桂西大型岩溶地下河系统离子来源及碳稳定同位素信息——以坡心地下河流域为例

Chemical ions source analysis and stable isotope implications of different water bodies in large karst underground river system: A case study of Poxin groundwater basin in Guangxi

为探讨坡心地下河系统内地表和地下水体的水化学离子特征、来源及其控制因素,无机碳来源及其稳定性。运用水化学计量法和同位素法对采自坡心地下河流域的38个采样点的水化学和碳稳定同位素样品数据进行分析。结果表明,地下河干流沿程受到局部岩性和支流稀释作用的影响明显,各水化学离子均有所变化。化学离子比例分析发现:大气降水对部分泉水的Cl^-和Na^+影响较大;碳酸盐岩溶解类型主要以灰岩溶解为主,地表水和地下河天窗的Mg^(2+)/Ca^(2+)摩尔比值与HCO_3^-呈负相关,说明在宏观上灰岩溶解程度越强烈,HCO_3^-值就越高,并且H_2SO_4和HNO_3积极参与流域内碳酸盐岩风化。硅酸盐岩的风化对地表和地下水体的Ca^(2+)、Mg^(2+)、Na^+、K^+有一定的贡献。此外,人为采矿活动和农业活动对SO_4^(2-)和NO_3^-的产生有较大的影响。质量平衡正推模型结果显示:受到区域岩性和水文条件的影响,地表和地下河天窗水体主要受碳酸盐岩溶解影响,硅酸盐岩溶解和大气输入也有一定的贡献,三大来源的相对比例在空间上变化较大。水体内的可溶性无机碳(DIC)主要来源于碳酸盐岩的溶解和土壤内CO_2的贡献。地表水和地下水的DIC浓度和δ^(13)C_(DIC)值差别较大,DIC值与δ^(13)C_(DIC)呈反相关关系,这说明来自土壤CO_2贡献的DIC越多,其对碳酸盐岩矿物的溶解能力越强。根据本研究区的数据与前人在西江干流的上、中、下游进行对比,结果表明碳酸盐岩风化产生的DIC可以被西江干流的水生植物利用,从而形成稳定的碳汇。

To explore the characteristics, sources and main controlling factors of water chemical ions in surface or groundwater of Poxin underground river system and the implications of carbon stable isotope, the authors collected hydrochemical and carbon stable isotopic data from 38 water sites in the Poxin underground river basin and did analysis with stoichiometry and isotopic method. The results show that spatially, the chemical ions in the mainstream of underground river vary significantly due to the effect of the regional rock types and the dilution of the tributaries. Ion proportional analysis shows that the precipitation has great influence on Cland Na^+ in some springs, and the main type of carbonate dissolution seems to be limestone in the study area . The Mg^2+/Ca^2+ molar ratios of surface water and water in ground river skylight are negatively correlated with HCO3^-, indicating that,at the macroscopic scale, the dissolution will become more intense with the higher HCO3 - value, and H2SO4 and HNO3 will participate during the weathering of carbonate rocks actively. Silicate rocks weathering has contributed to Ca2 ^+, Mg2 ^+, Na^+ and K^+ of surface water and groundwater. Mining activities and agricultural activities have a great impact on the generation of SO4 ^2- and NO3^-. The forward model of mass balance shows that, owing to regional rock types and hydrological conditions, water in surface and underground river skylights is mainly dominated by carbonate dissolution, whereas silicate dissolution and atmospheric input also contribute some components, and hence the ratios of three sources vary greatly spatially. The dissolved inorganic carbon (DIC) in the water is mainly derived from the dissolution of carbonate rocks and the soil CO2. DIC and the δ^13CDIC values are obviously different between surface water and groundwater, and are negatively correlated with δ13CDIC, implying that the more DIC from the soil CO2, the more intense the dissolution capacity of the carbonate minerals. Based on the data from this study area and previous study in upper, middle and lower reaches of Xijiang River, the authors detected that both of data support the argument that the aquatic algae can apply DIC transport to organic carbon, and forms a stable carbon sink.