Hydrogeochemistry and environmental meaning of Chongqing subterranean karst streams in China

The subterranean karst stream services as an important water source to the groundwater supply system in the karst areas.There are approxinately 380 subterranean karst streams that supply groundwater to the areas of Chongqing municipalities.The hydrogeochemistry is studied for 61 subterranean karst streams in Chongqing.The result shows that the solutes are mainly originated from the dissolution of carbonate rocks through the water-rock interaction process.The groundwater chemistry is found to be of Ca-HCO₃ type or Ca(Mg)-HCO₃ type.However,the Na+Ca-HCO₃、Na+Ca-SO₄、Na+Ca-Cl or Ca-SO₄+HCO₃ type water have been detected in some of subterranean karst streams due to the influence of human activities.The impacts of agriculture activities and city sewages on hydrogeochemistry are more common than that of industrial and mining activities.The groundwater temperature gradually decreases with the increase of elevations.For the same geological setting,the presence of Ca²⁺、Mg²⁺、HCO₃^- exhibitssignificant regionality due to the discrepancy of karst processes in different areas.The same regionality is also found in the distribution of K⁺、Na⁺、SO₄^2-、NO₃^-、Cl^- because of human activities.The study concludes that the hydrogeochemical condition of Chongqing karst subterranean streams is deteriorating due to the industrial and agricultural activities as well as the discharge from the city sewage system.