Study On The Well-Pond Combined Model Of Managed Aquifer Recharge (MAR)

To mitigate the shortage of groundwater resources,the concept of Managed Aquifer Recharge(MAR)has been proposed.Combining the characteristics of two types of Managed Aquifer Recharge facilities,namely,infiltration ponds and vadose zone infiltration wells,the combined well-pond model can take full advantage of the large recharge depth and fast rate of vadose zone infiltration wells and the large infiltration area of infiltration ponds,which is of great significance for improving groundwater recharge efficiency in areas with large groundwater depths.In order to explore the application potential of the well-pond combined model,a two-dimensional axisymmetric model was constructed using HYDRUS software,and the accuracy of the constructed model was verified by replicating the model with the well without pond model for the actual Fort Irwin site and with the pond without well model for the El Paso site in the existing literature.Then,the recharge effects of the well-pond combined model,the well without pond,and the pond without well model were compared,and the superiority of the well-pond combined model as an aquifer recharge management facility was determined.On this basis,the effects of different vadose zone textures and well depths on the infiltration and recharge effects of the well-pond combined model are discussed,and the infiltration and recharge efficiency of the well-pond combined model under different inhomogeneous conditions(stratified heterogeneity and random inhomogeneity)in the vadose zone are analyzed in depth.The following main conclusions were obtained from the above study:(1)The combined well-pond mode of infiltration recharge was better than the well-without-pond and well-without-pond modes.The model replication shows that the model constructed in this paper is reasonable and feasible.The cumulative recharge at 730 d for different infiltration pond radii(r_b=5m,15m,25m,and 35m)was 1.02,1.25,1.75,and2.6 times higher than that of the well-pondless model,respectively;and the cumulative infiltration of the well-pond combination model increased linearly with time and the saturated permeability coefficient K_s of the gas envelope.and tends to increase linearly with time and the saturated permeability coefficient K_s of the envelope;the coarser the texture in the initial recharge stage,the smaller the cumulative recharge,and the opposite in the later recharge stage;an appropriate increase in well depth can help achieve rapid and substantial recharge to groundwater.The cumulative recharge at 365 d for the combined well-pond model is 4.39 and 6.04 times higher than that for the ponded-well model,respectively,under different well depth conditions(45 m and 70 m).(2)When the lowly permeable layer exists in the vadose zone,the well-pond combined model is more suitable to be arranged in sites with deeper burial,thinner thickness and coarser texture of the lowly permeable layer.The results show that with the increase of the depth of the lowly permeable layer(15m,20m,25m and 30m),the cumulative recharge of the well-pond combined model increases,and when the depth of the lowly permeable layer increases by 5m,the cumulative recharge increases by 6.4%,13.4%and 16.1%,respectively,compared with the scenario of the lowly permeable layer depth of 15m.On the other hand,with the increase of the thickness of the lowly permeable layer(1m,2m,3m and 4m)and the finer texture(clay,siltly clay and silt),the cumulative recharge of the well-pond combined model showed a decreasing trend.Specifically,the cumulative recharge decreased by 16.5%,25.4%and 32.4%for each 1m increase in the thickness of the lowly permeable layer compared with the 1m thick lowly permeable layer scenario,and by 18.4%and 25.7%for the siltly clay and silt lowly permeable layer compared with the clay scenario,respectively.By exploring the influence of soil moisture characteristics parameters of different lowly permeable layers on the infiltration recharge results of the well-pond combined model,it was found that K_s played the largest role,andαand n had almost no effect.In addition,the maximum height value of the groundwater mound calculated according to the classical theoretical equation is larger because of the larger effect of lateral infiltration from the vadose zone permeable wells in the vadose zone.(3)Simulation results under different stochastic heterogeneous scenarios show that the well-pond combined model is more suitable for sites with a larger range of variation in saturated hydraulic conductivity K_s(larger standard deviationσ)and longer correlation lengths in the X and Z directions.Compared with the homogeneous case,the overall mean cumulative infiltration and recharge fluxes through the infiltration pond,the pack vadose zone infiltration well,and the combined well-pond model are all greater than the corresponding amounts in the homogeneous case.σincreases the formation of a highly permeable lens that contributes to the infiltration recharge effect of both the infiltration pond and the vadose zone infiltration well,increasing the mean location of the wetting front reaching the recharge area and reducing the corresponding recharge time.The increase in X-directional correlation length forms a longer lateral high permeability lens that promotes the lateral infiltration recharge effect in the permeable wells of the inclusion zone,thus increasing the average location of the wetting front reaching the recharge area,but delays the corresponding recharge time.On the contrary,the increase of Z-orrelation length to form thicker vertical high permeability lens will promote the vertical diffusion of water flow,thus speeding up the average time for the infiltrated water flow to reach the recharge area,but will reduce the average position of the wetting front to reach the recharge area.