Experimental Study On Influence Mechanism Of Aquifer Permeability Performance Under Brackish-water Alternating

In this article, we take semi-arid area such as Jiyang and Shanghe as the study area. On thebasis of the understanding of salt water which is formed by geological features of ore body andhydrogeological conditions in the study area, we research the variation characteristics ofaquifer permeability in the brackish-water alternate process, and analyzes the influence factorsof hydrogeological parameters such as coefficient of permeability of aquifer. Under theinfluence of climate, groundwater dynamic condition and the ancient sedimentary environment,salt water are mainly distributed in the dry North China, northwest and coastal zone. At present,the research on the invasion of seawater （coastal brackish） is relatively mature for both at homeand abroad, but the study of continental salinization salty water is relatively weak. Continentalsalinization salt water is different from the coastal salty water, which is formed under thecondition of certain geological history. Although shallow inland freshwater is long-term underthe influence of rainfall leaching and human activity in the development system, but which isstill coexist with saltwater. Brackish-water transition zone is bound to swing back and forth（salt water and fresh water displace each other）, which is accompanied by a complexhydrogeochemical action and the colloid process, so this kind of move can form a lowpermeability zone in brackish interface perpendicular to the groundwater flow. While theexisting interface movement theories mostly treate the permeability of the aquifer as constant,which is obviously not scientific.Therefore, this topic take semi-arid area as the research object, selecting soil samples andwater samples, and design the brackish-water alternate constant head test apparatus, to simulatethe mutual displacement process of salt water area and fresh water area in the field. Throughthe simulation control tests under different conditions, we systematically studied the influenceof brackish-water interface clay release, temperature, etc on the changes of permeability ofaquifer, which has very important theoretical significance and practical value for the saltwaterintrusion prediction and prevention in the semi-arid region, which is the main content of thistopic to study, is also the innovation of this paper. Through the brackish-water alternatingdynamic experiment, combining field investigation and laboratory test, we explore the influence of the brackish-water alternate process on the penetrating quality of aquifer, and candraw the conclusion as follows:（1） There is a close relationship between the permeability coefficient and the effectivepore volume of the aqueous media, the bigger the effective pore volume of the aqueous mediais, the greater the permeability coefficient is. In the experimental study of this paper, thevolume of aquifer medium is1645.97cubic centimeter, and porosity is45.47%, and porevolume is748.46cubic centimeter.（2） In the process of salt water through the soil column, the dispersion coefficient is4.88×10^-8m²/s; In the process of fresh water displace salt water, the dispersion coefficient is3.12×10^-7m²/s. Research shows that: the dispersion coefficient is closely related with thechange of the permeability coefficient, and the changes of the permeability coefficient affectthe dispersion coefficient, the faster the permeability coefficient changes, the larger thedispersion coefficient is.（3） In the phase of salt water displace fresh water, the coefficient of permeability increaseslowly along with the exudate which carries most clay particle flowing from pillar. As theexperiment progressed, owing to the release of the clay decreases, the permeability coefficientof aquifer plateaus. Known that viscous coefficient is inversely proportional to the temperature,viscosity coefficient will increase as the test temperature drop, thereby resulting inpermeability coefficient of water-bearing media decreases. However the test data shows thatthe permeability coefficient is up instead of down. So in the phase of salt water displace freshwater, the temperature changes effects on permeability which do not play leading role.（4） When the experiment entered the stage of fresh water displace salt water, at thebeginning, the coefficient of permeability is from5.0to5.73m/d, with the growth of14.7%.When the flow velocity is greater, the clay particle of the aqueous media is easier to fall offfrom the pore wall, meanwhile the release amount of the clay particle also increasesubsequently. As the clay particle transfer and combine again in the aqueous media, the porousmedia blocking, eventually lead to the permeability coefficient of the aqueous media plummetwith a decreasing amplitude of56.2%. In21to31days of the stage of fresh water displace saltwater, the drop of the experiment environment temperature accelerate the decrease of thepermeability coefficient. And then, the permeability coefficient rise and fall alternately, and fluctuate upward trend as a whole, and the steady permeability coefficient is5.45m/d finally.Therefore, at this stage, the temperature changes effects on permeability which also do not playleading role.