Volume 6

Hoori
Ajami
, in


Encyclopedia of Geology (Second Edition), 2021

Littoral Aquifers and Sea Water Intrusion


Littoral aquifers, the domains …

where continental fresh groundwater and seawater meet
(Mail 2005), are important water supply regions for more one billion people living in coastal regions (Ferguson and Gleeson, 2012). Submarine groundwater discharge from coastal aquifers includes the discharge of both fresh and saline water to marine waters (Befus et al., 2017). As a result, submarine groundwater discharge impacts coastal water quality and geochemical cycles by transporting heat, nutrients, contaminants, and dissolved ions (Sawyer et al., 2016). On boilerplate, the volumetric flux of fresh submarine groundwater discharge per unit length of shoreline is 420
 
mtwo/yr. However, fresh submarine groundwater discharge along the coast varies considerably, such that xiv% of coastlines contribute 50% of fresh submarine groundwater belch (Sawyer et al., 2016). A contempo modeling investigation has revealed contributing recharge surface area of 175,000
 
km2
for submarine fresh groundwater discharge from unconfined aquifers along the coast of the eastern United States. This effect illustrates that changes in country use practices and climatic condition extending several kilometers inland have the potential to impact coastal water quality and ecosystem dynamics (Befus et al., 2017).

Coastal aquifers are too susceptible to sea level ascent and over extraction of groundwater. In improver to excessive groundwater withdrawal, alterations to land utilize, and sea-level ascension generated by global climate change tin all pb to seawater intrusion. Defined as the landward incursion of seawater (Werner et al., 2013), seawater intrusion will dethrone water quality of inland freshwater groundwater systems. For a range of hydrogeological conditions in coastal aquifers, it has been shown that coastal aquifers are more than susceptible to groundwater extraction than projected body of water-level rise estimated past Global Climate Models (Ferguson and Gleeson, 2012). Most investigations of seawater intrusion processes have been focused on numerical modeling experiments and sand-tank experimentations (Werner et al., 2013). Future efforts should focus on intensive field investigations and measurement campaigns to provide a more robust understanding of coastal aquifer response to diverse ecology conditions (Werner et al., 2013).

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Water Quality and Sustainability

Due north.P.
Nikolaidis
, …
A.
Cristina Cardoso
, in


Comprehensive Water Quality and Purification, 2014

four.11.3.ane

Seawater Intrusion


Coastal aquifers
in the Mediterranean are under pressure because of the concentration of the Mediterranean population in coastal areas, to intensive tourism (especially during the summer period) as well as to the large extent of irrigated agriculture. Groundwater abstractions for agricultural purposes and securing drinking h2o supply have acquired seawater intrusion in many coastal aquifers of the Mediterranean surface area (

EEA, 1999; Mediterranean Groundwater Working Group, 2007). Seawater intrusion has been reported all along the Mediterranean coast.
Wriedt and Bouraoui (2009)
adult a big-scale approach to delineate areas with seawater intrusion affected by groundwater overexploitation. The application is illustrated in
Figure 8
for the Spanish declension, where agronomics is responsible for more than 50% of water abstractions.


Figure 8.
Risk of intrusion acquired by over-brainchild of aquifers.



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Status and Trends of Water Quality Worldwide

Y.
Shevah
, in


Comprehensive Water Quality and Purification, 2014

1.4.four.ii.ii

Coastal aquifer

The
coastal aquifer
is a quaternary sand and calcareous sandstone aquifer underlying the littoral manifestly of Israel and the Gaza Strip. The natural replenishment and the return flows to the aquifer are estimated at approximately 0.5 billion m

3
per year, of which approximately 57 to 113 1000000 chiliadthree
per year are a contribution to the aquifer within the Gaza Strip. The aquifer is exploited through thousands of shallow and deep private and public wells, making information technology hard to command the optimal withdrawal, especially under astringent drought and h2o scarcity weather condition. In the Gaza Strip, water extraction is estimated at 180 million chiliad3
per year, a arrears of approximately 75 to 90
 
meg yard3
per yr.

Overexploitation of the coastal aquifer has resulted in seawater intrusion, contributing to the salinity of the water torso. The littoral aquifer is too susceptible to Eocene saline intrusion from adjacent and deeper aquifers, while the decline in pressure results in the movement of seawater front into the freshwater body. To a higher place the aquifer, the coastal plain region is densely populated and intensively cultivated. Thus, the aquifer is under an accelerated deterioration process, as reflected past the trend of increasing levels of chlorides and nitrates and other organic and inorganic pollutants. Salts accumulate in groundwater at a rate of two
 
mg
 
l−ane 
year−1, in terms of chloride, increasing from 110
 
mg
 
l−1
in 1963 to approximately 200
 
mg
 
l−1
in 2008. Fertilizers and pesticides applied generously in the study area bear on the natural ecosystems and h2o resources, every bit described in
Section 1.four.five.ii.5
below.

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Model Dimensionality and Setting Boundaries

Mary P.
Anderson
, …
Randall J.
Hunt
, in


Practical Groundwater Modeling (Second Edition), 2015

iv.2.i.4

Freshwater–Seawater Interface

In
coastal aquifers, fresh groundwater discharges to the ocean along the freshwater–seawater interface (

Fig. 4.10). Under field conditions, the interface moves in response to tidal fluctuations, groundwater pumping, and changes in recharge, creating a transition zone, or zone of dispersion. When the interface is relatively stable, the zone of dispersion is narrow and a no-menstruum purlieus could exist specified at a representative boilerplate position of the interface (Fig. 2.3). Withal, if the interface is non stable (or may non be stable under atmospheric condition represented in a forecasting simulation), a static boundary is likely not appropriate. When it is not appropriate to simulate the freshwater–seawater interface every bit a no-menses boundary, special purpose codes may exist used to simulate a sharp interface without mixing betwixt freshwater and seawater (Box 4.4) or to simulate mixing and flow in the interface by including density furnishings and dispersion (
Sections 12.2 and 12.iii

Section 12.two


Section 12.3

).



Figure 4.10.
Freshwater–seawater interface in a coastal aquifer showing the transition from freshwater to seawater in the zone of dispersion. The interface acts equally a barrier to groundwater flow; freshwater flows upwards along the interface and discharges to the ocean.



Barlow, 2003


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Management of Coastal Groundwater Resources

Manivannan
Vengadesan
,
Elango
Lakshmanan
, in


Coastal Direction, 2019

four.one

Reduction in Pumping

Overextraction of groundwater in the
coastal aquifer
leads to a reduction in the availability of freshwater. Additional pumping of groundwater induces further move of seawater into the aquifer toward the groundwater extraction. The mixing of seawater with groundwater affects the quality and normal usefulness of groundwater. Hence, overpumping of groundwater should be reduced in the littoral aquifer. This method is cost-costless and more than effective toward the mitigation of seawater intrusion. A decrease in saltwater upconing due to a reduction in groundwater pumping in the littoral aquifer of Cebu City was studied past

Scholze et al. (2002). An increase in groundwater level was observed by a reduction in pumping in the coastal aquifer of Chennai (Pandian et al. 2016).

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Evaluation of Vulnerability Zone of a Littoral Aquifer Through GALDIT GIS Alphabetize Techniques

G.
Gnanachandrasamy
, …
Due south.
Selvam
, in


GIS and Geostatistical Techniques for Groundwater Science, 2019

15.five

Conclusions

The assessment of SWI in the
coastal aquifers
of the Cauvery deltaic region was successfully carried out through the application of the GALDIT index method. Using the GALDIT alphabetize method the results indicated that the aquifer is within a high-to-moderate category of vulnerability for SWI. The GALDIT index map of the proposed area clearly showed that the area near to the coast had high vulnerability and areas further away from the coast had moderate vulnerability. The GALDIT index map is shown in

Fig. 15.vii. From the results, the full study area was classified every bit 92.eight% being highly vulnerable and 7.2% being moderately vulnerables. The modern technique of GALDIT alphabetize aquifer vulnerability mapping is used for the identification of SWI in coastal areas. Finally, the vulnerability maps tin can exist used as a tool for the management of the coastal groundwater resources and to predict SWI.

Fig 15.7


Fig 15.7.
GALDIT alphabetize map.



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Groundwater Flow to the Littoral Ocean

A.Due east.
Mulligan
,
M.A.
Charette
, in


Encyclopedia of Bounding main Sciences (Second Edition), 2009

Groundwater Flow at the Coast

Several forces drive groundwater flow through
coastal aquifers, leading to a complex catamenia regime with significant variability in space and time (

Effigy ii). The main driving force of fresh SGD is the hydraulic gradient from the upland region of a watershed to the surface h2o discharge location at the declension. Freshwater flux is also influenced past several other forces at the littoral purlieus that also drive seawater through the sediments. For example, seawater circulates through a littoral aquifer nether the force of gravity, from oceanic forces such as waves and tides, equally a result of dispersive apportionment forth the freshwater–saltwater boundary within the aquifer, and from changes in upland recharge. Several other forcing mechanisms be, simply they are mostly only present in specific settings. For case, tidal elevation differences across many islands tin can drive flow through the subsurface. All of these forcing mechanisms affect the rate of fluid menstruation for both fresh and saline groundwater and are ultimately of import in decision-making the submarine discharge of both fluids.


Figure ii.
Simplification of an unconfined coastal groundwater arrangement. H2o flow is driven past the inland hydraulic gradient, tides, beach runup and waves, and dispersive apportionment. Other forcing mechanisms can bulldoze fluid through coastal sediments, including seasonal changes in recharge to the inland groundwater organisation and tidal differences across islands.



The analysis of coastal groundwater menstruum must account for the presence of both fresh- and saline water components. When appropriate, such as in regional-calibration assay or for coarse estimation purposes, an assumption tin can be made that there is a sharp transition, or interface, between the fresh and saline groundwater. While this assumption is non strictly truthful, it is often appropriate, and invoking it results in simplifying the assay. For example, we tin approximate the position of the freshwater–saltwater interface past assuming a sharp interface, no catamenia inside the saltwater region, and but horizontal flow within the fresh groundwater. Invoking these assumptions means that the pressures at adjacent points along the interface on both the freshwater and saltwater sides are equal. Equating these pressures and rearranging, the depth to the interface can be calculated equally follows:

[5]


Interface

depth
=




ρ

one





ρ

2




ρ

ane



z
=
40
z

where
ρ
i
is the density of fresh water (1000
 
kg
 
g−3) and
ρ
2
is the density of seawater (1025
 
kg
 
1000−iii). This equation states that the depth of the interface is forty times the elevation of the water table relative to hateful ocean level. While eqn
[v]
is simply an approximation of the interface location, information technology is very helpful in thinking nigh freshwater and saltwater movement in response to changes in fresh groundwater levels. As recharge to an aquifer increases, water levels increment and the interface is pushed down. This is also equivalent to pushing the interface seaward and the cyberspace effect is to force saltwater out of the subsurface and to replace information technology with fresh water. The contrary flows occur during times of little to no recharge or if groundwater pumping becomes excessive.

While the abrupt-interface approach is useful for conceptualizing menstruum at the coast, particularly in large-scale problems, the reality is more complex. Not merely does the saline groundwater menses but also a zone of intermediate salinity extends between the fresh and saline end members, establishing what many refer to as a ‘subterranean estuary’. Like their surface water counterparts, these zones are hotbeds of chemical reactions. Considering the water in the interface zone ultimately discharges into coastal waters, the flow and chemic dynamics within the zone are critically of import to sympathise. Research into these problems has only just begun.

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Sea Interfaces & Human Impacts

Ann East.
Mulligan
, …
Nils
Moosdorf
, in


Encyclopedia of Ocean Sciences (Third Edition), 2019

Groundwater Menses at the Coast

Several forces bulldoze groundwater flow through
coastal aquifers, leading to a complex menses regime with significant variability in space and fourth dimension (

Fig. 2). The chief driving force of fresh SGD is the hydraulic slope from the upland region of a watershed to the surface water discharge location at the declension amplified by the permeability of the aquifer. SGD is also influenced by several other forces at the littoral boundary that also drive seawater through the sediments. For example, seawater circulates through a coastal aquifer nether the strength of gravity, from oceanic forces such equally waves and tides, as a result of dispersive circulation along the freshwater–saltwater boundary within the aquifer, and from changes in upland recharge. Several other forcing mechanisms exist, but they are more often than not only present in specific settings. For case, tidal tiptop differences beyond many islands can drive menstruum through the subsurface. All of these forcing mechanisms affect the charge per unit of fluid flow for both fresh and saline groundwater and are ultimately important in controlling the submarine discharge of both fluids.

Fresh SGD may extend tens of kilometers offshore with highly continuous geologic units. Saline catamenia paths may also be driven farther offshore by geothermal convection and density gradients (Fig. iii). For example, along the southeastern coast of the United states of america, cold ocean water penetrates the limestone aquifer at depth, where it is heated every bit it migrates toward the interior of the platform. With warming, the water becomes less buoyant and rises, discharging along the continental shelf.

Fig. 3


Fig. 3.
(A) Simplification of a homogeneous unconfined coastal groundwater system. Groundwater menses is driven past the inland hydraulic gradient, tides, embankment runup and waves, and dispersive circulation. In this simplified model, SGD rates are highest about the shoreline and decrease exponentially offshore (top of figure). (B) Simplification of a heterogeneous unconfined coastal groundwater organization, where groundwater flow farther offshore may exist driven past geothermal convection, density gradients and geologic heterogeneities. Other forcing mechanisms can drive fluid through coastal sediments, including seasonal changes in recharge to the inland groundwater system or bounding main level, and tidal differences beyond permeable barriers.


Modified from Michael, H. A. et al. (2016). Geologic influence on groundwater salinity drives large seawater circulation through the continental shelf.
Geophysical Research Messages
43, 10782–10791.


The analysis of coastal groundwater menses must account for the presence of both fresh and saline h2o components. When advisable, such as in regional-calibration analysis or for fibroid estimation purposes, an assumption can exist made that at that place is a precipitous transition, or interface, between the fresh and saline groundwater. While this supposition is not strictly true, information technology is frequently appropriate, and invoking information technology results in simplifying the assay. For instance, nosotros tin can guess the position of the freshwater–saltwater interface by bold a abrupt interface, no flow within the saltwater region, and merely horizontal flow within the fresh groundwater. Invoking these assumptions means that the pressures at adjacent points along the interface on both the freshwater and saltwater sides are equal. Equating these pressures and rearranging, the depth to the interface can be calculated as follows:

(v)

Interface depth
=


ρ
1



ρ
ii



ρ
one



z
=
40
z

where
ρ
1
is the density of fresh water (1000
 
kg
 
1000

 
three
) and
ρ
2
is the density of seawater (1025
 
kg
 
k

 
3
). This equation states that the depth of the interface is twoscore times the elevation of the h2o table relative to hateful sea level (Fig. 3A). While Eq.
(v)
is only an approximation of the interface location, it is very helpful in thinking about freshwater and saltwater movement in response to changes in fresh groundwater levels. As recharge to an aquifer increases, water levels increment and the interface is pushed down. This is also equivalent to pushing the interface seaward and the net effect is to strength saltwater out of the subsurface and to replace it with fresh water. The contrary flows occur during times of lilliputian to no recharge or if groundwater pumping becomes excessive.

While the sharp-interface approach is useful for conceptualizing flow at the declension, particularly in large-scale problems, the reality is more circuitous (Fig. 3B). Not only does the saline groundwater catamenia only also a zone of intermediate salinity extends between the fresh and saline cease members, establishing what many refer to as a “subterranean estuary.” Similar their surface h2o counterparts, these zones are hotbeds of chemical reactions. Because the water in the interface zone ultimately discharges into coastal waters, the flow and chemical dynamics inside the zone are critically important to understand.

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Deformation, Storage, and General Flow Equations

Charles R.
Fitts
, in


Groundwater Scientific discipline (Second Edition), 2013

6.viii.iii

Storage in Aquifers with Fresh–Salt Interfaces

Fresh–common salt interfaces exist in
coastal aquifers
that may exist unconfined (eastward.g.,

Effigy 3.26) or confined. Where an interface is present, the interface has a huge impact on storage. As fresh h2o heads drop, the interface shifts upward and table salt water replaces fresh h2o, removing fresh water from storage.

An unconfined interface aquifer has a dynamic upper boundary (the water table), and a dynamic lower boundary (the fresh–salt interface). For two-dimensional catamenia models that fail the vertical resistance to flow in the fresh water as well every bit in the salt h2o, the storativity parameter is given equally

(half dozen.33)

S
=


S


y


+


north


e






ρ


f






ρ


s





ρ


f






lpar

 unconfined interface aquifers

)

where
northdue east

is effective porosity,
ρf

is the fresh h2o density, and
ρs

is the table salt water density. For typical fresh and salt water densities,



ρ


f


/

lpar



ρ


s





ρ


f



)


4

, so the 2d term on the right side of
Eq. 6.33
dwarfs the first and

(6.34)

Due south



northward


e






ρ


f






ρ


due south





ρ


f




In a confined aquifer with an interface, there is rubberband storage and storage contributed by interface motion. The interface storage is many orders of magnitude greater than elastic storage, and the storativity parameter is also given by
Eq. vi.34.

When a coastal interface aquifer responds to changing heads, the three possible types of storage reply at much different rates. Elastic storage response is rapid, water tabular array storage response is slower, and interface storage response is the slowest. These varied response rates are due to differences in the amount of water that must flow in society to arrange to the changing head. For interface motion, large volumes of both fresh and salt water must move every bit heads change. Such movements can’t exist instant and are limited past the hydraulic conductivity of the aquifer. The more movement is required, the more than delayed the response is. For case, when a well is turned on in an unconfined interface aquifer, the elastic storage changes happen almost instantly, the water table changes occur slowly, and the interface movements are slower still. For short-term transient flow problems, these differences must be taken into business relationship. For more detailed coverage of interface storage and modeling, run into
Bear (1999).

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Adaptation Strategies to Address Ascent H2o Tables in Littoral Environments Under Future Climate and Bounding main-Level Rise Scenarios

Alex K.
Manda
,
Wendy A.
Klein
, in


Coastal Zone Direction, 2019

Abstract

Climate modify and ocean-level ascent will impact
coastal aquifers
by facilitating saltwater intrusion and/or changing the quantity of water recharging the groundwater organisation. Some other less obvious, merely every bit of import result of climatic change and sea-level rise is rising water tables and/or groundwater inundation. Shallow water tables and groundwater inundation are likely to impact various types of infrastructure (e.g., septic systems, building foundations, roads, etc.) in littoral regions. Here, we written report on the need to critically account for shallow water tables and groundwater inundation in research studies that accost climate change and body of water level. We highlight the need for consideration of adaptive and innovative groundwater direction strategies that will address futurity changes to the groundwater systems, particularly those in coastal regions.

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