吉兰泰盆地及周边沙漠地区地下水补给与古气候环境记录研究

The Jilantai Basin and Ulan Buh Desert have recently been the focus of attention due to increasing stress on its water resources and environmental degradation. The environmental challenges have also become apparent from high rates of desertification, falling water tables and soil salinization. At present, few data are available for the hydrogeology of the Jilantai Basin and Ulan Buh Desert, and much about the sources and characteristics of shallow groundwaters remain poorly understood owing to the area's isolation and relatively low water demand. The project considers a wide selection of geochemical indicators to aid understanding of the groundwater recharge and evolution. Specific targets are the recharge sources, timing of recharge and recharge history, especially since much of the water being exploited may have been recharged under different climatic conditions to the present day. The research investigates age of groundwater using tritium, CFCs, radiocarbon and clorine-36, and also multi-element approaches linked with simulation modeling to resolving some of the key questions in hydrogeology such as surface water and groundwater intraction and renewable periods of groundwater in this arid basin. We plan to use a novel medium - the moisture percolating through dune sands - containing in its chemistry, the record of wetter and drier years with a resolution of about 5-10 years. Sand samples are recovered by augur to approx. 50m. Moisture content, geological and physical properties are logged, then samples are eluted with high purity deionised water for Cl analysis and other inert tracers such as NO3. Radioisotopes- tritium and chlorine-36 will also be applied to investigation of modern recharge. Geochemical and isotope results are interpreted in relation to the regional hydrogeology and water quality to validate the results in terms of modern and palaeowaters. We plan to compare the groundwater results with the palaeoenvironmental records from lake sediment, ice core and historic documents to reveal the impact of climate change on 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 the fragility of the resource.

针对西北干旱平原区地下水补给循环研究的重要性与气候环境信息直接提取研究的难度，选择对气候变化较敏感的吉兰泰盆地及乌兰布和沙漠为研究对象，通过大气降水、地表水、包气带与地下水水化学、同位素的综合监测研究，综合利用地下水环境示踪剂与包气带气候档案进行干旱区古气候信息的提取。通过氯累积结合同位素校正建立包气带地下水补给年龄与地下水的时序剖面；利用Sr及其同位素示踪地下水的补给来源与路径，以氢氧稳定同位素与惰性气体重建古补给环境；以包气带氯质量平衡(CMB)计算地下水补给量并反演重建降水变化过程。通过与吉兰泰盆地湖泊沉积记录、史料等相互印证，全面揭示该地区过去的气候环境变化，突出显示重大气候事件的表现特征及其对地下水补给的影响，进一步研究地下水形成演化及其对气候变化的响应，为认识区域地下水资源属性和实现可持续开发战略提供依据，为恢复干旱化环境形成的自然过程与预测未来发展趋势提供相关分析与基础资料。

为更好地理解中国西北半干旱地区第四纪地下水演化规律及其气候环境变化，项目对第四纪含水层中的主要离子、氢氧稳定同位素、Sr含量及同位素、放射性碳年代及“吉兰泰-河套”古大湖的形成进行了研究。以吉兰泰盆地水文地球化学信息了解地下水的补排关系及其水岩相互作用；基于氯质量平衡法，重建区域地下水补给历史和短时间尺度的气候变化过程；利用Sr及其同位素示踪地下水的补给来源与路径；根据估算溶解无机碳（DIC）的14C含量及其影响因素来校正地下水年龄；通过简化的第四系含水层补给的概念模型及地下水补给年龄，确定含水层的特性及参数；利用现代地形地貌确定不同古大湖湖可能的古湖表面积和体积，构建分布式流域水文模型与古大湖水量平衡模型的耦合模型，以反演重建古大湖期的气温及降水变化。究结果表明：碳酸盐溶解与沉淀、石膏溶解与硅酸盐的风化及离子交换是吉兰泰盆地主要的水岩相互作用；上层承压含水层与潜水含水层具有一定的水力联系，表明上层承压水向上补给是平原和沙漠中浅层潜水的主要补给来源；中间层承压水完全来自贺兰山；地下水年龄范围从现在到15ka，简化的物理概念模型估算的垂直渗漏率为0.01～0.03m/天，长期补给率为15mm/年(约占年降雨量的5%)；降水对沙漠地区的地下水补给非常小，区域地下水补给呈现出由西向东逐渐增大的趋势, AD1455-1500，1600-1650是气候较为干旱的时期，自19世纪初以后，干旱化进程进一步加剧，尤其是AD1920-1930，区域极端干旱。晚更新世和全新世早期的气候比现在更湿、更冷；古气候模拟结果表明，MIS 5e期间降水量明显高于现今。根据这些结果，我们推断气候因素对于古大湖的形成是至关重要的，只有将气候因素和古大湖的构造驱动机理综合考虑，才能全面地理解古大湖的形成和演化。该研究不仅对于了解古大湖的形成机理及区域古气候环境变化意义重大，而且也为认识区域地下水资源属性和实现可持续开发战略提供依据，为恢复干旱化环境形成的自然过程与预测未来发展趋势提供相关分析与基础资料。