Characteristics of heavy metal element distribution in the groundwater system of typical karst regions: A case study in Poyue underground river, Guangxi, China

Underground river is an important part of karst groundwater system. The purpose of this work is to reveal the concentrations and spatial variation of heavy metals represented by zinc (as Zn), aluminum (as Al) and Manganese (as Mn), in particular by the five toxic heavy metals, under the typically hydrogeochemical environment in the Poyue underground river system, Guangxi. Water sample were collected at the localities along defined cross section lines in the field and the concentrations of heavy metals were analyzed in laboratory. The results show that pH value of the Poyue groundwater is between 7.34 and 8.10, with an average of 7.67. Toxic heavy metals detected in the underground river have very low concentrations, much less than the level of III water quality category recommended by the national environmental quality standards. Therefore, the karst groundwater is suitable for a variety of purposes. The results also show that higher concentrations of these elements occur at the localities where the underground river exposes on the surface or surface water in the recharge area, with the maximum values at the underground river outlet located at slope toe. The reason is that in the slope area, industrial, agricultural and mining activities are extremely strong, which is characterized by the dumps of industrial and mine (such as gold mine) tailing wastes on the slope. This causes the occurrence of the heavy metals in the karst river system.