Carbon isotopes as tracers of dissolved organic carbon sources and water pathways in headwater catchments

Stable carbon isotopes (δ13C) are assessed in further detail for their potential to (i) trace the relationship between spatial variations in the source of dissolved organic carbon (DOC) in soils and temporal variability of both DOC concentration and composition in streams, and (ii) elucidate water pathway changes during storm events in headwater catchments. For this purpose, we investigated δ13CDOC values in a wetland soil (0–50 cm), in deep groundwater (until 6 m) and during a storm flow event with high-resolution monitoring (⩽hourly basis) in a small, lowland catchment in western France (Kervidy-Naizin catchment). The results show a combined increase of stream DOC concentration (from 4 to 14 mg L−1) and decrease of stream δ13CDOC (from −27 to −29‰) with increasing discharge, suggesting a change in DOC sources between base flow and storm flow periods. Such an interpretation is consistent with the δ13CDOC values in soils that show a 6‰ vertical variation, with δ13CDOC values of the uppermost soil horizons (0–10 cm) of the wetland domains being close to those measured in the stream channel during the ascending limb of the hydrograph. Overall, the results presented in this study are consistent with a model in which the water-table rise and wetland runoff caused by rainfall lead to a flushing of the DOC stored in the uppermost soil horizons of the wetland domains near the channel network. Subsequently, these wetland soils become the dominant DOC source during storm events (ca. 70% of the total DOC flux). In this way, the stream DOC isotopic composition reflects the combined effects of the vertical variation of soil organic matter composition as well as the changes in water routing through time. This study demonstrates the ability of the stable isotopes of carbon to serve not only as a tool for the location of stream DOC sources in landscapes but also the reconstruction of water pathways in headwater catchments.