The Research On Groundwater Quality Evolution Under Exploitation Conditions In Dezhou

Since the 1980s, the problems of groundwater resources shortage, water quality deterioration and environmental geology in the alluvial plain of downstream of Yellow River has been more and more severe, the conflict between water resources supply and need is also obvious, which have a close relationship to the climate change and mans activity. As Dezhou District chosen the experimental objects, where locates in alluvial plain of downstream of Yellow River, groundwater level monitoring, groundwater quality monitoring and Isotopic samples analysis is main technology for acquisition source information. The research confirms the source of groundwater recharge, underground runoff and groundwater discharge, reveals formation and evolution of Dezhou confined water depression cone, identifies and determines dominant factors inducing formation and evolution of depression cone. The study discusses the evolution progress and the developing trend of groundwater at spatial and temporal scales as well as predicts the evolution mechanism of groundwater, and then illustrates the main reasons for bad evolution of groundwater quality. According to the groundwater quality evolution tendency in study area, water adjustment measures put forward to make for benign evolution of groundwater quality.Dezhou City locates in the middle and eastern part of North China Plain, which is a typical Yellow River downstream alluvial plain pore water hydrogeology area, where the salt and fresh water coexists with each other and forms a unique aquatic environment. In such a complex groundwater system, the natural aquatic environment condition of fresh water is rather sensitive and fragile. A large-scale excessive exploitation of groundwater influenced the hydrodynamic force field and hydrochemical field in this area and intensified the trend of water quality deterioration in this area. Aiming at solving this serious aquatic environment security problem, the research of the groundwater quality evolution can provide scientific basis for several areas which will benefit the sustainable usage of water resources in this area and prevent the fragile aquatic environment from deteriorating under continuous exploitation.Based on the dynamic monitoring data of groundwater quality in the study area from 1996²009, the research analyzes the water quality evolution of groundwater by the groundwater dynamic analysis method, hydrogeochemical method, isotope technology, gray model and numerical model etc. Results indicate that the water quality of phreatic water evolves constantly influenced by combined action of exploitation condition and natural conditions, in which the water quality index of total hardness, degree of mineralization and Cl^- changes obviously. The total hardness and degree of mineralization decreases to some extent and the water quality has a tendency of desalinization in the area of Qihe County and Suliuzhuang Town of Xiajin County; whereas the total hardness and degree of mineralization increases to some extent and the water quality has a tendency of salinization in Ningjin County and Zhangzhuang Town of Yucheng City. Influenced by varying degrees of both natural processes and anthropogenic activity, the changing range of ion content is different, which the anions vary more than the cations. The greatest changing range is HCO₃^- and in the next place is SO₄^2-. The variation of ion content brings about changes of the hydrochemical type as well as changes of groundwater quality. The change of the hydrochemical type is a complicated hydrogeochemical process, on which the impacts of human activities are becoming more and more obvious besides relative to natural processes.In the confined water of the study area, there is a superiority of anion HCO₃^- that displays the feature of atmosphere water leaching character. The cation is mainly Na⁺. It can indicate that cations exchange happen, which more Ca²⁺ and Mg²⁺ are replaced by Na⁺ through runoff process, or many times of marine transgression happen. In the heartland of the confined water drawdown, the hydrochemical type of is mainly HCO₃–Na. At the farthest from the center of the funnel, the hydrochemical type is HCO₃·SO₄–Na, HCO₃·Cl–Na or HCO₃·SO₄·Cl–Na. The hydrochemical type changes from HCO₃·Cl–Na·Mg to Cl·SO₄–Na·Mg with increasing depth. The concentration of Na⁺, Ca²⁺, Mg²⁺, Cl^- and SO₄^2- increase, gradually decrease through the saline water aquifer, and then recover slightly in deep part. Total hardness and degree of mineralization have emerged a strengthening peak when the water level buried depth is 200 meters, and then gradually fall.Confined water depression cone of Decheng district is selected as simulated zone as well as the degree of mineralization and Cl^- taken as the simulated factors which can reflect the changing tendency of groundwater quality evolution well. Based on the hydrogeological condition and the dynamic monitoring data of groundwater, the issue establishes water quality evolution prediction models of the confined water. Water equilibrium calculations results indicate the deep aquifers are over-exploited. As is predicted, the center of confined water depression cone will fix at Zhangzhuang of Chenzhuang village in Decheng District in 2015. The groundwater level of the simulated zone descends gradually by year, meanwhile there is an increasing tendency of groundwater level from the center of confined water depression cone to its outskirts. The groundwater level elevation of funnel center decreases from -126.80m (Buried depth is 147.91m) in 2009 to -137.21m (Buried depth is 158.31m) in 2015, which descending at 1.76m/a. Across the whole region in 2009²015, areas with the degree of mineralization of less than 1000mg/L will decrease to 93.48km². Areas with the degree of mineralization of more than 1200mg/L will increase to 21.22km². Areas with the concentration of Cl^- of less than 60mg/L will decrease to 32.02km². Areas with the concentration of Cl^- of less than 90mg/L will decrease to 194.47m². Areas with the concentration of Cl^- of less than 110mg/L will increase to 112.02m². The water quality tendency of confined water in the simulated zone is that the degree of mineralization and the concentration of Cl^- show an increase to a certain extent over time whereas the hydrochemical type of confined water has not obviously changed and little seasonal variation. It represents that the hydrochemical type of confined water has not changed with the season. As a result of slow and small underground runoff, the water quality has not yet been obviously affected by periphery water in the process of the exploitation in current 20 years. With the groundwater levels falling and the pressure drop by over-exploitation of the groundwater resource, sharp hydraulic gradient causes damage to natural balance between salt and fresh water, and then the degree of mineralization and the concentration of Cl^- of the deep fresh groundwater in aquifers tends to increase gradually. It indicates that saline water aquifer on the top of deep fresh water aquifer can invade fresh water by leakage under continuous exploitation condition.The main reasons for bad evolution of groundwater quality are sustained groundwater over-exploitation, unreasonable exploitation distribution, further expanded depleted space in aquifer, regional spread of saline water and continuous deterioration of groundwater environment. According to the groundwater quality evolution tendency in study area, water quality regulation should be implemented for the aquifer's restoration and conservation where the groundwater has been excessively exploited in study area, including confined water exploitation according to the stages and layers, exploitation and utilization of the shallow slight saline groundwater resource, saline water desalination and so forth.