Distribution And Assessment Of Heavy Metals In The Surface Sediments Of Hangzhou Bay And Its Adiacent Sea Waters

In order to study the transformation processes of heavy metals, objective evaluate the heavy metal contamination and effective control the ecological risk of Hangzhou Bay and its adjacent sea waters, the spatial distribution characteristics and historical trend, as well as the controlling factors of heavy metals (Cu, Pb, Zn, Cd, Hg and As) in surface sediments were analyzed basing on the data obtained from Hangzhou Bay and its adjacent sea waters in spring from 1996 to 2012. At the same time, the study tried to distinguish the sources of heavy metals by the principal component analysis (PCA) and assess the heavy metals contamination by the geoaccumulation index of, the sediment quality criterias and the potential ecological risk index. The main conclusions were as follows:(1) The mean values of Cu, Pb, Zn, Cd, Hg and As in surface sediments from Hangzhou Bay and its adjacent sea waters of 2012 were 23.2±8.48 mg/kg,26.3± 6.05 mg/kg,78.4±17.8 mg/kg,0.112±0.043 mg/kg,0.080±0.031 mg/kg,8.27± 3.99mg/kg.(2) Cu, Pb, Zn, Cd, and As presented similar overall spatial distribution patterns though differed in smaller scales, with concentrations peaked around Nanhuizui waters and decreased along north-east and south-west directions, while Hg concentrated in southeast waters of Zhoushan, instead of the southern branch of Yangtze River Estuary or northern Hangzhou Bay waters.(3) The temporal variation of heavy metals displayed different patterns, i.e., Pb concentration kept rising during 1996 to 2012, and overall concentrations of Cu, Zn, and As increased during 2001-2009 and then decreased since 2009, whereas Cd and Hg remained relatively stable with slight fluctuations during the studied period.(4) The environmental background values were 20.6,21.0,80.5,0.102,0.039, and 9.99 mg/kg for Cu, Pb, Zn, Cd, Hg, and As concentrations, respectively, which were assumed based on mathematical statistics methods according to the historical records.(5) The heavy metal elements were significantly correlated (P<0.05 or even 0.01), suggesting possibly similar sources. And transport of terrigenous clastic particles was a decisive determinant of composition and distribution, while transition mechanisms of heavy metals in different substrates and sulfide enrichment were also important contributors to the sources of heavy metals in this area, based on the Principal Components Analysis.(6) The overall potential ecological risk level was moderate, with Hg and Cd being the prominent proportions of which Hg showed the level of potential risk as strong. The relatively high values of potential ecological risk index were determined in the surface sediments of Zhoushan southeast waters, southern branch of Yangtze River Estuary, and northern waters of Hangzhou Bay.