Coastal Hydrogeology is a branch of

Hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rock (geology), rocks of the Earth's crust (ge ...

that focuses on the movement and the chemical properties of

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidat ...

in coastal areas. Coastal Hydrogeology studies the interaction between fresh groundwater and

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...

, including

seawater intrusion Saltwater intrusion is the movement of saline water into freshwater aquifers, which can lead to groundwater quality degradation, including drinking water sources, and other consequences. Saltwater intrusion can naturally occur in coastal aquifers, ...

, sea level induced groundwater level fluctuation,

submarine groundwater discharge Submarine groundwater discharge (SGD) is a hydrological process which commonly occurs in coastal areas. It is described as submarine inflow of fresh-, and brackish groundwater from land into the sea. Submarine Groundwater Discharge is controlled by ...

, human activities and groundwater management in coastal areas.Jiao, J., & Post, V. (2019). ''Coastal hydrogeology''. Cambridge University Press. The freshwater-seawater interface is a dynamic boundary where

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does in ...

mixes with

. An interface in Coastal Hydrogeology refers to the location that freshwater from

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteri ...

meets seawater. Steady freshwater-seawater interface is an equilibrium stage where the boundary locates in a relatively fixed location, while

or a strong recharge rate breaks the equilibrium, leading to an unsteady freshwater-seawater interface. Mixing of groundwater and seawater creates a special chemical system that is a good indicator to show the interaction and the interface. Human activities such as pumping of

and

land reclamation Land reclamation, usually known as reclamation, and also known as land fill (not to be confused with a waste landfill), is the process of creating new land from oceans, seas, riverbeds or lake beds. The land reclaimed is known as reclam ...

break the equilibrium, leading to

, development of a seepage zone or pollution of the

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...

. The interaction between groundwater system and the ocean is complex. Preventive actions and

engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...

measurements are adopted to mitigate the impacts.

Freshwater-Seawater Interface

Freshwater – Seawater interface or saltwater interface is an essential aspect in Coastal Hydrogeology that tries to figure out the location of the transition zone or mixing zone between

and

. A sharp interface is formed when the transition zone is thin or narrow. Coastal aquifers can be classified in the same three categories as all aquifer systems: Sedimentary Aquifers, Hard Rock Aquifers and Limestone Aquifers.

Ghijben – Herzberg Principle

The Ghijben – Herzberg Principle formulated the

water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. Th ...

and the boundary of groundwater in an

island An island or isle is a piece of subcontinental land completely surrounded by water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island in a river or a lake island may be ...

unconfined aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...

should form a lens-shape. This principle can estimate the bottom boundary of the interface which provides a general idea of the capacity of an aquifer. By estimating the capacity of an

, we can find out the available groundwater resource in some regions.

Freshwater-Seawater Interface in Coastal Aquifers

It is assumed that the aquifers below are

homogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

in the different models so that the

hydraulic conductivity Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on ...

of each aquifer is uniform. The above figures simulate possible coastal aquifers. In reality, it is complex. Due to complex geology - non-uniform rock layers and weathering, both confined and unconfined aquifers can be found within a coast. It is possible to have multiple confined aquifers at the bottom and an unconfined aquifers at the top of a coast.

Seawater Intrusion

Seawater intrusion Saltwater intrusion is the movement of saline water into freshwater aquifers, which can lead to groundwater quality degradation, including drinking water sources, and other consequences. Saltwater intrusion can naturally occur in coastal aquifers, ...

is a process where seawater intrudes into a freshwater aquifer. Natural or anthropogenic factors can cause seawater intrusion.Klassen, J., & Allen, D. M. (2017). Assessing the risk of saltwater intrusion in coastal aquifers. ''Journal of Hydrology (Amsterdam)'', ''551'', 730–745. Salinization of a freshwater aquifer can be caused by the inflow of seawater due to a change in groundwater pressure, reduction in water recharge, reduction in groundwater discharge, or other sources of

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...

entering the aquifer. The transition zone or the interface would shift either to the land or shift vertically.

Sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...

flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...

ing or

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...

also leads to

saltwater intrusion Saltwater intrusion is the movement of saline water into freshwater aquifers, which can lead to groundwater quality degradation, including drinking water sources, and other consequences. Saltwater intrusion can naturally occur in coastal aquifers, ...

into the freshwater aquifer.

Pumping-Induced Saltwater Up-coning

Pumping of groundwater from a well near the interface could cause seawater up-coning where seawater intrudes vertically to the aquifer. When the pumping rate exceeds the maximum pumping rate or critical pumping rate, seawater would be pumped out.

Effects of Heterogeneity and Anisotropy

Aquifers should be anisotropic and

heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

. A high permeability layer is associated with the freshwater flow. If the

of an aquifer in a vertical direction is lower than the horizontal direction, the transition zone would shift more in a horizontal direction.

Tides

Tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can ...

s can push the transition zone towards the land and widen the transition zone.

s can strengthen the freshwater seawater mixing due to

dampening Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples in ...

of the tidal pressure forming the non-uniform flow of groundwater. The greater the tidal amplitude, the greater the mixing effect. Less dampening in an aquifer would lead to a more uniform flow.

Other factors

Geological structures that cross-cut different layers such as faults or dykes can affect the direction and velocity of

. Dykes, an

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

unit that is impermeable or low in

might stop the intrusion of seawater. Faults also affect the direction of water flow which is studied by

Fault Zone Hydrogeology Fault zone hydrogeology is the study of how brittlely deformed rocks alter fluid flows in different lithological settings, such as clastic, igneous and carbonate rocks. Fluid movements, that can be quantified as permeability, can be facilitated o ...

. Both regular and catastrophic flooding can cause downward intrusion of seawater recreating the transition zone of the fresh-salt water.

Diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

would also be responsible for seawater intrusion, as there is a net flow of solutes from high concentration to low concentration. For example, a complete salinization of a 10 m thick layer of freshwater takes thousands of years through

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

. Other factors can be pumping-induced seawater intrusion.

Submarine Groundwater Discharge

Submarine groundwater discharge Submarine groundwater discharge (SGD) is a hydrological process which commonly occurs in coastal areas. It is described as submarine inflow of fresh-, and brackish groundwater from land into the sea. Submarine Groundwater Discharge is controlled by ...

(SGD) is the groundwater flows across the interface of the aquifer and the sea. It refers to the flow of water shifting towards the sea. Nearshore submarine groundwater discharge is defined as a range of 0 to 10 m, embayment SGD is defined as 10 m to 10 km and offshore SGD is defined as more than 10 km.

Topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary s ...

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

convention and tidal pumping are responsible for the driving force for the

Topography-Driven Flow

The

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary s ...

and

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

of an area affect the permeability and flow network of groundwater. For an

groundwater discharge Groundwater discharge is the volumetric flow rate of groundwater through an aquifer. Total groundwater discharge, as reported through a specified area, is similarly expressed as: :Q = \fracKA where :''Q'' is the total groundwater discharge ( 3·T� ...

s would be near shore and discharge decreases with the propagation towards the sea. For confined aquifers, groundwater can flow further towards the sea to the embayment zone or even develop submarine springs.

Groundwater Tidal Dynamics

Periodic sea level changes by

tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can ...

s would cause a fluctuation in the groundwater level of a coastal aquifer system. The tidal signal of sea tides becomes more attenuated and delayed with increasing distance to the land. Water level fluctuations in the wells can be caused by the degree of connection between the ocean and the aquifer; groundwater flows well connected to the sea; alternating loading and unloading of the sea tide leads to

plastic deformation In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strai ...

. Tidal efficiency of the magnitude of the oscillation of water level in a well to the oscillation of sea level is about 42% to 44%. For example, if the magnitude of the oscillation of the sea level is 1 meter, the magnitude of the oscillation of water level will be 0.42 to 0.44 meter. However, the further away from the coast, the groundwater fluctuation would be lower.

Chemistry of Coastal Groundwater Systems

The coastal groundwater system consists of terrestrial (

)

and a mixture of two.

Rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water ...

fall is the main source of the recharge of terrestrial groundwater. In the mixing zone,

dilution Dilution may refer to: * Reducing the concentration of a chemical * Serial dilution, a common way of going about this reduction of concentration * Homeopathic dilutions, Homeopathic dilution * Dilution (equation), an equation to calculate the rate ...

occurs that results in the different chemical compositions of water there.

Salinity

Marine, natural terrestrial and anthropogenic terrestrial is the source of salinity. The

total dissolved solids Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular ( colloidal sol) suspended form. TDS concentrations are often repor ...

(TDS) of the ocean are between 33 and 36.5 gl⁻¹. The TDS of standard seawater at 25^oC is 36gl⁻¹. TDS of seawater would be lower near the coast as there is fresh water supply through a river. The charge in TDS of seawater can indicate the existence of groundwater supply, submarine spring and the transition zone.

Electrical Conductivity

Electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

(EC) is another way to explain

. Electrical conductivity shows the ability of water to carry

electrical current Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...

. Higher electrical conductivity reflects a higher concentration of dissolved

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s. Electrical conductivity increases by 2% when the temperature increases by 1^oC.

Chemical Composition of Terrestrial Groundwater

Terrestrial groundwater is dominated by

cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosp ...

(K⁺),

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

(Na⁺),

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

(Ca⁺) and

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

(Mg⁺) and

anions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

(Cl^-), bicarbonate (HCO^3-) and

sulfate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...

(SO₄^2-). Each ion has a concentration of >1mgl⁻¹. The chemical composition highly depends on the geology which is the composition of local rocks and the chemical composition of recharge sources like rainfall and rivers. Fresh groundwater is likely alkaline as there is

and

Chemical Composition of Seawater

Seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...

is dominated by

(Na⁺) and

(Cl^-). The chemical composition of seawater has small variability between different oceans due to the long residence time which facilitates mixing.Millero, F. J., Feistel, R., Wright, D. G., & McDougall, T. J. (2008). The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale. ''Deep-Sea Research. Part I, Oceanographic Research Papers'', ''55''(1), 50–72.

Chemical Processes in Coastal Aquifer

Physical Physical may refer to: *Physical examination In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient for any possible medical signs or symptoms of a medical condition. It generally cons ...

and

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...

s occur in coastal aquifers, including oxidation-reduction reactions, mineral dissolution and precipitation, acid-base reactions,

ion exchange Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...

, and gases dissolution and exsolution.Appelo, C.A.J., & Postma, D. (2005). Geochemistry, Groundwater and Pollution. A. A. Balkema, London. Those chemical processes also happen in terrestrial aquifer system. The rate of different chemical processes depend on temperature and pressure of different part of the aquifers. Rain is the major recharge for different aquifers. Thus, groundwater would be acidic. Groundwater becomes less acidic with increases flow path or flow distance.

Ion Exchange

mainly contains

and

while fresh groundwater is dominated by

and

bicarbonate In inorganic chemistry, bicarbonate ( IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial bioch ...

Cation exchange Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...

occurs in the transition zone given by the chemical equation: Na⁺+1/2Ca-X₂ → 1/2Ca²⁺+Na-X, where X is the exchange site on the soil particles. Compared to seawater, the water in the transition zone would have excess

and be depleted in

. Compare with fresh water, it is the opposite case. Other cations like

and

exchange as well.

Magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

and

would be exchanged for

In a limestone aquifer,

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scra ...

dissolves due to the acidity of groundwater. With the presence of

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

may form in the transition zone.Hanshaw, B. B., Back, W., & Deike, R. G. (1971). A geochemical hypothesis for dolomitization by ground water. ''Economic Geology'', ''66''(5), 710-724.

Mineral Dissolution and Precipitation

As rain is acidic, it dissolves different

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...

s. For example, rain dissolves

inside the aquifer. In the transition zone where fresh groundwater meets seawater,

dolomitization Dolomitization is a geological process by which the carbonate mineral dolomite is formed when magnesium ions replace calcium ions in another carbonate mineral, calcite. It is common for this mineral alteration into dolomite to take place due to e ...

occurs due to abundant

of seawater. Lead to

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...

Reduction-Oxidation Reactions

Reduction-Oxidation reactions (Redox) takes place in the recharge areas where

organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...

is available.

Oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

dissolves into freshwater when rainfall or river penetrates soil with organic matter.Abesser, C., Shand, P., & Ingram, J. (2005). Millstone Grit of Northern England. ''Baseline Report Series 18''. Environment Agency, Bristol, UK. Oxygen may lost under

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

reactions and microbiological processes. Oxidation of

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

or sulphide minerals also consume the dissolved oxygen inside water. Dissolved oxygen concentration level in groundwater decreases during long travel distance. Anaerobic conditions occur in deep

confined aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...

. Under anaerobic conditions, sulphate reduction,

methanogenesis Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a gro ...

and

ferric iron In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. The adjective ferric or the prefix ferri- is often used to ...

reduction might occur. Leading to dissolve of

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy u ...

nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . It is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the productio ...

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...

and

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The und ...

into groundwater.

Chemical and Isotopic Indicators

The change in the chemical composition of groundwater is an indicator of

. It prevents the multi-sources of

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...

, leading to a change in

of groundwater. Chemical concentration ratios including Na/Cl, Ca/Cl, Mg/Ca, Cl/Br, Ca/Mg and Cl/HCO3 can be used to distinguish the seawater intrusion.

Anthropogenic Impacts

Reclamation

Land reclamation Land reclamation, usually known as reclamation, and also known as land fill (not to be confused with a waste landfill), is the process of creating new land from oceans, seas, riverbeds or lake beds. The land reclaimed is known as reclam ...

is a way to create flat land in

coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...

al areas for coastal urban development. Reclamations affect regional groundwater flow systems, the location of groundwater discharge zones, seepage zones, groundwater divide, the interface between seawater and fresh groundwater and the water chemistry.Hu, L., & Jiao, J. J. (2010). Modeling the influences of land reclamation on groundwater systems: A case study in Shekou peninsula, Shenzhen, China. ''Engineering Geology'', ''114''(3), 144–153. It takes years to decades to reach a new equilibrium after reclamation.

Reclamation – Chemical and Groundwater Change

Land-fill materials would change the equilibrium of the coastal areas. Land-fill materials interact with

and

marine sediment Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles have their origins in soil and rocks and have been transported from the land to the sea, mai ...

s chemically and physically. Fill materials can be

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...

, completely weathered

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...

or even

waste Waste (or wastes) are unwanted or unusable materials. Waste is any substance discarded after primary use, or is worthless, defective and of no use. A by-product, by contrast is a joint product of relatively minor economic value. A waste pr ...

. The placement of fill materials and diversity of fill materials can make the aquifer heterogeneous.Jiao, J.J. (2002). Preliminary conceptual study on impact of land reclamation on groundwater flow and contaminant migration in Penny’s Bay. ''Hong Kong Geology 8'': 14–20. It is difficult to determine the chemical change in groundwater in a general way. It depends on the fill materials. The

after

reclamation Reclaim, reclaimed, reclaimer, reclaiming or reclamation means "to get something back". It may refer to: * Land reclamation, creating new land from oceans, riverbeds, or lake beds * Dedesertification, reversing of the land degradation in arid ...

decreases as the fill increases subsurface flow path length. As a result, the water level elevation in the upstream or attitude would rise. Rainfall recharges the new land and increases the storage capacity of the land. Increasing the water level and seaward groundwater discharge and shifting the water divide in the future. Lengthening of the groundwater flow paths can dissolve the pollutants inside the marine mud and bring the pollutants to the ocean.

Sea Level Changes to Coastal Hydrogeology

Sea level rise in a unconfined system coloured

and

ing would push the interface towards the land. Thus, seawater intrudes the land vertically and horizontally.Barlow, P. M. (2003). ''Ground water in freshwater-saltwater environments of the Atlantic Coast / by Paul M. Barlow''. U.S. Dept. of the Interior, U.S. Geological Survey, 2003. A reduction in the recharge area leads to a reduction in the discharge of groundwater and groundwater level. For islands, sea level rise would reduce the freshwater volume resulting in a smaller freshwater lens. Sea level rise also increases the coastal erosion rate leading to

shore A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water p ...

line retreat.

Seawater Intrusion Management

Seawater intrusion leads to social and economic loss. It is important to promote seawater intrusion management to prevent seawater intrusion including increases in freshwater recharge, pumping control, well design improvement, building barriers and land and water management.

Increase of Freshwater Recharge

For areas that have sufficient fresh water supply, fresh water can be injected into the aquifers named

aquifer storage and recovery Aquifer storage and recovery (ASR) is the direct injection of surface water supplies such as potable water, reclaimed water (i.e. rainwater), or river water into an aquifer for later recovery and use. The injection and extraction is often done by ...

(ASR). Apart from injection to pumping wells,

infiltration basin An infiltration basin (or recharge basin) is a form of engineered sump or percolation pond that is used to manage stormwater runoff, prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay. It i ...

s or

canal Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface fl ...

s are used for

infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...

and the replenishment of groundwater.

Well Design Improvements

Improvements on the location and design of a well can minimize the effect of seawater intrusion. For example, building a well that is close to the water table, having a well that far away from the transition zone. Multiple wells with low pumping rate, horizontal pumping wells and radial wells can be built to reduce the chance of seawater intrusion. As well as calculating the maximum pumping rate and critical pumping rate of a well. Prevent the pumping rate from exceeding the limit which is over-pumping.

Engineered Barriers

Types of barriers to control seawater intrusion modified

Engineered barriers can be built to reduce freshwater flow to the sea and seawater intrusion into the aquifer. Engineered barriers can be hydraulic barriers or physical barriers. Hydraulic barriers operate by injecting fresh water into the well or pumping saltwater from the well. Water from

river A river is a natural flowing watercourse, usually freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the ...

, and

treated wastewater Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environmen ...

can be injected into the well. The location of the injection well should be far away from the pumping well to prevent the neutralization of pumping and injection of water. Physical barriers are impermeable walls, that cut off the interaction between fresh groundwater and seawater. Cut-off walls have been built since the 1970s.

Slurry wall A slurry wall is a civil engineering technique used to build reinforced concrete walls in areas of soft earth close to open water, or with a high groundwater table. This technique is typically used to build diaphragm (water-blocking) walls surr ...

s and grout walls are a type of cut-off walls.

s are made of

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

and

bentonite Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-m ...

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...

, forming an impermeable wall.Paul, D. B., & Davidson, R. R. (1992). ''Slurry walls: Design, construction, and quality control'' (Vol. 4). ASTM International. Grout walls are made of

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement m ...

or silicate reagents.

References

Hydrology Oceans