Denitrification hotspots in intertidal mixing zones linked to geologic heterogeneity

Heiss, James W.; Michael, Holly A.; Koneshloo, Mohammad

doi:10.1088/1748-9326/ab90a6

Cited by 40 publications

(37 citation statements)

References 56 publications

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“…Indeed, Tamborski et al (2017b) observed significant radon, radium, and dissolved Mn 2+ enrichments at this depth horizon. Geologic heterogeneity can act to both modify groundwater flow paths and enhance biogeochemical transformations within the STE (Heiss et al, 2020). However, denitrification cannot explain the differences in δ 15 N-NO 3 − and δ 18 O-NO 3 − compositions (Figures 3A, 4).…”

Section: Subsurface Processes and Variabilitymentioning

confidence: 99%

Investigating Boron Isotopes for Identifying Nitrogen Sources Supplied by Submarine Groundwater Discharge to Coastal Waters

Tamborski

Brown

Bokuniewicz

et al. 2020

Front. Environ. Sci.

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Stable isotopes of oxygen, nitrogen, and boron were used to identify the sources of nitrate (NO 3 −) in submarine groundwater discharge (SGD) into a large tidal estuary (Long Island Sound, NY, United States). Potential contaminants such as manure, septic waste and fertilizer overlap in δ 15 N and δ 18 O but have been shown to have distinctive δ 11 B in non-coastal settings. Two distinct subterranean estuaries were studied with different land-use up gradient, representative of (1) mixed medium-density residential housing and (2) agriculture. These sites have overlapping δ 15 N and δ 18 O measurements in NO 3 − and are unable to discriminate between different N sources. Boron isotopes and concentrations are measurably different between the two sites, with little overlap. The subterranean estuary impacted by mixed medium-density residential housing shows little correlation between δ 11 B and [B] or between δ 11 B and salinity, demonstrating that direct mixing relationships between fresh groundwater and seawater were unlikely to account for the variability. No two sources could adequately characterize the δ 11 B of this subterranean estuary. Groundwater N at this location should be derived from individual homeowner cesspools, although measured septic waste has much lower δ 11 B compared to the coastal groundwaters. This observation, with no trend in δ 11 B with [B] indicates multiple sources supply B to the coastal groundwaters. The agricultural subterranean estuary displayed a positive correlation between δ 11 B and [B] without any relationship with salinity. Binary mixing between sea spray and fertilizer can reasonably explain the distribution of B in the agricultural subterranean estuary. Results from this study demonstrate that δ 11 B can be used in combination with δ 15 N to trace sources of NO 3 − to the subterranean estuary if source endmember isotopic signatures are wellconstrained, and if the influence of seawater on δ 11 B signatures can be minimized or easily quantified.

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Section: Subsurface Processes and Variabilitymentioning

confidence: 99%

Investigating Boron Isotopes for Identifying Nitrogen Sources Supplied by Submarine Groundwater Discharge to Coastal Waters

Tamborski

Brown

Bokuniewicz

et al. 2020

Front. Environ. Sci.

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“…Reactive transport simulations in homogeneous intertidal aquifers show that denitrification rates are highest within the freshwater-saltwater mixing zone (Heiss et al, 2017). However, observations of reaction rates in intertidal aquifers show spatial heterogeneity (Kim et al, 2017(Kim et al, , 2019, and simulations indicate that nitrate removal rate through denitrification may be 80% greater in heterogeneous aquifers compared to their homogeneous equivalents (Heiss et al, 2020). Thus, it is critical to understand both mixing in intertidal aquifers and the effects of heterogeneity in aquifer sediments.…”

Section: Introductionmentioning

confidence: 99%

Heterogeneity Affects Intertidal Flow Topology in Coastal Beach Aquifers

Geng

Michael

Boufadel

et al. 2020

Geophysical Research Letters

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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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“…Numerical simulations of water flow toward the root layer were conducted with randomized heterogeneous fields; a simple linear correlation was assumed between height of the capillary fringe and saturated K. Their results revealed that an increase in heterogeneity at the root-soil interface leads to high root water uptake rates. While a number of previous studies have explored the effects of geologic heterogeneity on flow and transport processes in coastal aquifer systems, the studies only considered spatial variability in K (Geng et al, 2020b;Heiss, Michael, & Koneshloo, 2020;Lu et al, 2013;Michael et al, 2016;Pool et al, 2015;Sebben et al, 2015). In coastal aquifers, due to oceanic forcing (e.g., tides and waves), variably saturated conditions are ubiquitously observed in nearshore beach systems; therefore, it is important to consider heterogeneity of sediment capillarity when investigating coastal flow and transport processes.…”

Section: Introductionmentioning

confidence: 99%

Groundwater Flow and Moisture Dynamics in the Swash Zone: Effects of Heterogeneous Hydraulic Conductivity and Capillarity

Geng

Heiss

Michael

et al. 2020

Water Resources Research

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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. Heterogeneous K greatly enhances the seawater infiltration into the swash zone and modulates its spatial distribution along the beach surface. Due to heterogeneous K and 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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Denitrification hotspots in intertidal mixing zones linked to geologic heterogeneity

Cited by 40 publications

References 56 publications

Investigating Boron Isotopes for Identifying Nitrogen Sources Supplied by Submarine Groundwater Discharge to Coastal Waters

Investigating Boron Isotopes for Identifying Nitrogen Sources Supplied by Submarine Groundwater Discharge to Coastal Waters

Heterogeneity Affects Intertidal Flow Topology in Coastal Beach Aquifers

Groundwater Flow and Moisture Dynamics in the Swash Zone: Effects of Heterogeneous Hydraulic Conductivity and Capillarity

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