Heavy metal toxicity levels in the coastal sediments of the Arabian Sea along the urban Karachi (Pakistan) region

摘要

Heavy metals are stable and persistent environmental contaminants since they cannot be degraded or destroyed. Therefore, they tend to accumulate in the soils and sediments. Excessive levels of metals in sediments affect marine biota and pose a risk to human health through the consumption of seafood (Adams et al., 1992; Rowlatt and Lovell, 1994; Mucha et al., 2003; Feng et al., 2004; Zhang et al., 2007). The main anthropogenic sources of heavy metals are various industrial point sources, including present and former mining activities, foundries and smelters, and diffuse sources such as piping, combustion by-products, traffic, etc. (NRC, 1989; Bryan and Langston, 1992; Long, 2000; Pekey, 2006; Hyun et al., 2007). Relatively volatile heavy metals and those that become attached to air-borne particles can be widely dispersed on very large scales. Heavy metals in aqueous and sedimentary transport (e.g., river run-off) enter the normal coastal biogeochemical cycle and are largely retained within near-shore and shelf regions in proportion to geochemical controls (McAlister et al., 2005; Chaparro et al., 2005). Sediment analyses play a crucial role in assessing the degree of heavy metal pollution and the resulting health risk associated with the food chain.