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

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.

农田面源污染会导致地表水与地下水中残留痕量浓度的农药及其转化产物，进而降低饮用水水质。潜流带（hyporheic zone）作为连接地表水与地下水的过渡带，被视为高效生物反应器，其在农药的水生环境归趋与分布中可发挥关键作用。本研究针对一条以地下水补给型（下行流）水文条件为主、全长20公里的受农业活动影响河流，探究该河流潜流带沉积物中MCPA（甲氯苯氧乙酸）、甲草胺（metolachlor）与丙环唑（propiconazole）的细菌降解潜力。研究同时测定了该河段及地下水的水流特征与理化参数、农药在沉积物上的吸附行为，以及细菌群落组成。采用环境相关浓度（10 μg·kg⁻¹）的农药，在6个采样点位（5个下行流点位、1个上行流点位）设置降解与矿化批次实验。计算得到的降解半衰期（DT50）存在显著空间变异：MCPA为11~44天，甲草胺为11~27天，丙环唑为60~147天。尽管下行流与上行流条件未对降解过程产生显著影响，但我们观测到上行流条件下甲草胺的矿化作用有所减弱。MCPA的降解半衰期与沉积物总有机碳（TOC）、吸附行为及细菌丰度呈显著相关，而甲草胺的降解半衰期则与上述参数无明显相关性。丙环唑的降解半衰期仅与总有机碳呈弱相关。较高且相似的生物多样性无法解释观测到的降解半衰期差异。本研究数据证实，潜流带是一类高效生物反应器，其降解潜力可与农田土壤相媲美。从甲草胺生成2种已知转化产物、从丙环唑生成1种持久性转化产物的结果表明，潜流带可能是受地表影响的无压含水层中转化产物的来源之一——如今转化产物已被视为水生环境中的一类新兴污染物。