Heavy metal pollution in sediments of a typical mariculture zone in South China

Abstract

The heavy metal inventory and the ecological risk of the estuarine sediments in Hailing Bay, an important maricultural zone along the southern coast of China, were investigated. Results show that the surface sediments were mainly polluted by As (2.17–20.34 mg/kg), Ni (1.37–42.50 mg/kg), Cu (1.21–58.84 mg/kg) and Zn (11.69–219.22 mg/kg). Furthermore, the aquafarming zone was significantly more polluted than the non-aquafarming zone, and cluster analysis suggested additional sources of heavy metal input in the aquafarming zone. As, Cr, Cu, Ni, Pb and Zn were mainly present in the non-bioavailable residual form in the surface sediments, whereas Cd was predominantly in the highly mobile acid soluble and reducible fractions. The ecological risk of the polluted sediments stemmed mainly from Cd, and from As, Cu and Pb to less degrees. The highest potential risks occurred near the aquaculture base, indicating the need to control heavy metal inputs from aquafarming activities.

Introduction

Sediments are effectively the sink for trace metals in marine and estuarine ecosystems, but also act as a source of metals for aquatic organisms (Chapman et al., 1998, Breslin and Sanudo-Wilhelmy, 1999, Dural et al., 2007, Qiu et al., 2011). Arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) are some of the most common heavy metal pollutants (Hakanson, 1980, Chen et al., 1991, Wong et al., 2001). These metals can enter estuaries and coastal waters through the discharge of industrial and municipal wastewaters, storm run-off, dust deposition, mine discharge, waste incineration, and other diffuse sources (Tanner et al., 2000, Dai et al., 2007, Meng et al., 2008, Zhang et al., 2008, Cheng et al., 2009, Besser et al., 2009, Cheng and Hu, 2010a, Cheng and Hu, 2010b, Hosono et al., 2010, Hosono et al., 2011). Sediment-bound heavy metals have a tendency to adsorb and accumulate on fine-grained particles that eventually move into the depositional areas (Zhang et al., 2001, Morillo et al., 2002, Jain, 2004, Man et al., 2004, Qiu et al., 2005, Singh et al., 2005, Mendiguchía et al., 2006). Sediment pollution by heavy metals has been regarded as a critical problem in marine environment because of their toxicity and bioaccumulation (Chapman et al., 1998, Islam and Tanaka, 2004, Singh et al., 2005, Todd et al., 2010). Many studies have shown that heavy metals in sediments could significantly impact the health of marine ecosystem (Snodgrass et al., 2008, Zheng et al., 2008, Besser et al., 2009). The toxicity and mobility of heavy metals in sediments vary greatly among different chemical forms (Perin et al., 1985, Cuong and Obbard, 2006, Fan et al., 2008, Yu et al., 2010). Thus, the simple measurement of heavy metal content cannot provide sufficient information about their mobility and bioavailability, and consequently the risk to the marine ecosystem. In contrast, heavy metal speciation offers a more realistic evaluation of their actual environmental impact (Perin et al., 1985, Cuong and Obbard, 2006).

Hailing Bay, located in Yangjiang city, Guangdong Province, is a major culture fishery base and a well-known tourist attraction in South China (Wen, 2011). Aquaculture and tourism industries are the main drivers of economic growth in Yangjiang, and its gross domestic product (GDP) per capita has grown by 2.3 times from 2005 to 2010. Fish farming activities in Hailing Bay have been growing fast to meet the demands for seafood from the Pearl River Delta and other regions of China over the past two decades. Trash fish and fish meals are mainly used for marine fish farming, which usually contribute to fecal mass and food residues, and raise the loading of heavy metals (Chang and Chen, 2006, Mai et al., 2006). Deteriorations of sediment quality around fish farming zones have been reported in several studies (Wong et al., 2001, Zhang et al., 2007, Meng et al., 2008, Yu et al., 2008, Qiu et al., 2011). Pollution of marine sediment in aquacultural zones and the subsequent bioaccumulation of heavy metals pose a potential risk to the public health (Dural et al., 2007, Man et al., 2010).

A comprehensive study on both the status of heavy metal pollution and the bioavailability (and mobility) of heavy metals in the sediment environment of Hailing Bay is important for the sustainable development of the regional economy and for the long-term improvement of public health. Qiu and co-workers had measured the total contents of heavy metals in the surface sediments from Hailing Bay in 2001, 2002 and 2007, and found that they were slightly polluted by Cd, Cu, Zn and Pb (Qiu and Zhu, 2004, Qiu et al., 2005, Qiu et al., 2011). This study was conducted to further investigate whether the fast growth in aquafarming in Hailing Bay had accelerated the heavy metal pollution, and to assess the potential ecological risk of the sediment pollution caused by heavy metals. Specifically, the objectives of this study were: (1) to determine the levels of heavy metals (As, Cd, Cr, Cu, Ni, Pb and Zn) in the sediments from the non-aquafarming zone and the aquafarming zone and to identify the possible sources of these metals via cluster analysis; (2) to infer the bioavailability (and the mobility) of heavy metals in the sediments by analyzing their chemical speciation using the modified BCR (Community Bureau of Reference) sequential extraction procedure; and (3) to assess the potential ecological risk of heavy metal pollution in the sediments through calculating the relevant ecological risk indices. It is noted that chemical speciation of heavy metals and assessment of their ecological risks had not been addressed in any previous studies focusing on Hailing Bay (Qiu and Zhu, 2004, Qiu et al., 2005, Qiu et al., 2011).