Groundwater flow and salt transport in a subterranean estuary driven by intensified wave conditions

Abstract

[1] A numerical study, based on a density-dependent variably saturated groundwater flow model, was conducted to investigate flow and salt transport in a nearshore aquifer under intensified wave conditions caused by offshore storms. Temporally varying onshore hydraulic gradients due to wave setup were determined as the seaward boundary condition for the simulated aquifer. The results showed a rapid increase in influxes across the aquiferocean interface in response to the wave event followed by a more gradual increase in effluxes. The upper saline plume first widened horizontally as the wave setup point moved landward. It then expanded vertically with recirculating seawater pushed downward by the wave-induced hydraulic gradient. The time for the salt distribution to return to the prestorm condition was up to a hundred days and correlated strongly with the time for seawater to recirculate through the aquifer. The pathways of recirculating seawater and fresh groundwater were largely modified by the wave event. These pathways crossed through the same spatial locations at similar times, indicating significant salt-freshwater mixing. The flow and salt transport dynamics were more responsive to wave events of longer duration and higher intensity, especially in more permeable aquifers with lower fresh groundwater discharge. Despite their larger response, aquifers with higher permeability and beach slope recovered more rapidly postevent. The rapid recovery of the flows compared with the salinity distribution should be considered in field data interpretation. Due to their longlasting impact, wave events may significantly influence the geochemical conditions and the fate of chemicals in a subterranean estuary.

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…Waves drive large volumes of water exchange across the CUA-ocean interface [Li et al, 1999]. It is challenging to quantify wave-driven recirculation from field data alone, but estimates have been provided through numerical studies [e.g., Xin et al, 2010;Bakhtyar et al, 2011;Geng et al, 2014;Robinson et al, 2014;Xin et al, 2014].…”

“…Robinson et al [2014] simulated an isolated period of intensified wave conditions with a storm surge. The simulations showed that while the USP rapidly expands as the wave height increases, it may take ~100 d for the salinity distribution in the STE to recover to its initial pre-storm state after the waves subside.…”

“…Both SGD and the salt distribution attributed to a particular force must be determined with consideration of the influence of other forces both at the present and in the past. The response of SGD and saltwaterfreshwater mixing zone to the different forces is not instantaneous but delayed [Michael et al, 2005;Lu and Werner, 2013;Robinson et al, 2014;Xin et al, 2014;Liu et al, 2016;Yu et al, 2017]. This complicates the assessment of interactions between multiple forces.…”

“…This is likely in part due to the challenges inherent in investigating irregular and longer period forcing via field experiments as well as the difficulties of disentangling the various forcing effects. Irregular forcing, particularly varying wave conditions and episodic events such as overland seawater inundation, may have a great impact on the salinity distribution, mixing and geochemical processes within a CUA due to potential large-scale salinization combined with the slow recovery (freshening) of the aquifer [Smith et al, 2008;Robinson et al, 2014;Yu et al, 2016]. Large-scale salinization of a CUA may also, for instance, lead to mobilization of legacy metals and organic contaminants, although these potential impacts have not been explored.…”

Nitrogen transformations along a shallow subterranean estuary

“…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).…”