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Creators/Authors contains: "Geng, Xiaolong"

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  1. Free, publicly-accessible full text available November 1, 2024
  2. Free, publicly-accessible full text available October 1, 2024
  3. Abstract

    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.

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  4. Abstract

    Preferential flow can result in rapid contamination of groundwater resources. This is particularly true in aquifers with connected, high permeability geologic structures and in coastal systems where the oceanic source of contamination is ubiquitous. We consider saltwater intrusion due to pumping in volcanic aquifers with lava tubes represented as connected high‐K structures and compare salinization responses to those of heterogeneous aquifers with different structure and equivalent homogeneous systems. Three‐dimensional simulations of variable‐density groundwater flow and salt transport show that conduits formed by lava flows create preferential groundwater flow in volcanic aquifers. These conduits allow fresh groundwater to extend further offshore than in other systems. However, onshore pumping causes saltwater to migrate landward quickly through the conduits relative to the other models, resulting in more severe saltwater intrusion, particularly at shallow depths. The geometry of geologic heterogeneity in volcanic aquifers leads to increased risk of salinization of fresh groundwater as well as substantial uncertainty due to significant spatial variation in saltwater intrusion. The findings illustrate the importance of considering geologic heterogeneity in assessing the vulnerability of coastal freshwater resources in volcanic and other aquifers with connected high‐permeability geologic structures.

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  5. Abstract

    The intertidal zone of beach aquifers hosts biogeochemical transformations of terrestrially derived nutrients that are mediated by reactive organic carbon from seawater infiltration. While dissolved organic carbon is often assumed the sole reactive organic carbon component, advected and entrapped particulate organic carbon (POC) is also capable of supporting chemical reactions. Retarded advection of POC relative to groundwater flow forms pools of reactive carbon within beach sediments that support biogeochemical reactions as dissolved solutes move across them due to transient groundwater flow. In this work, we simulate the contribution of POC to beach reactions and identify parameters that control its relative contribution using a groundwater flow model (SEAWAT) and reactive transport model (PHT3D). Results show transient contributions of POC to denitrification, as the spatial extent of the saline circulation cell varies over time due to changing hydrologic factors. A decrease in POC retardation and an increase in tidal amplitude during POC deposition resulted in POC expansion, which increased the relative contributions of POC to beach reactivity. Decreased hydraulic conductivity and increased tidal amplitude post‐deposition decreased the utilization of POC for denitrification by allowing the oxic, saline water to completely encompass the pool of POC. Results highlight that POC is an intermittently utilized source of carbon that displays complex spatial relationships with groundwater flow conditions and overall beach biogeochemistry. This work demonstrates that POC may be a periodically important but overlooked contributor to biogeochemical reactions in carbon‐poor beach aquifers.

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  6. 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.

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  7. Abstract

    Intertidal aquifers are hotspots of biogeochemical cycling where nutrients and contaminants are processed prior to discharge to the ocean. The nature of the dynamic subsurface mixing zone is a critical control on mitigating reactions. Simulation of density‐dependent, variably saturated flow and salt transport incorporating realistic representations of aquifer heterogeneity was conducted within a Monte Carlo framework to investigate influence of nonuniform permeability on intertidal groundwater flow and salt transport dynamics. Results show that heterogeneity coupled with tides creates transient preferential flow paths within the intertidal zone, evolving multiple circulation cells and fingering‐type salinity distributions. Due to heterogeneity, strain‐dominated (intense mixing) and vorticity‐dominated (low mixing) flow regions coexist at small spatial scales, and their spatial extent reaches peaks at high tide and low tide. Such topological characteristics reveal complex tempo‐spatial mixing patterns for intertidal flow with localized areas of high and low mixing intensities, which have implications for intertidal biogeochemical processing.

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