Submarine groundwater discharge and seawater circulation in a subterranean estuary beneath a tidal flat

Abstract

Abstract:A numerical simulation and stationary geophysical survey at Omaehama, Japan, described the hydrogeology in a tidal flat and adjacent sea water column. The simulation of a tidally influenced aquifer with inland fresh groundwater discharges showed three circulations in the subterranean tidal flat estuary: a small, tide-induced recirculation (TIR) near the sloping beach, a deeper circulation (DC) across the interface between saltwater and freshwater, and a large, tidal flat-induced circulation (TFIC) in the offshore subterranean tidal flat and slope. The horizontal distribution of the simulated submarine groundwater discharge (SGD) showed two peaks, depending on the distance from the coast along the transect line of tidal flat; the offshore peak was quantitatively consistent with the previously observed peak based on automated seepage measurements. Time-series of the observed resistivity profiles using a marine cable (140 m) showed the structure of the fresh water/salt water interface, and indicated a salt wedge intruding from the bottom aquifer and infiltration at the surface in the beach and nearshore tidal flat. The qualitative structure of the resistivity hardly changed at each tidal stage. These results qualitatively validated the circulation patterns shown by the numerical simulation. The source of this discharging groundwater in the tidal flat is a complex mixture of fresh groundwater derived from the landward aquifer, as well as locally recycled seawater, which can contribute to the purification of the nearshore seawater.

Submarine Groundwater Discharge-Derived Nutrient Loads to San Francisco Bay: Implications to Future Ecosystem Changes

“…The tidal stage (ebbing or flooding) may also impact the residence time, as it is well-known that seawater infiltration occurs during high tide and drainage during low tide (Nielsen 1990). Infiltration typically occurs faster than draining, resulting in variable porewater residence time during tidal stages for water to react with sediments and build higher concentrations (Nakada et al 2011). Another explanation of lower-end nutrient concentrations in the groundwater samples may be the influence of pumping of groundwater during our study.…”

“…At AI, the SGD flux based on Rn was higher compared to the SGD flux based on Ra, suggesting some contribution of freshwater inputs at this site (indeed fresher groundwater was encountered). Conditions such as topographic relief can play an important role in SGD, seawater infiltration, subsurface flow, and the overall impact on transport mechanisms (Nakada et al 2011). Saltwater infiltration can be intensified at the beach face of a low-relief estuary, but infiltration has been shown to be even stronger at tidal flat settings (Mao et al 2006).…”

Estimation of seawater–groundwater exchange rate: case study in a tidal flat with a large-scale seepage face (Laizhou Bay, China)

“…Moore 1996;Li et al 1999). Ignoring seepage face, Nakada et al (2011) reported the seawater circulation in a subterranean estuary beneath a tidal sand flat using SEAWAT-2000. The tidal contribution to SGD was estimated to be ∼20 m 3 d −1 m −1 .…”

Hydrogeological effects of dredging navigable canals through lagoon shallows. A case study in Venice

“…Despite the large research effort dedicated to the understanding of the freshwater-groundwater exchange in coastal aquifers (e.g., Li et al, 1999;Michael et al, 2005;Nakada et al, 2011;Qu et al, 2014), studies developed in the past have never addressed the evaluation of possible effects of excavating navigable canals through tidal flats on the underlying hydrogeological system. However, in-depth investigations using direct measurements (isotopes, benthic chambers), geophysical surveys, and modeling simulations revealed that submarine groundwater discharge (SGD) may provide considerable freshwater inputs to coastal waterbodies (e.g., Rapaglia et al, 2010;Wang et al, 2015) and may be the primary pathway for nutrients and other contaminants to enter coastal lagoons (e.g., Rapaglia, 2005;Rocha et al, 2016;Santos et al, 2008;Tait et al, 2013).…”