Groundwater Recharge And Evolution In Shiyang River Basin And Tengger Desert, Northwestern China

Groundwater is the most valuable natural and strategic resources in arid areas, which plays an important role in the local ecological environment and living, and economic development. Identifying a reasonable and effective evolution of groundwater recharge and groundwater residence time is particularly necessary to understand groundwater resource properties, the formation mechanism of drought and its development, and the promotion of water resource management and sustainable development. Shiyang River Basin and its adjacent Tengger Desert area, located in northwest China, is a typical fragile ecosystem area. It is extremely sensitive to climate changes, which can provide an important evidence for revealing changes of drought/wet transition and mechanisms in drought arid regions of Northwestern China;its specific geological features and hydro geological conditions, and the relatively independent system of water cycle in the northwestern arid area of inland are typically representative. Strenthening the research and investigation work on the groundwater recharge evolution and renewability in Shiyang River Basin and the adjacent to the desert is able to provide a new scientific field and study direction for the global changes and continent hydrological cycle; meanwhile, it is also necessary to assist prediction of the future ecosystem interactions within the hydrological system process and mechanism.As an issue of concern about the importance and difficulty in the arid hydrology research work of domestic and international science front porch, this thesis attempts to characterize groundwater circulation (atmosphere-surface water-groundwater) and hydrochemical evolution on an intermediate scale for water resource investigations of arid area, using a combination of theoretical and field based methods in unsaturated aquifers at Shiyang River Basin and Tengger Desert, Northwestern China. Identifying the location of active hydrogeology, flow zones and hydrochemical information along the flow paths are required as well as its complicacy and variability. Groundwater system contacts with its neighborhood water systems by mass and energy transfer, which is influenced by many factors, such as condition of weather, hydrology, geology and tectonics, hydrogeology and vegetation. So the chemical and isotopic components in groundwater are used to provide an effective method to trace the process of groundwater circulation. Furthermore, this work presents a new corrected model for the calculation of the initial 14C concentration that is the primary step to estimate the renewal capability of groundwater and reliable age. Above all, it is necessary to understand groundwater circulation and renewability as the key objective for the water resources management improvement. In addition, the hydrochemical characteristics provide unique information carriers for the global change and terrestrial hydrologic cycle.The main results of the present study are followed:(1) Samples of precipitation (rain and snow) were one-year collected at three local weather stations (Jiutiaoling, Nanying, Hongyashan) for stable isotope studies (δ2H andδ18O) and major ion analysis. Generally the precipitation of the study catchment has dilute chemistry; the total dissolved solid(TDS) of the snow from mountains above 3000 m is lower than 50 mg/L, which increases as the altitude decreases. The Cl concentrations range from 2.14-11.7 mg/L and average value is 3.17 mg/L, the composition of rainfall anions presents like HCO3->SO42->Cl-; the major cation is Ca2+ with the concentration of 9.72 mg/L while Na+ only presents 4.05 mg/L in the rainfall samples. The salinity increases significantly from upstream to downstream because of evaporation-induced saline enrichments that water of HCO3-type evolves to HCO3--SO42- type and Ca2+-Na+type for cations. This distribution variation of facies may also be influenced by rainfall duration, snow deposition time and the surface materials of catchment. Meanwhile, precipitation contains a relatively large magnitude of variations inδ18O andδ2H, withδ18O ranging from 20.6%o to 3.8%o (average of-7.3‰) andδ2H from-158‰to 33.1‰(average of-46.9%o). The Local Meteoric Water Line (LMWL) of Shiyang River Basin is defined asδ2H=7.618δ18O+4.398%o VSMOW, which lower gradients (7.13-7.92) to GMWL indicates a significant dry and evaporation effect controlling in the arid area of northwestern China. The temperature effect is the most important effect toδ18O in precipitation and the relationship between them is described asδ18O=0.593T-12.59‰.(2) The relationship between regional structural elements and the hydrochemical evolution of groundwater are determined as another important objective. The characteristics of groundwater chemistry and mechanisms are distributed horizontally zone by zone from mountain front to desert basin. The total dissolved solid (TDS) is increasing along the flow paths, with average value of 1149.6 mg/L; however, the vertical distribution of TDS and other hydrochemical characteristics are not significant. As well, Cl concentration also increases gradually (2 mg/L-5216 mg/L) along the groundwater flow paths from the Piedmont-Plains-Desert. Due to the stability chemical properties of Cl element, the mixing ratios with the other elements (e.g. SO42-/Cl-,NO3-/Cl-,Na+/Cl-, etc.) and other special ratios of the major ion species (e.g. Mg2+/HCO3-,Ca2+/HCO3-,Ca2+/SO42-, etc.) could be a good indicator for understanding hydrochemical evolution and mechanisms. Across the whole aquifer, groundwater evolves gradually from Ca·Mg-SO4·HCO3 type water (TDS≥203 mg/L) from mountain front aquifers to more mineralized Ca·Mg·Na-HCO3 and then Na·Ca·Mg-SO4·Cl type water in the alluvial plain, and then becomes SO4·Cl type water (TDS≤5952 mg/L) below the desert plain. The chemical composition of the water and the relationship between ions reflect that the direct infiltration of precipitation is not the recharge sources to the plain and desert, which chemistry strongly influenced by evaporation and subsequent dissolution of minerals during recharge in the rainy season, as showed in the ionic plots and saturated index. Other processes such as cation-exchange and weathering also contribute to the water composition.(3) The water-rock reactions were simulated using the PHREEQCI inverse model and the results showed a good agreement with the measured groundwater quality. The proposed reactions are plausible for explaining the observed concentrations in groundwater that finally evolves to become mineralized and single chemical composition water type. Based on three sections of Gulang-Wuwei, Hongyashan-Minqin and Helianshan-Yaoba flow paths, the constraints and phases used in modeling were selected on the potential reactions between water and the principal mineralogical species existing in the aquifers. The ionic exchange reaction occurring among the cations is interpreted as a result of flow of relatively dilute water through a highly saline medium, where the concentrations of Na+ and K+ are slightly reducing as the Ca2+ and Mg2+ releasing for carbonate reactions. The model requires the input of CO2 in the groundwater; however, the precipitation of calcite, gypsum, halite and dedolomitisation reaction was simulated out in the flow paths, giving rise to the production of output CO2 as well. This process leads to the existence and evolution of the most mineralized water.(4) The results of isotopic measurements for groundwater of Shiyang River Basin and Tengger Desert show a normal arid magnitude of variations, ranging between-11.5‰and-1.5‰inδ18O (average of-8.84%o), and between-90.9‰and-27%o inδ2H (average of-65.1‰). A comparison of stable isotope compositions of groundwater with local modern values of precipitation (i.e. the annual average values ofδ18O andδ2H in Zhangye station are around -6.5‰o and-43.9‰, respectively) indicates the direct infiltration of precipitation is not an important source of recharge to the groundwater in the study area. From the discussion ofδ18O and Cl, it is clear that the least isotopically-depleted waters indicates a wetter condition of the past recharge duration. As an independent indicator of ancient environments and salt constraints, the lower Cl concentration (with average of 316 mg/L) is considered to characterize a long-term sustainability of a phase of humid climate cycle, suggesting it may still retains the cooler and wetter supply of oxygen water under the ancient climate.(5) Radiocarbon (14C) is used as the important tracer element to calculate the groundwater age. Theδ13C-DIC contents and fractionation factor could firstly be used as the standard of the recharging water that can identify the enrichment of dissolved carbonate in the closed-system. The 14C contents in the closed-system below the water table possibly indicative of influence by the dilute DIC of the recharging water considering the amount of carbonate interfering sources, where presents much more significantly in Shiyang River basin withδ13C value of-4.73‰. However, it may not be representative of the eastern area of Tengger desert in the less closed system (δ13C of-10.8‰). A mean residence time in the range of 40-12 ka for the deserts is inferred, revealing that some replenishment to desert aquifer was only occurring in late Pleistocene and Holocene when some of the upriver characterize as the modern water. A modified version of the Pearson (1965) isotopic correction model is used to account for isotopic dilution from incongruent dissolution of carbonates. The radiocarbon dilution factor q is calculated based on the change inδ13C-DIC between the sample and the recharge zone that the advantage of this model is the reliability and practicability for arid area of northwestern China.