Spatial Distribution Of Groundwater Hydrochemistry And Bacterial Diversity Around Lake Taihu

With the rapid development of society and the intensification of human activities,surface water is seriously polluted,and the discharge of excessive pollutants has led to an increase in eutrophication of water bodies.Since there is a large amount of water exchange between groundwater and surface water,the serious pollution of surface water can directly or indirectly have some impact on groundwater,which in turn can cause serious damage to the whole aquatic ecosystem.Groundwater is a globally important freshwater resource.Especially in arid or semi-arid areas,groundwater is the only water source,and its quality is directly related to the water security of the people.A large amount of organic matter contained in eutrophic lakes that enters groundwater can provide energy for bacteria to multiply,which in turn can led to the proliferation of bacteria and pose a treat to human health.In addition,the degradation process of organic matter consumes a large amount of dissolved oxygen,thus changing the redox conditions of groundwater,and further affecting the biogeochemical cycle of nitrogen,phosphorus,sulfur and other substances.Therefore,the relationship between water quality,organic matter and bacterial community structure during groundwater recharge from eutrophic lakes needs to be paid high attention.A well-studied shallow freshwater eutrophic lake,Taihu(China),was selected as the study area.Groundwater recharge from Taihu Lake dominated the sampling period In this study,the spatial variability of water quality indicators,water chemical ion characteristics,heavy metals,dissolved organic matter(DOM)and bacterial community structure were analysed and related relationships were investigated to clarify the overall impact of eutrophic lakes on groundwater ecology during groundwater recharge,and to provide an ecological basis for the exploitation of groundwater resources and the management,protection of the basin ecosystem.The main conclusions of this study are as follows:(1)The rock dominance was the major mechanism controlling the hydrochemical composition of groundwater around Lake Taihu,and the dominant hydrochemical type of groundwater was Ca·Mg-HCO3 type.The input of large amounts of organic matter in the study area affects the ion concentrations in the groundwater.The ion correlation results indicate that N,P,K+,As and Cd in groundwater in the study area were mainly derived from the application of chemical fertilizers,and the sources of As and Cd could be similar to that of K+.In addition,Pb in groundwater around Lake Taihu was mainly originated from the water-rock interactions in the aquifer.(2)In the aquifer surrounding Lake Taihu,six fluorescent components of groundwater DOM were identified,of which C1 was indentifed as an UVA humic-like aubstance closely related to biology,C2 was an UVC humic-like substance with a high molecular,C3 was indentified as an UVA humic-like substance from terrestrial sources,C4 was the tryptophan-like substance degraded by microorganisms,C5 was protein-like substance which free or bound to proteins.And the humic-like substance C6 was similar to a terrestrial photodegradation product.Moreover,the Fmax of four fluorescent DOM components(except for C2 and C6)in groundwater gradually decreased with increasing distance to Lake Taihu and then became stabilized.The results showed that the influence of Taihu lake on DOM in surrounding aquifers can extend to 15 km from the lake shore,and in the process of groundwater migration around Taihu Lake,microorganisms decompose DOM continuously and release new secretions.(3)The diversity of bacterial communities was high in the study area.In total,33 bacterial phyla and 119 bacterial genera were identified from the surrounding aquifer of Lake Taihu.And the bacterial community structure of the groundwater in the study area was found to be different from the composition of the bacterial community in Taihu Lake,suggesting that the bacterial community composition of the groundwater is mainly the result of environmental selection.In addition,the input of large amounts of DOM from eutrophic lakes can cause changes in the structure and function of bacterial communities associated with the cycling of nitrogen,sulphur and phosphorus in groundwater,which in turn can affect the biogeochemical cycling of groundwater systems.This study systmatically analysed the water quality indicators,DOM and bacterial community structure of the aquifer around Lake Taihu,revealed the migration patterns of major solutes and DOM in the groundwater around Lake Taihu,clarified the influence of Lake Taihu on the spatial distribution of water chemistry characteristics and microbial diversity in the surrounding aquifer,and provided a scientific basis for groundwater exploitation and protection in eutrophic watersheds.