【作者】 丁贞玉；

【导师】 马金珠；

【作者基本信息】 兰州大学 ， 自然地理学， 2010， 博士

【摘要】 地下水是干旱地区最宝贵的自然资源及战略资源,对生态环境及生活、经济发展等方面起着重要的作用。合理有效判别地下水补给演化过程以及地下水的更新时间,对于认识地下水资源属性、认识干旱化环境形成机制与发展规律、促进水资源的科学管理及可持续发展具有重要作用。石羊河流域及毗邻的腾格里沙漠是我国西北干旱区典型的生态系统脆弱带,对气候变化极为敏感,能为揭示西北干旱地区干湿变化的自然规律和机制提供重要依据；其特殊的地质地貌与水文地质条件、相对独立的流域水循环系统在西北干旱区内陆区均具有典型的代表性。开展石羊河流域及毗邻沙漠地区地下水补给循环演化与更新能力研究,能够为全球变化及大陆水文循环研究提供新的研究方向与领域,对认识整个内陆河流域生态系统与水文系统相互作用的过程和机理有重要意义。论文着眼于国内外干旱区地下水研究的重要性与难度,重点应用水文地球化学理论与方法,并与自然地理学、第四纪地质学、气候学和古水文地质学等多学科领域交叉,全面系统地研究石羊河流域与腾格里沙漠地区水文循环过程及补给规律,分析对比大气降水与地下水水化学及同位素特征之间的差异,判断地下水补给与大气降水的关系；同时,强调主要离子及化学相的指示作用,结合西北干旱区特有的地质条件及气候特征,提取地下水中的多方面历史及年代信息,确定地下水水流补给历史及路径,建立区域地下水14C年龄模型。主要成果如下：(1)根据石羊河流域三个气象站点(九条岭,南营水库,红崖山水库)为期一年的水化学与稳定同位素(δ2H和δ180)监测结果,石羊河流域降水中主要化学离子总量不高,且随流域海拔降低而有所增加。在海拔3000 m以上的降雪中离子总量小于50 mg／L,其中Cl-介于2.14～11.7 mg／L,平均为3.17 mg／L,阴离子组成HCO3-＞SO4 2-＞Cl-;阳离子组成以Ca2+主,为9.72 mg／L,Na+仅为4.05 mg／L。中下游地区,降水中离子总量增加,水化学类型由HCO3-型变为HCO3--SO4 2-型,阳离子以Ca2+-Na+为主,受降雨持续时间、降雪沉积时间及地表物质影响差别有所不同。石羊河流域的大气降水稳定同位素δ180值介于-20.6‰～3.8‰,平均为-7.3‰,δ2H介于-158‰～33.1‰之间,平均为-46.9‰。区域降水线(LMWL)为δ2H=7.6δ180+4.4,与全球大气降水线相似但斜率稍低(7.13-7.92)反映了西北干旱区强烈的蒸发作用。影响降水稳定同位素特征的主要因子为温度,总体上石羊河流域δ18O与温度相互关系为δ18O=0.593T-12.59。(2)石羊河流域及腾格里沙漠地下水的水化学分布具有较明显的水平分带特征,但浅层水与深层水矿化度及水化学类型并无明显的垂直性分布特征。由出山口向细土平原至荒漠地区,地下水矿化度(TDS)逐渐增大,平均含量为1149.6mg／L,Cl-在地下水径流途径上呈现规律性变化,浓度从山前-平原-荒漠以水平分带性由2 mg／L增加到5216 mg／L。由于Cl-化学性质的稳定作用,与其它离子比值(如SO4 2-／Cl-、NO3-／Cl-、Na+／Cl-等)以及其它特殊离子间比值(Mg2+／HCO3-、Ca2+／HCO3-、Ca2+／SO4 2-等)具有较好的指示作用,能够全面反映含水层介质矿物组成对地下水化学演化及补给来源调查的水文地球化学影响作用。具体的水化学类型为：山前平原区地下水以Ca·Mg-SO4·HCO3类型为主,多为浅层水且地下水水质较好,TDS含量不高,最低值仅为203 mg／L,经平原区后阳离子分异性特征逐渐弱化,以Ca·Mg·Na-HCO3型水过渡Na·Ca·Mg-SO4·Cl型水,最终形成Na-SO4·Cl型水,尤其石羊河流域终端民勤盆地附近,地下水水位较深,矿化程度高,TDS含量最高达5952 mg／L。与区内降水的化学离子组分相比,地下水各离子显著的浓度差异及比值关系反映了大气降水不是平原区及荒漠区地下水的直接补给来源。(3)利用PHREEQCI反向模拟地下水一维恒定流的情况下补给演化过程及矿物质转移量的变化情况。基于石羊河流域古浪-武威剖面、红崖山-民勤剖面及腾格里沙漠东缘贺兰山-腰坝剖面地下水流动系统的模拟结果表明,沿水流路径阳离子中Na+及K+参与大量的离子交换作用而损失,Ca2+与Mg2+多在地下水中参与碳酸盐的形成与溶解,CO2气体在水流路径上游为输入项后参与水化学作用逸出,方解石(文石)、白云石、石膏(硬水)、岩盐等选定矿物相以沉淀为主,进入下游区域溶滤作用减弱,蒸发浓缩作用控制强烈,导致主要水化学离子沿地下水水流路径不断从水体中析出剥离,最终演化成矿化度偏高、水化学成分单一的地下水水体。(4)石羊河流域及腾格里沙漠的地下水水分子稳定同位素δ18O值范围为-11.5‰～1.5‰,平均为-8.84‰,δ2H介于-90.9‰～-27‰之间,平均为-65.1‰。其相较于现代降水(如张掖站δ18O为-6.5‰左右,δ2H为-43.9‰左右)同位素特征值较低,表明地下水基本上没有受到现代大气降水的直接补给。同位素δ18O比较负的数值伴随较低的Cl-浓度主要表征了过去湿润的补给条件。作为独立的古环境及含盐量指标,地下水较低的Cl-浓度值(平均316 mg／L)表征了长期持续性的湿润气候周期,也说明研究区内仍然保留有古代较冷较湿气候环境下补给的有氧水。(5)14C可以作为地下水测年的主要示踪证据及手段,以无机碳δ13C含量标准与标准物质VPDB(海成碳酸盐)的富集程度及碳酸盐的溶解量作为系统指标,说明腾格里沙漠西部石羊河流域地下水含水层封闭性(δ13C含量为-4.73‰)较好于东缘区域(δ13C含量为-10.8‰),地下水封存的14C含量干扰稀释程度基本只受补给区DIC稀释影响。区域年龄校正模型结果表明研究区内荒漠盆地区主要具有轻同位素的晚更新世及全新世时期的水(40～12ka)特征,少部分腾格里东缘中上游区地下水可能具有重同位素现代水特征干扰。该区域模型14C初始水校正源于对Pearson模型进行配合西北干旱区区域性修正。基于样品和补给区DIC的δ13C的变化,放射性碳稀释因子q值计算简便,增强14C年龄模型在我国西北干旱区的适用性及可靠性。更多还原

【Abstract】 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.更多还原