Are dissolved organic carbon concentrations in riparian groundwater linked to hydrological pathways in the boreal forest?

• Published in: Hydrology and earth system sciences Vol. 24; no. 4; pp. 1709 - 1720
• Main Authors: Ploum, Stefan W, Laudon, Hjalmar, Peralta-Tapia, Andrés, Kuglerová, Lenka
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 09-04-2020; Copernicus Publications
• Subjects: Banks (Finance); Biogeochemistry; Boreal forests; Buffers (chemistry); Carbon; Carbon content; Catchment area; Catchments; Chemistry; Dissolved organic carbon; Distance; Ecosystems; Electrical conductivity; Electrical resistivity; Environmental changes; Environmental management; Forest management; Forest Science; Forests; Geologic depressions; Groundwater; Groundwater chemistry; Groundwater discharge; Groundwater hydrology; Headwater catchments; Headwaters; Hydrology; Inflow; Oceanografi, hydrologi, vattenresurser; Oceanography, Hydrology, Water Resources; Organic carbon; Questions; Riparian forests; Riparian land; Riparian waters; Riparian zone; Riverbanks; Rivers; Runoff; Seasonal variations; Seasonality; Skogsvetenskap; Soil; Soil carbon; Soil chemistry; Soils; Spatial variability; Spatial variations; Strategic management; Stream banks; Stream flow; Streams; Sustainable forestry; Terrestrial environments; Topography; Variability; Vegetation; Water
• ISSN: 1607-7938; 1027-5606; 1607-7938
• DOI: 10.5194/hess-24-1709-2020

The riparian zone (RZ), or near-stream area, plays a fundamental role in the
biogeochemistry of headwaters. Here, wet, carbon-rich soils can change
groundwater chemistry before it enters the stream. In the boreal forest, the RZ plays an especially important role in the export of dissolved organic carbon (DOC) to streams. However, the RZ is not uniform, and spatial variability of riparian groundwater hydrology and chemistry can be large. Terrestrial topographic depressions create hydrological pathways towards focal points in the RZ, which we refer to as “discrete riparian inflow points” (DRIPs). Combining the chemical function of the RZ and the convergence of hydrological pathways, we hypothesize that DRIPs play a disproportionally large role in conveying DOC to small streams. Earlier work has demonstrated that runoff from DRIPs can make up the majority of riparian flow contributions to streams, but it is currently unknown how their groundwater chemistry differs from the rest of the RZ. Therefore, we ask the following question: are DOC concentrations in
riparian groundwater linked to hydrological pathways in the boreal forest? To answer this question, we sampled riparian groundwater during six campaigns across three boreal headwater streams in Sweden. The groundwater wells were distributed into 10 DRIP and non-DRIP pairs (60 wells), following transects from the upland (20 m lateral distance from the stream bank) to the near-stream area (<5 m lateral distance from the stream bank). The variability in DOC, pH, and electrical conductivity (EC) was analyzed using linear mixed-effects models (LMMs). We explained the variability using three factors: distance from the stream, seasonality, and DRIP/non-DRIP. Our
results showed that DRIPs provided DOC-rich water (34 mg L−1) with relatively low EC (36 µS cm−1). The “non-DRIP” riparian water had 40 % lower DOC concentrations (20 mg L−1) and a 45 % higher EC (52 µS cm−1) on average. Moreover, groundwater chemistry from DRIPs was spatially and temporally relatively homogeneous. In contrast, non-DRIP water transformed distinctly in the last 25 m towards the stream, and the chemical variability was also larger between seasons. We concluded that hydrological pathways and spatial variability in riparian groundwater DOC concentrations are linked, and that DRIPs can be seen as important control points in the boreal landscape. Characterizing DRIPs in headwater catchments can be useful for upscaling carbon inputs in boreal stream ecosystems and for delineating hydrologically adapted buffers for forest management practices.