Hydrogeochemical characterization of groundwater and surface water and their influencing factors based on principal component analysis: An example in the Shuicheng basin of Guizhou

As the major water resources for most of the settlements in karst regions, karst water could be more easily polluted in a shorter time by human activity than water in non-karst aquifers. Assessing current hydrochemical status and understanding the mineralization process in karst water are of significance to analyze the influence factors from natural evolution and human activities. The Shuicheng basin, a tectonic basin with well-developed karst topography, is located at the center of the Yunnan-Guizhou Plateau, southwestern China. The NW-SE oriented basin is an area of approximately 22.5 km long, 5.5 km to 9 km wide and 208 km2 in size, with the elevation ranging from 1,700 m to 1,800 m above mean sea level. Climatically, it falls in the north subtropical monsoon zone with plentiful precipitation which is a major source recharging the karst water. Average annual precipitation in this area is 1,200 mm/year to 1,500 mm/year. Geologically, the basin predominantly consists of the Permian and Carboniferous carbonate rocks, accounting for nearly 90% of the total rock exposure.This work investigated the hydrogeochemical characteristics and influence factors of groundwater and surface water in different regions of the Shuicheng basin, with the intension of providing references for helping to significantly improve the quality of the aquifer. Sampling campaigns were carried out in September 2015. Variable parameters of the sampled water were measured using a portable instrument and HCO3- was titrated with 0.01 mol/L HCl in the field. All water samples were filtered through 0.45 μm cellulose acetate membrane filters and then collected into 50 mL high-density polyethylene bottles. The samples for cation analysis were immediately acidified with pure HNO3 to a pH lower than 2. The cations and anions of the samples were analyzed by the methods of atomic absorption spectrometry and ion chromatography, respectively, with the test accuracy of less than 5%.The interpretation of hydrochemical data was performed by using various methods, i.e. Piper diagram, correlation analysis, multivariate principal component analysis, and saturation mapping. The results show two hydrochemical types (HCO3-Ca and SO4-Ca) that characterize the groundwater and surface water. Natural dissolution of carbonate rock (water-rock interaction) is the primary source of Ca2+and HCO3- in groundwater and surface water. Domestic sewage and the wastewater from livestock and poultry industry may be the main contribution of Cl-,NO3-and Na+ to surface water. The main factor affecting on the quality of groundwater is the water-rock reaction. The urban sewage water from Zhongshan district and coal industrial wastewater from Shuicheng County and discharge from the groundwater are the main sources affecting the surface water quality of the basin. In conclusion, the evolution of water quality is determined by the lithology of the aquifer (dissolution and base exchanges) and by anthropogenic processes (urbanization, coal mining activities and agriculture).