Dominant factors controlling hydrochemical variation of karst underground river in different period, Qingmuguan, Chongqing

Hydrochemical variation of Qingmuguan karst underground river in the typical karst valley area was monitored by using multi-parameter water quality analyzer. Dominant factors of the chemical dynamics were studied by principal component analysis method with monitoring data derived from three different time intervals, including that of monthly monitoring, storm event and farming period. Results showed that on a monthly scale, because of the dilution effect due to rainfall, the major underground river constituents such as pH and the concentrations of Ca2+, Mg2+, K+, Na+ and HCO3- were lower at flooding or wet season than those of dry season. The hydrochemical variation was affected by five main components with an accumulated variance contribution rate of 90.75%, of which the dominant factor was identified as the rock-water interaction. In farming period, rainwater carried residual manure and nitrogenous, phosphate and potash fertilizers into the underground river, which resulted in the increase of electrical conductivity (EC) and Cl-, NO3-, K+ and Na+ contents. The hydrochemical variation was affected by three main components, with an accumulated variance contribution rate of 87.81%, of which the dominant factors were rock-water interaction and fertilization. During the storm event, the change of the ionic concentrations could be divided into four stages, including stationary, rising, stable and special periods. During the rising period, rainfall obviously effected on underground river and caused more active rock-water interaction and the increases of EC and the concentrations of Ca2+ and HCO3-. Significant impact of erosion and leaching due to rainfall on topsoil and fertilizers resulted in obvious raises in undergroundriver Al,Fe, Cl-, K+, Na+ and NO3- contents. The hydrochemical variation was affected by four main components with an accumulated variance contribution rate of 80.09%; and the dominant factors could be rock-water interaction, soil erosion and soil nutrient leaching.