Investigation on bacterial community and diversity in the multilayer aquifer-aquitard system of the Pearl River Delta, China.

Bacteria play an important role in groundwater chemistry. The groundwater resource in the Pearl River Delta (PRD) is responsible for 50 million people's water requirement. High amount of ammonium, arsenic and methane had been reported in groundwater of the PRD, which was considered as the result of intensive bacterial metabolism in the multilayer aquifer-aquitard system. To investigate bacterial community in this system and its relation with groundwater chemistry, sediment and groundwater samples were taken from representative locations in the PRD at different lithological units. Bacterial 16S rRNA gene clone libraries were constructed for microbial identifications and community structures in different strata. Canonical correlation analysis between bacterial linages and environment variables (Cl(-), PO4(3-), SO4(2-), NH4(+)) showed that community structures were significantly modified by geological conditions. Higher bacterial diversity was observed in samples from the Holocene aquitard M1 and aquifer T1, while in the older aquitard M2 and basal aquifer T2, bacterial diversity was much lower. Chloroflexi, γ-proteobacteria and δ-proteobacteria were the dominant phyla in the aquitard sediment. β-proteobacteria was the dominant phylum in sediment which was strongly influenced by fresh water. The results of this study demonstrated that bacterial community contains information of geological events such as sea transgression and deltaic evolution, and microbes in the aquitards have great potential in dominating groundwater quality in aquifers.