Reactive transport modelling to assess pesticide dissipation at the sediment-water interface

Rivers that are hydrologically connected to an agro-ecosystem act as a source or sink of pollutants transported by surface runoff and subsurface water. The Sediment-Water Interface (SWI) of rivers is a critical boundary for river dynamics where hydrological and biogeochemical processes tightly control pesticide dissipation. Transport processes govern pesticide transit time and distribution across the SWI depending on the water flow and hyporheic exchanges. Simultaneously, reactive processes such as sorption and biodegradation are responsible for retardation or actual degradation of pesticides within the porous sediment. However, knowledge on the interplay of these processes at the SWI remain sparse mostly because a physically-based generalized framework to model transport and reactivity at fluid-porous interfaces is still lacking. Here, we combine model development and laboratory experiments to investigate the effects of representative hydrological conditions on pesticide transport at the SWI.

An innovative discrete flow-transport model accounting for sorption was developed to consider the pure fluid layer (via Navier-Stokes model) and the porous medium (via Darcy-Brinkmann model). Advanced and appropriate numerical techniques are implemented to solve the coupled models (Navier-Stokes and Darcy-Brinkmann) without any interface conditions or empirical transfer functions. Conservative (NaCl) and non-conservative (Foron Blue 291 – sorptive) tracer experiments were performed within a 15 cm long and 10 cm deep recirculated river model (3 < equivalent length < 30 km) to characterize pesticide transport at the SWI. Configurations with the contaminant in the sediment (sediment as contaminant source) or contaminations from the overlying water (sediment as a sink) were tested. Simulated tracer concentrations fitted well to measured concentrations over a range of laminar flows representative of low Strahler order rivers (Re < 700, bulk velocities 10 <  U < 100 mm.s^-1). In all flow conditions, the first few mm of sediment constituted the most dynamic layer, which was controlled by advective processes. In contrast, the tracer in deeper sediment layers undergone diffusive transport with lower exchange rates. Sorption was also observed to significantly increase residence time within the sediment and to slow down the progression of the tracer plume into the sediment. The times required to reach the bottom of the river model rose up to 12 times as compared with the non-sorptive tracer, indicating limited hyporheic exchanges with increasing sorption.

To account for biodegradation at the SWI, the model is further extended to include degradation kinetics and stable isotope fractionation of organic micropollutants. Changes of stable isotope ratios of the remaining, non-degraded pool of pollutants over time or across the sediment layer is used as a proxy of in situ biodegradation. Biodegradation is interpreted as a function of oxygen zonation within the sediment. This model is eventually tested against tracer experiments with caffeine, which is used here as a fast degrading anthropogenic micropollutant. Patterns of micropollutant dissipation at the SWI arising from these developments will be further extrapolated at river reaches within an agricultural catchment (Souffel catchment, France). Altogether, this study will help understanding how rivers influence pesticide transport, storage and degradation at the catchment scale.