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Submarine groundwater discharge and associated chemical input to a coastal sea
Water Resources Research
Abstract
This paper presents a theoretical model of flow and chemical transport processes in subterranean estuaries (unconfined brackish groundwater aquifers at the ocean- land interface). The model shows that groundwater circulation and oscillating flow, caused by wave setup and tide, may constitute up to 96% of submarine groundwater discharge (SGWD) compared with 4% due to the net groundwater discharge. While these local flow processes do not change the total amount of land-derived chemical input to the ocean over a long period (e.g., yearly), they induce fluctuations of the chemical transfer rate as the aquifer undergoes saltwater intrusion. This may result in a substantial increase in chemical fluxes to the ocean over a short period (e.g., monthly and by a factor of 20 above the averaged level), imposing a possible threat to the marine environment. These results are essentially consistent with the experimental findings of Moore [1996] and have important implications for coastal resources management.
... Mixing and reaction processes occur between freshwater, seawater and other chemical constituents (e.g., nutrients, carbon, metals, etc.) within coastal aquifers, and these influence groundwater-based chemical loads to the sea. Additionally, tides and aquifer heterogeneities create complex pathways of coastal groundwater flow, with profound effects on groundwater residence times, groundwater-borne chemical loads, and the location of groundwater outflows Heiss et al., 2017;Kim et al., 2020;Li et al., 1999). A comprehensive understanding of these (and other) flow and transport processes within coastal aquifers is a precursor to the sustainable management of fresh groundwater resources and the ecosystems of coastal zones. ...
... Homogeneous coastal aquifers are often assumed in conceptual models when exploring fundamental mechanisms of flow and transport in coastal aquifers subject to tidal fluctuations (Li et al., 1999;Robinson et al., 2009). In coastal regions containing layered aquifer systems, deeper aquifers may be semi-confined, creating the opportunity for inter-aquifer fluxes to occur (Kim et al., 2006;Love et al., 2000). ...
Coastal aquifers are commonly layered, and thus, a clear understanding of groundwater flow and salt transport in layered coastal aquifers is important for managing fresh groundwater. However, the influence of leakage between adjacent aquifers on flow and transport processes remains largely unknown where the influence of tides is considered. This study used laboratory experiments and numerical simulation to examine the processes of flow and transport within a tidal aquifer‐aquitard system (i.e., an unconfined aquifer underlain by a semi‐confined aquifer, with an intervening thin aquitard). The laboratory‐scale observations of the current study are the first observations of offshore fresh groundwater within a semi‐confined coastal aquifer. The numerical and laboratory results are in close agreement, revealing that upward leakage from the semi‐confined aquifer into the saltwater wedge of the overlying unconfined aquifer caused buoyant instabilities to form. The development of freshwater fingers created complex saltwater‐freshwater mixing, leading to mixed saltwater influx‐efflux patterns across the sloping aquifer‐ocean interface. Compared with non‐tidal conditions, tidal forces reduced the net upward leakage from the semi‐confined aquifer to the overlying unconfined aquifer. This increased the horizontal flow toward the sea, which in turn reduced the extent of the saltwater wedge in the semi‐confined aquifer. The higher rates of both fresh and saline submarine groundwater discharge (SGD), caused by tides, led to lower groundwater ages in the semi‐confined aquifer. These findings have important implications for unveiling the complex characteristics of seawater intrusion, SGD and geochemical hotspots within layered coastal aquifers.
... Thenceforth, the term "iron curtain" was proposed and has since gained widespread use to describe its environmental functionalities in coastal areas. So far, SGD has been studied intensively with their response to continental processes (e.g., inland hydraulic heads) (Liu et al., 2016;Kuan et al., 2019;Mo et al., 2021) and oceanic oscillations (wave and tide particularly) (Li et al., 1999;Robinson et al., 2006;Xin et al., 2010;Shen et al., 2018). In contrast to the extensive numerical studies addressing coastal hydrogeological processes, limited research has been conducted on major ions and metals, and fieldbased studies on iron curtains are also scarce. ...
The high concentration of dissolved iron (Fe) in coastal waters triggers Lyngbya blooms in the Moreton Bay region of Southeast Queensland, Australia. Previous studies have provided a restricted understanding of how land-derived Fe is transported and then transformed into other forms (e.g., Fe oxides) before its release into the ocean. Here, a field investigation was conducted at a sandy beach on the northern end of Deception Bay, Queensland, Australia, focusing on porewater exchange and Fe transformation. This study revealed that tides provided a significant mechanism for driving the groundwater-seawater mixing in the intertidal area. Such forcing formed an upper saline plume (USP) with high dissolved oxygen (DO), creating a dynamic reaction zone for Fe oxidation and precipitation beneath the USP. The spatial distribution of Fe oxides highlighted a substantial Fe content in the subsurface, providing concrete evidence for the transformation of Fe from an aqueous state to a solid form. It also exhibited a low-permeable area that served as a geochemical barrier, absorbing chemical components like phosphate. These findings can assist in constructing a more accurate transport model that couples physical and geochemical processes to quantify the mechanisms driving Fe transformation in coastal areas and further deepen our comprehension of the hydrogeochemical functionalities in land-ocean connectivity via groundwater.
... Such zones with Fe precipitates were observed in the intertidal area of Waquoit Bay, Massachusetts, USA [12], and additional research revealed that the accumulation of these precipitates can act as a geochemical barrier to retain dissolved chemicals transported to the ocean [13][14][15][16][17]. Thenceforth, the term "iron curtain" was proposed and has since gained widespread use to describe its environmental functionalities in coastal groundwater systems. While numerous studies have focused on the response of groundwater to physical processes, such as inland hydraulic head [18,19], sea level rise [20,21], wave and tide [22][23][24][25], there has been limited attention directed toward geochemical processes associated with redoxsensitive Fe and iron curtain-like features. Despite significant research efforts providing valuable insights into Fe speciation, phosphate (PO4 3-) removal, and sulfate (SO4 2-) reduction in intertidal areas [11,17,21,[26][27][28][29][30], our understanding of hydrogeochemical processes and spatiotemporal variations of the "iron curtain" remains incomplete. ...
The mixing of terrestrial groundwater and seawater creates dynamic reaction zones in intertidal areas, where land-derived Fe(II) is oxidized to Fe(III) and then precipitates as Fe hydroxides at groundwater-seawater interface. These hydrogeochemical processes contribute to the formation of iron curtains beneath soil surface, which in turn influences the evolution of coastal aquifers. This paper provides a comprehensive review of physical and geochemical processes at field scale in coastal areas, explores the impact of mineral precipitation on pore structure at pore scale, and synthesizes reactive transport modeling (RTM) approaches for illustrating continuum-scale soil physio-chemical parameters during the evolution of porous media. Upon this review, knowledge gaps and research needs are identified. Additionally, challenges and opportunities are presented. Therefore, the incorporation of observational data into a comprehensive physico-mathematical model becomes imperative for capturing the pore-scale processes in porous media and their influence on groundwater flow and solute transport at large scales. Furthermore, a synergistic approach integrating pore-scale modeling and non-invasive imaging can provide detailed insights into intricate fluid-pore-solid interactions for future studies, as well as facilitate the development of regional engineering-scale models and physio-chemical coupled models with diverse applications in submarine science and engineering.
... Por otra parte, la Laguna Costera de la Ensenada de La Paz, tiene aportes de agua pluvial en el periodo estacional de verano (Ficha informativa de los humedales de Ramsar, 2007), es decir que aportan fuentes de agua dulce (Mejía- González, et al., 2012). Así mismo, se ha determinado que las aguas subterráneas pueden proveer a algunas lagunas costeras de nutrientes, entre los que destacan el nitrógeno y fosforo (Li, et al., 1999). ...
Radioisotopes from the U/Th decay series are used routinely as tracers for submarine groundwater discharge (SGD) worldwide. We present the main principles of radon and radium mass-balance approaches used for quantifying SGD in coastal areas and discuss some challenges. For example, modeling exercises can substantially help interpret field measurements and reduce uncertainties. We showed how the stable isotopes of nitrogen, carbon, and pigments were used to determine the impacts of SGD's quality on coastal water and biota. Finally, we suggest that the novel deep-learning modeling approaches using radioisotopes are projected to be an important future direction in SGD research.
Keywords: Machine learning in future SGD studies; Modeling SGD; Radiotracers: radon, radium; SGD biological impacts; SGD-derived emerging contaminants; Submarine groundwater discharge (SGD)
The mixing of terrestrial groundwater and seawater creates dynamic reaction zones in intertidal areas, where land-derived Fe(II) is oxidized to Fe(III) and then precipitates as Fe hydroxides at the groundwater-seawater interface. These hydrogeochemical processes contribute to the formation of iron bands at the saltwater wedge (SW) and beneath the upper saline plume (USP). This study provides a comprehensive review of physical and geochemical processes at field scale in coastal areas, explores the impact of mineral precipitation on pore structure at pore scale, and synthesizes reactive transport modeling (RTM) approaches for illustrating continuum-scale soil physio-chemical parameters during the evolution of porous media. Upon this review, knowledge gaps and research needs are identified. Additionally, challenges and opportunities are presented. Therefore, we reach the conclusion that the incorporation of observational data into a comprehensive physico-mathematical model becomes imperative for capturing the pore-scale processes in porous media and their influence on groundwater flow and solute transport at large scales. Additionally, a synergistic approach, integrating pore-scale modeling and non-invasive imaging, is equally essential for providing detailed insights into intricate fluid-pore-solid interactions for future studies, as well as facilitating the development of regional engineering-scale models and physio-chemical coupled models with diverse applications in marine science and engineering.
The need for estimation of direct groundwater discharge to seas is shown and some concepts of this process are presented. Methods for regional evaluation of submarine groundwater discharge are briefly discussed. The estimates of groundwater and dissolved solids discharge from the Australian continent to the ocean are given. The main regularities of this process and its connection with natural factors are discussed. The dependence of submarine discharge values on the physiographic zonality is shown.-Authors
The dispersion of salts produced by reciprocative motion of the salt-water front in a coastal aquifer induces a flow of salt water from the floor of the sea into the zone of diffusion and back to the sea. The head losses that accompany the landward flow tend to lessen the extent to which the salt water occupies the aquifer.
Tidal motions of the water table height inside a sloping beach are
investigated via field measurements and theoretical considerations. Only
the movements forced by the tide are considered, so a beach with
negligible wave activity was chosen for the field measurements. The data
show that even in the absence of precipitation the time averaged inland
water table stands considerably above the mean sea level. Also the water
table at a fixed point inside the beach is far from sinusoidal even
though its variation is forced by an essentially sinusoidal tide. This
latter effect is due to the boundary condition along the sloping beach
face which acts as a highly nonlinear filter. The observed behavior of
the water table is explained in terms of perturbation extensions to the
classical "deep aquifer solution." One extension deals with the
nonlinearity in the interior, the other with the boundary condition at
the sloping beach face.
Two forms of the equation of flow of a compressible liquid in an elastic porous medium are derived by considering mass conservation in (1) a control volume whose boundaries are fixed in space and (2) a control volume that deforms and moves through space when the material deforms. The first method yields a form of the equation that necessarily involves the velocity of the grains of the medium. The second yields a form that does not involve the grain velocity. Jacob's equation is found to be correct when negligible terms are omitted.
Waves approaching a sloping beach induce a tilt in the mean water level within the surf zone. The existence of this `set-up' is here demonstrated by observing the mean flow in a straight tube laid parallel to the incoming waves; also by showing that the waves induce a siphon in a U-tube laid on the sloping bottom. It is argued theoretically, and confirmed by experiment, that the set-up should help to drive an offshore bottom current (the undertow) between the shoreline and the breaker line. Seawards from the breaker line the bottom current is reversed. The consequent convergence of the bottom currents may contribute to building up the `breaker bar'. Further experiments show that the mean onshore pressure gradient drives a circulation of water within a porous beach. The associated pattern of streamlines also extends into the land, inshore from the run-up line. Theoretically, the injection of dye at the sediment-water interface might be used to probe the porosity of the beach material.
Water flows from the land to the sea in rivers - that is the classroom
picture that seems too self-evident to question. But there is evidence
that a comparable amount may flow underground directly into coastal
waters.
Mixing between meteoric water and sea water produces brackish to saline water in many coastal aquifers. In this mixing zone, chemical reactions of the salty water with aquifer solids modify the composition of the water; much as riverine particles and suspended sediments modify the composition of surface estuarine waters. To emphasize the importance of mixing and chemical reaction in these coastal aquifers, I call them subterranean estuaries. Geochemical studies within subterranean estuaries have preceded studies that attempt to integrate the effect of these systems on the coastal ocean. The mixing zone between fresh ground water and sea water has long been recognized as an important site of carbonate diagenesis and possibly dolomite formation. Biologists have likewise recognized that terrestrial inputs of nutrients to the coastal ocean may occur through subterranean processes. Further evidence of the existence and importance of subterranean estuaries comes from the distribution of chemical tracers in the coastal ocean. These tracers originate within coastal aquifers through chemical reactions of the saline water with aquifer solids. They reach the coastal ocean as the surface and subterranean systems exchange fluids. Exchange between the subterranean estuary and the coastal ocean may be quantified by the tracer distribution in the coastal ocean. Examples from the east and Gulf coasts of the U.S., as well as the Bay of Bengal, will be used to evaluate the importance of these unseen estuaries in supplying not only chemical tracers, but also nutrients, to coastal waters. Anthropogenic effects on subterranean estuaries are causing significant change to these systems. Ground water mining, sea level rise, and channel dredging impact these systems directly. The effects of these changes are only beginning to be realized in this vital component of the coastal ecosystem.
Urine-affected areas can lead to considerable losses of N by leaching, ammonia volatilisation and denitrification from dairy pastures in the southeast of South Australia. Potable groundwater supplies are considered to have become contaminated by nitrate as a result of leaching from these leguminous pastures. Dairy cow urine, labelled with 15N urea, was applied to micro-plots and mini-lysimeters installed in two adjacent irrigated (white clover-rye grass) and non-irrigated (subterranean clover-annual grasses) paddocks of a dairy farm on four occasions representing different seasonal conditions. These experiments allowed measurement of nitrogen transformations, recovery of 15N in the pasture and soil, and leaching below various depths. Gaseous losses were calculated from the nitrogen balance.