The Study of Hydrogeochemical Environments and Microbial Communities along a Groundwater Salinity Gradient in the Pearl River Delta, China

The salinization of groundwater is an issue in coastal areas because it causes the deterioration of freshwater resources, significantly impacting human livelihoods and ecosystems. This study integrated isotopic geochemical measurements with high-throughput sequencing of 16S rRNA gene amplicons to evaluate the source of groundwater salinity and the influence of hydrogeochemical variations on microbial communities under different salinity gradients in the Pearl River Delta of China. Results showed that the groundwater salinity in this area varied from fresh water in the inland area to brackish water, and then to saline water close to the southeast shoreline. The major ions (Na+, K+, Ca2+, Mg2+, Cl−, NO3−, SO42−, 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. A further 14C analysis showed that the salinity of the groundwater was likely attributed to the Holocene transgression. Analysis of the microbial community showed that γ-proteobacteria were frequently observed in all the groundwater samples, while the other main microbial community at class level varied greatly, from β-proteobacteria in the freshwater wells to ε-proteobacteria in the brackish wells and to Bacilli in the saline wells. Exiguobacterium and Acinetobacter were dominant in saline water and the brackish water sample of Q144, while Sulfuricurvum dominated in the brackish water sample of Q143. Aeromonas, no rank Gallionellaceae, no rank Methylophilaceae, Acidovorax, and Comamonas unevenly thrived in the freshwater samples collected from different locations. Therefore, the distribution of microbial communities reflected the salinity and hydrogeochemical characteristics of a groundwater aquifer, and can be regarded as a potential environmental indicator in the groundwater.