Trace Element and Pesticide Dynamics During a Flood Event in the Save Agricultural Watershed: Soil-River Transfer Pathways and Controlling Factors

Agricultural practices are the main source of water contamination in rural areas. Rainfall events, and subsequently, soil leaching and storm runoff are mainly controlling the transfer of pollutants from diffuse sources in watersheds during floods. These periods are also very important to better understand their dynamics, particularly their different soil-river transfer pathways (surface runoff SR, subsurface runoff SSR, and groundwater flow GF). This study focuses on riverine transfers of both pesticides and trace elements. High-resolution monitoring of water discharge and water sampling were performed during a flood event that occured in May 2010 in an agricultural catchment of SW France. Chemical composition of major and trace elements, silica, alkalinity, pH and conductivity, DOC and POC, TSM, and commonly used pesticides were analyzed with a high sampling frequency. The different stream flow components (SR, SSR, and GF) were assessed using two independent hydrograph separation methods: a hydrological approach based on Maillet’s formula (1905) for the recession period and a chemical approach based on physico-chemical tracers, TSM for SR and PO₄³⁻ for GF. Both methods exhibited important contributions of SR (33 %) and SSR (40 %) to the total riverine pollutant transfers. The contribution of different components was also visible using concentration-discharge relationships which exhibited hysteresis phenomenon between the rising and the falling limbs of the hydrograph. Higher concentrations during the rising period (clockwise hysteresis) were characteristic of pollutants mainly exported by SR (trifluralin, Cd). Anticlockwise hysteresis with higher concentration during the recession period showed pollutants mainly exported by SSR (metolachlor, Cu). Moreover, significant relationships were highlighted between the controlling factors (DOC, POC, and TSM) and SR, SSR, and GF contributions: DOC and the complexed pollutants were highly correlated to SSR while POC, TSM, and the adsorbed pollutants were linked to SR. During the flood, K d of most pollutants increased, particularly at the beginning, and therefore, future studies should investigate their availability to living organisms and thus their toxicity. An additional characteristic equation between K d and K ₒw of the different pesticides was proposed to help future management, modelling, and estimation of pollutant transfers during floods.