Effect Of Surface Water-groundwater Interaction On Arsenic Mobilization In Shallow Groundwater Of Jianghan Plain

High arsenic groundwater is widely distributed in the world and the formation mechanism and distribution pattern of which have received increasing attention and emerged as hot topics in the field of pollution hydrology.The key to solve the problem of high arsenic groundwater is to identify the mechanism of arsenic mobilization and enrichment in groundwater systems and the distribution pattern of high arsenic groundwater.It has been generally believed that adsorption-desorption,evaporation-enrichment,and reductive dissolution dominated the mobilization and enrichment of arsenic in groundwater.However,due to the complex groundwater environment and arsenic properties,the mechanisms of arsenic mobilization and release in groundwater are still not well understand.In fact,surface water-groundwater interaction（SW-GW）,that is the exchange of water and materials between surface water and groundwater at different spatial and temporal scales,can affect arsenic transport in groundwater system.High arsenic water sources were first discovered in the Jianghan Plain in 2005.Strong surface water and groundwater interaction can be found in the Jianghan Plain and arsenic concentration in the groundwater fluctuated seasonally.Therefore,it is necessary to clarify the effect of SW-GW interaction on arsenic trasnport in groundwater systems.Shahu field site has been selected as a typical high arsenic groundwater area in Jianghan Plain.Based on field monitoring and numerical simulation,the SW-GW interaction patterns and intensities and the amount of arsenic exchange between surface water and groundwater can be valid identification,respectively.The main insights and results are summarized as follows:（1）Groundwater water chemical indicators,such as Ca²⁺,Mg²⁺,HCO₃^-,SO₄^2-and As are basically higher in the rainy season than those in the dry season,which may be related to SW-GW water exchange.Arsenic is mainly located between 20 and 25 m below the surface.The closer to the river,the higher concentrated arsenic in groundwater.（2）Surface water recharges groundwater during the rainy season and vice versa.Changes in aquifer redox conditions due to the groundwater flow changing are the main reason for the seasonal variation of As.The As mobilization is specifically manifested as follows:vertical mixing controls As mobilization in shallow groundwater;lateral recharge and/or mixing with adjacent aquifers determines As mobilization in deep aquifers.（3）SW-GW has different mixing intensities in different seasons,thus affecting the mobilization and enrichment of arsenic in groundwater.Based on the mass conservation equation,it can be seen that the mixing intensity of SW-GW is higher in the rainy season,which has a significant scouring effect on the aquifer,resulting in the rapid transport of arsenic in the aquifer and an increase in concentration;on the contrary,the SW-GW interaction intensity decreases in the dry season,which has a weaker scouring effect on the aquifer,resulting in the slowing down of arsenic mobilization in the aquifer system and its retention and relative weakening in concentration.As is enriched in the adjacent rivers（with the strongest SW-GW）in both wet and dry seasons.（4）There are also seasonal differences in groundwater flow velocity and direction.The maximum horizontal groundwater flow velocity in phreatic aquifer and confined aqfueir during the rainy season（from July to September）is 0.0026 m/d and 0.18 m/d,respectively,and which are 0.0021 m/d and 0.1 m/d respectively in the dry season.SW-GW interaction significantly affects the variation of groundwater flow.（5）The maximum vertical arsenic exchange during 10,25 m and 28 m below the ground surface are 457.24 mg/day and 4379.98 mg/day,respectively,in the rainy season,and which are 191.25 mg/day and 1385.63 mg/day respectively in the dry season.The amount of arsenic exchange between surface water and groundwater is higher in the wet season than that of in the dry season,and which higher at 25 m depth below the ground surface than that of 50 m.The innovation of this paper is to quantitatively characterize the influence of SW-GW interaction patterns and intensity on the arsenic mobilization in groundwater systems through high-precision monitoring and numerical simulation in the study area.