Pe (1984) are mainly nondetects, whereas the large body of information on
organics generally is found in EMAP, 1998. These high-quality data were entered most
recently, and processing is underway at this writing. |
| LDEQ |
Although most metals are stated to have been
analyzed by atomic absorption graphite furnace, whether these refer to total metal or acid
extraction is not unambiguously indicated in the analytical notes (p. 23) |
| Tarver |
The samples are available only in a generalized
grid location. |
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