Links of ground- and streamwater in discrete riparian inflow points in boreal Sweden – DOC mobilisation and the role of reducing conditions during snowmelt and summer

Mobilisation of dissolved organic carbon (DOC) links fluxes from terrestrial ecosystems via streams to the oceans. The increase in mobilisation that has been observed as a browning of headwaters during the last decades, resulted in ecosystem change of receiving waters and had implications for drinking water production and carbon storage. Riparian soils at the groundwater/ surface water interface are hotspots of biogeochemical transformations shaping water entering the streams. Preferential flow paths, where larger areas of the watershed drain through a distinct point to the stream, have been described as discrete riparian inflow points (DRIP). DRIPs have high watertables, mostly organic soils and strongly influence stream discharge and chemistry. They have been identified as major sources of DOC to streams, making them key areas for studying DOC mobilisation mechanisms. High watertables connect highly conductive and organic rich top soil layers to streams, but also influence redox conditions in the ground. If oxygen and nitrate availability decreases, ferric iron gets reduced and could release DOC previously bound to iron (oxy) hydroxides. Reduction processes consume protons and thus increase pH, in turn increasing solubility for negatively charged organics.

We hypothesized that redox induced mobilisation of DOC plays an important role especially after drying and rewetting cycles occuring after warm and dry summers with the onset of late summer rains. During snowmelt, we hypothesized redox induced mobilisation to be less important due to cold conditions and a large fraction of surficial flow paths. In this study, data from sampling campaigns in a small forested headwater stream with adjacent riparian wetlands (DRIPs) located in the Krycklan Catchment Study in boreal Sweden, conducted during snowmelt 2024 and two late summer seasons in 2023 and 2024, is presented. Samples were analysed for DOC quantity and quality, iron speciation and concentration, oxygen saturation and pH, among others. We show that stream- and groundwater have distinct chemical properties. The role of riparian soils as source areas of solutes differs between seasons with a more diluting effect during peak discharge at snowmelt and concentrations being transport limited in summer and autumn. In groundwater, DOC and iron are co-mobilised with higher concentrations under reducing conditions. Oxygen saturation changes with watertables depending on whether they exceed ground level, resulting in different effects of watertable changes depending on small scale topography. We find some indication of DOC mobilisation due to redox induced pH increase in some DRIPs especially during snowmelt. DOC concentrations are higher pre- and during early snowmelt in the stream, maybe due to release of older, more reduced groundwater before the diluting effect of freshly melted snow dominates.

In conclusion ground- and streamwater chemistry relate differently dependent on season. Small scale topography results in non-uniformal effects of elevated watertables and thus groundwater chemistry is to some degree site specific. However, iron and DOC are jointly mobilised especially under low oxygen availability. In spring water that might have been subject to reducing conditions in late autumn, might still be present in the groundwater and could be released early on during snowmelt.