Geochemical Characteristics Of Lateral Hyporheic Zone Between The River Water And Groundwater In Red Bed Hill

With the rapid development of economy in our country,water pollution problem has become one of the major environmental problems,red bed region has a wide distribution in our country,meanwhile the problem of water pollution and water resources is relatively serious in the red bed region which distributes throughout our country widely.For a long time,the water pollution control and water resources development and utilization of surface water and groundwater in red layer regionare independent of each other.Nowadays,the interaction of layers and interface process are received more and more attention,so the research of surface water and groundwater interaction also needs to be strengthened.The lateral hyporheic zone between the river water and groundwater offers a buffer zone to the dynamic changes of groundwater on water level,temperature and chemical changes,in which many pollutants were purified.lateral hyporheic zone has more complex interaction flow movement and geochemical process than vertical hyporheic zone,which plays an important role in the water quality protection,therefoer the research on the interactive movement mechanism and geochemical characteristics in the lateral hyporheic zone is of great significance.Taking on the research at the lateral hyporheic zone between the river water and groundwater in redbed mound,this study has sampled the subsurface flow,river water and ground water samples on December of 2015 to November 2016,and the indices of water temperature,dissolved oxygen,pH,electrical conductivity were measured at the hyporheic zone of Maanxi.The concetrations of ions in water and hydrogen and oxygen stable isotope,organic carbon were also analyzed aimed at study the migration mechanisms of lateral subsurface flow and geochemical characteristics of lateral hyporheic zone in river and ground water.The results show that: The superficial sediment of the off shore area of lateral hyporheic zone is relatively loose,making for the rainfall infiltration and subsurface flow evaporation,and the clay is much more in hyporheic zone.A large number of ions adsorbed make the subsurface flow movement slow at the same time.Besides,the changes of temperature,hydrodynamic condition,hydrogen and oxygen stable isotopes and Cl-on subsurface flow movement also has a certain instruction function.From the dry season to the early stage of rainy season,the river water is recharged from the groundwater.Groundwater from the river bank within 30 cm is mixed with river water,precipitation,groundwater recharge to further strengthen the early rainy season.In the process of groundwater recharging river water,the mixing action of precipitation,river water and groundwater may become intensive within 30 cm away from the banks of river,and groundwater recharge become strong in the early stage of rainy season.Then,under the influence of precipitation increasing and flood plain submerged river by the reservoir flood many times,the water infiltration at the place of about 10 cm away from the riverbank is relative big,then the water recharge to the groundwater.After water-break,the water will be in riched in hyporheic zone and recharge river.In the summer drought period,groundwater are supplied by interaction flow.At the end of rainy season,the subsurface flow was influenced increasingly by precipitation,seepage and water in the flood plain.Because the period of river water infiltration and the summer drought is short,the river water is recharged mainly by groundwater during the study period.Hydrodynamic conditions,temperature and concentration of organic carbon which influence each other and work together,affecting the activity of microorganism,make hyporheic zone in the reductive environment as a whole during the study period.Reduction in the rainy season has been further strengthened,and redox zone is Less than 10 cm away from the banks of the river.During the study period,not only the hyporheic zone is turning into partial acid environment,the lowest point of pH is also moving to the Banks of the river gradually.Geochemical characteristics of lateral hyporheic zone are affected by migration mechanisms of lateral subsurface flow and environmental changes of lateral hyporheic zone.Ca²⁺,Mg²⁺ and HCO₃^-is main ions in the research area,it influence the change of the EC of the monitoring stations.The change of riparian zone condition such as river flooded,water lettuce and hyacinth accumulation,vegetation destruction has important influence on ion changes in lateral hyporheic zone and the influence of acid and alkali environment changes are small.Human activities influence on the ion change of lateral hyporheic zone.The protecting of riparian zone have great significance.The river has a higher concentration of K⁺,Na⁺,NH₄⁺-N,NO₃^-,P and SO₄^2-.During the dry season,lateral hyporheic zone have high NO₃^-concentration,and affect NO₃^-concentration of river water under the condition of groundwater recharge of the river.During the rainy season,the effect affected by denitrification is abate,Under the mineralization of organic sulfur,compared with groundwater,lateral hyporheic zone have high concentrations of SO₄^2-,especially in the summer drought period is more apparent,and affect the SO42� concentration in river.Lateral hyporheic zone may be the river pollution source of the NO₃^-and SO42�.And in addition to the nutrition of pollutants,in the process of groundwater recharge of the river,lateral hyporheic zone may be the river pollution source of the Zn,Cd and other heavy metals,make the potential impact on the river water quality.Under the condition of groundwater recharge of the river,the purification of lateral hyporheic zone to groundwater is not obvious,it makes the groundwater water quality deterioration in turn,and impacts on the river water quality to some extent.