Several watersheds in the Louisiana Pontchartrain Basin are listed as impaired, with dissolved Cu as one of the impairments. In this study we determine Cu levels in sediment of Lake Pontchartrain (which comprises part of one of the largest estuaries in the Gulf Coast region) which receives water from impaired watersheds. Sediment samples were collected along an 8 km sampling grid (174 sites) and analyzed for total Cu in sediment. The average Cu content in sediment was 13.6 ± 6.7 mg.kg−1 ranging from 0 to 31.5 mg.kg−1 (n = 174). Results show that there was little Cu pollution in the lake sediment. Cu level was correlated with Al content (r = 0.888**) and Fe content (r = 0.907**) of sediment. Higher Cu was also found in sediment with high clay content (r = 0.429**). Cu was inversely correlated with sand content of sediment (r = −0.520**).
Cu is an essential nutrient element to aquatic organisms. However, it can cause toxicity at elevated concentration (Santore et al., 2001). Both oxidation states, Cu1 + and Cu2 + can exist in aqueous systems, although the latter is much more dominant. The Cu toxicity is proportional to the concentration of Cu2 + rather than total Cu in aquatic ecosystems. As low as 40 μ g·l− 1 of Cu in the water has been found to be toxic to many fish (Erickson, 1996). In general, 3 μ g·l− 1 of Cu concentration has been considered as the level for “reference water”; whereas Cu found in “typical” seawater is about 0.9 μ g·l− 1. Cu concentration in the estuaries can be high due to contribution from industrial sources and municipal waste treatment facilities. The Cu in surface water is often strongly associated with organic colloids so that almost no Cu free ion concentration would exist when dissolved or particulate organic carbon is present (Pagenkopf, 1983). The Cu associated with organic colloids is also significantly impacted by the salinity level of a water body due to relatively easy flocculation of Cu-associated colloidal phases.
In 2002, USEPA mandated the addition of several watershed subsegment of Louisiana Lake Pontchartrain basin to the 303(d) list of impaired water bodies and identified one impaired cause as dissolved Cu although the exact source was unknown. Louisiana Department of Environmental Quality (LDEQ) reported some of these subsegments located on the Northshores of Lake Pontchartrain in the Category 1 of a 2004 Integrated Report. There are also sources of Cu entering Lake Pontchartrain from the New Orleans urban area bordering the southshores of Lake Pontchartrain. For example, the estimated annual loading of Cu in run-off from Jefferson Parish is 2054 kg·y− 1 (NPDES Storm Water Permit, 1993).
Therefore the recently reported high Cu levels in watershed subsegments of Lake Pontchartrain Basin are likely caused by loading of sources other than within the Lake system. In this study Cu levels in Lake Pontchartrain bottom sediment in relation to sediment properties were determined.
The objective of this study was to (1) determine Cu content of bottom sediment in Lake Pontchartrain and (2) determine the relationship between Cu and sediment properties. To achieve this objective, grid samplings were carried out using GPS and the sediment samples were analyzed for total Cu.
Sediment samples were collected for various analyses from predetermined sampling sites (8 km grid) in Lake Pontchartrain (Figure 1). Lake Pontchartrain and adjacent lakes in Louisiana form one of the largest estuaries in the Gulf Coast region. A total of 174 sites were sampled using a global positioning system (GPS). The samples were collected from the surface 10 cm of sediment using a hand operated Peterson dredges. A sub sample of the sediment was placed in 1 liter wide mouth jars, sealed and placed on ice until transported to the laboratory for storage at 5°C until the various analysis were carried out.
After transport to the laboratory an aliquot from each jar was removed and used for the determination of Cu, total Fe and Al content, organic matter, salinity, and grain size. Sediment redox potential and pH were measured using platinum electrodes and an Ag/AgCl reference electrode. Four replicate electrodes were inserted in the sediment and allowed to equilibrate for 6 hours before measurement. The pH was measured using a combination glass-reference electrode.
Surface water salinity and conductively were taken at the same time the sediment samples were collected using a portable salinity/conductivity meter. Sediment particle size analysis was determined using the Sedigraph technique (Coakley et al., 1991). The sediment was pretreated to remove organic matter and calcium before analysis. Organic matter content was also determined in the sediment using loss on ignition method (Davis, 1974).
Cu analysis (total analysis)
Sediment was dried (100°C), ground, and thoroughly mixed prior to analysis. Sediment was digested using standard EPA method 3050 with HNO3 and H2O2. The Cu, Fe and Al in digested sediment sample were determined using ICP. The “Clean” sampling technique was employed, and negative and positive controls were used to assure quality of sampling and analytical process. Analysis was calibrated against known standard of Cu. Data were compiled and statistically analyses performed using Microsoft Excel program available in Microsoft Professional 2000 on an IBM PC-AT.
Results and discussion
The concentration of Cu in Lake Pontchartrain sediment was not elevated and was within the normal range for the metal in sediment. Average Cu concentration was 13.6 ± 6.7 mg·kg− 1 and ranged from 1 to 31.5 mg·kg− 1 (n = 174). The measured Cu concentration in sediment was similar to results from a previous study along the Louisiana Coastal zone where Cu concentration ranged between 7.4 to 30.3 mg·kg− 1 (Pardue et al., 1992). DeLaune et al. (2008) reported Cu level of 17.3 ± 10.4 mg.kg− 1 (n = 220) for soils and sediment of Chenier Plain region of the Southwest Louisiana coastal zone. Byrne and DeLeon (1987) reported average Cu content of 5.3 mg·kg− 1 for a limited number of sites in Lake Pontchartrain sediment. Manheim et al. (1997) and Manheim and Hayes (2002), from an analysis of 11 different data sets, reported mean concentration of 17 mg·kg− 1 Cu for Lake Pontchartrain.
Regression analysis was performed on the data set comparing concentrations of Cu with sediment properties. The regression of Cu and toxic metals to Al, Fe, organic matter, sand, silt and clay are shown in Table 1. A strong relationship was found between Al and Cu (r = 0.888**) (Figure 3). Al is a major constituent of aluminosilicate minerals that are important metal-bearing phases in coastal sediments (Windom et al., 1989). Strong correlations with Al in these sediments suggest that metals found in mineral sediment material rather than pollution is source of metal in lake sediment. Metal/Al correlations remain significant despite likely repartitioning into various sediment phases (e.g. sulfide or Fe oxide phases) following deposition (Pardue et al., 1992).
Cu was also statistical correlated with Fe and Al content of the lake sediment (Table 1). Figure 2 shows typical relationship between Cu and Fe. Figure 3 shows typical relationship between Cu and Al. The correlation for Cu and Fe was r = 0.907** and for Cu and Al was r = 0.888** (Table 1). Cu was also correlated with clay content of the sediment (r = 0.429**) (Table 1). The higher concentration of Cu was found in sediment with high clay content (r = 0.429**). Sandy sediment was inversely correlated with Cu (r = −0.520**) (Table 1). Correlations between Cu and other sediment properties are also shown in Table 1.
Even though the total Cu level in sediment was not elevated, it is difficult to predict its bioavailablity. The most widely used approach to predict metal bioavailability in sediment is based on the tendency of toxic metals (Cd, Cu, Pb, Ni and Zn) to form highly insoluble metal sulfides. Metals including Cu are predicted to be unavailable (and non toxic) if the molar sum of the concentration is less than the molar concentration of AVS (Ankley et al., 1996). In addition the adsorption and/or release of metal is strongly influenced by sediment texture and organic matter content.
Results from this study show that overall there was little Cu pollution in sediment of the lake. The levels are well below contaminated levels and in general within sediment background levels (as supported by correlations with Fe and Al content of sediment). Results will serve as a baseline for future studies to document Cu pollution in Lake Pontchartrain. Areas where possible Cu contamination may not have been adequately documented by this study but deserve future investigation include, urban waterways such as drainage canals in New Orleans and sites near industry sources.
The research presented in this paper was supported by the Lake Pontchartrain Basin Foundation, 3338 North Causeway Blvd, Matairrie, La 70002.