Impact of Human Activities on Water-Rock Interactions in Surface Water of Lijiang River

Abstract

The influence of human activities on karst surface water quality has received increasing attention. To explore the impact of human activities on water-rock interactions in karst surface water, water samples were collected from July to November in 2016 in the Lijiang River, which is dominated by human activities. The samples were analyzed for the hydrochemistryand the characteristics of δ13CDIC. The results show that:①The Lijiang River is mainly characterized by HCO3-Ca chemical composition.. The SO42- and NO3- concentrations are inferior to HCO3- concentrations in the anion composition; besides, the concentrations of SO42- and NO3- in the river between Guilin and Yangshuo are higher than in the other sampling sites, caused by tourism and urban development. ②Equivalent ratios of[Ca2++Mg2+]/[HCO3-] are between 1.01 and 1.51 and the equivalent ratios of[Ca2++Mg2+]/[SO42-+NO3-+HCO3-] are between 0.85 and 1.12, indicating that the sulfuric acid and nitric acid have participated in the water-rock interaction. ③The proportions of carbonic acid dissolving carbonate rocks range from 32.10%-98.01% with an average of 75.89%. The proportions of sulfuric acid and nitric acid dissolving carbonate rocks vary from 1.99%-67.90% with an average of 24.11%. The proportions of sulfuric acid and nitric acid dissolving carbonate rocks in the rainy period with an average of 24.31% is higher than in the brief rain period with an average of 23.88%. In addition, the proportions of sulfuric acid and nitric acid dissolving carbonate rocks present a fluctuated increasing trend from upstream to downstream and the maximum strength and the minimum strength of the sulfuric and nitric acid dissolving carbonate rocks occur in Lingchuan and Huajiang, respectively. ④ HCO3- mainly originated from the carbonate rocks dissolved by carbonic acid, with an average of 83.58%, followed by the carbonate rocks dissolved by sulfuric acid and nitric acid, with an average of 14.24%. The contribution of carbonic acid weathering silicate rocks is the smallest with an average of 2.18%. ⑤ δ13CDIC of the Lijiang River ranges from -11.95‰--7.61‰; the calculated δ13CDIC-rock using the mixture endmember model ranges from -14.24‰--7.23‰. δ13CDIC is close to δ13CDIC-rock in the rainy period and δ13CDIC is significantly higher than δ13CDIC-rock in the brief rain period. The difference between δ13CDIC and δ13CDIC-rock is caused by the influence of aquatic photosynthetic organisms.