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 delta Fe-56 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 delta Fe-56 data ranged between -0.8 +/- 0.1 and 1.8 +/- 0.1 parts per thousand with the lowest values in summer and maximum delta Fe-56 ratios in late autumn or spring, indicating an influx of more oxidized, less Fe(II) rich groundwater during those seasons. Local differences in delta Fe-56 ratios between the headwater streams, seemed to be driven by the different soil redox status of the catchments. The streams with the lowest delta Fe-56 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.

Keywords

Runoff generation; summer drought; wetlands; iron isotope ratios; pH; TOC; water chemistry

Published in

ACS Earth and space chemistry
2019, Volume: 3, number: 22, pages: 2816-2823

SLU Authors

• Löfgren, Stefan

  • Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences
    

Associated SLU-program

Forest
Acidification
Use of FOMA data


Sustainable Development Goals

Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss


UKÄ Subject classification

Geochemistry
Oceanography, Hydrology, Water Resources
Environmental Sciences


Publication identifier

DOI: https://doi.org/10.1021/acsearthspacechem.9b00237

Permanent link to this page (URI)

https://res.slu.se/id/publ/103073