CHAPTER 7 GEOCHEMISTRY
7.3. DISCUSSION
7.3.2. Enrichment and Contamination
Sediments are excellent indicators of contaminants as these are integrated into sediments which are in constant flux with the water column (Binning and Baird, 2001). It is estimated that 90%
of contaminants transported to estuaries and coastal areas are deposited there (Binning and Baird, 2001). Chemicals are introduced mainly through runoff into rivers which then flow into estuaries (Chapman and Wang, 2001; Harrison et al., 2001). Estuaries serve as sediment and nutrient sinks (Harrison et al., 2001; Kennedy, 1984) and provide a record of pollution levels.
No base line for pre-anthropogenic levels has been established, making it difficult to evaluate contamination in the Amatikulu system. For pre-anthropogenic element levels, strategically placed cores will have to be undertaken together with XRF analysis, similar to the study of the
uMgeni Estuary done by Leuci (1998). Coring was not conducted as this fell outside the scope of this study, though it is recommended for follow up studies.
Both Leuci (1998) and Tinmouth (2009) used pre-anthropogenic levels to compare present day contamination levels. These two studies provide a proxy for a preliminary desktop contamination study of the Amatikulu Estuary as the two estuaries have similar catchment geology. Using other estuaries as a proxy for contamination studies is useful where there is a lack of baseline data. Examples of studies of this nature are Callow (1994) where chemical analysis done in St. Lucia Estuary, a presumed pristine estuary, was used as base line data for the chemical analysis of Durban harbour sediments.
Using pre-anthropogenic data from the uMgeni Estuary (Leuci, 1998), enrichment values were constructed for the Amatikulu system. The levels of contamination using the enrichment equations varied. Shi et al., (2010) considers values >2 as being enriched and contaminated, whereas Tinmouth (2009) and Bourennane et al., (2010) used values greater than 1.5 as indicative of enriched and contaminated. The parameters of Tinmouth (2009) and Bourennane et al., (2010) are used here. Three levels of contamination are recognised; 0-0.5 no contamination, 0.5-1.5 negligible contamination, >1.5 indicates that large proportions of the trace metal are not derived from a natural source. Only the mean metal values presented by Leuci (1998) are used for enrichment calculations.
The enrichment factor was calculated by dividing the metal content by the Al content of each sample of the Amatikulu Estuary, this was than divided by the mean metal content of the uMgeni Estuary, divided by the mean Al content of the uMgeni Estuary:
Enrichment = (ME/Al) Amatikulu / (MEmean/Al) uMgeni (following Liaghati et al., 2003; Fang et al., 2006; Bourennane et al., 2010; Shi et al., 2010)
The distribution plots of enrichment factors (Fig 7.7) indicate 5 distinct patterns:
1) Enriched throughout the estuary (Co, Pb and Sn) (Fig. 7.7 b, f and g).
2) Barrier enrichment (Ni, V and Zn) (Fig. 7.7,e, i and h).
3) Amatikulu River enrichment (Cu) (Fig. 7.7 d).
4) Enrichment at the Casuarina forest (As) (Fig. 7.7 a).
5) Scattered enrichment (Cr) (Fig. 7.7 c).
Figure 7.7 Enrichment plots of the Amatikulu Estuary using the uMgeni Estuary’s (Leuci, 1998) mean pre-anthropogenic values as a proxy for the Amatikulu Estuary. Significant enrichment is only present for As, Cr, Sn and V. Tin indicating that the entire estuary has under gone significant enrichment where as As, Cr and V all have isolated pockets of enrichment in the central areas of the Amatikulu Estuary towards the Casuarina forest and inlet. Five distinct distribution patterns for the sources of enrichment are distinguishable: Co, Pb and Sn show enrichment through out the estuary. Nickel, Zn and V are enriched from the barrier. Cu shows enrichment from Amatikulu River. Arsenic shows enrichment at the Casuarina forest. Scattered enrichment indicated by Cr.
Enrichment based on the mean pre-anthropogenic metal values form the uMgeni Estuary indicates that As, Cr, Sn and V have areas of significant enrichment. Along the Amatikulu River the main source of anthropogenic influence will be from pesticides and fertilisers used in the sugar cane fields along the banks of the Amatikulu River and influxes from the sea through the inlet and barrier overwashing. This is what is expected when the catchment is less than 1%
urbanised (the town of Eshowe), 33% is considered natural and 60% is agricultural with the majority comprising of sugar cane farming and small percentage subsistence farming (Harrison et al., 2001). Chemicals are routinely sprayed on the crops and these accumulate in the soil which is transported to the Amatikulu Estuary via the Amatikulu River. The anthropogenic contamination is considered to be from pesticides and inorganic wastes which may contain the elements Cd, Pb, As, Ni, Cu, Zn and Mn (Brady, 1974). Inorganic waste bio-accumulates in the food chain and the adsorption of contamination is directly linked to the organic matter content of the sediments (Brady, 1974). Finding a direct source for any pollution in the estuary is difficult due to the influence of overwashing, which introduces sediment from the Thukela River, and the additional input from the Nyoni River. Munksgaard et al., (2003) found that the longshore sediment transport on the eastern Gulf of Carpentaria results in a large sediment supply and masks the ability to distinguishing different populations within the sediments and therefore their provenance in the area. Liaghati et al (2003) feel that elevated values within estuaries may be purely natural and found that Cr tends to be elevated due to the immobility of the metal.
In comparison to Long et al.,’s (1995) international sediment contamination study, the only element that can be considered to be an environmental health risk is Zn which exceeds Long et al.,’s (1995) Ecological Response Medium (ERM) level. As and Cd both exceed the ERL (Ecological Response Low) indicating that there may be a slight contamination. Leuci’s (1998) pre-anthropogenic As, Cr, Cu and Ni values for the uMgeni all exceed the ERL values. Due to the pre-anthropogenic sediments exceeding the ERL it suggests that a source rock to the uMgeni and Amatikulu Estuaries contains higher trace metals than what is described by Long et al., (1995). This highlights the importance of individual site specific studies and evaluation of site specific pre-anthropogenic studies.