An original approach combining biogeochemical signatures and a mixing model to discriminate spatial runoff-generating sources in a peri-urban catchment

Abstract. Hydrograph separation using biogeochemical data is a commonly used method for the vertical decomposition of flow into surface, subsurface and groundwater contributions. Such approach is not yet widely used for the spatial decomposition of flow. However, it has great potential for estimating contributions linked to specific geological, pedological or land-use characteristics, or to particular anthropogenic contaminant sources, in addition to a vertical decomposition. A mixing model approach was applied to the Ratier peri-urban sub-catchment of the OTHU Yzeron observatory. Eight sources were identified and sampled, corresponding to different land uses (e.g. forest, grassland, breeding), hydrological compartments (e.g. aquifer) and urban point discharges (e.g. sewer system, urban and road surface runoff). A wide range of biogeochemical parameters were analysed including classical (i.e., major chemical compounds, dissolved metals) and innovative tracers (i.e., dissolved organic matter characteristics, microbial indicators). A Bayesian mixing model method was used to decompose streamwater compositions sampled at the outlets of two sub-catchments, under contrasted hydro-meteorological conditions. Results showed distinct biogeochemical signatures mostly linked to the land-use and the geological compartments. The estimated contributions were contrasted and strongly influenced by the hydro-meteorological conditions. The inferred contributions were used to improve an existing perceptual hydrological model of the Ratier and Mercier catchments, at the hillslope scale. This confirmed the potential of biogeochemical data to discriminate spatial runoff-generating sources according to land use, in addition to a more traditional vertical decomposition.