Terrestrial-originated submarine groundwater discharge through deep multilayered aquifer systems beneath the seafloor

Abstract

Abstract:A large quantity of submarine groundwater discharge (SGD) of about 1000 m 3 day À1 m À1 of the 600-km-long shoreline of South Atlantic Bight has been estimated by Moore (Global Biogeochemical Cycles, 2010b, 24, GB4005, doi:10.1029. However, there is great uncertainty in estimating the percentage of net, land-originated groundwater recharge of SGD. Moreover, most previous studies considered the homogeneous case for the coastal superficial aquifers. Here, we investigated the terrestrial-originated SGD through a multilayered submarine aquifer system, which comprises two confined aquifers and two semi-permeable layers. The inland recharge includes a constant part representing the annual average and a periodical part representing its seasonal variation. An analytical solution was derived and used to analyse the distributions of the terrestrial-originated SGD from the multilayered aquifers along the Winyah Bay transect, South Atlantic Bight. It is found that the width of the zone of SGD from the upper aquifer ranges from 0.8 to~8.0 km depending on the leakance of the seabed semi-permeable layer. A head of the upper aquifer at a coastline 1.0 m higher than the mean sea level will cause a SGD of 1.82-18.3 m 3 day À1 m À1 from that aquifer as the seabed semi-permeable layer's leakance varies from 0.001 to 0.1 day

Geologic influence on groundwater salinity drives large seawater circulation through the continental shelf

“…While there are many mechanisms that drive offshore land-sea water exchange [Wilson, 2005;Santos et al, 2012], most, for example benthic exchange driven by tides and waves, produce flow paths that are too short to accumulate high longlived Ra concentrations and flow rates too low to sustain the observed levels of Ra and other solutes in the ocean [Santos et al, 2012]. Extensive confined aquifers that outcrop on the shelf can result in local, primarily fresh, discharge [e.g., Guo and Li, 2015], and geothermal gradients can drive significant, large-scale flow across the continental slope [Wilson, 2003] mechanisms do not explain large saline groundwater discharges all along the shelf. Deep, long-time scale saline groundwater circulation is also driven by density gradients along the freshwater-saltwater interface [Cooper, 1959;Kohout, 1960], with discharge rates that rival those of fresh SGD [Smith, 2004].…”

Contaminant transport in a small deformation aquitard affected by the delayed drainage phenomenon

“…Aquitards consist of fine sediments such as clay and silt in the low‐energy depositional environments, and they are natural hydrogeological barriers and protect regional aquifers from contamination (Neuzil, ; Yan, Kerrich, & Hendry, ; Zhuang, Zhou, Zhan, & Wang, ). Aquitards are distributed over large land areas across the globe, covering the entire basins or alluvial plains, and sometimes even extending under the sea, such as the Dakota aquifer system in the United States and the Yangtze Delta in China (Cihan, Zhou, & Birkholzer, ; Guo & Li, ; Guo, Li, Zhou, & Huang, ; Konikow & Neuzil, ; Zhou, Guo, & Dou, ). Once the contaminants enter into an aquitard, they cannot be easily extracted due to its high porosity and low permeability and the binding force between the contaminant ions and the negatively charged clay surface (Barbour, Hendry, & Wassenaar, ; Hendry, Barbour, Boldt‐Leppin, & Wassenaar, ).…”

A Joint Analytic Method for Estimating Aquitard Hydraulic Parameters

“…In this part, we assume that the aquitard is subject to long‐term stable, cyclic changes in hydraulic heads within adjacent aquifers. Cyclic hydraulic head variations usually occur in coastal aquifers in response to tidal fluctuations (Li and Jiao ; Guo and Li ) and in a system where groundwater pumpage and recharge changes seasonally (Zhang et al ; Shi et al ; Xue et al ). Under this assumption, the aquitard only undergoes elastic deformation, thus K v and S ske are two hydraulic parameters that control flow and deformation behaviors within the aquitard.…”