| Lake Pontchartrain Basin: Bottom Sediments and Related Environmental Resources |
Table B2. Interpretive notes
[for consolidated table as well as full database, appendixes C and D]| Methodologies | |
| Leachate vs. total dissolution analysis | All metal analyses reported by Demcheck and others (1996), Schurtz and St Pé (1984), Byrne and DeLeon (1986, 1987), Landrum (1994), LDEQ (1989), and Marot and Holmes (in review) were performed by leachate methods. Data by Flowers and Isphording (1990), Macauley and Summers (1998), and Sarkar (1995) were performed by total dissolution techniques. Aluminum, iron, and barium values, in particular, are much lower when analyzed by leachate methods than those analyzed by total dissolution methods. Results from these two methods should not be combined without normalization. Variable water content complicates interpretation of data for Byrne and DeLeon (1986, 1987), and Voyksner (1985). See Discussion for further information. |
| Dry vs. wet weight |
The following data are reported on a
wet weight basis (and, therefore, will be lower by the proportion of
water in the samples): - As, Hg, and Se data of Byrne and DeLeon (1986), - organics data of Ferrario and others (1985), - organics data of Schurtz and St. Pé (1984), and - the single sample of Voyksner (1985). All others are on a dry weight basis. |
| Organics | |
| 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 ("ERM") PCB levels. Long and others (1995)
report toxicity ranges for PAH congeners in the 0.5 to 9.6 ug/g range, with Total PAH
having much higher values. Total PCBs have a range in the vicinity of .027, and pesticides
0.005 - 0.025.
The large body of information on organics found in Macauley and Summers (1998) was analyzed using more recent, high-sensitivity methods at the Texas A&M Geochemical and Environmental Research Group (GERG), which serves several Federal agency programs. These data showed no internal evidence of quality problems. |
| Inorganics | |
| Ag/Zn | Ag/Zn ratio refers to Ag/Zn x 1000. Note that data of Schurtz and St. Pé (1984), Demcheck and others (1996), and Byrne and DeLeon (1986, 1987) 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 used in Flowers and Isphording (1990), Macauley and Summers (1998), and Sarkar (1995). |
| As/Zn | As/Zn ratios have particularly wide scatter. Some data, analyzed by leachate techniques, showed low but consistent ratio patterns. These are attributed to low recovery of non-contaminant As. Data sets with anomalously high discriminant levels that could not be validated are marked with "W". |
| Ba | As noted in the text, barium values are particularly affected by analytical methods. Reported values by leachate method are highly variable. Elevated concentrations of barium in the vicinity of New Orleans waterways appear to be confirmed by analyses from several sources, but assessment of true values requires reevaluation by total dissolution methods. |
| 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 showed 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). See Discussion for comments on Cr analyzed by leachate methods. |
| Mo | Elevated Mo values are exclusively found in Bayou Trepagnier, where this metal may have been leached from Mo-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. |
| Sources | |
| Demcheck and others (1996) | Metals were extracted using standard digestion
method 3050 RCRA, hot acid leach. Most were then analyzed using Inductively Coupled Plasma
(ICP) spectrometry, USEPA method 200.7 (Atomic Emission Spectrometry). The exceptions are
arsenic (As), selenium (Se), and thallium (Tl), which were analyzed using Graphite Furnace
Atomic Absorption Spectrometry, method 200.9; and mercury (Hg) which was analyzed using
Flameless (Cold Vapor) Atomic Absorption Spectrometry, USEPA method 245.5.
Nitrogen (nitrate, nitrite) is reported in bottom material (sediments) as "mg/kg as N" (mg/kg = ug/g). These parameters are normally reported for water column data, not sediments. The specific methods are not reported. |
| LDEQ (1989) | 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 clearly indicated in the analytical notes (p. 23 in the LDEQ report). |
| Tarver and Dugas (1973) | Coordinates for sample locations were derived from a sketch map with a probable error of +/- 1minute. |
| Tarver and Savoie (1976) | The sample locations were only given in generalized terms (grid squares drawn on a map with sides of approximately 3 minutes in length). The coordinates used in this database have been approximated by calculating the coordinates of the centroids of these grid squares. Locations for stations along the edge of lakes are the approximated centroids of the irregular shape formed by the square and the shoreline. |
Back to Appendix B |
Forward to Table B3  |