环境同位素特征对滨海岩溶地区海水入侵过程的指示意义

Environmental Tracers(δ~2H-δ^(18)O,δ^(34)S,δ^(13)C) as Indicators of Seawater Intrusion Processes in the Coastal Karst Area

大连大魏家水源地位于中国北方典型滨海岩溶地区。近30年来,地下淡水的不合理开采造成的地下水位降落漏斗引发了严重的海水入侵。以大魏家水源地为研究对象,通过大量的水文地质调查和水化学及同位素采样测试分析,探讨海水入侵形成的水动力条件,通过分析滨海岩溶含水层中地下水主要水化学和多种同位素(δ2H-δ18O,δ34S,δ13C)组成特征,识别了海水入侵过程中发生的主要水文地球化学作用,并对其进行了定量模拟,从而阐明了岩溶含水层中的海水入侵机理。研究结果表明:大连大魏家海水入侵主要通道为大魏家地区存在的导水断裂、岩溶裂隙以及第四系松散地层。对δ2H-δ18O同位素的组成分析表明,研究区地下水主要来自大气降水补给,结合Cl-浓度分布,认为除海水入侵淡水含水层后增加了地下水中的盐分外,浅层地下水的蒸发也对地下水中盐分的累积起到了重要作用。根据不同水体中δ34SSO4,δ13CHCO3等同位素特征,结合水化学成分(如SO2-4,Cl-)分析认为,研究区微咸水和咸水并不是地下水淡水和海水简单混合而成。利用反向水文地球化学模拟揭示了控制滨海岩溶含水层中水化学演化的主要水文地球化学反应有方解石、蒙脱石和石膏的溶解作用,伊利石的沉淀作用以及Ca-Na离子交换作用,伴随着CO2的释放。

Daweijia wellhead field is located at the coastal karst area in north China. Unsustainable groundwa-ter exploitation in recent decades has resulted in severe seawater intrusion in this area. Employing hydrogeological investigations, hydrodynamic monitoring and hydrogeochemical and isotope data analysis, this study analyzed the groundwater hydrodynamic conditions partially controlling seawater intrusion processes, and delineated the mecha- nism of seawater intrusion in this area. The characteristics of major hydrochemical composition and multi-stable-iso- topes （δ^2H-δ^18O, δ^34S,δ^13C） in groundwater have been used to identify the main hydrogeochemical behaviors; these were also quantitatively simulated by inverse hydrogeochemical reactions. Seawater intrusion occurs primarily along the permeable fault zone, karst fissures and loose Quaternary strata. The compositions of δ^18O and δ^2H in groundwater show that the critical groundwater recharge source is precipitation. Combined with the C1- distribution, it can be identified that besides salinity increase accompanying seawater intrusion into fresh water aquifer, evapora- tion of shallow groundwater also plays an important role in the accumulation of salt in groundwater. Joint analysis of δ^34SSO4 δ^13CHCO3 composition with special anions （e. g. SO4^4-, C1-） reveals that brackish and saline groundwater are not the results of a simple mixture between seawater and fresh groundwater. Inverse hydrogeochemical modeling can be used to reveal the main hydrogeochemical reactions controlling the hydrochemical evolution of coastal karst aquifers in the study area, including the dissolution of calcite, montmorillonite and gypsum, the precipitation of il- lite and Ca-Na cation exchange with CO2 extrication.