Effects of hydrogeochemical conditions on the distribution of pesticides in the karst river system.

Karst aquifer systems are tended to be polluted compared to other types of aquifers because pollutants are able to enter aquifers through developed conduit systems. To identify the effects of hydrogeochemical conditions on the distribution of pesticides in a karst river system in Kaiyang, southwest China, a typical pollution mode combining intermittent infiltration with intrusion was constructed. Twelve aqueous samples were collected along the karst river, and a total of 24 pesticides were detected. The results showed that the pesticide ubiquity and the dominant organophosphate (40%) and organonitrogen pesticides (49%) were both observed. Based on the spatial distribution, the attenuation of pesticides was found in the underground conduit and surface river. The wastewater treatment plant and the rural dump were the two important point sources releasing pesticides. In addition, ten core pesticides were identified by clustering analysis and regional characteristics of three types of pesticides in Songnen Plain, North China Plain, and Southwest karst areas were also summarized. With correlation analysis between pesticides and environmental factors, the significant correlations of pesticides with ammonium ion and dissolved oxygen were found, which indicated that rapid developing urbanization and long-term agricultural practices could remarkably affect the spatial distribution of pesticides. The calculation of ecological risk quotients showed that organophosphate pesticides had the highest risk to invertebrate, followed by organonitrogen pesticides, and finally organochlorine pesticides. Invertebrates were the most vulnerable aquatic organisms. These findings fill a gap in the multiple pesticides' pollution in the karst areas of China. Graphical abstract.