Study On Water Sensitivity Of The Seawater-freshwater Transition Zone And Its Applications

Basing on the investigations of the natural geography, geology and hydrogeology conditions downstream the Dagu River, combining with the collecting, processing, chemical composition and physical properties analyzing of the samples( sand sample, water sample and clay mineral), the water sensitivity of the seawater-freshwater transition zone and its applications are studied by laboratory batch and column experiments in this paper.At first, the variation of the permeability for the aquifer's media during abrupt and gradual salinity decrease processes is studied. At the same time, with the changes of clay minerals'physical forms and chemical properties during abrupt salinity decrease, the mechanism that different clay minerals (kaolin and smectite) have different influences on the permeability of porous media in salinity decrease process is explored and explained according to colloid and surface chemistry theories. Then, the impacting factors on the water sensitivity, such as critical fluid velocity ,critical salt concentration, critical ratio of salinity decrease, critical ionic strength and pH etc. are analyzed deeply. In addition, for first time, a feasibility study on heavy metal and organic pollutants'removal by the process of particle release and transfer is researched. Finally, combining with the engineer example of saline water resumption downstream the Dagu River, under the consideration of water sensitivity, the resumption scheme (pumping scheme) is simulated by the groundwater simulation soft-Visual-MODFLOW 3.1.The replacing experiments of freshwater and seawater indicate that abrupt salinity decrease will result in obvious water sensitivity in the aquifer media with the permeability's rapidly and drastically decline and hydraulic conductivity decreased by 80.9%.But, in the gradual salinity(seawater ,50% seawater and 50% groundwater, 10% seawater and 90% groundwater, 5% seawater and 95% groundwater, and groundwater replacing in turn) decrease experiments, the permeability declines slowly and the hydraulic conductivity only decreases by 36%.Clay minerals'influence experiments on water sensitivity indicate that, during abrupt salinity process,the hydraulic conductivity of the sand column decreases from 0.81 m/d to 0.39 m/d when the weight content of kaolin increases from 1.5% to 12%, and the hydraulic conductivity decreases from 10-8cm/s to 0cm/s when the weight content of smectite increases from 3% to 4%. Photos scanning by electron microscopy show that most of the kaolin minerals are in a decentralized state and stack loosely together which attach to the pore wall or as the filling in the sand pores, but for the smectite minerals, crystal swelling and flocculation effects result in the form of a large amount of the micelle, which mostly block in the pores and bring about the hydraulic conductivity decrease by logarithmic grade.Particle release experiments show that the fluid shear stress can lead to the occurring of release phenomenon when the fluid velocity of seawater and freshwater is over 90m/d and 55m/d respectively, and there are only few particles released. For the NaCl solution, its critical salt concentration which can cause particle release is 0.06±0.005mol/L, and the critical ratio of salinity decrease is 0.012 mol/L. Ca2+ has a strong restraining effect on particle release process, for mixed solution of NaCl and CaCl2, the critical ionic strengths which lead to particle release decline to 0.02mol/L and 0.007mol/L when Ca2+ molar percentages are 5% and 10% respectively, which is far below the critical concentration of NaCl solution. Tests also indicate that the pH has significant impact on particle release and the release time and particle quantity is far less at acidic condition than alkaline one.Pollutant removal by the particle release experiments show that there are 89.77% of heavy metal pollutant (divalent mercury ion)and 85.71% of organic pollutant (phenol) move out of the sand columns under the effects of water leaching and particulate releasing, where there are 59.5% and 30.0 % pollutants remove due to particle release respectively.Numerical simulation results of the resuming schemes (pumping schemes) for underground saline water indicate that, under consideration of water sensitivity of the aquifer, the efficiency of the pumping resume scheme will be improved greatly, and the resuming time shorts by 1/3, the area of residual low concentration seawater body decreases by 81.3% and the total pumping quantity reduces by 20.9%.