The pollutant infiltration pathways and measures to control groundwater pollution in the Guijiang River drainage system

The investigation of karstic groundwater pollution in areas of Guijiang River has recently been completed; and our research findings show that the pollution sources can basically be categorized into nitrogenous, heavy metal and organic compounds. Among them the nitrogenous (nitrite, nitrate and ammonia) pollutants were mainly derived from industrial activity, livestock farming, domestic sewage and waste and the use of nitrogenous fertilizers, while heavy metal pollutions are attributed to industrial and mining activities and organic compounds to the impact of industries and livestock farming. In this case, the patterns of pollutants that migrate downward to reach the karst groundwater include diffuse and concentrated infiltrations, of which the former usually happens at isolated peak karst plains , rocky hill karst valleys. Concentrated infiltrations largely take place at the sink points in isolated-peak karst plain,the ponors in peak-cluster depressions and karst windows in peak-cluster and hoodookarst valleys. Systematic tests and analyses were performed to evaluate the rate of groundwater pollution. Results show that nitrogenous pollutions occur in 36.5% of the water samples which are broadly distributed over the karst autogenic recharge area in a diffuse infiltration manner. In the meantime, heavy metals in 15.3% water samples are above the directives of water quality regulations; and these point pollution sources enter groundwater via concentrated infiltration largely happens at the localities as sink points and ponors in peak-cluster poljes and via diffuse infiltration in the karst valleys. As heavy metals are easily absorbed by sediments, karst groundwater around the pollution sources has been more likely contaminated. Organic compounds were detected in 3.49% of the water samples, which are distributed at the downstream of the alluvial plain; and the pollutants that infiltrate into groundwater demonstrate a feature of short linear or point pollution because of the low hydraulic gradients of the groundwater.