Herbicide occurrence in riparian soils and its transporting risk in the Songhua River Basin, China

A riparian zone is the interface between land and a river or stream. Riparian zones are major elements of ecosystems. However, human conversion of riparian land to agricultural uses has reduced the ecological benefits of riparian land such as water and pollutant filtration. Over 80 % of the original riparian area has been lost from North America and Europe over the past 200 years. Intensive land use along riparian areas has increased soil erosion and, in turn, sedimentation in rivers. Intensive land use has also increased the transfer of unused fertilizers, e.g., nitrates and pesticides in river water. We therefore studied pesticide transfer and soil erosion in riparian zones of the Songhua River Basin in China. For that, we identified land use type and measured herbicide residues of acetochlor and atrazine in riparian soils and soil columns of samples from 248 stations. We also modeled the risk of herbicide transfer. Our results show that herbicides were detected in 97 % of the samples, evidencing a widespread contamination. Acetochlor residue concentrations in soils are 26.10 μg/kg for farm land, 1.76 μg/kg for grass land, 1.46 μg/kg for forest land, and 2.48 μg/kg for bare land on average. Atrazine residue concentrations in soils are 11.28 μg/kg for farm land, 0.51 μg/kg for grass land, not detected in forest land, and 0.13 μg/kg for bare land on average. Atrazine residue concentration increased in riparian agricultural soils below 25 cm; whereas atrazine residue concentration of grassland soils showed a decrease. We found also that the highest risk of soil erosion and herbicide transfer is located in the middle reach of the Songhua River.