Investigation and evaluation on heavy metal pollution in the waste water collecting depression in a lead-zinc mine of karst area

The study area is in southwest Guangxi karst area. The lead-zinc ore is low temperature hydrothermal filling deposit that has mined over 50 years. The major strata include the upper Cambrian, the Devonian, the lower Cretaceous and the Quaternary systems. The shallow karst develops strongly but the deep karst weak in the mining and the surrounding areas, with obvious stratified development of caverns. The karst depression collecting the lead-zinc mine wastewater is land for farm. In order to find the quality of soil environmental and evaluate potential ecological risk of heavy metal pollution in the soil, the contents of cadmium, copper, lead, zinc, chromium, arsenic and mercury in the soil are investigated and analyzed by means of single pollution index, comprehensive pollution index and Hakanson potential ecological index. The results show that pollution index of cadmium, zinc, mercury, lead are greater than 1, and cadmium pollution index has reached 88.4, followed by zinc and mercury, lead the minimum. All of the soils, 0-30 cm layer(A),30-60 cm layer(B)and 60-90 cm layer(C), have been polluted by heavy metals with the degree of as layer(A)> layer(B) > layer(C).The comprehensive pollution index of layer(A), layer(B) and layer(C) is 67.0, 11.9 and 8.8 respectively. Potential ecological risk index of heavy metal pollution in layer(A), layer(B), layer(C) is 2 921.0,543.4 and 421.2 respectively. Potential ecological risk degree is very strong, strong, strong level.It follows that the soil in karst depression is polluted by cadmium, zinc, mercury, lead strongly, which is of high ecological risks. So it is suggested that inedible industrial crops should be produced and edible plants should be forbidden growing before the heavy metals in the soil being cleared away.