Degradation potential of MCPA, Metolachlor and Propiconazole in sediments of an agricultural impacted river. undefined

Diffuse, non-point pollution of agricultural land results in trace concentrations of pesticides and transformation products in surface- and ground-water, and therefore reduces drinking water quality. The hyporheic zone, considered as a highly efficient bioreactor, could play an important role in the environmental fate and distribution of pesticides in the aquatic environment, as this transition zone links surface and groundwater. This study evaluates the bacterial degradation potential of MCPA, metolachlor and propiconazole in sediments from the hyporheic zone, along a 20 km long stretch of an agriculturally impacted river dominated primarily by downwelling conditions. River flow and water physico-chemical parameters in the river and groundwater were assessed as well as pesticide sorption to sediments and bacterial community composition. Degradation and mineralization batch experiments were set up from six locations (five downwelling, one upwelling) using environmentally relevant concentrations of pesticides (10 µg kg-1). Highly variable DT50 values from 11-44 days for MCPA, 11-27 days for metolachlor (MTC) and 60-147 days for propiconazole were calculated. While downwelling and upwelling conditions did not impact degradation, we observed a reduced mineralization of metolachlor in upwelling conditions. MCPA degradation (DT50) correlated with sediment TOC, sorption, and bacterial abundance, whereas metolachlor DT50 values did not correlate with these parameters. DT50 for propiconazole correlated slightly with TOC. High but similar biodiversity did not explain observed DT50 differences, Our data confirms that the hyporheic zone is a highly efficient bioreactor with degradation potentials comparable to agricultural soils. The formation of two know transformation products from metolachlor and a persistent transformation product from propiconazole suggests that the hyporheic zone may be a source of transformation products in surface impacted unconfined aquifers – transformation products are nowadays considered an emerging contaminant of aquatic environments.