银川平原深层地下水14C年龄校正

地下水14C年龄的校正精度取决于对地下水溶解无机碳演化过程中影响地下水14C活度主要因素的准确识别及其影响程度的定量评价。以银川平原为例,在进行银川平原承压水反向地球化学反应路径模拟的基础上,识别出影响区内深层地下水碳酸演化的主要作用并进行深层地下水14C年龄的校正。研究认为,控制银川平原深层地下水化学演化的主要反应路径为碳酸盐矿物的沉淀和长石、角闪石、石膏等矿物的溶解以及Ca-Na离子交换。地下水流路径上所发生的水文地球化学反应对路径上14C的浓度变化影响较小。地下水14C年龄校正结果表明,除补给区和承压水水位漏斗区外,银川平原承压水年龄均在2000a以上,属“古水”,天然条件下径流缓慢、地下水更新速度小。在承压水水位漏斗区外,沿地下水流方向,地下水年龄逐渐增大;而在承压水集中开采区,承压水的年龄明显小于路径上游地下水,潜水与承压水之间的垂向水力联系比较密切。

14C Age Correction of Deep Groundwater in Yinchuan Plain

The accuracy of ~ 14 C age correction mainly depends on the identification of the factors affecting the variation of ~ 14 C activity in the process of formation of dissolved inorganic carbon and its evolution. Yinchuan plain has been taken the as case study and mass equilibrium reaction model and isotope mass transfer model have been employed to simulate the inverse geochemical reaction path of deep groudwater in Yinchuan plain and then to correct ~ 14 C age. The simulation result shows that the main reactions controlling the geochemical evolution of confined water from the initial point to the final point in the groundwater flow path are the dissolution of plagioclase,horn blende,gypsum, Ca-Na exchange and the precipitation of carbonate minerals. The above reactions have little effect on the transfer time in the groundwater flow path. ~ 14 C correction result shows that deep groundwater in Yinchuan plain is paleo-groundwater with ~ 14 C age more than 2 000 a except in the recharge area and the circulation rate is very slow and has low renewing capacity. Outside the depression cone of deep groundwater, the age of deep groundwater is becoming older along the groundwater flow path.However, in the depression cone,the age of deep groundwater is younger than the incoming water,which signifies that there is a close hydraulic relationship between deep groundwater and shallow groundwater.