Abstract
The concentrations of select metals (Cd, Co, Cu, Fe, Pb, Ni and Zn) within the water column and sediment of the permanently open Kariega Estuary and temporary open / closed Riet and East Kleinemonde Estuaries were investigated during a dry and a wet season. Enrichment factors (EFs), using Fe as a reference element, and baseline linear regression models for metals vs Fe were used to assess the extent of metal enrichment in the sediments. The results of the study indicate that Cd, Co Ni and Pb were enriched above baseline concentrations (1.0 < EF < 4.1) in the sediments of all three estuaries. Co, Pb and Ni enrichment in the Kariega Estuary sediments was significantly higher during the dry season, and the mean concentrations of Pb and Cd in the water column were 19-fold and 66-fold higher in the dry season. The elevated concentration of metals during the dry season could be related to accumulation of diffuse pollution from human activities within the catchment area. Conversely, inflow of freshwater into the estuary had the net effect of reducing the concentration and enrichment of these metals within the Kariega Estuary due to scouring and outflow of estuarine water and sediment into the marine environment. The temporal variations in metal concentrations and enrichment factors were less pronounced in the temporary open / closed estuaries than the Kariega Estuary. The observed trend can probably be related to the low anthropogenic impact within the catchment areas of these systems, and the relatively smaller size of the catchments. Significant spatial variations existed in metal enrichment in the sediment of both the East Kleinemonde and Riet Estuaries, with the highest degrees of enrichment occurring in the sediments from the marine environment and lower reaches.
Highlights
Estuaries may be the repositories of contaminants generated by human activities (Kennish, 1997)
The mean concentration of Cd decreased from 3.323 ± 4.093 μg·l-1 in the dry season to 0.0503 ± 0.991 μg·l-1 in the wet season (Fig. 5B), and the mean concentration of Pb decreased from 34.13 ± 42.56 μg·l-1 in the dry season to 1.753 ± 1.096 μg·l-1 in the wet season (Fig. 5A). Both the mean concentrations of Cd and Pb were above the target concentrations (4 μg·l-1 and 12 μg·l-1 respectively) recommended for South African coastal waters (DWAF, 1995). These results suggest that the inflow of freshwater into the Kariega Estuary had the net effect of reducing the concentrations of Pb and Cd in the water column by flushing estuarine water to the marine environment, or via dilution with freshwater
The increased freshwater inflow led to a reduction in Co, Ni and Pb enrichment in the sediments of the Kariega Estuary
Summary
Estuaries may be the repositories of contaminants generated by human activities (Kennish, 1997). Metals typically bind to fine grained sediments, such as mud and organic matter. Concentrations of metals in the sediment are typically several orders of magnitude higher than those in the water column (Bryan and Langston, 1992), and the levels of metals within the water column may meet water quality guidelines for water use and ecological health but the sediment may not (Binning and Baird, 2001). The concentrations of metals in the water column are subject to greater temporal and spatial variability than in the sediment, which makes it difficult to obtain water samples that are representative of the contaminant status of the water-body. On the other hand, are long-term integrators of metals and provide a more accurate reflection of the extent of metal contamination within a particular system (Binning and Baird, 2001)
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