Study On The Groundwater Pollution Migration Of A Waste Disposal Site

The thesis is based on a scientific research project "XX low-medium radioactive solid waste disposal plant and the surrounding's hydrogeology, engineering geology additional engineering research " to carry out the disposal site's hydrogeological research. Build hydrogeological conceptual model, and use Visual Modflow2.8.2 the 3-dimensional numerical simulation software to carry out the simulation and prediction of groundwater pollution transfer.Through detailed field geological and hydrogeological investigation, field test and the groundwater sample analysis, finding out that the disposal site's geological structure is complex, depth of groundwater level is superficial, groundwater resource is abundant, the aquifer medium include unconsolidated rock pore and bedrock weathering fracture and its spatial distribution characteristics, all type of groundwater's recharge, runoff, drainage characteristics, etc;analyzing the chemical characteristics of groundwater, the dynamic variation characteristics of groundwater and its influencing factors, and the main flow direction and velocity of groundwater, etc.Based on surface modeling method to build the three dimensional geological model, generalizing its vertical structure, then on the basis of this, a conceptual model of hydrogeological geology is established. And through a variety of field experiments to determine the hydrogeological parameters of aquifer The hydrogeological parameters of the disposal plant were stratified?zone?.Use the Visual Modflow 2.8.2 software to establish groundwater flow model of the disposal plant, and through the Modflow2000 module to deal with the natural flow field prediction. And through the MT3 DMS module to carry out groundwater pollution migration's three dimensional numerical simulation, analyzing the pollution transfer regular pattern of tracer factor Cl^-. It provides a theoretical basis for the safety assessment of the disposal plant and the protection and utilization of groundwater resources.The main research results of the paper are as follows:?1?According to the research on the regional geological environment and on the hydrological geology characteristics, establish a hydrogeological structure of the superimposed and water-resisting layer in the complex hydrogeological conditions.The aquifer system including quaternary loose rocks aquifer, Silurian Longmaxi group bedrock weathering fissure aquifer, weakly weathered bedrock relative water-resisting layer.?2? Basic hydrogeological characteristics of the disposal site1) The disposal site groundwater is weak alkaline, low mineralization. The overall characteristics is soluble.Among them, the loose rock pore water's chemistry types is HCO₃-Ca and HCO₃-Ca·Mg. The groundwater's eluviation function is strong.The chemical type of bedrock fissure water is HCO₃- Ca·Na·Mg, which is influenced by the deep structure fissure water, it is of strong water rock action.2) The overall groundwater dynamic type of the disposal plant is infiltration-runoff pattern, and part of them is infiltration- runoff- evaporation pattern.The dynamic change of groundwater is the result of rainfall condition, landform, geological structure and human activities.3) Through a field tracer test to determine the groundwater mainly flow direction and the velocity: The loose rocks pore water's average diffusion flow speed is 1.04 1.87 m/d.The runoff velocity of bedrock fissure water is different, characterized by 3.81 4.84 m/d and 23.13 m/d. This is controlled by the terrain, the bedrock fissure and the rock tendency.There are two advantages direction of the groundwater flow in the disposal plant. One is South West 247 degrees and the other is North West 339 degrees, the latter is the main direction of runoff.?3? Related hydrogeological parameters of the disposal site1) Through the water penetration test and water injection test to determine the Quaternary artificial soil layer permeability is weak permeability, permeability coefficient K is in the range of 10-5¹0-4 cm/s.2) Through the water packer test to determine the permeability of bedrock is weak, permeability coefficient K is in the range of 10-4¹0-2 cm/s.3) By water pumping test to determine the Silurian mudstone fractured bedrock aquifer is a low permeability, permeability coefficient K is in the range of 3.9×10-3⁴.9×10-3 cm/s.?4? The simulation results of the pollution migration of the tracer factor Cl^- in groundwater :1) The prediction results of the natural seepage field of the disposal plant show that the groundwater level change of disposal site in the future 20 years is relatively small, and it is basically maintain in a stable state.2) The tracer factor Cl^- in underground water is migrate in all directions and mainly horizontal direction, which is the results of hydrodynamic dispersion effect. The greater the hydrodynamic gradient, the larger tracer factor migration velocity. On the contrary, hydrodynamic gradient is small, tracer factor migration rate correspondingly slow.3) The tracer factor Cl^- pollution migrates with the movement of groundwater. Its main pollution migration direction controlled by the runoff direction of groundwater. In the leakage pollution accident, Cl^- migrates mainly from south to north,which is the result of the dominant fracture and terrain. In the action of building impervious curtain case, Cl^- migrates mainly from west to east, which is the result of the development of fissure.4) The differences of formation lithology and permeability, lead to the migration of the tracer factor Cl^- in vertical direction, show the tendency of decreasing over time.5) The impervious curtain has a significant blocking effect on the Cl^- in the aquifer along with the groundwater migration, which indicates the effectiveness of the seepage control measures.