Groundwater Vulnerability Assessment Based On Numerical Simulation

Groundwater vulnerability reflects the sensitivity of groundwater system to the outside influence. Currently, there are many groundwater vulnerability assessment methodologies including overlay and index methods, fuzzy mathematics methods, statistical methods, methods employing process-based simulation models, etc. Overlay and index methods are widely adopted for its advantages, such as simplicity, low-cast, easy-access data, etc. Besides, it is an empirical methodology, which is featured by qualitative analysis in combination with quantitative analysis. So it has a strong subjectivity. Most of researches are carried out in temperate climates. But in arid areas, the formation condition of groundwater and function and structure of system are distinct from other climates, so, the factors effecting groundwater vulnerability are different. Meanwhile, it is quite necessary to carry out the researches on groundwater vulnerability in arid climates whose ecological system is more fragile.Through analyzing and studying the arid climate characteristics in Xinjiang, agriculture activities, the formation conditions and distribution of groundwater, the recharge and burial conditions, etc, the groundwater assessment indicator system is constructed to fit for the arid climate followed construction principles of the target system. Evaporation from phreatic water, function of groundwater system, soil salinization, etc are brought into assessment system. The entropy method is used to overcome the subjectivity in determination of index weights. The groundwater vulnerability results in study area will be obtained eventually. For verifying the evaluation results, it is necessary to choose the measured groundwater quality data to check the consistency of the evaluation results.Methods employing process-based simulation models, quantitative analysis methods, are water and solute movement models based on physical models, which depict the truth of groundwater pollution process. Classifying the groundwater vulnerability results is not necessary because the assessment processes are objective. If all processes relevant to the contaminant transportation are taken into consideration, the over-load and high-cost investments are so impossible that it is difficult to achieve. So, it is enough to consider one or more main processes. Take the water table as the interface, the contamination transportation process is divided into two parts:the vadose zone and saturated zone. The HYDRUS-1D software is applied in vadose zone assuming the contaminant types and density in the surface of the soil profile. According to measured data and fitting parameters, the paper simulates the water movement and solute transportation in vadose zone to calculate the water flux, solute concentration in real time between the vadose zone and saturated zone.Then the simulation results are assigned to the PMWIN software-a simulation system for modeling groundwater flow and transportation processes. The author compels HYDRUS-1D and PMWIN to make comprehensive evaluation of groundwater vulnerability. The contaminant densities and pollution ranges of different times can be found to reflect spatial and temporal distribution of contaminant. The groundwater vulnerability is time-dependent.Some main conclusions were obtained in this study.(1)The results by overlay and index methods showed that about 92% of study area located in the area whose groundwater vulnerability is moderate to high, indicating that the overall protection conditions of groundwater was poor in YanQi County. Low-vulnerability areas were mainly distributed in the northwest of seed multiplication farm, in the west of Qigexing town and Sishilichengzi, and scattered on the north bank of the KaiDu River; Moderate-vulnerability areas were mainly distributed in most parts of the north bank of the Kaidu River and in some areas of Sishilichengzi,Qigexing and Qikecaikai; High-vulnerability areas were mainly distributed in the downstream areas of the river alluvial plain on the south bank of KaiDu River.(2)The total dissolved solids of groundwater was in good agreement with the groundwater vulnerability zoning map by comparing the measured groundwater quality data with the assessment results by overlay and index methods. It indicated that the salinization was the most important factor to groundwater quality.(3) The results by methods employing process-based simulation models showed that the polluted phreatic aquifer recharged the micro artesian water, but the artesian water would not be polluted. In the northwest of Beidaqu and Wuhaiqu on the north bank of Kaidu River, the concentration of pollutants was the highest, and the low salinity water was near the north bank of River. The groundwater whose concentration was 3～10g/L was mainly distributed in the coverage of silt. In southwest of Yanqi County there had fresh groundwater. Comparing the simulation results of ten years, the scope of 1～3g/L,3～10g/L were increasing gradually and that of the fresh water were continuing to shrink. There was almost no fresh water on the north bank of River, and it only existed in piedmont in the fourth year. The groundwater of more than 10g/L began to appear in the groundwater on the south bank of River in the seventh year. And as time went on,the polluted scope expanded continuously. (4) Comparing the results by the different methods and combining with the actual situation. it showed that the overlay and index methods had some limitation in the practical application. Meanwhile, it also showed that methods employing process-based simulation models had universal applicability.(5) This study proposed that the assessment methods should be choosed according to the research scale, the quality and quantity of data and the purpose of research. For a small scale area and the artesian water, the methods employing process-based simulation models was suggested to applied; For a large scale and the phreatic aquifer, other methods were used to evaluated; If the conditions of the study area satisfied the demand of data, numerical simulation methods should be adopted; For the water source protection, numerical simulation methods should be used to identify the groundwater vulnerability accurately.(6) According to the results of groundwater vulnerability assessment, strategy of water pollution control was proposed combined with the main influencing factors of groundwater vulnerability in arid areas. Such as changes of irrigation methods, increasing the exploitation of shallow groundwater appropriately, improving drainage measures, scientific fertilization, rational use of pesticide, setting up the protection zone of the drinking water source, establishment of non-point source pollution monitoring network.