Land-use change caused microbial pollution in a karst underground river, Chongqing, China

The Qingmuguan underground river of the Qingmuguan karst valley was used as a source of local drinking water until 2010. However, it was abandoned in 2011 due to pollution caused by a new horse farm. Horse feces polluted sub-surface water, especially after storm events, causing a bad odor and discoloration. In this study, data on hydrochemical and microbial variables, including water discharge and fecal bacteria, from a May 2011 storm event are analyzed. The results of these hydrochemical and microbial analyses show that different recharge sources were responsible for different contamination substances, occurring in three stages: (I) small quantities of recharge water near the outlet meant only minor increases in the river discharge and in the hydrochemical variables, but a large increase in bacterial numbers. (II) Large quantities of surface water recharged the underground river through sinkholes, causing an increase in discharge and significant contamination of the river water by sewage and soil erosion. With the exception of fecal coliform, PO43-, and dissolved oxygen, soil erosion was responsible for variations in most variables. (III) At the end of the storm event, recharge from sinkholes was reduced, but water seepage from soil and fissures, which were contaminated by horse feces and fecal bacteria, led to a significant increase in the fecal bacteria and other variables related to fecal pollution in the river water.