Contaminant transport behavior in a karst subterranean river and its capacity of self-purification：A case study of Lihu, Guangxi

Lihu subterranean river in South China was selected to study the contaminant transport behavior and self-purification capacity of a karst aquifer. The subterranean river was investigated through water sampling along the stream reach for one year. Analysis of inorganic ions and trace elements in different seasons and at different locations showed the major contaminants are NO₂^--N, NH₄⁺-N, COD, As, Cd, and Hg, due to domestic waste, mining industrial waste, and fertilizer application in the recharge area. Along the course of the karst conduit, a self-purification process was observed because of significant decrease in some ions concentration. The mechanism of self-purification was caused by dilution and nitrification, which was determined by the suterranean river's structure and hydrogeological condition. By comparing the attenuation quantity and percentage of ions, it was found that ammonium had the highest, while chloridion had the lowest percentage. According to the attenuation rate, four types of attenuation character could be classified. Firstly, the ions decayed completely after they entered the karst aquifer, such as TFe, Zn, and Al; secondly, the ions had the highest attenuation rate when they traveled underground through the aquifer, such as K, Na, Mg, F, Cl, HPO₄, and TP; thirdly, the concentration gradually decreased from upstream to downstream, which was represented by NH₄⁺-N, COD_Cr, and BOD₅; and finally, they had the highest attenuation rate when transformed from surface stream to underground stream, but secondary suspension of contaminants could happen in the cave, such as Mn, Ba, and Hg.