Characteristics Of Groundwater System In Dongting Lake And Its Influence On Spatial Differentiation Of Hydrochemistry Based On Tectonic Perspective

With the operation of the Three Gorges Project since the 21st century,hydrological conditions such as water level and discharge in the middle and lower reaches of the Yangtze River have changed.Dongting Lake,an important over-water lake in the middle reaches of the Yangtze River,is the first to be affected.Seasonal water shortage happened to partial areas,especially in County Huarong,Nanxian and Anxiang.Dongting Lake area is rich in groundwater resources,which is a potential water supply source.However,in the past,the understanding of groundwater system in the lake area was insufficient,and the groundwater quality factors in the lake area also exceeded the standard in a large area.It is of great significance to deepen the understanding of groundwater system in the lake area and analyze the spatial distribution law of water quality factors on this basis for regional groundwater development and utilization.In this paper,the spatial distribution characteristics of groundwater system and hydrochemical factors in Dongting Lake area are studied from the perspectives of geological structure pattern and hydrogeochemistry,in order to provide theoretical reference and basis for the development and utilization of groundwater resources in this area.Based on the collected geological data,combined with the data of 158 geological boreholes and geographic elevation data of Dongting Lake area,the geological structural pattern and water control mechanism of structural pattern have been analyzed.The results show that Dongting Lake area has been affected by a series of tectonic events since Cretaceous,which has formed the present situation that the bedrock subsidence in the center of the lake area is negative topography,and the bedrock topography around the lake area rises to expose the bedrock layer.Quaternary sediments are widely distributed in the upper layer of the Cretaceous bedrock,which provides an exceptional water storage environment for Dongting Lake area.However,influenced by some secondary tectonic activities,a series of secondary geological structures,such as Huarong uplift,Chishan uplift,Lixian sag,etc.,have formed in Dongting Lake area,leading strong impact on the connection between the original groundwater.To explore the internal mechanism of water controlled by these secondary structures,an indoor experiment was designed according to the geological settings of Dongting Lake area.The simulation results show that the geological structure pattern impacts directly on the groundwater flow field,in which the tectonic uplift actives as a blocking on the groundwater flow field,resulting in the phenomenon of "water blocking effect" and "seepage around the dam".On the basis of study on the structural pattern and structural water control mechanism in Dongting Lake area,the groundwater system is divided.The results are verified by flow field and hydrochemistry with measured flow field data and water quality data of 1258 shallow groundwater samples.Finally,the groundwater system in Dongting Lake area is divided into seven subsystems,i.e.,Lixian Basin Subsystem(LBS),Lishui-Yuanjiang Subsystem(LYS),Yuanjiang South Bank Subsystem(SYB),Lishui-Ouchi River Subsystem(LOS),Datong LakeSouth Dongting Subsystem(DSS),Xiangjiang River-Zishui Subsystem(XZS)and East Dongting Subsystem(EDS).On the basis of the study on groundwater system regionalization,Surfer11 software is valuable to interpolate the main groundwater quality factors such as Eh,nitrogen,iron,manganese and pH in lake area,and analyze the spatial differentiation characteristics.Internal mechanism of spatial differentiation of each water quality factor is analyzed with the geological structure pattern and groundwater system regionalization.The results show that Eh value and NO3-concentration are higher in the periphery of the lake area and decreasing towards the lake center,while Fe2+,Mn2+and NH4-N concentrations are higher in the lake center and decreasing towards the periphery of the lake area,while pH value is decreasing towards the southeast along the northwest of the lake area.According to the analysis,these spatial distribution laws are influenced by geological structure and human factors.Induced by geological structure,groundwater in the periphery of the lake area is oxidizing and the groundwater in the center of the lake area is reducible.In this study,three sensitive oxidation-reduction environment factors,NO3-,Fe2+ and Mn2+,are selected as oxidation-reduction zoning indicators,and oxidationreduction environment of shallow groundwater of Dongting Lake area is divided into three zones from outside to inside,i.e.,nitrate stabilization and reduction zone,manganese(?)reduction zone and iron(?)reduction zone.