Examining suspended sediment sources and dynamics during flood events in a drained catchment using radiogenic strontium isotope ratios (87Sr/86Sr)

Abstract

Soil erosion is recognized as one of the main processes of land degradation in agricultural areas. High suspended sediment loads, often generated from eroding agricultural landscapes, are known to degrade downstream environments. Accordingly, there is a need to identify suspended sediment sources and to investigate their dynamics. Here, soil and sediment strontium isotopic ratios were used to examine suspended sediment sources and dynamics in a lowland drained catchment in France.Suspended sediment (n=14) was collected in stream and at tile drain (n=4) outlets during three flood events between December 2013 and February 2014. Potential source soils (n=28) representative of the carbonate and silicate substrates found in the catchment were sampled and analyzed. Strontium isotopic ratios (87Sr/86Sr) were measured in different particle size fractions (<2μm, <63μm and <2mm).Soil 87Sr/86Sr ratios significantly discriminated between source samples classified as carbonate (0.712625 to 0.717815) and silicate (0.719287 to 0.724631) soils. 87Sr/86Sr ratios measured in suspended sediment (0.713660 to 0.720571) reflect variations in source contributions during different hydrological conditions. The 87Sr/86Sr ratios varied in the different fractions of soil samples (i.e. <2μm, <63μm and <2mm) and in suspended sediment samples. Suspended sediment was interpreted as a mixture of two end-members, with the dominant contribution of the <2μm fraction of soil samples occurring during small scale flood events and the <2mm fraction during more intense flood events. Modelling results indicate that carbonate source soil contributions are variable, with their highest contributions occurring during the two first flood events compared to the last flood event.The results also show that the tile drainage system enhances the connectivity between cultivated hillslopes and the river network, providing a preferential pathway for fine (<2μm) particles. This study demonstrates the potential of using strontium isotopic ratios to examine the variations of suspended sediment sources in drained catchments with contrasting carbonate and silicate sources. More importantly, the results highlight the need to improve the management of sediment exported from tile drains in similar agricultural environments, as they were demonstrated to transfer very fine material to the riverine environment.