Risk assessment of heavy metals in Meretrix meretrix culture

 area of Qidong City based on multivariate evaluation method 

Abstract: The seas with high population density are submitted to numerous anthropogenic pressures. Chemical pollution by heavy metals coming from continental human activities are transported through rivers or by air and accumulate in seas where they mainly affect coastal areas. Heavy metals exist widely in soil, the atmosphere, and watersediment systems, heavy metals can accumulate and become magnified in living organisms due to their persistence, high enrichment, and ease of migration. As a consequence, even low concentrations of metals pose potential threats to organisms, including humans. Qidong City, Jiangsu Province is located on the coast of the Yellow Sea at the mouth of the Yangtze River. It has a long coastline and rich tidal flat resources. It is a famous clam breeding base in China. In recent years, with the expansion of the breeding scale, the discharge of sewage from various sources along the coast, the occasional largescale death of shellfish cultured on the beach, pollution in the breeding area has become a common occurrence in marine aquaculture. In this study, the content and distribution characteristics of heavy metals Cu, Zn, Pb, and Cd in the water and sediment of Qidong clam culture area during 2017—2019 were systematically analyzed and summarized. In order to study the status of heavy metals and ecological risks of Meretrix meretrix culture areas of Qidong City, the probabilistic ecological risk assessment method was employed to evaluate the ecological risks of the water body，while sediment quality guidelines (SQG) was applied to analyze the biological toxicity of sediment. This study showed that the content of four heavy metals in the sea water of Qidong clam aquaculture was relatively low. The threeyear average concentrations of Cu, Zn, Pb and Cd from high to low were: Cu (3.6 μg··L-1), Zn (0.2 μg··L-1), Pb (0.18 μg··L-1), Cd (0.06 μg··L-1). Since there were no largescale enterprises around the sea area, combined with the low heavy metal content in this sea area, it showed that the water body in the study area was not significantly polluted by heavy metals. The concentrations of Cu, Zn, Pb, and Cd were relatively higher in station 1 and station 2, which were near the shore. The main reason for this distribution was that there were many electroplating, steel wire rope and textile dyeing and finishing enterprises in Haian and Tongzhou in Nantong City, and the near shore was affected by various sources of heavy metal contaminants from industrial production discharge, domestic sewage, agricultural water or ship discharge, etc.  In sea areas far from the shore, after landbased pollutants entered the ocean, with the flow of water and strong water exchange capacity, terrestrial pollutants migrated and diffused. Coupled with the selfpurification of water, the concentration of heavy metals was low. The exposure concentrations of Cu, Zn, Pb, and Cd corresponding to the cumulative probability of 90% were 4.77, 2.33, 1.91, 1.75 μg··L-1, respectively. The acute toxicity data of Cu, Zn, Pb, and Cd corresponding to the cumulative probability of 10% were 4.81, 2.51, 3.13, 3.52 μg··L-1，the ecological risks assessment of four kinds of heavy metals showed that the value of MOS10 was more than 1, indicating that these areas had low ecological risk of heavy metals pollution. The probabilities of four heavy metals affecting more than 10% of aquatic organisms were 50.05%, 25.38%, 24.01% and 19.09%, respectively. In the sea water the ecological risk of four main heavy metals was in order of Cu>Zn>Cd>Pb. In surface sediments, the contamination degree of heavy metals was Zn＞Pb＞Cd＞Cu. The potential biological toxicity was analyzed using sediment quality guidelines (SQG), the guideline values showed that the content of Cd at each station was lower than that of TEL, indicating no Cd potential biological toxicity. The Cu content of 33.33% site and the Pb content of 46.70% site had no potential biotoxic effects, the Cu content of 66.67% site, the Zn content of 50% site and the Pb content of 53.30% site were between TEL and PEL. As for sediment it showed occasional potential biological toxicity in every sampling station, Cu, Pb, Cd showed potential biological toxicity in some sampling stations. Longterm exposure will have toxic effects when the pollutant content in sediment samples is higher than TEL, and acute toxic effects will occur when the pollutant content is higher than PEL. Among them, the potential biological toxicity of Zn was the most frequent. The value of ∑TUs of four heavy metals was above 6 in station 1 and station 2, indicating that there was obvious acute toxicity in these two stations, and the ∑TUs value was mainly contributed by Zn, which was consistent with the high probability of the toxic effect of Zn on organisms. For the other stations the values of ∑TUs were less than 4. Basically, there were no acute toxic effects on aquatic organisms. Following the value of ∑TUs, it indicated that there was heavy metal acute toxicity in only two of the sampling stations (station 1 and station 2), no heavy metal acute toxicity in the other stations. It was recommended that sitespecific risk assessments be conducted to assess possible ecological effects at these sites. Different potential biotoxic effects of the four heavy metals were related to their respective heavy metal contents and their critical effect concentration (TEL). The concentration of Cd was significantly lower than that of TEL, the content of Cu and Zn were significantly higher than that of TEL, and the content of Pb was not much different from that of TEL. The toxicity and bioaccumulation of heavy metals to aquatic organisms can be affected by various environmental factors, such as pH value and salinity of the water environment, which will directly affect the form of heavy metals in water and change the toxicity of heavy metals. It is very important to focus on the functions and roles of various organisms in the ecological environment and to evaluate the risk effect of heavy metals on aquatic organisms in further research. The analysis results provide a scientific basis for the risk management and evaluation of heavy metals in shellfish farming areas.