Study On The Dynamic Change Of Groundwater Quality And Comprehensive Assessment Of Water Quality In Minqin Oasis

Water resources are not only the indispensable natural resources to human production andlife but also the environmental resources other biological live by. Groundwater is an importantpart of water resources and it plays a crucial role in guaranteeing residential water、supporttingthe development of the social economy、maintaining ecological balances and so on, especiallyin the arid regions and the arid and semi-arid regions, the groundwater plays an irreplaceablerole. In the case of the surface water pollution and the pollution trend is aggravating, the rolegroundwater played is becoming more and more important. In recent years, the improperutilization of the groundwater has caused a series of ecological problems. In order to promoteregional sustainable development, the researchers at home and abroad have regarded thegroundwater as a research focus. Finding out the spatial and temporal evolution of thegroundwater and evaluating groundwater quality reasonably are the basic premise and effectivebasis for making plans for groundwater utilization and protection.Minqin is located in the northeast of the Hexi Corridor, downstream of the Shiyang RiverBasin and the climate is a typical arid climate. Water shortage and unreasonable groundwaterutilization have caused a series ecological environment problems in Minqin Oasis, such as asharp decline in groundwater levels, deterioration of water quality, desertification, sandstorm,vegetation degradation and so on. The study area in this paper is Minqin Oasis downstream theShiyang River Basin in Gansu Province. Based on the groundwater monitoring data from1983to2008,The piper TLn、ArcGIS、Matlab and some other tools were used to find out the mainchemical composition of groundwater、hydrochemical type of the gooundwater、the spatial andtemporal evolution of the main chemical elements in groundwater、the groundwater quality andthe spatial and temporal evolution of groundwater quality. The main conclusions are as follows:（1） The evolving feature of water chemistry indicators in Minqin Oasis:â‘ Thehydrochemical type of the gooundwater is mainly SO₄^2--Cl^--Na⁺-Ca²⁺、SO₄^2--HCO₃^--Na⁺-Ca²⁺,and some are SO₄^2--Cl^--Na⁺-Ca²⁺-Mg²⁺、SO₄^2--HCO₃^--Na⁺-Ca²⁺-Mg²⁺.At the same time, the hydrochemical type is developping from SO₄^2--Cl^--Na⁺to SO₄^2--HCO₃^--Ca²⁺.â‘¡The gooundwater in Minqin Oasis is alkalescent. The most anion are SO₄^2-and Cl^-and theleast are NO₂^-and F^-. The most cation is Na⁺and the next are Ca²⁺and Mg²⁺. The Na⁺、K⁺、Ca²⁺、Mg²⁺、Cl^-、SO₄^2-and HCO₃^-were relatively stable from1983to2008, but the NO₂^-、F^-、NO₃^-increased significantly during this period.â‘¢The main water chemical elementschanged significantly during the year: with the turn of the season, precipitation, temperature,evaporation and other natural elements changes significantly, so from February to Novemberthe major chemical elements in groundwater experienced a change process that increases firstlyand then decreases and reaches the maximum in May, June, July or August.â‘£The annualvariations of major chemical elements in groundwater is obvious: some major chemicalelements increased significantly in1984,1995and1998and decreased significantly after2005.The possible reason of the decreasing may be the water-saving measures were taken in Minqinand some wells were closed in this period.⑤The main water chemical elements showing angeneral increasing trend from south to north along the groundwater flow direction in space,except for a small part of wells influenced by the mountain lateral recharge and the wellsthemselves.（2） Comprehensive evaluation of water quality: The Fuzzy Neural Network based on T-Smodel and the SVM generally have same results except for a little wells when evaluate thegroundwater quality in the study aera, which illustrates that the water quality in the south isbetter than in the north and this two methods all can be effectively applied to the comprehensiveevaluation of groundwater quality, with some applicability. They get the over80%same â…¤classin the wells numbered108、113、117、118、131、141、147、149and154. On the other hand,their results all show that the water quality of the wells numbered142、147、170、175has aimproving trend. In the exceptional wells the quality got by the T-S model is better than that gotby SVM.（3） Spatial analysis of the water quality.The overall groundwater quality of Minqin Oasis ispoor, with a â…£orâ…¤ class. Most Ⅰ、Ⅱor â…¢water is in the south of Minqin Oasis, but the waterin the north is almost â…¤ class, that is to say that the groundwater quality in the south aeras ofMinqin Oasis is generally better than that in the north aeras. Because of the surface waterleakage, the water near the canal is mainly â…¡or â…¢ class. The distance is longer away from thecanal, the water quality is poorer and the water at the tail in the south of Minqin Oasis is almostall â…¤ class. The statistical results showed that the IV and â…¤class water reduced from71%to58%from1983to2008and the â… water increased from0to4%at the same time. Cmobinedwith the IDW diagram of the water grade, it was indicated that the improving trend was just in the south of Minqin Oasis and the water quality in the north is still poor.（4） By studying the dynamic change of the water quality, some recommendations weremaken in order to improve the water quality and develop the groundwater resources resonly:water-saving, closed some wells planning, improving drainage system, strengthening the unityof the surface water groundwater planning, improving the water supply equipment,strengthening the construction of ecological environment, and so on.