BayCEER Workshop 2020

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

P 3.7 in Research Poster Kaleidoscope

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. But how is it mobilized and where exactly does it come from? Are there hydrological and topographic controls?

We have been sampling two different topographical positions within a Bavarian Forest headwater catchment, one in its flat downstream part and one in its steep upstream part, starting in May 2020, using piezometers, pore water samplers (peepers) and spectrometric devices. Data collection will continue until November 2020. Additional to snapshot sampling data every 3 weeks, continuous stream DOC data during rainfall events will be available. We have been analyzing DOC concentrations as well as DOC absorbance (A₂₅₄/A₃₆₅ and SUVA₂₅₄) and fluorescence (fluorescence and freshness indices) characteristics to better understand the source areas of DOC within the catchment.

High DOC concentrations (30-80 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. Stream DOC concentrations increased during events, with maximum concentrations lagging the discharge peak at the downstream sampling location. This increase was accompanied by changing DOC characteristics, for example a change to more recently produced DOC compared to base-flow, approaching DOC freshness values of the DOC-rich small ponds. Our preliminary results point to a depletion of initial DOC sources and/or a change of flow pathways during events and suggest DOC-rich small ponds as important DOC sources. Here, iron may interact in the mobilization process. DOC mobilization from upper soil layers may be dominant during events, being in line with previous studies that have described Transmissivity Feedback as an important mechanism of DOC mobilization.

Keywords: dissolved organic carbon (DOC), source area, mobilization mechanism, DOC absorbance and fluorescence, DOC-rich small ponds, Transmissivity Feedback