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Groundwater Carbon Within a Boreal Catchment: Spatiotemporal Variability of a Hidden Aquatic Carbon Pool
Author(s) -
Nydahl Anna C.,
Wallin Marcus B.,
Laudon Hjalmar,
Weyhenmeyer Gesa A.
Publication year - 2020
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1029/2019jg005244
Subject(s) - dissolved organic carbon , groundwater , environmental chemistry , weathering , surface water , carbon fibers , fractionation , isotopes of carbon , chemistry , total inorganic carbon , carbon dioxide , hydrology (agriculture) , environmental science , soil science , total organic carbon , geology , geochemistry , environmental engineering , materials science , geotechnical engineering , organic chemistry , composite number , composite material
Groundwater is an essential resource providing water for societies and sustaining surface waters. Although groundwater at intermediate depth could be highly influential at regulating lake and river surface water chemistry, studies quantifying organic and inorganic carbon (C) species in intermediate depth groundwater are still rare. Here, we quantified dissolved and gaseous C species in the groundwater of a boreal catchment at 3‐ to 20‐m depth. We found that the partial pressure of carbon dioxide ( p CO 2 ), the stable carbon isotopic composition of dissolved inorganic carbon (δ 13 C‐DIC), and pH showed a dependency with depth. Along the depth profile, a negative relationship was observed between p CO 2 and δ 13 C‐DIC and between p CO 2 and pH. We attribute the negative p CO 2 ‐pH relationship along the depth gradient to increased silicate weathering and decreased soil respiration. Silicate weathering consumes carbon dioxide (CO 2 ) and release base cations, leading to increased pH and decreased p CO 2 . We observed a positive relationship between δ 13 C‐DIC and depth, potentially due to diffusion‐related fractionation in addition to isotopic discrimination during soil respiration. Soil CO 2 may diffuse downward, resulting in a fractionation of the δ 13 C‐DIC. Additionally, the dissolved organic carbon at greater depth may be recalcitrant consisting of old degraded material with a greater fraction of the heavier C isotope. Our study provides increased knowledge about the C biogeochemistry of groundwater at intermediate depth, which is important since these waters likely contribute to the widespread CO 2 oversaturation in boreal surface waters.