Perennial flow through convergent hillslopes explains chemodynamic solute behavior in a shale headwater catchment

Elizabeth M. Herndon, Grit Steinhoefel, Ashlee L.D. Dere, Pamela L. Sullivan

Research output: Contribution to journalArticlepeer-review

26 Scopus citations

Abstract

Stream chemistry reflects the mixture of complex biogeochemical reactions that vary across space and time within watersheds. For example, streams experience changing hydrologic connectivity to heterogeneous water sources under different flow regimes; however, it remains unclear how seasonal flow paths link these different sources and regulate concentration-discharge behavior, i.e., changes in stream solute concentration as a function of discharge. At the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in central Pennsylvania, USA, concentrations of chemostatic solutes (K, Mg, Na, Si, Cl) vary little across a wide range of discharge values while concentrations of chemodynamic solutes (Fe, Mn, Ca) decrease sharply with increasing stream discharge. To elucidate controls on chemodynamic solute behavior, we investigated the chemistry of surface water and shallow subsurface water at the SSHCZO in early autumn when discharge was negligible and concentrations of chemodynamic solutes were high. Dissolved ions, colloids, and micron-sized particles were extracted from hillslope soils and stream sediments to evaluate how elements were mobilized into pore waters and transported from hillslopes to the stream. During the study period when flow was intermittent, the stream consisted of isolated puddles that were chemically variable along the length of the channel. Inputs of subsurface water to the stream were limited to an area of upwelling near the stream headwaters, and the water table remained over a meter below the stream bed along the rest of the channel. Chemodynamic elements Fe and Mn were preferentially mobilized from organic-rich soils as a mixture of dissolved ions, colloids, and micron-sized particles; consequently, subsurface water draining organic-rich soils in the upper catchment was enriched in Fe and Mn. Conversely, Ca increased towards the catchment outlet and was primarily mobilized from stream sediments as Ca2+. Concentrations of chemostatic solutes were relatively invariable throughout the catchment. We conclude that chemodynamic behavior at SSHCZO is driven by seasonally variable connectivity between the stream and hillslope soils. During the dry season, stream water derives from a shallow perched water table (interflow) that upwells to generate metal-rich stream headwaters. High concentrations of soluble Fe and Mn at low discharge occur when metal-rich headwaters are flushed to the catchment outlet during periodic rain events. Interflow during the dry season originates from water that infiltrates through organic-rich swales; thus, metals in the stream at low flow are ultimately derived from convergent hillslopes where biological processes have concentrated and/or mobilized these chemodynamic elements. In contrast, high concentrations of Ca2+ at low discharge are likely mobilized from stream sediments that contain secondary calcite precipitates. We infer that chemodynamic solutes are diluted at high discharge primarily due to increased flow through planar hillslopes. This study highlights how spatially heterogeneous biogeochemistry and seasonally variable flow paths regulate concentration-discharge behavior within catchments.

Original languageEnglish
Pages (from-to)413-425
Number of pages13
JournalChemical Geology
Volume493
DOIs
StatePublished - Aug 20 2018
Externally publishedYes

Funding

Research was supported by subaward 5058-KSU-NSF-1726 (E. Herndon) from NSF EAR-1331726 (S. Brantley) for the Susquehanna Shale Hills Critical Zone Observatory. Logistical support and/or data were provided by the NSF-supported Susquehanna Shale Hills Critical Zone Observatory. This research was conducted in Penn State's Stone Valley Forest, which is funded by the Penn State College of Agriculture Sciences, Department of Ecosystem Science and Management and managed by the staff of the Forestlands Management Office. The Fall 2014 Hydrogeochemistry class in the Department of Geology at Kent State University is acknowledged for surface and groundwater collection and analysis.

FundersFunder number
Department of Ecosystem Science and Management
NSF-supported
Penn State College of Agriculture Sciences
National Science Foundation1331726
Department Ecosystem Science and Management, Texas A and M University
National Science FoundationEAR-1331726

    Keywords

    • Colloids
    • Concentration-discharge behavior
    • Interflow
    • Metals
    • Surface-groundwater interactions

    Fingerprint

    Dive into the research topics of 'Perennial flow through convergent hillslopes explains chemodynamic solute behavior in a shale headwater catchment'. Together they form a unique fingerprint.

    Cite this