Elucidating source areas of in-stream DOC by characterizing stream-groundwater exchange in a low mountain forested headwater catchment

Export of carbon from terrestrial catchments remains a poorly understood flux within the global carbon budget. In order to better understand the release and in-stream transport of dissolved organic carbon (DOC) in a small forested headwater catchment within the Bavarian Forest National Park (Germany), we characterized the stream-groundwater exchange along the headwater stream. We divided a 3000 m long stretch of the headwater stream into three topographically delineated sections that consisted of a steep upstream section (length 620m, elevations 888-967 m a.s.l.), a transition section (length 770 m, elevations 805-855 m a.s.l.) and a flat and wide downstream section (length 1330 m, elevations 770-805 m a.s.l., outlet of headwater catchment). Using sequential tracer injections of known masses of sodium chloride, we determined stream discharge and lateral exchange fluxes between stream and the surrounding riparian zone in the three stream sections and evaluated the effects of the resulting hydrologic turnover on stream water composition. We also compared the calculated lateral exchange fluxes with previously measured longitudinal profiles of Radon activities that can be used to locate groundwater inflow points. Discharge increased over the investigated 3000 m stretch in downstream direction, as expected, although the transition section did not show any change in discharge. The analysis of recovered tracer masses revealed that in the steep upstream section, the exchange fluxes consisted mainly of inflow of water into the stream. The transition section was characterized by an absence of exchange fluxes. In the downstream section, however, large inflows were offset by slightly lower outflows, resulting in a very pronounced exchange between stream water and riparian zone groundwater in this flat valley bottom. The spatial pattern of exchange fluxes was supported by the longitudinal Radon profiles, which pointed to a marked groundwater inflow in the downstream section. Our results suggest that the dominant source area of in-stream DOC at the outlet of our headwater catchment is the flat and wide valley bottom in the downstream section as most of the DOC released into the stream along the steep upstream section will be removed from the stream during the passage through the downstream section.