Effect of urbanization on the hydrogeochemical evolution of karst groundwater system：A case of the Laolongdong watershed, Chongqing, China

The underground river is in Nanshan district of Chongqing City where the karst groundwater is well-developed. There are karst depressions, sinkholes, underground rivers and exposed springs, which has the features of sensitivity and vulnerability of typical karst groundwater systems. Due to urbanization and human activities, groundwater in this area has been seriously polluted in recent years. In this paper, an underground river conduit in the karst area is taken as an example. Through analyzing the evolution of the underground, water quality deterioration and pollution sources of the groundwater, the main controlling factors of groundwater pollution in the study area are discussed, in order to protect the groundwater and provide a scientific basis for groundwater governance in this area. From 2008 to 2012, the outlet of Laolongdong underground river was chosen as a water quality minoring locality where pH value, conductivity, water temperature, potassium, sodium, calcium, magnesium, bicarbonate, chloride, nitrate, sulfate and phosphate were measured on monthly basis. The principal component analysis (PCA) was used to analyze the variation of water quality during the rainfall event in August 2011. Results indicated that in the process of urbanization, the ion concentrations of sodium, chloride, phosphate, calcium, magnesium, bicarbonate increased because of human activities. Meanwhile, the ion concentrations of nitrate, sulfate decreased due to the urbanization effect and agricultural activities. Moreover, water-rock interaction, human activities, and soil erosion played a crucial role in the hydrochemistry compositions and variation of the groundwater. Among them, dump of domestic sewage and solid wastes and the excessive uses of pesticide/fertilizers in urban residential areas are the main sources of pollutants such as potassium, sodium, chloride, sulfate, nitrate, phosphate, iron, manganese and aluminum in underground rivers. The water quality in this region varies because of the improvement of the urbanization levels and regional environment change. From the annual variation of nitrates and sulfates, the groundwater quality has been greatly improved. The karst area is sensitive and vulnerable in ecological environment. For resources protection purpose, it is essential to develop and utilize groundwater resources reasonably and reduce negative effects of groundwater pollution caused by human activities for realizing a win-win situation between the local economic benefits and sustainable development.