| Siboshu well field is located at the northern loess plateau and the belt on the edge of Maowusu desert, JingBian County, Shaanxi Province. Due to the arid and semi-arid climate, the shortage of water resources has became a major factor that restrictes the construction of chemical energy base in the well field,or even affects local people's normal life. Groundwater, as an important composition of water resources, is themain water resources for agricultural and daily uses there. In recent years, urbanization and construction develop very rapidly. Howver, the pollutants made during this processes turn to be a significant threat to the Siboshu well field. As the population grows fast, local people's demands of ground water are increasing so sharply that the excessive exploitation leads to decline of watertable.Therefore, it's very crucial to establish a groundwater protection area there to maintain the quanlity and storage of groundwater there. In this research, the geological conditions and hydrogeological conditions are identified by investigating the secondary data and doing field work. The groundwater samples collecting from study area are analyzed simplily and totally to assess the environmental quality of groundwater and the chemical characteristics are checked out. Combing the numerical simulation of groundwater flow with the distances from a certain point to the well point calculated by reverse particle tracer simulation method, the scope of different levels of protection areas are determined under the time standards. Based on the comprehensive analysis of the chemical characteristics and assessment of water quality, this paper does lots of researches on delineanating Siboshu groundwater protection area and achieves the following results:(1) The hydrochemical characteristics are discussed on the two caterigries respectively, that of phreatic water located at the north of Shuanghuochang-Guanlupan-Luhe Line is very simple and mainly is heavy carbonated water, namely HC03-Mg·Ca or HCO3-Na·Ca, that of the south of this line in the area of Shuanghuochang and Dahewang is mainly HCO3·Cl-Mg·Na.The confined water's hydrochemical characteristics are HCO3-Ca·Na and SO4·Cl-Na·Mg respectively.(2) This research selects eighteen indicators including temperature, pH, total hardness, TDS, ammonia nitrogen, Mn,Pb,and so on,which can reflect the situationof groundwater environment.According to the groundwater quality standards (GB/T14848-93),this research evaluates fifty-two samples, including thirty-four phreatic water samples and twenty confined water samples by using single index evaluation and comprehensive evaluation methods. The results show that the quanlity of groundwater at the northern part of study area, namely the northern part of Shashimao-Huhuochang-Guojiaomiao-Haizepan line, is pretty good and its TDS is lower than 0.5g/L. That of the southern part of this line, which include Shuanghuochang, Guanlupan, County seat and both banks of Lu River is commonly poor and TDS is between 2-4.84g/L. In one word, the quanlity of confined water is commonly better than phreatic water in the study area.(3) This research establishes the numerical modle to simulate the groundwater flow, then, the Reverse tracer function of Modpath is applied to calculate the migration distances of pollutants from a certain point to the well in 100 days and 25 years, which are delineated as first and second level protection area, and third level area is the recharge and runoff area. Based on the calculation results, the area of first level protection area is 0.43 km2, which is the scope of radius 50m as the pumping well the center. The area of second and third level protection areas are 57.13 km2 and 137.34 km2. The protection measures are also proposed to maintain Siboshu water source field.
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