Study On The Dynamic Distribution Characteristic And The Feasibility Of Remediation In Coastal Salt Groundwater

Freshwater and saltwater are liquids which may mix, as a result of hydrodynamics dispersion, the contact zone will be in the form of mixed water transitional zone, which is saltwater-freshwater transitional zone. The distribution and form of saltwater-freshwater transitional zone are influenced by the human behavior and natural environment change in coastal hydrogeological condition. Seawater and saltwater intrusion become a serious geological disaster in coastal areas for people exploit groundwater excessively. And in the same time, the proper remedial measures have been taken to protect the groundwater. Therefore, studying on the dynamic distribution characteristic and the feasibility of remediation in coastal salt groundwater, supply important theoretical significance practical value for reasonable developing and using groundwater resources and forecast and prevention of saltwater intrusion.Based on the geohydrological investigation in the fields, physical and chemical character of the aquifer media and water samples which was collected from the downstream of Dagu River was analyzed. Through column and sand tank rainfall simulation experiment, dynamic distribution characteristic of different concentration of saltwater influenced by rainfall intensity and pattern is studied. The water sensitivity is researched by saltwater and freshwater displacing each other in the sand tank experiment, the change law and mechanism in hydraulic conductivity of aquifer medium are deeply analyzed in salinity abrupt changed and gradual changed process. Using column and sand tank experiment, factors such as flowing velocity,injecting salinity of solution and remedial measures that have effect on remedial effect of saltwater are studied ,and feasibility of remediation of saltwater is discussed. On the basis of a series of research above, the major conclusions presented in this paper can be summarized as follows:(1) The salinity of upper layer aquifer medium is more easily influenced by rainfall intensity. When rainfall stopped, the salinity of upper layer aquifer medium is rising due to molecular diffusion, and increased extent is greater under intensity of heavy rainfall. The pattern of rainfall has effect on salinity variation rate and extent of the aquifer medium, but does not influence the salinity variation trend. The lower the initial solution concentration of the aquifer medium, the rainfall has greater influence on variation of salinity.(2) There are significant changes in hydraulic conductivity of aquifer medium when the saltwater and freshwater displace each other. Mechanism of permeability variation appears different in salinity abrupt changed and gradual changed process. The reason for the reduction of hydraulic conductivity in salinity abrupt changed process should be clogging the pore necks by released particles. The reduction of hydraulic conductivity in salinity gradual changed process may due to reactive clay (as montmorillonite) swelling.(3) Changes in hydraulic conductivity is irregular: In various horizontal distances from water let and various heights, the change law in hydraulic conductivity of aquifer media is obviously different.(4) Injecting freshwater to displace the saltwater, the more rapid the fluid velocity, the more easily the particles are released. Accordingly, when the release amount increase, the larger the reduction degree of the permeability would be. The lower the injecting salinity of solution and the faster change rate of the salinity decrease, the larger the reduction degree of the permeability is.(5) Compared with fully penetrating well, adopting partially penetrating well restore the saltwater, the change range of hydraulic conductivity is smaller. Meanwhile, the change rate of the salinity is slow which would take more time on remediation when adopting partially penetrating well.(6) Using pumping-injection plan to displace the saltwater, the change rate of the salinity is faster. The clogged pore necks would be dislodged by rapid flow fluid velocity which is not avail to particle deposition. Leading to the decreasing range in hydraulic conductivity of pumping-injection plan is less than injection plan. Therefore, pumping-injection plan is worthy of use in the remediation of saltwater, which take less time on remediation and do less damage on permeability of aquifer medium.