Effects Of Seawater Intrusion On Hydrogeochemistry And Microbial Communities Of Groundwater

The salinization process of groundwater caused by seawater intrusion is one of the major natural disasters in coastal areas,which not only leads to the pollution of underground fresh water,but also has a great impact on the underground microbial communities and destabilizes the balance and stability of the coastal ecosystem.Microorganisms in aquifers are extremely sensitive to environmental changes and thus can be useful indicators for environmental process and ecosystem functioning.However,little attention has been paid to the effect of seawater intrusion on microbial communities,and the relationship between the microbial diversity,distribution characteristics and environmental factors of groundwater in seawater intrusion areas remains unclear.It is an important subject of ecological research to determine the microbial community diversity,composition and their responses to seawater intrusion.In the Pearl River Delta?PRD?region,there is not only modern seawater intrusion,but also the influence of paleo-seawater intrusion,and the sources of salinity in groundwater are complicated.The Laizhou Bay area is one of the most seriously invaded areas in China,which has a great threat to the drinking water safety,food security and life and health of local residents.It is of great theoretical and practical significance to understand the complex hydrogeochemical processes and effects on microbial communities in seawater intrusion areas.Therefore,two typical seawater intrusion areas?the PRD and Laizhou Bay?were selected in this study to explore hydrogeochemical processes of groundwater in seawater intrusion areas,and their linkages with the diversity,composition,and structure of groundwater microbial communities.For the seawater intrusion area of the PRD,the hydrochemical characteristics,sources of salinity,major biogeochemical effects and microbial community structure characteristics of groundwater were analyzed using various hydrogeochemical and isotopic methods as well as high-throughput sequencing of 16S r RNA gene amplicons.The results showed that the hydrochemical types of groundwater varied greatly,changing from HCO₃-Ca/HCO₃·SO₄-Ca type in the freshwater?0<TDS<1 g/L?to HCO₃·Cl-Ca·Na/HCO₃·Cl-Na·Mg type in the brackish water?1<TDS<10 g/L?,and then to Cl-Ca·Na/Cl-Na type in the saline water?10<TDS<100 g/L?.The major ions(Na⁺,K⁺,Ca²⁺,Mg²⁺,Cl^-,NO₃^-,SO₄^2-,and HCO₃^-)and isotope analyses(²H,³H,¹⁸O,and ¹⁴C)indicated that the groundwater in the confined aquifer was recharged by local precipitation and seawater.The ¹⁴C isotope and hydrochemical analysis further shows that the salinity of groundwater was likely attributed to the Holocene transgression,and the contribution rate of seawater to the groundwater salinity was<1%-92.8%.High-throughput sequencing results showed that seawater intrusion has a great influence on microbial community diversity,but no significant effect on?-diversity?P>0.05?.The results of geological survey,hydrochemistry and molecular analysis all indicated that sulfate reduction and methanogenesis existed in the aquifer examined,and the two microbial groups?Desulfovibrio and Methanococcus?,were the main microbial groups for producing H₂S and CH₄ gases in the groundwater.In addition,some biological groups existing in salt water,such as Alteromonadales and Marinobacter,which may be used as biomarkers for seawater invasion.For the seawater intrusion area of Laizhou Bay,the Leve Logger was used to monitor the groundwater quality changes for a long time,and the high-density resistivity method was used to continuously detect the changes of groundwater salinity along the section.The results showed that the monitoring areas were seriously affected by seawater intrusion,and as much as 70 percent of drinking water was polluted by seawater.The microbial community structure of drinking water in the seawater intrusion area was studied,and the main microbial communities were?-,?-and?-proteobacteria affiliated with Proteobacteria as well as Bacilli of Firmicutes,which were consistent with the main microbial communities in the underground aquifers of the seawater intrusion area of the PRD,but also showed some regional differences.The main physical and chemical factors affecting the bacterial community structure in drinking water included salinity,temperature and p H.Pseudomonas and Flavobacterium are potential pathogens,which may be related to the high endemic incidence in this region.The results on the microbial community structure of soil sediments in the seawater intrusion area showed that the microbial diversity and community structure are significantly different?P<0.05?with the depth.Compared with seawater intrusion,soil depth has a greater impact on microbial diversity and community structure.Soil microbial community structure was closely related to the environment condations,among which the most significant environmental factors were soil depth,p H,organic carbon?OC?and total nitrogen?TN?.To sum up,this research combines multi-disciplinary field,using multiple technical analysis methods included hydrology geochemistry,isotope technology,resistivity prospecting and high-throughput sequencing technologies.Investigate the hydrogeochemical processes,the source of groundwater salinity,and the microbial community structure of the groundwater,drinking water as well as soil sediments in the seawater intrusion areas.It provides a large amount of detailed data for the study of the diversity of groundwater microbial community in seawater intrusion areas,and provides a scientific theoretical basis for the rational utilization and protection of groundwater.