Saltwater‐freshwater mixing dynamics in a sandy beach aquifer over tidal, spring‐neap, and seasonal cycles

Heiss, James W.; Michael, Holly A.

doi:10.1002/2014wr015574

Cited by 197 publications

(175 citation statements)

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“…Under these hydrologic conditions, the saline circulation cell and its associated mixing zone are spatially limited, and the inland hydraulic gradient is the main control of total SGD (Heiss and Michael, 2014;Robinson et al, 2007a). Based on the stable isotopes of water along the STE, Chaillou et al (2017) confirmed the contribution of only two water endmembers (i.e.…”

Section: Study Areamentioning

confidence: 94%

“…tidal and wave pumping, hydrography, and density) induce temporal and spatial variability in biogeochemical conditions (see Santos et al, 2012, and references therein). The mixing zone is subject to oscillating conditions, with rapid changes in oxygen saturation, redox potential and organic matter input controlled by tidal stage and amplitude, sea level and seasonal water table fluctuations (Abarca et al, 2013;Gonneea et al, 2013;Heiss and Michael, 2014;Robinson et al, 2014). These physical processes are likely to impact the distribution and biogeochemical reactivity of many dissolved constituents (Beck et al, 2007;Kroeger and Charette, 2008).…”

Section: Introductionmentioning

confidence: 99%

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Nitrogen transformations along a shallow subterranean estuary

et al. 2017

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Abstract. The transformations of chemical constituents in subterranean estuaries (STEs) control the delivery of nutrient loads from coastal aquifers to the ocean. It is important to determine the processes and sources that alter nutrient concentrations at a local scale in order to estimate accurate regional and global nutrient fluxes via submarine groundwater discharge (SGD), particularly in boreal environments, where data are still very scarce. Here, the biogeochemical transformations of nitrogen (N) species were examined within the STE of a boreal microtidal sandy beach located in the Magdalen Islands (Quebec, Canada). This study revealed the vertical and horizontal distribution of nitrate (NO, dissolved organic nitrogen (DON) and total dissolved nitrogen (TDN) measured in beach groundwater during four spring seasons (June 2011(June , 2012(June , 2013(June and 2015 when aquifer recharge was maximal after snowmelt. Inland groundwater supplied high concentrations of NO x and DON to the STE, whereas inputs from seawater infiltration were very limited. Non-conservative behaviour was observed along the groundwater flow path, leading to low NO x and high NH + 4 concentrations in the discharge zone. The long transit time of groundwater within the beach (∼ 166 days), coupled with oxygen-depleted conditions and high carbon concentrations, created a favourable environment for N transformations such as heterotrophic and autotrophic denitrification and ammonium production. Biogeochemical pathways led to a shift in nitrogen species along the flow path from NO x -rich to NO x -poor groundwater. An estimate of SGD fluxes of N was determined to account for biogeochemical transformations within the STE based on a N-species inventory and Darcy's flow. Fresh inland groundwater delivered 37 mol NO x yr −1 per metre of shoreline and 63 mol DON m −1 yr −1 to the STE, and NH and DON. Our study shows the importance of tidal sands in the biogeochemical transformation of the terrestrial N pool. This local export of bioavailable N probably supports benthic production and higher trophic levels leading to its rapid transformation in surface sediments and coastal waters.

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Section: Study Areamentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

Nitrogen transformations along a shallow subterranean estuary

et al. 2017

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“…SEAWAT has been developed by the United States Geological Survey (USGS), and numerous studies have used the code to simulate variably-density, transient groundwater flow (Heiss and Michael, 2014;Herckenrath et al, 2011;Rasmussen et al, 2013). In SEAWAT the governing flow and solute transport equations are coupled and solved with a cell-centred finite difference approximation.…”

Section: Variable-density Groundwater Modelmentioning

confidence: 99%

Fresh groundwater resources in a large sand replenishment

Huizer

Essink

Bierkens

2016

Hydrol. Earth Syst. Sci.

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Abstract. The anticipation of sea-level rise and increases in extreme weather conditions has led to the initiation of an innovative coastal management project called the Sand Engine. In this pilot project a large volume of sand (21.5 million m 3 ) -also called sand replenishment or nourishment -was placed on the Dutch coast. The intention is that the sand is redistributed by wind, current, and tide, reinforcing local coastal defence structures and leading to a unique, dynamic environment. In this study we investigated the potential effect of the long-term morphological evolution of the large sand replenishment and climate change on fresh groundwater resources. The potential effects on the local groundwater system were quantified with a calibrated three-dimensional (3-D) groundwater model, in which both variable-density groundwater flow and salt transport were simulated. Model simulations showed that the long-term morphological evolution of the Sand Engine results in a substantial growth of fresh groundwater resources, in all adopted climate change scenarios. Thus, the application of a local sand replenishment could provide coastal areas the opportunity to combine coastal protection with an increase of the local fresh groundwater availability.

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“…These studies can be classified into two categories: field or laboratory experiments and analytical or numerical models. Field or laboratory experiments rely on measurements to delineate the hydrodynamics of near-shore aquifer systems (e.g., Westbrook et al 2005;Trefry et al 2007;Kuan et al 2012;Heiss and Michael 2014), whereas analytical or numerical models solve governing equations that can approximately reproduce system responses according to input parameter variations (Ataie-Ashtiania et al 1999, 2001Mao et al 2006;Werner and Lockington 2006;Li et al 2008;Bakhtyar et al 2013). The greatest challenges hindering most experimental approaches involve obtaining sufficient data for resolving detailed mechanisms of flow and transport among various scales of interest.…”

Section: Introductionmentioning

confidence: 99%

Effects of hydrogeological properties on sea-derived benzene transport in unconfined coastal aquifers

Tsai

et al. 2016

Environ Monit Assess

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This paper presents numerical investigations on quantifying the hydrodynamic effects of coastal environment factors, including tidal fluctuations, beach slopes, hydraulic conductivity, and hydraulic gradients on sea-derived benzene transport in unconfined coastal aquifers. A hydrologic transport and mixed geochemical kinetic/equilibrium reactions in saturated-unsaturated media model was used to simulate the spatial and temporal behaviors of the density flow and benzene transport for various hydrogeological conditions. Simulation results indicated that the tidal fluctuations lead to upper saline plumes (USPs) near the groundwater and seawater interfaces. Such local circulation zones trapped the seaward benzene plumes and carried them down in aquifers to the depth depending on the tide amplitudes and beach slopes across the coastal lines. Comparisons based on different tidal fluctuations, beach slopes, hydraulic conductivity, and hydraulic gradient were systematically conducted and quantified. The results indicated that areas with USPs increased with the tidal amplitude and decreased with the increasing beach slope. However, the variation of hydraulic conductivity and hydraulic gradient has relatively small influence on the patterns of flow fields in the study. The increase of the USP depths was linearly correlated with the increase of the tidal amplitudes. The benzene reactive transport simulations revealed that the plume migrations are mainly controlled by the local flow dynamics and constrained in the USP circulation zones. The self-cleaning process of a coastal aquifer is time-consuming, typically requiring double the time of the contamination process that the benzene plume reach the bottom of a USP circulation zone. The presented systematic analysis can provide useful information for rapidly evaluating seaward contaminants along a coastal line with available hydrogeological properties.

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Saltwater‐freshwater mixing dynamics in a sandy beach aquifer over tidal, spring‐neap, and seasonal cycles

Cited by 197 publications

References 83 publications

Nitrogen transformations along a shallow subterranean estuary

Nitrogen transformations along a shallow subterranean estuary

Fresh groundwater resources in a large sand replenishment

Effects of hydrogeological properties on sea-derived benzene transport in unconfined coastal aquifers

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