Scale-dependent groundwater contributions influence patterns of winter baseflow stream chemistry in boreal catchments

Abstract

Understanding how the sources of surface water change along river networks is an important challenge, with implications for soil-stream interactions, and our ability to predict hydrological and biogeochemical responses to environmental change. Network-scale patterns of stream water reflect distinct hydrological processes among headwater units, as well as variable contributions from deeper groundwater stores, which may vary nonlinearly with drainage basin size. Here we explore the spatial variability of groundwater inputs to streams, and the corresponding implications for surface water chemistry, during winter baseflow in a boreal river network. The relative contribution of recent and older groundwater was determined using stable isotopes of water (δ 18 O) at 78 locations ranging from small headwaters (0.12 km 2 ) to fourth-order streams (68 km 2 ) in combination with 79 precipitation and 10 deep groundwater samples. Results from a two end-member mixing model indicate that deeper groundwater inputs increased nonlinearly with drainage area, ranging from~20% in smaller headwater subcatchments to 70-80% for catchments with a 10.6 km 2 area or larger. Increases in the groundwater contribution were positively correlated to network-scale patterns in surface stream pH and base cation concentrations and negatively correlated to dissolved organic carbon. These trends in chemical variables are consistent with the production of weathering products and the mineralization of organic matter along groundwater flow paths. Together, the use of stable isotopes and biogeochemical markers illustrate how variation in hydrologic routing and groundwater contributions shape network-scale patterns in stream chemistry as well as patchiness in the relative sensitivity of streams to environmental change and perturbation.

Seasonal resource limitation of heterotrophic biofilms in boreal streams

“…The temperature dependence (and activation energy) of biotic reactions increases with greater molecular complexity (Davidson and Janssens ), and DOC in boreal streams is generally humic with a high‐molecular weight (Thurman ). Moreover, a reduction in the supply of freshly leached C resources may also occur during winter, when soil frost isolates riparian and upland hydrological pathways and the relative contribution of deep, older groundwater increases (Ågren et al ; Peralta‐Tapia et al ). Reduced insolation for ice‐covered streams may also eliminate the production of labile C from both photochemical degradation (Köhler et al ) and exudation from aquatic autotrophs (e.g., Baines and Pace ) during winter months.…”

From soil water to surface water – how the riparian zone controls element transport from a boreal forest to a stream

“…A prerequisite for this is that pH increases with increasing catchment area, while the TOC concentration decreases, which causes both Fe and Al to precipitate (Köh-ler et al, 2014;Lidman et al, 2014). This is in turn related to less organic matter in the riparian soils of larger streams and an increasing influence from deeper groundwater, which entirely may bypass the riparian soils (Peralta-Tapia et al, 2015;Lidman et al, 2016).…”

Spatial variability in specific discharge and streamwater chemistry during low flows: Results from snapshot sampling campaigns in eleven Swiss catchments

“…For example, Karlsen et al () showed that for subcatchments of the Krycklan basin in Sweden, specific discharge during the summer low‐flow period increased with catchment area and with the fraction of area characterized by deeper sediment deposits, but summer low flows were smaller in catchments with denser forest cover. Also, at Krycklan, Peralta‐Tapia et al () reported increasing contributions from deeper groundwater sources with increasing catchment area during winter baseflow. Similar increases in groundwater contributions with increasing catchment area during baseflow were observed in a forested catchment in Japan by Egusa, Ohte, Oda, and Suzuki () and in the southern Rocky Mountains by Frisbee, Phillips, Campbell, Liu, and Sanchez ().…”