敦煌盆地地下水补给及其对气候变化的响应

Groundwater recharge is the most significant issue in the water resources exploitation, and recently years, its response to the climate change has been a focus in hydrogeology study. Dunhuang basin is a typical arid watershed in Northwest China, and increasing attention has been paid to Dunhuang recently because its famed Crescent Lake has been rapidly shrinking into the desert sand due to groundwater depletion. In addition, the environmental challenges have also become apparent from high rates of desertification and soil salinization. However, characteristics of groundwater beneath Dunhuang basin remain poorly understood, such as the timing of the groundwater recharge and its response to the past climate change.. The object of this study is to obtain the process of the groundwater recharge as well as its response to the climate change using the combination of the precipitation, groundwater, unsaturated zone and lake sediments. The input status of water resources under the modern climate system is examined by the stable isotope and 3H. which would provide clear sight of balance of the water resource in current atmospheric circulation. The chlorine mass balance(CMB) method is appiled to estimate groundwater recharge rate and its history in the vadose zone. The circulation and dynamics of groundwater is investigated using tritium and radiocarbon, with the result of construction of groundwater timing sequence in the aquifer, and then we plan to explore the groundwater recharge environment by combining hydrogen and oxygen isotopes as well as the dissloved inert gas. Lake sedments from the Qingyanchi are obtained to recover the process of regional climate change in forms of the sequence of the precipitation and temperature. Finally, we intend to compare the groundwater results with the palaeoenvironmental records form the lake sediment, unsaturated zone and historic documents to reveal the impact of climate change on the groundwater system. The results are then discussed in terms of their application to the improved water resources management in the region by providing a deeper understanding on the slow accumulation of water resources and raising a greater appreciation of the value and fragility of the resource..

针对干旱区地下水补给循环研究的重要性和对气候变化响应研究的难点，选取敦煌盆地及其周边地区为研究对象，综合降水、地下水、包气带和湖泊沉积记录，以同位素识别现代气候条件下地区水资源输入状况；采用包气带氯质量平衡法估算历史以来地下水的补给量，建立降水对地下水的补给序列；以地下水14C年代、高精度3H研究地下水循环年代学及动力学，构建地下水时序序列，结合氢氧稳定同位素、惰性气体探讨地下水形成环境与更新变化过程；应用青盐池沉积记录反演区域气候变化过程，重建区域古水文气候信息。最终通过包气带、地下水、湖泊沉积记录和史料记载相互对比验证，揭示地下水对气候变化的响应机制，为认识气候变化和人类活动背景下区域水资源演化，理解地下水资源属性和实现地下水资源的可持续开发提供理论依据。

开展干旱区地下水补给演化研究工作，对提升地下水形成演化机制的理解和维护区域生态经济安全具有重大意义。本项目以敦煌盆地及其周边地区为研究对象，通过降水、地下水、包气带的水化学、同位素记录研究，探明了现代气候条件下地区水资源输入状况；识别了区域地下水的化学演化和形成更新过程；探讨了沙丘包气带剖面水分和离子对环境因素的响应关系，最后构建了区域地下水补给循环的水文地质概念模型。主要认识如下：1）研究区大气降水δ18O平均值为-8.1‰；δ2H平均值为-58.6‰，降水线方程为：δ2H=7.647δ18O+1.284（R2=0.971, P<0.05）。降水同位素温度效应明显，无降水量效应，δ18O的垂直递减率为0.2‰/100m。研究区全年降水以西风水汽为主，局地再循环水汽占据较大比重，西南季风水汽和北部极地水汽也有部分贡献。2）研究区地下水与现代当地降水具有完全不同的同位素特征。南湖灌区、敦煌盆地上游潜水及山前地带承压水与地表水特征值接近。盆地中下游潜水同位素值升高，可能受蒸发或是农业灌溉水的回归水的影响。剩余部分承压水具有很低的同位素值。14C年代结果显示潜水和山前地带承压水较为年轻，盆地北部边缘深层承压水十分古老。3）强降水事件后包气带水分含量最大湿润位置随时间向下迁移，包气带水分得到补给。沙丘不同植被覆盖条件下包气带剖面氯离子、水分的分布、运移特征及组合关系显示，干湿沉降、植物对营养元素的选择性吸收、蒸散发以及包气带水溶质浓度梯度对离子和水分起到了再分配作用。4）敦煌平原区地下水几乎无当地现代降水的补给。南湖灌区，三危山和鸣沙山山前地带地下水来自祁连山区的降水和冰雪融水快速下渗进入含水层，形成具有现代特征的地下水。敦煌盆地中下游地区潜水接受河流及灌溉回归水的补给，为现代和次现代地下水。山前深层承压水接受了来自山区地下水的侧渗和河流下渗，补给时段从全新世持续到现在。盆地中下游的承压水形成于晚更新世气候寒冷湿润时期，循环交替十分缓慢。