Sedimentary History Of Trace Metals Over The Past Half-Century In Songkhla Lake,Southern Thailand:Anthropogenic Impacts And Contamination Assessment

Coastal lagoons are affected by the joint action of sea and land,and are sensitive to natural processes and anthropogenic activities.Heavy metal contamination in lagoon sediments is one of the environmental problems faced by countries all over the world.Analyzing the impact of human activities on heavy metals in coastal lagoons is of great significance to the utilization and protection of lagoons and the sustainable development of economy and society.Songkhla Lake,connected with the Gulf of Thailand,is the second largest lake in Southeast Asia and supports the development of the fishery,transportation and tourism industries in southern Thailand.Here,this study provides the first temporal record of ²¹⁰Pb activity,particle size,color,magnetic susceptibility,TOC,TN and elements(Al,Ca,Cl,As,Hg,Pb,Cr,Ni,Cu and Zn)concentrations of the sediment core SKL8-2 and SKL3-2.Then,based on those data,we established the chronological framework of Songkhla Lake sedimentary core,reconstructed the sedimentary history in the past few decades and evaluated the contamination degree of heavy metals.The record of sediment core SKL8-2 collected from southern Songkhla Lake,reveals a three-stage sedimentary and trace metals'input history:(1)From 1964 to1982,the channel connecting Songkhla Lake and the Gulf of Thailand was closed,and it was determined as a natural terrigenous input period with a relative reduction environment.The deposition rate was high during this period,about 2.3±0.6 cm/yr;trace metals and organic carbon content at high values,with extremely high correlation and the mean TOC/TN value reached as high as 21.7.The yellowness and magnetic susceptibility was marked low.(2)Trace metals concentration,organic carbon content,b*value,and magnetic susceptibility changed abruptly at 1982.The sedimentary environment changed to be oxidizing,and the deposition rate decreased to 1.6±0.6 cm/yr.During 1982-2000,the sources of trace metals were more complicated than before,which mainly came from urban wastewater,industrial effluent and fishery discharge.The extremely high calcium content in this period suggested the booming shrimp farming.(3)From 2000 to 2019,pollution signals of Pb,Hg,As,Zn,Ni emerged at the first decade owing to the rapid development of the cities and the backward effluent treatment,and was pronounced alleviated after 2010.Northern Songkhla Lake holds more stable sedimentary environment than that of southern Songkhla Lake,and the deposition rate is higher than the southern.It was2.6±0.6 cm/yr from 1956 to 1973,and then the average deposition rate dropped to1.7±0.6 cm/yr.Since 1956,the hydrodynamic force in the lake area has increased slowly,which is reflected in the gradual coarsening of sediment particle size.The material in the northern Songkhla Lake was mainly from inorganic sources,that the proportion of organic matter was less than 15%.Organic matter was a mixed source of lake authigenic and terrigenous input.Combined with the PCA results,Cr,Ni,Cu and Zn in the northern Songkhla Lake were mainly input by land weathering,while Pb and Hg were mainly input by human emission.The sediment contamination of Songkhla Lake kept in low level,generally.The order of enrichment degree of heavy metals in southern Songkhla Lake was Pb>Hg>As>Zn>Ni>Cr>Cu.Among them,Pb,Hg,As,Zn and Ni were slightly enriched,while Cr and Cu were not enriched at all.The enrichment degree of heavy metals in northern Songkhla Lake was Pb>Hg>Ni>Zn>As>Cr>Cu.Although the concentrations of heavy metals in Songkhla Lake is within a reasonable range,there have been records of heavy metal contamination caused by human activities in the past,such as fishery and laundry.Hence the situations of heavy metals still need to be monitored in the long run.