| There are widespread Paleozoic and Mesozoic limestone formations in China,which constitute the important karst aquifers.The karst aquifers with strong karst development can be generalized into two parts,matrix and conduit.Matrix is generalized as an equivalent porous medium system where the groundwater is transported by seepage.The subterranean rivers and karst pipeline are generalized into conduit system,which is the main way of drainage in karst groundwater system.Studying and simulating the process of karst groundwater and solute transport have great significance in the utilization of karst groundwater as well as the prevention of pollution.MODFLOW-CFP is a hybrid model based on the fissure-conduit dual structure theory MODFLOW cell grid represents an equivalent permeable medium system,and then create pipe nodes within the MODFLOW cell grids.A discrete pipe network is formed by connections between pipe nodes.Two systems coupling by exchange flow,which is directly proportional to the difference of the water head.MODFLOW-CFP considers the two kinds of motion state of groundwater turbulence and fast laminar flow in pipelines.Xianghualing is located in Chenzhou City,Hunan province.It is a typical karst peak-cluster depression area.In this paper,the MODFLOW-CFP model is used to simulate the hydrological process in the area.By analyzing the results of previous studies,it is found that the dual structure model can recharge the conduit and matrix system by two recharge ways,those are concentration recharge and infiltration recharge.But the simulation results are still disappointing because simulation flow curve always is gentler than the observation curve.Therefore,the nonlinear method is used to deal with the recharge of the conduit which improves the effect of the simulated flow curve.Two different precision models of the monthly and diurnal stress periods are established to analyze the applicability of the CFP model in two cases of large time scale and rainstorm.Nash coefficients of simulated flow results are 0.91 and 0.68respectively.It shows that the simulation results of the CFP model are credible in these two situations.The change of exchange flow is analyzed and discussed.It is found that the conduit system does not recharge the equivalent permeable medium system during the rainy season or rainstorm.When the dry season or rainfall decreases,the attenuation rate of the underground river flow gradually slowdown.And the flow is subsequently equal to the exchange flow rate.It is assumed that there are two cases of pollution source migration with conduit flow and seepage in fracture.Two cases that(1)use the CFP model to output the flow velocity in the pipeline.In this case diffusion and dispersion are unconsidered.Calculation of the solute transport time is compared with the tracer test results.(2)And an equivalent porous medium(EPM)model is established to simulate the migration velocity and pollution plume range of heavy metal Mn in the model with MT3DMS.It is found that the solute transport velocity in the conduit system is 4 to 5 orders of magnitude bigger than that in the equivalent permeable medium system.The groundwater resources in the study area were evaluated,and the recharge volume was 1700*10~4 m~3/a.According to the characteristics of topography and hydrology in this area,two plans for development and utilization are put forward. |
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