Environmental Atlas of the Lake Pontchartrain Basin

	Environmental Issues: Coastal Land Loss | Shoreline Change and Rates | Urbanization | Aggregate Dredge Holes | Dredge Pit Characterization | Bonnet Carré Diversion | Water Quality | Sediment Quality & Dispersal

Environmental Issues - Sediment Quality and Dispersal

• Trace Metal Distribution in Surficial Sediments
• Sediment Geochemistry: Aluminum
• Sediment Geochemistry: Silicon
• Sediment Geochemistry: Magnesium and Trace Metals

Figure 94: Sediment sample sites for chemical and grain size data. Click on the image for a larger version.

Numerous studies over the past three decades have collected surficial sediment samples to determine chemical composition of the Lake bottom (Figure 94). As a part of this project, the studies have been compiled into a comprehensive database (Manheim and Hayes, 2000). A toxic constituent in these sediments is trace metals associated with the input of pollutants to the Lake. Distribution of these metals indicate increased concentrations along the south shore and in the outfall canals (Figure 95 a, b, and c). Average concentrations of trace metals, however, fall below widely-used bulk-sediment toxicity criteria (SQG's) as noted in Table 26.

In 1996 and 1997 surficial sediments were sampled over a 3/4 kilometer grid across the Lake. The frequency distribution of various metals measured within these sediments (Figure 96 a, b, and c) indicate an increase in constituents such as Fe and Cu within the surficial sediments, whereas Pb remained relatively static. The increase in trace metals over this time period is attributed by Waters et al., 2000 to diversion of Mississippi River floodwater into the Lake.

Table 26: Average Trace Metal Concentrations
Lake Pontchartrain1				Miss. River TSS2		Toxicity criteria levels
Constituent	Mean	SD3	N4	Mean	N4	Low5	Medium6
Ag (µ/g)	0.148	0.085	127			1	3.7
Al (%)	5.05	1.82	127	8.00	73	NA	NA
As (µ/g)	7.02	3.93	127			8.2	7.0
Ba (µ/g)	482.1	187.6	115			NA	NA
Cd (µ/g)	0.211	0.162	127	1.3	7.3	1.2	9.6
Co (µ/g)	9.41	2.43	121	21	73	NA	NA
Cr (µ/g)	26.3	19.8	285	80	73	81	370
Cu (µ/g)	17.5	8.99	285	45	73	34	270
Fe (%)	2.43	1.07	248	4.61	73	NA	NA
Hg (µ/g)	0.42	0.021	114			0.15	0.71
Mn (µ/g)	526.2	398.7	127	1300	73	NA	NA
Ni (µ/g)	18.2	7.86	248	55	73	20.9	51.8
Pb (µ/g)	17.5	6.78	164	46	73	46.7	218
Sb (µ/g)	0.62	0.36	127
Se (µ/g)	0.34	0.22	122
Sn (µ/g)	1.53	0.76	126
V (µ/g)	78.3	31.3	121
Zn (µ/g)	73.3	31.1	285	193	73	150	410
NA = these elements are not toxic even at high concentrations. 1 Mean chemical composition of Lake Pontchartrain sediments (from comprehensive database). 2 Data for Mississippi River total suspended solids (TSS) from Presley et al., 1980. 3 Standard deviation. 4 Number of samples. 5 Sediment quality guidelines (SQG) for potential toxicity levels ("Low"), Long et al., 1995. 6 Sediment quality guidelines (SQG) for potential toxicity levels ("Medium"), Long et al., 1995.

Table 26: Average trace metal concentrations for Lake Pontchartrain and Mississippi River.

	Figure 95: a) (top left) Copper concentrations in sediments from Lake Pontchartrain and Maurepas. b) (top right) Lead concentrations in sediments from Lake Pontchartrain. c) (bottom left) Zinc concentrations in sediments from Lakes Pontchartrain and Maurepas. Click on the maps for a larger version.

Figure 96: a) (top) Frequency distribution of Iron (weight %) within sediment fine fraction preceding and following spillway event. b) (middle) Frequency distribution of Copper (weight %) within sediment fine fraction preceding and following spillway event. c) (bottom) Frequency distribution of Lead (weight %) within sediment fine fraction preceding and following spillway event.