城市发展对岩溶地下水质影响的地球化学示踪——以重庆南山老龙洞地下河系统为例

GEOCHEMICAL TRACERS TO CHARACTERIZE EFFECTS OF URBANIZATION ON KARST GROUNDWATER QUALITY FROM NANSHAN UNDERGROUND RIVER SYSTEM,SW CHINA

随着城市化的快速发展,大量生活废水的排放严重影响岩溶地下水质。本文尝试利用重庆南山老龙洞流域2012年每月地下水、雨水和生活污水的水化学数据与环境同位素^(87)Sr/^(86)Sr、δ^(34)S-SO_4^(2-)和δ^(13)C_(DIC)来研究城市发展对岩溶地下水质的影响,结果表明:1)城市化过程带来的硫酸改变了岩溶地质过程,不同月份碳酸溶蚀碳酸盐岩对地下水中(Ca^(2+)+Mg^(2+))和HCO_3^-的贡献率分别变化于38%~68%(平均贡献率为53%)和55%~81%(平均贡献率为68%),而硫酸溶蚀碳酸盐岩对地下水(Ca^(2+)+Mg^(2+))和HCO_3^-的贡献率分别变化于32%~62%(平均贡献率为47%)和19%~45%(平均贡献率为32%);2)生活废水的输入改变了流域地下水锶同位素的地球化学特征,地下水^(87)Sr/^(86)Sr变化于0.70800~0.70900,明显高于雨水和石灰岩溶解来源的^(87)Sr/^(86)Sr值,而低于生活废水的^(87)Sr/^(86)Sr值,表明流域地下水质受水-岩作用和污水输入的共同影响;3)地下水δ^(34)S-SO_4^(2-)变化于7.8‰~12.0‰之间,SO_4^(2-)主要来源于生活污水和雨水的输入与石膏的溶解;4)地下水δ^(13)C_(DIC)变化于-11.2‰^-7.3‰之间,且地下水(Ca^(2+)+Mg^(2+))/HCO_3^-摩尔比率介于0.60~0.71,流域地下水质一方面受控于CO_2对碳酸盐岩溶解的自然过程,另一方面则受城市化过程产生的硫酸对碳酸盐岩溶解的控制。

With the expansion of urbanization,population in cities or suburbs has increased in number where sewage discharge from residential areas has deteriorated karst groundwater quality of drainage basins across the globe.The chemical and stable isotopic compositions of strontium,sulfur and carbon from groundwater are used to evaluate the contributions of urbanization to karst groundwater quality of Laolongdong Underground River in the Nanshan karst trough valley （29°28′ ～ 29°33′N,106°35′ ～ 106°37′E） of Chongqing,SW China.The groundwater monitored from January to December in 2012,was characterized by hydrochemical facies of HCO3-SO4-Ca.The contributions of carbonate dissolution by carbonic acid to total （Ca2＋＋Mg2＋） and HCO3-in groundwater ranged from 38％ to 68％ （average 53％）,and from 55％ to 81％ （average 68％）,in different months.While the contributions of carbonate dissolution by sulphuric acid to total （Ca2＋ ＋Mg2＋） and HCO3-in groundwater varied from 32％ to 62％ with an average of 47％,and 19％ to 45％ with an average of 32％,respectively.The 87Sr/86Sr ratios of groundwater ranged from 0.70800 to 0.70900,higher than that derived from the dissolution of calcite,suggesting the natural 87Sr/86Sr ratios of groundwater controlled by water-rock interaction has been significantly elevated by detergents derived from domestic sewage water.The δ34S-SO24-of the groundwater varied from ＋ 7.8‰ to ＋ 12.0‰ with high SO42-concentrations,indicating that the sulfate was mainly originated from detergents （domestic sewage water）,rainwater and the dissolution of gypsum.The δ13CDIC in groundwater,varying from-11.2‰ to-7.3‰ with a changeable molar ratio between （Ca2＋＋Mg2＋） and HCO3-of 0.60 to 0.71,suggested that the groundwater quality was controlled by processes that carbonate rocks are dissolved by CO2 from C3 vegetation under open system conditions which the δ13CDIC of groundwater has a value of around-14‰ with a molar ratio between （Ca2＋ ＋Mg2＋） and HCO3-of around 0.5 in groundwater,and H2SO4 introduced by urbanization which the δ13CDIC of groundwater approaches a value of around 0‰ with a molar ratio between （Ca2＋ ＋Mg2＋） and HCO3-of around 1 in groundwater.