Seasonal Variations of Redox State in Hemiboreal Soils Indicated by Changes of δ56Fe, Sulfate, and Nitrate in Headwater Streams

Document identifier: oai:DiVA.org:ltu-77862
Access full text here:10.1021/acsearthspacechem.9b00237
Keyword: Natural Sciences, Earth and Related Environmental Sciences, Geochemistry, Naturvetenskap, Geovetenskap och miljövetenskap, Geokemi, Runoff generation, Summer drought, Wetlands, Iron isotope ratios, PH, TOC, Water chemistry, Applied Geochemistry, Tillämpad geokemi
Publication year: 2019
Relevant Sustainable Development Goals (SDGs):
SDG 15 Life on landSDG 6 Clean water and sanitation
The SDG label(s) above have been assigned by OSDG.ai

Abstract:

During recent decades, much focus has been put on the iron (Fe) isotope ratios in soils, rivers, and oceans, while studies on the variation in headwater streams are scarce. Here we assess seasonal water chemical data from 104 hemiboreal headwater streams. Between summer and late autumn, decreasing Fe concentrations and simultaneously increasing sulfate and nitrate concentrations suggest a shift from reduced to oxidized conditions in the soils along the main groundwater flow paths. Fe isotope data, obtained from a subpopulation of 16 streams, show low δ56Fe ratios during summer drought, indicating an important influx of reduced groundwater to the streams with primarily Fe(II) as an important Fe source. In total, the δ56Fe data ranged between −0.8 ± 0.1 and 1.8 ± 0.1‰ with the lowest values in summer and maximum δ56Fe ratios in late autumn or spring, indicating an influx of more oxidized, less Fe(II) rich groundwater during those seasons. Local differences in δ56Fe ratios between the headwater streams, seemed to be driven by the different soil redox status of the catchments. The streams with the lowest δ56Fe ratios during summer are characterized by a small share (4.4 ± 6.6%) of wetlands, indicating discharge of reduced groundwater from mainly anoxic, moist, organic-rich mineral soils during drought. Relatively high total organic carbon (TOC) concentrations (2.4 ± 1.1 mM) and low pH (5.2 ± 0.8) may have restricted efficient Fe(II) oxidation in streamwater especially during the late autumn survey. Our results from hemiboreal headwater streams reveal the importance of climatic, pedogenic, and land cover-derived controls on the provenance of stream Fe loads that is likely broadly applicable to similar streams elsewhere.

Authors

Sarah Conrad

Luleå tekniska universitet; Geovetenskap och miljöteknik
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Stefan Löfgren

Department of Aquatic Sciences and Assessment; Section for Geochemistry and Hydrology, Swedish University of Agricultural Sciences, Uppsala, Sweden
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Susanne Bauer

Luleå tekniska universitet; Geovetenskap och miljöteknik
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Johan Ingri

Luleå tekniska universitet; Geovetenskap och miljöteknik
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