The Evolution Of Water Chemicals And Quality In The Liyuan River Basin Of Gansu Province

Because of rapid population growth and the development of industries and agriculture, humans require a higher amount of water resources. Therefore, managing and conserving limited water resources has been a significant issue for society. In the arid inland areas of China, especially the northwest China, surface water is scarce and not in a stable state, so groundwater has gradually become more important in supporting human lives and local ecosystems. However, the excessive use of groundwater resources and the mishandling of discharge waste water have harmful effects on water resources. At present, the increased salinity of groundwater and the decreased groundwater level has created a bottleneck in agricultural and domestic water use, which also influences the future use of water resources. The research area, the Liyuan River Basin, is located to the west of Zhangye City in Gansu Province, which is a first-order tributary of the Heihe River and an important inland river in the Hexi Corridor. However, when compared to research of the Heihe River Basin, the degree of research of the Liyuan River Basin is low. It is necessary to research the condition of groundwater resources in order to draw up governmental outlines for the protection and rational utilization of water resources, and for the sustainable development of ecology. Through the combined analysis of hydrochemistry, stable isotopes in water, hydrochemical modeling and related mathematical methods, the paper investigated the evolution of water chemicals and the current water environment condition. The main research results are as follows:(1) For surface water usage amount, agricultural irrigation took the largest proportion; forestry and grass irrigation took the second-largest proportion; while industrial use, drinking water for humans and animals, tertiary industry and public greening took the smallest proportion. However, the amount of agricultural irrigation water usage decreased from450million m3in1998to360million m3in2007, while the amount of industrial water usage increased from0.3million m3in1998to0.65million m3in2007. For groundwater usage, agricultural irrigation and drinking water for humans and animals took the largest proportion. For example, the amount of agricultural irrigation usage increased from20millionm3in1998to53million m3in2007.(2) The isotopic analysis showed that the values of σ18O and σD in surface water were-8.75--7.01‰and-49.71--47.37%o, while their average values were-7.89%o and-48.42%o, respectively; the values of518O and5D in deep groundwater were-9.37--6.89%o and-65.22--47.31‰, which were more depleted when compared to that of shallow groundwater. The average d-excess of shallow groundwater in the Liyuan River Basin was15.84%o, while that of deep groundwater was12.94%o, both were higher than that of the Heihe source area and lower or close to the value of snowmelt from the Qilian Mountains. This inferred that the possible recharge sources of groundwater were ice-snowmelt runoff and precipitation in the source area of the Heihe River.(3) The hydrochemical type of the surface water was Ca-HCO3; while the dominant cation of the groundwater changed from Ca2+to Mg2+, and the dominant anion changed from SO42-to HCO3". Water-rock interactions, mixture and cation exchange processes all occurred in the groundwater. Hydrochemical modeling results showed that gypsum in shallow groundwater was under-saturated, while calcite and dolomite were over-saturated. In deep groundwater, the precipitation and dissolution conditions of the rocks were similar. If calcite and dolomite are over-saturated to a large extent, industrial facilities, such as electric power plants and boiler factories, would be negatively affected. Additionally, the coexistence of gypsum, calcite and dolomite in groundwater led to the common-ion effect, where the dissolution of gypsum encouraged the precipitation of calcite and dolomite.(4) The groundwater quality in most regions was suitable for human and animal drinking water and for irrigation, with a TDS value of less than1000mg/L; only a few sites had high levels of nitrate and sulphate, which were higher than the standard values of45mg/L and250mg/L, respectively. For example, the content of nitrate of sample17was119.3mg/L and the content of sulphate of sample21was580.8mg/L. The two samples were taken from shallow groundwater, so the high contents of these ions indicate the influence of irrigation return flows.The hydrochemical types of surface water and groundwater have had no obvious changes in the last three years. The Synthetic Pollution Index of surface water was zero, which indicates that the surface water quality at all sites was within standard limits. Thus, the water environments in the research area have not been endangered by human activity. The maintenance of the current water quality and the avoidance of water pollution, by means of reasonably controlling and organizing human activity, are both needed to realize a harmonious coexistence and the sustainable development of water environments, local ecosystems and economic society.