Long-term study of heavy metal pollution in the northern Hangzhou Bay of China: temporal and spatial distribution, contamination evaluation, and potential ecological risk.

Coastal ecosystem is vulnerable to heavy metal contamination. The northern Hangzhou Bay is under intensifying impact of anthropogenic activities. To reveal the heavy metal pollution status in the coastal environment of the Hangzhou Bay, a long-term investigation into the heavy metal contamination during 2011 to 2016 was initiated. Seawater and sediment samples of 25 locations depending on the sewage outlet locations in the northern Hangzhou Bay were collected to analyze the concentrations and temporal and spatial distribution of Cu, Pb, Zn, Cd, Hg, and As. Pollution condition, ecological risk, and potential sources were additionally analyzed. Results show that the annual mean concentrations of Cu, Pb, Zn, Cd, Hg, and As were 2.13-4.59, 0.212-1.480, 7.81-20.34, 0.054-0.279, 0.026-0.090, and 1.08-2.57 μg/L in the seawater, and were 16.34-28.35, 16.25-26.33, 67.32-97.61, 0.084-0.185, 0.029-0.061, and 6.09-14.08 μg/L in the sediments. A decreasing trend in Cu, Pb, Zn, Cd, and Hg concentrations and an increasing trend in As of the seawater were observed. However, in the sediment, the heavy metals demonstrated a rising trend, except for Hg. The single-factor pollution index showed an increasing trend in Cd and As in the seawater, depicting an enhanced pollution of Cd and As, while in the sediments, Cu, Pb, and As were in pollution-free level (average Geo-accumulation index (I) values below 0) in general, and only occasional slight pollution occurred in individual years, e.g., As with 0.403 in 2016. The mean I values of Cd ranged from - 0.865 to 0.274 during 2011 to 2016, indicating that the pollution level of Cd was slight, but is likely to increase in the forthcoming years. The level of heavy metal contamination in sediments was low in 2011 (5.853) and 2012 (5.172), and moderate during 2013 to 2016 (in the range of 6.107 to 7.598), while the degree of potential ecological risk was low in the study period, except moderate in 2013 (125.107). The highest contamination degree and potential ecological risk appeared in 2013 (Cd = 7.598; RI = 125.107), while Cd and Hg contributed over 75% of the ecological risk. Overall, the results show low pollution level and low potential ecological risk in the northern Hangzhou Bay; however, more attention should be paid to the potential ecological risk due to Hg and Cd. Graphical abstract Spatial distribution of the heavy metal levels in the sediment of the coastal environment of the northern Hangzhou Bay on a long-term basis.