Geochemical Exports to River From the Intrameander Hyporheic Zone Under Transient Hydrologic Conditions: East River Mountainous Watershed, Colorado

Abstract

To understand how redox processes influence carbon, nitrogen, and iron cycling within the intrameander hyporheic zone, we developed a biotic and abiotic reaction network and incorporated it into the reactive transport simulator PFLOTRAN. Two‐dimensional reactive flow and transport simulations were performed (1) to evaluate how transient hydrological conditions control the lateral redox zonation within an intrameander region of the East River in Colorado and (2) to quantify the impact of a single meander on subsurface exports of carbon and other geochemical species to the river. The meander's overall contribution to the river was quantified by integrating geochemical outfluxes along the outside of the meander bend. The model was able to capture the field‐observed trends of dissolved oxygen, nitrate, iron, pH, and total inorganic carbon along a 2‐D transect. Consistent with field observations, simulated dissolved oxygen and nitrate decreased along the intrameander flow paths while iron (Fe2+) concentration increased. The simulation results further demonstrated that the reductive potential of the lateral redox zonation was controlled by groundwater velocities resulting from river stage fluctuations, with low‐water conditions promoting reducing conditions. The sensitivity analysis results showed that permeability had a more significant impact on biogeochemical zonation compared to the reaction pathways under transient hydrologic conditions. The simulation results further indicated that the meander acted as a sink for organic and inorganic carbon as well as iron during the extended baseflow and high‐water conditions; however, these geochemical species were released into the river during the falling limb of the hydrograph.

Hyporheic Zone Microbiome Assembly Is Linked to Dynamic Water Mixing Patterns in Snowmelt‐Dominated Headwater Catchments

“…Previous work on the lateral distribution of microbial respiration rates also found that DO and nutrient concentrations influence spatial and temporal “hot spots” of biogeochemical activity within the hyporheic zone (King et al, ; Reeder et al, ). In contrast to the vertical exchange measured here at the East River site, the introduction and rapid depletion of DO along lateral hyporheic flow paths through the floodplain have also been modeled at East River, indicating that complete oxygen consumption occurs over ~1‐m distances into the floodplain (Dwivedi et al, ).…”

Shale as a Source of Organic Carbon in Floodplain Sediments of a Mountainous Watershed

“…Cores were collected from the cut bank (upstream location; MOP‐1, MCP‐1), middle (MOP‐2, MCP‐3), and point bar (downstream location; MOP‐3, MCP‐5) in both meanders and a wetland section of the oxbow located in the former river channel (MOP‐4). The official SFA project designations (numbers) are included here to allow the reader to reference other data and information from these locations that may be available currently (e.g., Dwivedi et al, ) or in the future; the reader is referred to the SFA website (watershed.lbl.gov) for current data availability and publications. GPS coordinates for all sampling sites can be found in Table S1 in the supporting information.…”

“…The location of the hillslope shale core (PLM-6) is also indicated. from these locations that may be available currently (e.g., Dwivedi et al, 2018) or in the future; the reader is referred to the SFA website (watershed.lbl.gov) for current data availability and publications. GPS coordinates for all sampling sites can be found in Table S1 in the supporting information.…”

Evidence of Spatio‐Temporal Variations in Contaminants Discharging to a Peri‐Urban Stream

“…We suggest instead that an inter‐channel flow driven by local variations of river stage resulted in a complex contaminant transport from source zone B, located at the junction of the two channels, which otherwise seems to be small. Such flow dynamics and resulting contaminant transport is closely related to the meander‐driven hyporheic exchange, or transient hydrological conditions at the channel junction scale as described and modeled, for example, by Boano et al (), Dwivedi et al () and Han and Endreny (). To date, we could not find many other published studies describing point pollutant source dynamics with a source located in the near vicinity of a stream junction (but see Fryar et al [] for a discussion on the influence of tributary flow and interaction with a contaminant plume).…”