skip to main content

Title: Impacts of Evaporation‐Induced Groundwater Upwelling on Mixing Dynamics in Shallow Wetlands

Groundwater mixing dynamics play a crucial role in the biogeochemical cycling of shallow wetlands. In this paper, we conducted groundwater simulations to investigate the combined effects of evaporation and local heterogeneity on mixing dynamics in shallow wetland sediments. The results show that evaporation causes groundwater and solutes to upwell from deep sediments to the surface. As the solute reaches the surface, evaporation enhances the accumulation of the solute near the surface, resulting in a higher solute concentration than in deep sediments. Mapping of flow topology reveals that local heterogeneity generates spatially varied mixing patterns mainly along preferential flow pathways. The upwelling of groundwater induced by surface evaporation through heterogeneous sediments is likely to create distinct mixing hotspots that differ spatially from those generated by lateral preferential flows driven by large‐scale hydraulic gradients, which enhances the overall mixing in the subsurface. These findings have strong implications for biogeochemical processing in wetlands.

more » « less
Award ID(s):
Author(s) / Creator(s):
 ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    A density‐dependent, variably saturated groundwater flow and solute transport model was used to investigate the influence of swash motions on subsurface flow and moisture dynamics in beach aquifers with heterogeneous distributions of hydraulic conductivity (K) and capillarity. The numerical simulations were performed within a Monte Carlo framework using field measurements conducted in the swash zone of a sandy beach. Our results show that heterogeneous capillarity causes spatially variable capillary rise above the groundwater table. In response to swash motions, heterogeneity creates capillary barriers that result in pockets of elevated moisture content beneath the swash zone. These moisture hotspots persist within the unsaturated zone even at ebb tide when the swash motions recede seaward. Heterogeneous capillarity also results in highly tortuous preferential flow paths and alters the flow rates from the sand surface to the water table. HeterogeneousKgreatly enhances the seawater infiltration into the swash zone and modulates its spatial distribution along the beach surface. Due to heterogeneousKand capillarity, complex mixing patterns emerge. Both strain‐dominated and vorticity‐dominated flow regions develop and dissipate as tides and waves move across the beach surface. Complex mixing patterns of seawater percolating from the swash zone surface to the water table, with localized areas of high and low mixing intensities, are further demonstrated by analysis of dilution index. Our findings reveal the influence of geologic heterogeneity on swash zone moisture and flow dynamics, which may have important implications for sediment transport and chemical processing in beach aquifers.

    more » « less
  2. Abstract

    Riverbank groundwater discharge faces are spatially extensive areas of preferential seepage that are exposed to air at low river flow. Some conceptual hydrologic models indicate discharge faces represent the spatial convergence of highly variable age and length groundwater flowpaths, while others indicate greater consistency in source groundwater characteristics. Our detailed field investigation of preferential discharge points nested across mainstem riverbank discharge faces was accomplished by: (1) leveraging new temperature‐based recursive estimation (extended Kalman Filter) modelling methodology to evaluate seasonal, diurnal, and event‐driven groundwater flux patterns, (2) developing a multi‐parameter toolkit based on readily measured attributes to classify the general source groundwater flowpath depth and flowpath length scale, and, (3) assessing whether preferential flow points across discharge faces tend to represent common or convergent groundwater sources. Five major groundwater discharge faces were mapped along the Farmington River, CT, United States using thermal infrared imagery. We then installed vertical temperature profilers directly into 39 preferential discharge points for 4.5 months to track vertical discharge flux patterns. Monthly water chemistry was also collected at the discharge points along with one spatial synoptic of stable isotopes of water and dissolved radon gas. We found pervasive evidence of shallow groundwater sources at the upstream discharge faces along a wide valley section with deep bedrock, as primarily evidenced by pronounced diurnal discharge flux patterns. Discharge flux seasonal trends and bank storage transitions during large river flow events provided further indication of shallow, local sources. In contrast, downstream discharge faces associated with near surface cross cutting bedrock exhibited deep and regional source flowpath characteristics such as more stable discharge patterns and temperatures. However, many neighbouring points across discharge faces had similar discharge flux patterns that differed in chloride and radon concentrations, indicating the additional effects of localized flowpath heterogeneity overprinting on larger scale flowpath characteristics.

    more » « less
  3. Abstract

    Considering heterogeneity in porous media pore size and connectivity is essential to predicting reactive solute transport across interfaces. However, exchange with less‐mobile porosity is rarely considered in surface water/groundwater recharge studies. Previous research indicates that a combination of pore‐fluid sampling and geoelectrical measurements can be used to quantify less‐mobile porosity exchange dynamics using the time‐varying relation between fluid and bulk electrical conductivity. For this study, we use macro‐scale (10 s of cm) advection–dispersion solute transport models linked with electrical conduction in COMSOL Multiphysics to explore less‐mobile porosity dynamics in two different types of observed sediment water interface porous media. Modeled sediment textures contrast from strongly layered streambed deposits to poorly sorted lakebed sands and cobbles. During simulated ionic tracer perturbations, a lag between fluid and bulk electrical conductivity, and the resultant hysteresis, is observed for all simulations indicating differential loading of pore spaces with tracer. Less‐mobile exchange parameters are determined graphically from these tracer time series data without the need for inverse numerical model simulation. In both sediment types, effective less‐mobile porosity exchange parameters are variable in response to changes in flow direction and fluid flux. These observed flow‐dependent effects directly impact local less‐mobile residence times and associated contact time for biogeochemical reaction. The simulations indicate that for the sediment textures explored here, less‐mobile porosity exchange is dominated by variable rates of advection through the domain, rather than diffusion of solute, for typical low‐to‐moderate rate (approximately 3–40 cm/day) hyporheic fluid fluxes. Overall, our model‐based results show that less‐mobile porosity may be expected in a range of natural hyporheic sediments and that changes in flowpath orientation and magnitude will impact less‐mobile exchange parameters. These temporal dynamics can be assessed with the geoelectrical experimental tracer method applied at laboratory and field scales.

    more » « less
  4. Abstract

    Nitrous oxide (N2O) is a potent ozone‐depleting greenhouse gas produced by incomplete denitrification. Recent works on riverine N2O emissions focus mainly on contributions from in‐channel, benthic, and fluvial hyporheic environments under assumptions of steady‐state conditions and homogeneous sediment hydraulic conductivity (K). However, riparian floodplains are also a potentially important N2O source characterized by complex sediment heterogeneity and dynamic surface and groundwater interactions. We use numerical flow and reactive transport models to investigate the influence of complex sedimentary architecture and high‐flow events (e.g., storms) on N2O production. We interpret the correlation between flow and solute fields with the flow topological Okubo‐Weiss metric (OW) and the scalar dissipation rate weighted by soil organic matter (OM) fraction and soil saturation. We model a heterogeneous riparian floodplain based on field observations from the Theis Environmental Monitoring and Modeling Site, Ohio, USA. N2O production is greatest within intermediate‐Ksediments (e.g., sands) where denitrification rates are highest, and emissions increase by more than an order of magnitude during storms. Sensitivity analysis reveals that the denitrification rate is most influential for N2O flux, accounting for nearly 46% of the variance in production rates. Denitrification rates adapt to spatial changes in the flow topology (measured by OW) related to sediment heterogeneity and are strongly influenced by subsurface mixing dynamics. Mixing is greatest in shear strain‐dominated regions, while vorticity promotes OM dissolution and prolongs residence times. Accurate lithologic representation is imperative for characterizing subsurface N2O production dynamics, especially given growing concern regarding climate change driven hydrologic changes within watersheds worldwide.

    more » « less
  5. Abstract

    Sediment cores were collected under ice‐cover in late winter from three wetlands located along a subsurface hydrologic gradient within the Prairie Pothole Region of North America. Within each core, sediment porewaters were analyzed byin situvoltammetry for a suite of redox active species as a function of depth and revealed shifts in complex oxidation‐reduction dynamics related to ice cover in these wetlands. We observed a reduced sulfur boundary that is close to or above the sediment‐water interface (SWI) under ice cover. In contrast, the reduced sulfur boundary retreats several centimeters deeper in the sediments under ice‐free conditions. These findings are analogous to previous observations in shallow lakes that show anoxia at the SWI during ice cover but not under ice‐free conditions. Further, biogeochemical processes varied depending upon wetland type. During winter, sulfide levels in sediment porewaters in groundwater fed “flow‐through” wetlands were significantly lower than under ice‐free conditions. The converse applied to groundwater discharge wetlands where reduced sulfur concentrations in porewaters increased under ice cover. Decreases in ice cover extent and duration due to climate change coupled with profound landscape changes due to agriculture will affect the biogeochemical cycles of these wetlands and could lead to increased carbon emissions in the future.

    more » « less