Elucidating sources and pathways of dissolved organic carbon in a Bavarian Forest headwater catchment - A qualitative assessment of stream, soil and shallow groundwater

Dissolved organic carbon (DOC) constitutes the biggest portion of carbon that is exported from soils. During the last decades, widespread increases in DOC concentrations of surface waters have been observed, affecting ecosystem functioning and drinking water treatment. However, hydrological controls of DOC mobilization and source areas within catchments are still poorly understood.

From May to October 2020, we sampled two different topographical positions within a Bavarian Forest headwater catchment, one in its flat downstream part (770 m.a.s.l.) and one in its steep upstream part (880 m.a.s.l.), using piezometers, pore water samplers (peepers) and in-stream spectrometric devices. We analyzed the concentrations of DOC and several ions as well as DOC absorbance (A₂₅₄/A₃₆₅ and SUVA₂₅₄) and fluorescence characteristics (fluorescence and freshness indices) of soil water, shallow groundwater and stream water to better understand the DOC source areas during baseflow and during precipitation events. In addition to snapshot sampling data in an interval of three weeks, continuous stream DOC data were collected.

High DOC concentrations (up to 80 mg/l) were found in soil water from cascading sequences of small ponds in the flat downstream part of the catchment that fill up temporarily. Increasing stream DOC concentrations during events were accompanied by changing DOC characteristics at both locations, pointing to a change of flow pathways during events. In the flat downstream sampling location, stream DOC freshness values and SUVA₂₅₄ as well as aluminium and nitrate concentrations were approaching the values found in the DOC-rich small ponds, suggesting these ponds as important DOC sources during precipitation events.