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

Abstract

Heavy metals are stable and persistent environmental contam-inants since they cannot be degraded or destroyed. Therefore, theytend to accumulate in the soils and sediments. Excessive levels ofmetals in sediments affect marine biota and pose a risk to humanhealth 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 met-als are various industrial point sources, including present and for-mer mining activities, foundries and smelters, and diffuse sourcessuch 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 at-tached to air-borne particles can be widely dispersed on very largescales. Heavy metals in aqueous and sedimentary transport (e.g.,river run-off) enter the normal coastal biogeochemical cycle andare largely retained within near-shore and shelf regions in propor-tion to geochemical controls (McAlister et al., 2005; Chaparro et al.,2005). Sediment analyses play a crucial role in assessing the degreeof heavy metal pollution and the resulting health risk associatedwith the food chain.Karachi is situated at the southern most part of Pakistan alongthe Arabian Sea and it is the largest business hub and the mostheavily populated urban centre in the vicinity. The coastal zoneof Karachi is about 167 km long. The estimated population of Kar-achiis morethan15 million.The rapidindustrializationandurban-ization of the city has altered the quality of the environment andcreated ecological disturbances and associated problems for the lo-cal community. Pollution in the Karachi coastal region is mainlyattributed to the Lyari and Malir rivers, which are served by vari-ous channels of untreated domestic and industrial waste, carryingmore than 300 million gallons per day (MGD) (1,125,000 m