Evaluation Of Groundwater Resources Quantity In Riverside Well Field Under The Condition Of Permeability Variation Of Riverbed Sediment During Scouring And Silting Process

For riverside well field,the infiltration of natural rivers is the most important source of recharge to the aquifer.In order to reasonably plan the pumping strategy of the riverside well field,it is necessary to obtain the dynamic information of river infiltration,that is,to grasp the characteristics of sediment properties such as the permeability of riverbed sediments and there variability over time.Due to the influence of river scouring and silting process,the lithology,structure and thickness of natural riverbed sediments change with time,resulting in strong spatial and temporal variability of riverbed sediment permeability,which in turn affects the recharge of groundwater on both sides of the river bank.At present,the mechanism of the influence of river scouring and silting process on the permeability of riverbed sediments is not fully understood.Therefore,the calculation of the amount of water supplied to the groundwater by infiltration of surface water is inaccurate,resulting in distortion of the regional groundwater resources assessment results.Based on this,in this paper,Gaojiadian riverside well field in Dehui City along the middle reaches of the second Songhua River was taken as the study area,and on the basis of identifying the geological and hydrogeological conditions of the area,field monitoring,laboratory tests,field tests were used to find out the temporal and spatial variation characteristics of riverbed sediment permeability and its influencing factors and mechanism.Numerical simulation of river dynamics was used to identify hydrodynamic characteristics and sediment distribution characteristics,based on which,the relationship between sediment distribution and riverbed sediment permeability was established.The riverbed sediment permeability was partitioned which was coupled with the numerical model of groundwater flow to evaluate and predict the groundwater resources of the riverside well field,in order to evaluate the groundwater resources of the the riverside well field more accurately,and to provide scientific basis for operating,monitoring and protecting the well field and ensuring the safety of local residents.Through this research,the following main insights have been achieved:1.Under the control of hydrodynamic conditions,there is obvious scouring and silting process on the riverbed.The hydraulic conductivity of riverbed sediment strongly varies spatially and temporally with a maximum change up to 7 orders of magnitude.The hydraulic conductivity of the main channel sediment is significantly higher than that of the heart beach,sandbank and river bank edge.Under the influence of silting effect,the hydraulic conductivity of riverbed sediments in heart beach,sandbank and river bank edge is lower.Under the influence of scouring and silting process,the infiltration rate of river water in riverbed sediments fluctuates obviously and there is a significant positive correlation between the infiltration rate and the river water level.The infiltration rate of river water in the main channel is up to 6.455í10^-6m/s,while the infiltration rate of river water in the sandbank and heart beach or there edge is lower,with a maximum value of 3.000í10^-6m/s.2.The permeability of riverbed sediments is affected by sediment lithology,river hydrodynamic conditions and riverbed geomorphology.The hydraulic conductivity of riverbed sediments has a significant positive correlation with the particle size of sediments and river flow velocity.When the river flow velocity is greater than the critical starting velocity of the riverbed sediment particles,the change of river bed sediment permeability will be mainly affected by the change of sediment lithology caused by the bedload sediment transfer during the riverbed scouring and silting process;When the river flow velocity is smaller than the critical starting velocity of the riverbed sediment particles,the change of river bed sediment permeability will be affected by the formation of silt layer on the riverbed surface caused by suspended load sediment in the river.The dissolution of calcite and manganese minerals in riverbed sediments is not the main factor affecting the permeability change of riverbed sediments.By influencing their ability to decompose organic matter in riverbed sediments,the abundance and diversity of microbial community in riverbed sediments affect the organic matter content of riverbed sediments,which ultimately leads to the change of riverbed sediment permeability.3.Chemical factors and biological factors have little effect on the permeability of sediments relative to physical factors,so they are negligible in the study area where the scouring and silting process happens significantly.Stepwise regression was used to determine the most sensitive factors affecting the change of sediment permeability in riverbed,and the quantitative estimation formula of sediment hydraulicconductivitywasobtainedbygeneticalgorithm?GAS?:???.Based on the simulation results of river bed hydrodynamics and sediment transport on riverbed sediment lithology,the river bed sediment hydraulic conductivity can be calculated according to the empirical formula among riverbed sediment hydraulic conductivity and the effective particle size and median particle size.The hydraulic conductivity of sediment are partitioned into different areas based on its temporal and spatial distribution characteristics.According to the monthly calculation results,the average value of the hydraulic conductivity of each area is calculated and assigned into the groundwater flow simulation,which can accurately reflect the spatial distribution and variation trend of river bed sediment hydraulic conductivity in the study area,and provide a parameter basis for the groundwater resource evaluation of the riverside well field.4.Under the two conditions of not considering and considering the permeability change of riverbed sediments,the error rate between the change of groundwater storage calculated by the measured data and the calculated value of the model is 16.9%,respectively,is 4.2%.Considering the temporal and spatial changes of river bed sediment permeability during the scouring and silting process,it can more objectively reflect the actual situation of hydrogeological conditions in the study area.When the spatial and temporal variability of river bed sediment permeability during river scouring and silting is not considered,the quantity of infiltration recharge of rivers during the flood season will be underestimated.5.Under the exploration conditions of the riverside well field,the total aquifer recharge was 12709.3m³/d and the total aquifer discharge was 12080.2m³/d from April 2016 to December 2016.The regional groundwater was in a positive equilibrium with a balance difference of 629.1 m³/d.The infiltration of river water is recoverable potential 8940.6m³/d,accounting for 70.3%of the total recharge,which is the most important source of recharge in the riverside well field.The groundwater exploration potential of Gaojiadian riverside well field is much larger than the current exploration volume,so there is still certain exploitable potential of the aquifer.Under the conditions of partial abundance,flat and extremely dry years,the groundwater exploitation potential of Gaojiadian Weihe water source is 41,200m³/d,39,000 m³/d and 31,100 m³/d,respectively.The exploration plan of aquifer should be determined according to the hydrological characteristics of the river in order to avoid a series of ecological or environmental geological problems caused by the amount of exploration exceeding the amount of groundwater that can be explorated.