Simulation Of Groundwater Quality And Ariation Tendency In Shuizuishan Area, Ningxia, Northwest China

Shizuishan City, which is the second largest city in the Ningxia Hui Autonomous Region, isnamed for mouth-shaped projecting hillstone located in intersection between Helan mountainand the Yellow River. Benefiting from the Yellow River, the land is fertile and the agriculturehas been developed rapidly. Shizuishan is located in the arid and semi-arid area of theNorthwest China, and is the most arid and strongest evaporation city in Ningxia. Economicdevelopment of the city is restricted by water resources shortage. Meanwhile, there areproblems such as over-exploitation, serious groundwater pollution, soil salinization andeco-environmental deterioration and so on. So it’s necessary to simulate groundwater qualityand analyse its change trend in Shizuishan area. Based on the detailed investigation ofgeological conditions, hydrogeological condition and water environmental status,three-dimensional groundwater flow model and solute transport model are established ofstudy area in the paper. After identifying and verifying the groundwater flow model tillsatisfactory, taking Cl-for example, the variation trend of groundwater quality is forecastedand analysed. In order to exploit and protect groundwater resources sustainably andreasonably, some measures and advices to control groundwater pollution are proposed in thepaper. Some major conclusions obtained from the study are as follows:(1) The type of groundwater in the study area mainly is unconsolidated rock pore water，itcan be divided into single phreatic water area and multilayer structure area on the basis ofwater-bearing structure. According to the dynamic monitoring data of groundwater, dynamiccharacteristics of phreatic water and the first confined water are mainly discussed, andphreatic water is greatly influenced by natural factors and human factors, whose dynamiccharacteristics change relatively complex, moreover, dynamic characteristics of exploit andradial flow types are mainly showed in the first confined water.(2) Phreatic water horizontally flows along the north east direction, and it manly accepts therecharge from the leakage of Yellow River irrigation canal system and the field irrigation,while it discharges through the form such as evapotranspiration and drain drainage. Indepressions front of the northern fan, the capacity of groundwater alternate update weakens.Furthermore, confined water mainly accepts the recharge from lateral runoff and dischargesby artificial exploitation and transfluence, and vertical movement is the main form ofgroundwater. Groundwater balance calculation and analysis turn out that the groundwater isbasically in state of equilibrium. (3) The TDS of groundwater in the study area is mainly influenced by the chemicalproperty of recharge water source, water-rock interactions, mixing effect, evaporation andconcentration effects and soil salinization in the area. Phreatic water is shallow buriedaffecting by plain irrigation, and aggravates groundwater evaporation and concentration andsalt deposition process. While groundwater in deep confined aquifer keeps good water qualityof low salinity because of adequate water supply and positive hydrological circulationconditions.(4) The three-dimensional groundwater unsteady flow and solute transport model which istaking chlorine ion as target solute of the study area are established by Visual MODFLOW.Under the condition of the status quo of exploitation, the chlorine ion of phreatic water isconcentrated obviously along with the extension of simulation time. After the model running50years, the biggest chlorine ion concentration of phreatic water is within8000mg/L. Theincrease of chlorine ion concentration aggravates the occurrence and development of regionalsoil salinization. While the interface of chlorine ion concentration of250mg/L in the confinedaquifer is keep in the groundwater discharge area which is in the northeast of the study area.So in most area, the standard limit of chlorine ion concentration has not been achieved and thewater quality is good. Under the condition of the status quo of exploitation, water stage ofphreatic and confined water is basically stable, with the rate of average maximum drawdownis about0.03m/a.(5) Under the condition of increasing exploitation in confined water source, variationtendency of the chlorine ion concentration in groundwater is studied, and the change of theregional groundwater drawdown in the corresponding simulation period is predicted. With theextending of exploitation time, excessive range of chlorine ion in phreatic water relativelyrelieves, but chloride ion concentration grows rapidly in local area. Meanwhile, with theincrease of concentrate exploitation quantity of confined water, deep confined water isinvaded by the phreatic water containing of more chloride ion under the impetus ofhydrodynamic field, which lead to gradually increasing trend of chlorine ion concentration inconfined water in the center of the exploitation, the area exceed the standard range is berween10and36square kilometres. Moreover, by comparing the analysis of simulation results underdifferent exploitation schemes, it shows that the setting of well group density, spacing andsingle well exploitation quantity is of great importance for the design of water intaking incentralized water source, and water chemical composition change and regional waterdrawdown are need to be taken into account. The study can provide scientific reference forsustainable development, utilization and protection of groundwater.