Study On Water-rock Interaction In Saltwater And Freshwater Transitional Zone

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. And consequently, there is a salt concentration transitional zone between saltwater and freshwater that is saltwater-freshwater transitional zone which is not in the same position with the influence of human behavior and natural environment change in inshore hydrogeological condition. There is complicated hydrogeochemistry process which include cation exchange and mineral dissolution /precipitation in transitional zone when seawater intrusion and aquifer flushing occur. There is seawater intrusion phenomenon of different degree in almost all coastal areas when people exploit groundwater excessively. And in the same time, the proper measures have been taken to protect the groundwater from saltwater intrusion. Therefore, it is emergent that we should study deeply the water-rock interaction in saltwater-freshwater transitional zone. That study will do help for understanding mechanism of seawater intrusion and aquifer flushing, and meanwhile it will provide scientific basis for quantitative estimation and prevention of seawater intrusion. There is important theoretical significance and practical value about forecast and prevention of seawater intrusion in various academic fields.Hydrogeochemisty process such as cation exchange and mineral dissolution/precipitation in transitional zone is studied through field investigation and the measure of physical and chemical character in aquifer media and water samples in the downstream of Dagu river. The mixing erosion of saltwater-freshwater is studied systematically through batch mixing experiment and PREEQC soft. Using cation exchange dynamic and batch experiment, the equilibrium time of cation exchange is decided. At the same time, cation exchange process is modeled using different mass equilibrium equations and different factors such as temperature and sample size that have effect on the cation exchange are analyzed. Adopting medium-sand-column which stimulates the dynamic process of seawater and freshwater displacement, the multispecies components transport law and process of cation exchange and mineral dissolution/precipitation are discussed. On the basis of a series of research above, the major conclusions presented in this paper can be summarized as follows:(1) It is not a simple mechanically mix but complex physics-chemistry process when seawater mixes with freshwater in open system. Dolomite's mixed precipitation occurs when seawater mixes with freshwater in different ratio, and meanwhile that is strictly mixed precipitation. While, Calcite's mixed dissolution happens that is general mixed dissolution in some mix ratios and strict mixed dissolution in other ratios.(2) The equilibrium time of the main cation (Na+,K+,Ca²⁺,Mg²⁺) is about 90 minutes through sandy samples dynamic experiment which is different with the former study that indicates the equilibrium time is 15 minutes.(3) The amount of cation exchange in soil samples is that the fine sand is smaller than coarse silt which is smaller than clay. Gaines-Thomas equation can model the process of cation exchange accurately. The effect of temperature on cation exchange is negligible.(4) Saltwater-freshwater displacement experiments show that the phenomenon of cation exchange is obvious in water and solid matrix. Significant cation exchange exists between Na+ and Ca²⁺in the process of seawater intrusion. when this exchange terminates, Ca²⁺ won't increase. In order to maintain the former equilibrium, Mg²⁺ and Na+ exchange takes place. After equilibrium of these two exchange effect, cation exchange between Mg²⁺ and Ca²⁺ happens which Mg²⁺ is adsorbed. K+ is chemically absorbed by solid matrix. And When river displaces seawater, cation exchange between Na+ and Ca²⁺ takes place where Ca²⁺ is adsorbed and Na+ is released to the solution. And then exchange between Mg²⁺ and Ca²⁺ occurs that is Ca²⁺ is desorbed and Mg²⁺ is adsorbed in aquifer medium. The amount of K+ exchange is little.(5) It is found that Calcite and Dolomite begin to precipitate at 40% seawater-freshwater mix ratio until seawater intrusions finish. In the whole process of intrusion, aqueous is erosive for Gypsum. Although the amount of precipitation or dissolution is small that in the scope of 5mg/L, the effect can't be negligible.