Migration and Transformation Mechanism of Arsenic in Variable-permeability Clayey Aquitard at Jianghan Plain
The clayey aquitard is associated with the aquifer and is an important part of the groundwater system.Although the low hydraulic conductivity of aquitard,more and more studies have suggested that there is a close hydraulic relationship between the aquitard and the adjacent aquifer.Compared with aquifer,the permeability coefficient of clayey aquitard decreases with burial depth and sediments in clayey aquitard is rich in organic carbon,microorganisms and a variety of chemical components.These chemical composition can enter the aquifer through the vertical seepage and compaction(for example,aquiatrds compaction caused by overlying formation pressure or overdraft of groundwater)of pore water in the clayey aquitard.Arsenic(As)contamination in groundwater is a global problem affecting the health of millions of people,investigations have shown that the high arsenic aquifer is always associated with clayey aquitard of black clay or mud.Nevertheless,the migration and transformation mechanism of arsenic in variable permeability clayey aquitard and the contribution of clayey aquitard to arsenic enrichment in aquifer are still unclear.In accordance with the above questions,this study selected Jianghan Plain as a typical area.Four related studies were conducted in this work:1)the hydrogeochemical characteristics of arsenic in the clayey aquitard was found out through systemic survey.2)revealed the migration and transformation mechanism of arsenic from aquitard to aquifer.3)clarified the release mechanism of arsenic in the sediments of clayey aquitard compaction through compaction simulation experiment.4)a quantitatively evaluation was used to reveal the contribution of clayey aquitard to arsenic enrichment in high arsenic aquifer.The conclusions and understandings are as follows:(1)As concentration within 0-18 m depth of the aquitard increased with depth gradually,and decreased with the depth of more than 20 m of the confined aquifer.At the interface between the aquitard and aquifer(18-20m),the concentration of As reached the maximum,presenting a "bell-shaped" change curve.The variation trend of Fe,Mn,NH4-N,S2-,DOC and HCO3-was consistent with that of As,indicating that the reductive dissolution of Fe oxides/hydroxides under the reduction condition was the potential mechanism leading to the release of arsenic in the aquitard.(2)Based on the sediments,piezometer water and the simulation results in the three boreholes,we believed that there were two different arsenic migration mechanisms in the aquitard under natural conditions.The first was near the surface water,the clayey aquitard has a significant influence on the enrichment of arsenic in the aquifer.The second was nearby areas without surface water,the concentration of As in pore water was very low and had little effect on the enrichment of arsenic in the aquifer.In addition,the arsenic in the aquitard migrated with the pore water to the aquifer under compaction,thus affecting the water quality of the aquifer.(3)During the whole process of compaction(pressure is 0-0.9 MPa),reductive dissolution of iron oxides and desorption of arsenic from iron oxides mainly occurred in period 1(0-0.2MPa).In the early period 2(0.2-0.5MPa),reductive dissolution of iron oxides was the main cause of arsenic release,while in the later period 2(0.5-0.7MPa),desorption of arsenic from iron oxides was the main cause.During period 3(0.7-0.9Mpa),desorption of As mainly occurred on the surface of clay minerals and carbonate minerals,resulting in the release of the adsorbed arsenic.(4)When the compaction rate was relatively fast(0.1MPa /12h),rapid release of pore water occurred in clayey sediments,and the release of arsenic was controlled by water content and reaction time.However,When the compaction rate was relatively slow(0.1Mpa/24 h and 0.1Mpa/48h),slow release of pore water occurred,and the compaction rate has little effect on arsenic release in clayey sediments.The total release amount of arsenic under different compaction mode was shown as: acceleration compaction mode ? constant compaction mode ? deceleration compaction mode.(5)In the process of aquitard compaction,the amount of release arsenic contributed about 1/9 to the enrichment of arsenic in the underlying aquifer.At present,the total arsenic reserves of the aquitard in the study area were about 1.2 times that of the aquifer.The reserves of arsenic in the aquitard in the study area were more than that of the aquifer,suggesting that arsenic in the aquitard will continue to enter the aquifer through the modern hydrological cycle and natural compaction in the future,making a significant contribution to the enrichment of arsenic in the aquifer.(6)Compaction and modern hydrological cycle control the migration of arsenic from the aquitard to the aquifer and the concentration of arsenic in the aquifer.The migration of arsenic in the aquitard compaction is common under natural conditions,and it contributes to a certain concentration background value in aquifer.The migration of arsenic in the aquitard with the participation of modern hydrological cycle is the key factor to determine the arsenic concentration in the aquifer.