Study Of Distribution And Environmental Impact Of Radionuclides And Heavy Metals In Sediments In Nansha Sea Area,South China Sea

Nansha Sea area is located in the south of the South China Sea（the largest marginal sea in China）,which is rich in marine resources and has a special geographical location,so it has very important research value.Marine sediment is a"museum"that records the historical changes of the marine environment.The history of the evolution of the marine environment can be explored through the study of marine sediments.Activity concentrations of radionuclides(²³⁸U,²²⁶Ra,²³²Th,and ⁴⁰K)in the sediments of eight sampling stations and radionuclides(²²⁸Th,²²⁸Ra)in six short cores sediments from Nansha Sea area were obtained by high-purity germanium spectroscopy.The activity concentrations of ²³⁸U,²²⁶Ra,²³²Th,and ⁴⁰K in eight surface sediments were found to be（16.49±1.75）Bq·kg^-1to（43.87±3.02）Bq·kg^-1,（14.60±0.85）Bq·kg^-1to（38.51±0.73）Bq·kg^-1,（149.70±6.53）Bq·kg^-1 to（569.35±11.11）Bq·kg^-1,（15.81±0.60）Bq·kg^-1 to（49.21±1.05）Bq·kg^-1,respectively.The activity concentrations of ²²⁸Ra and ²²⁸Th in the sediments of six short cores were found to be（15.73±1.00）Bq·kg^-1 to（44.35±1.69）Bq·kg^-1 and（13.62±0.67）Bq·kg^-1 to 49.21±1.05)Bq·kg^-1.The predominant influencing factors of radionuclide activity concentrations were the sediment type,the water flow direction,and the station position depth.The ²³⁸U,²²⁸Th,²²⁸Ra,²³²Th,and ⁴⁰K concentrations in shallow waters were lower than those in deep waters,and they gradually decreased in the direction of the water flow.The concentration of ²²⁶Ra in shallow seas was higher than that in deep seas,and it gradually increased in the direction of the water flow.The depth profiles of ²²⁸Ra and ²²⁸Th can be roughly divided into two types,one was a slight increase with the increase of depth,but there is a sudden decrease in the middle depth;while the other showed a gradual decrease with the increase of depth,but there were greater changes in the deepest segment in two cores.Heavy metal concentrations（Cr,Co,Ni,Cu,Zn,As,Cd,and Pb）in the sediments of two long cores from Nansha Sea area were obtained inductively coupled plasma atomic emission spectrometry.The heavy metals concentrations varied with a range of（12.58 to 58.97）mg/kg for Cr,（11.73 to 21.58）mg/kg for Co,（23.57 to 95.29）mg/kg for Ni,（14.82to 50.25）mg/kg for Cu,（94.87 to 242.20）mg/kg for Zn,（19.00 to 163.80）mg/kg for As,（0.02 to 1.17）mg/kg for Cd,（22.72 to 40.13）mg/kg for Pb,respectively.The geo-accumulation index,pollution load index,potential ecological risk index,and multivariate statistical techniques were used to evaluate the pollution degree and possible sources of heavy metals in the sediments.The results of I_geo indicated that the overall pollution levels of heavy metals in the sediments were in the following orders:NS-7,Cr（-1.35）<Cu（-0.87）<Zn（-0.47）<Cd（-0.45）<Pb（-0.31）<Co（-0.05）<Ni（0.34）<As（3.13）;NS-8,Cr（-1.68）<Cu（-0.65）<Pb（-0.47）<Zn（-0.22）<Cd（-0.21）<Co（0.04）<Ni（0.77）<As（3.52）.Except for the serious pollution of As,the pollution degree of other heavy metals is relatively low.The results of pollution load index showed that Cr and Cu exhibiting a low contamination status,Co,Ni,Zn,Cd and Pb demonstrated a moderately contaminated status,As exhibiting a highly contaminated status,and the two sampling sites were polluted by heavy metals.The potential risk assessment results exhibited that the potential ecological risk coefficients of heavy metals ranked as follows:Cr<Zn<Cu<Pb<Co<Ni<Cd<As.In a correlation analysis between the radionuclides and heavy metals,²³²Th was positively correlated with As and Pb.²³⁸U demonstrated a significant positive correlation with Co,Ni,Cu,Zn,and Cd.⁴⁰K was positively correlated with Co,Ni,and Cu.The results of analysis of the sources of heavy metals suggested that Cr,Co,Ni,Cu,Zn,Cd and Pb were primarily derived from natural processes,such as rock weathering and soil erosion in water areas,while the sources of As may be related to natural processes and human activities.