7

CHAPTER 2

REVIEW OF LITERATURE

2.1 GENERAL

The quality of groundwater in coastal aquifers deteriorates as a

result of the over pumping of water that was previously discharged into the

sea (Gallardo and Marui 2007). Saltwater intrusion problems are common in

coastal areas throughout the world and the same has been reported by many

researchers (Appelo et al 1989; Gnanasundar and Elango 1999; Bakker 2000;

Shammas and Jacks 2007; Chenini and Khemiri 2009; Sathish et al 2009;

Praveena and Aris 2010; Bhuvana and Ramesh 2012). The intrusion of

seawater has been identified by many approaches such as isotope studies

(Ganyaglo et al 2011; Yechieli and Sivan 2008) and geochemical and

geophysical studies (Melloul and Goldenberg 1997; Gnanasundar and Elango

1999; Di Sipio et al 2006; Aris et al 2009; Korfali and Jurdi 2010; Sathish et

al 2011a; 2011b). Measurement, monitoring, modelling and modifications are

the four components to maintain the aquifer withstanding capability

especially in seawater intruded aquifer (Rahim and Ghani 2002). Since 1950’s

the analysis on groundwater composition has been providing information on

geological conditions (Chebotarev 1955). To maintain the quality of

groundwater and to prevent it from seawater intrusion, the flow and quality of

groundwater in coastal aquifer must be studied. Groundwater flow models

which are the duplication of the real field conditions are the useful tool to

understand the aquifer condition. Such studies carried out by various

researchers in the world are reviewed in this section. This chapter focuses on

the various literatures related to the local relationships among groundwater

8

chemistry, geo-electrical properties of the subsurface and modelling of

groundwater to understand the status of aquifer.

2.1.1 Geo-electrical Methods

Since 1920’s the geo-electrical methods are in use for the

identification of different subsurface formations, based on their electrical

resistance. Such surveys are useful in groundwater studies, numerous

environmental surveys and geotechnical investigations including preparing

for building, bridge and dam foundations, archeological prospecting and

landfill delineation (Zonge et al 1985; Garman and Purcell 2003; Srinivasa

Gowd 2004; Anderson and Tahsin 2006; Yadav and Singh 2007; Yechieli and

Sivan 2008; Idowu et al 2008; Chandra et al 2008; Aboh 2009; Balkaya et al

2009).

The geo-electrical resistivity survey holds promise to find out the

zone of contamination, especially seawater intrusion due to concentration

gradient between seawater and freshwater (Patra and Bhattacharya 1967;

Mooney 1980 and Barker 1990). The resistivity value for saturated aquifer in

Maryland varies between 100 Ohm-m and 250 Ohm-m which is the range for

basaltic aquifers in southern Idaho (Zohdy 1974). The resistivity values for

unconsolidated aquifer which holds saltwater are less than 1 Ohm-m and

several thousand Ohm-m for dry sands, gravels and basalt. Similarly, the

range of resistivity for a formation may differ from place to place.

The resistivity of ground is related to various geological parameters

such as the mineral present in the aquifer matrix, porosity and degree of

saturation of the aquifer. The advantages of resistivity survey are

9

(1) To avoid intrusive techniques and direct sampling

(2) Relatively inexpensive and can be used for rapid monitoring

of large areas and optimization of the required number of

monitoring wells

(3) Readily identify the degree of groundwater contamination

(Ebraheem et al 1990; 1997; El Mahmoudi 1999).

Bugg and Lloyd (1976) used resistivity method to delineate

freshwater lenses in the Cayman Islands of the north Caribbean. Due to lesser

density, freshwater lenses rest on the top of the saline water derived from sea.

Due to tidal effect, a transition zone has been developed along the boundary

between freshwater and seawater. Thus, resistivity method provides a rapid

and cheap method of mapping the transition zone. The empirical relationship

between hydraulic conductivity and aquifer electrical resistivity was studied

by Kelly (1977 using Vertical Electrical Sounding (VES) technique. In that

study two type of curves were identified with four layers including topsoil,

unsaturated, saturated and bed rock. Pumping tests were carried out in six

locations and the correlation between aquifer resistivity and hydraulic

conductivity was made and good results were obtained. Similar work was also

carried out by Warner (1969), Merkel (1972), Stollar and Roux (1975) and

Mazac et al (1987).

The geo-electrical survey was successfully used by Fretwell and

Stewart (1981) to delineate the seawater intrusion in west central Gulf coast

of Florida using geo-electrical survey. Four geologic layers were identified

and the first three layers were sand, carbonate zone and less porous carbonate

zone. The fourth layer was fully saturated with salt water showing resistivity

value less than 1 ohm-m. Towards the coastal boundary, the depth of interface

between freshwater and salt water was decreased and due to the presence of

seawater the further deep lithological information was hidden.

10

The coastal groundwater condition and problem due to seawater

intrusion in Mtwara town, south eastern Tanzania was studied by Gondwe

(1990). The aquifer in the study area comprised of alluvium, quaternary

sediments and marine neogene formation from Mtwara in north to Lindi Bay.

The apparent resistivity of the seawater intruded area was less that 10 Ohm-

m. The withdrawal of groundwater was compensated for by annual recharge

in the area. Hence the seawater problem in the area is not because of up-

coning of seawater by heavy withdrawal of groundwater but the residual

seawater after regression of sea. The presence of corallites confirms this area

was present under sea in past.

Recently, the electrical resistivity imaging survey has been widely

used to map groundwater contamination and for various environmental

surveys (Griffiths and Barker 1993; Sathish et al 2011a; 2011b; Satriani et al

2012). The contamination of groundwater due to improper sewage, improper

irrigation drainage system, leachate from old lagoon and seawater intrusion

was studied by Ebraheem et al (1997) with the help of VES and Total

Dissolved Solids (TDS) present in the groundwater. The vertical and lateral

salinity variation has been measured in the monitoring wells. The empirical

relationship between TDS and VES data were obtained. The relationship was

used to construct three zones of water bearing formation (fresh, brackish and

saline water zones). The depth to the fresh-brackish interface exceeds 150 m

at Tanta City and decreases northward to 40 m or less between Qutor and

Kafr El-Sheikh. Depth to the brackish-saline interface exceeds 180 m south of

Kafr-El-Sheikh and decreases northward to 70 m near Hadadi village.

Courteaud and Robineau (1998) has used electrical resistivity

survey in lower north eastern rift zone of Piton de la Fournaise to evaluate the

possibility of locating volcanic medium in areas with the help of known

subsurface geology. The shallow geological structures have been inferred by

11

using electrical interpretation. The relationship between groundwater

resources and geological structures present in this area was estimated. The

aquifer was broadly divided into two parts: southern section and northern

section. In the southern section, the freshwater lens was present on the

saltwater. In the northern section, the seawater wedge is disturbed by the

presence of clay and other less permeable materials.

Nassir et al (2000) have studied the saltwater intrusion boundary in

the Yan aquifer, Northwest Malaysia. The inversed resistivity image

delineated the boundary between seawater and freshwater because of its

inherent capability to detect the changes in electrical conductivity of

groundwater. This information of shallow and deep layers increases the

resolution of the image. It shows the usability of geo-electrical image for

groundwater investigation in coastal areas especially for seawater intrusion.

Shtivelman and Goldman (2000) studied the coastal aquifer of Israel along the

Mediterranean shore line. This aquifer is separated into subdivisions by an

impermeable layer. The groundwater quality within each sub aquifer was

measured using geo-electrical surveys. Choudhury et al (2001) conducted

electrical resistivity and seismic refraction surveys in the coastal belt of

Digha, Eastern India with the objective to find salinity contamination.

Different subsurface geological formations such as dune sand, top sandy soil,

saline sand and saline clay were identified on the basis of resistivity data and

velocity of seismic waves. The thickness of saline zone decreases towards

land and the presence of clay, which extends into the sea, acts as a barrier for

seawater intrusion into the deep aquifer.

Resistivity survey, along with geochemical analysis proved to be an

effective tool to identify the freshwater and saline water zones. The

integration was helpful to identify the narrow saline water zone. To avoid an

ambiguity due to same apparent resistivity from various formations such as

12

saline clay and saline sand, an integration of geophysical and hydro-

geochemical data has been applied by Choudhury and Saha (2004). Di Sipio

et al (2006) have compared geophysical studies in Venice lagoon with the

chemical and physical properties of the aquifer. There is a displacement due

to the lineament through which the saline water intrudes from the sea, lagoon

and from fossil brines that are mobilized due to the tectonic action and

infiltration of freshwater.

Shevnin et al (2006) described petro-physical properties in relation

with the resistivity value of formation. They discussed variation in the

resistivity value with respect to different proportion of sand and clay. If the

salinity is less than 5g/l in the sand and clay mixture, the resistivity curve is

not parallel to the water resistivity and also the parallelism with sand curve

depends on the clay content. Less clay content causes parallelism with the

sand curve very early. It states that sand has a Cation Exchange Capacity

(CEC) of less than 4 milli-equivalents per 100 g and clay has a CEC of more

than 25 milli-equivalents per 100 g. The presence of organic material

increases CEC. The porosity and presence of clay content is examined by the

known value of salinity and resistivity data.

Song et al (2007) studied the intrusion of seawater in coastal area of

Byunson peninsular, Korea. They demarcated the seawater wedge without

drilling borehole which is very expensive. The main objective is the potential

applicability of VES to delineate the seawater intrusion. The VES survey has

been carried out in thirty locations and the groundwater quality has been

monitored in fifteen nearby wells. Ionic ratios like Cl-/HCO3

-, Ca

2+/Na

+

confirms the intrusion of seawater and the zone affected by seawater can be

identified by low apparent resistivity. Lee and Song (2007a) have made use of

the of water level fluctuation and electrical conductivity to identify the zone

influenced by seawater and various ionic ratios were calculated and compared

13

with apparent resistivity data to suggest the usability of groundwater for

agricultural usage in the coastal area of Byunson peninsular, Korea. The

dominant type of water is Na+-Cl

- and it is followed by Ca

2+-Cl

-. Whereas the

former is fully affected by seawater, the latter is the leading edge of the plume

of seawater.

The seawater intrusion due to urbanization and over pumping was

studied by Cimino et al (2008). In this study VES survey with schlumberger

array was used in Acquedolci coastal aquifer, Northern Sicily, to delineate the

zone of seawater intrusion. The resulted output of the VES data was

correlated with chemical parameters of the groundwater and the narrow belt

of freshwater and saltwater mixing was demarcated. Previously, this area

undergone pollution risk studies and an intrinsic vulnerable zone was

identified and correlated with the presented result. The motivation of this

work is to integrate different methodologies to describe the degradation of

aquifers on the whole as well as to assess their pollution risk better.

The resistivity data is usually inversed with minimum residual error

and the number of layers and zone of contamination were identified. The

inversion algorithm used, is iterative and fully automated. This was used by

Adepelumi et al (2009) in Lekki peninsula, Lagos, Nigeria. Three traverses of

VES were carried out in the area and four layers were identified. The

resistivity values for each layer were described. The third layer comprised of

mixture of sand and sandy clay, shows apparent resistivity ranges from 8

Ohm-m to 130 Ohm-m. It is identified that two major saltwater intrusion

zones are generally high in the central part of the traverses. The suggested

reason for seawater intrusion is over pumping which disturbs the

hydrodynamic equilibrium and the reduction of groundwater gradients which

allows saline water to displace fresh water in the aquifer. The occurrence of

saline water could also be explained as the trapped saline water during the

14

transgressive and regressive movement of the sea during the quaternary

period.

The zone of brackish water is identified in north Kalantan plain

using electrical resistivity imaging (Islami 2010). The depth of investigation

is around 20 m below ground level and the zone of brackish water is

identified. Sathish et al (2011a) have studied three dimensional electrical

resistivity model and two dimensional electrical resistivity image in south of

Chennai, India. The objective of this work was to assess the zone of mixing

between seawater and groundwater by using advanced multi-electrode

resistivity survey with IRIS make SYSCAL Pro-96. The two dimensional and

three dimensional imaging of subsurface study started at the distance of 5 m

to 80 m away from the coastal boundary. The maximum length of the image

was 170 m. The zone of mixing is identified more towards northern part of

the study area due to the dense population.

Sathish et al (2011b) have carried out two dimensional High

Resolution Electrical Resistivity Tomography (HERT) survey perpendicular

to the coastal boundary, at the distance of 5 m to 80 m from it. The maximum

length of the HERT image is 170 m with the maximum depth of penetration

of 28.7 m. The four layers including unsaturated quaternary formation,

saturated quaternary formation, weathered charnockite and basement hard

rock were identified and the range of apparent resistivity for each layer is

identified. These layers are compared with the subsurface lithology model

obtained by available borehole logs using Rockworks software to confirm the

interpretation. The zone of low resistivity in each image perpendicular to the

sea has been delineated as a zone of mixing between seawater and freshwater.

The various concentration of groundwater in nearby wells includes electrical

conductivity, sodium, chloride and Cl-/HCO3

- have been measured to confirm

the influence of seawater. Similarly, the concentration profiles of the

parameters mentioned above were correlated with the HERT image. Asfahani

and Abouzakhem (2013) used integrated approach of geoelectrical and

15

hydrogeochemical studies for mapping of the zone of interface between

seawater and freshwater was done successfully.

2.1.2 Hydrogeochemical Studies

Hydrogeochemical studies of groundwater have been used to define

zone of contamination, zone of vulnerability and their sources and the

suitability of groundwater usage for various purposes. The coastal aquifer is

facing salinisation mainly due to human influences such as over exploitation,

destruction of beach dunes and irrigation and natural influences such as

evapotranspiration of irrigation water (Pinder 1973; Hajalilou and Khaleghi

2009; Lenahan and Bristow 2010). Daskalaki and Voudouris (2008) said that

an improper pumping and development of settlement, tourism places,

fertilizers from agricultural activities, disposal of untreated water are the

reason for groundwater pollution in various parts of Greece.

There are many ways by which seawater may contaminate

freshwater aquifer and are direct lateral intrusion, up-coning from deeper part

of the aquifer, downward intrusion from coastal waters (e.g. Estuaries and

embayments), saltwater flooding in to coastal lowlands. It can also occur

along open boreholes, abandoned wells, corroded wells and dredged channels

(Barlow and Reichard, 2009). Stuyfzand and Sturrman (1994) explained that

around eleven kinds of mechanisms take place, besides the process of

salinisation, in the Netherland (Fidelibus, 2003). Seawater intrusion changes

the groundwater composition and the geochemical process includes ion

exchange, redox reactions, dissolution, precipitation and sulphate reduction

(Beekman 1991). The evolution of freshwater facies (Ca2+

-HCO3-) in to

seawater facies (Na+-Cl

-) is described by Fiky (2010).

The causes for evolution of salinisation are often studied, whether it

is due to natural influences, such as infiltration of surface water or derived

16

from deep brines or dissolution of evaporates, or human influences such as

inward movement of seawater due to pumping (Howard and Lloyd 1984;

Howard and Mullings 1996; Kim et al 2006; Sdao et al 2011). The evolution

of groundwater chemistry is high with the increasing time of contact of

groundwater with aquifer matrix. Gimenez and Morell (1997) examined the

process of salinisation in the coastal aquifer of Oropesa, Spain. The evolution

of salinisation is due to the upconing of deep depleted paleo-seawater. The

old seawater composition has been modified by the increase of minor ions.

This is due to the long residential time with aquifer material in which the

composition undergoes modification by exchange process with aquifer

material.

The evaporation of surface water body or water within the

unsaturated zone changes the constituent of groundwater (Drever 1982). The

estuary which is the junction of fresh surface water and saline surface water

will affect the hydrodynamic equilibrium between freshwater and saltwater in

the aquifer (Abam 2001). He also said that the disturbance of surface water

bodies by dredging the channel or construction of dams upstream side cause

changes in ecological system. The presence of saline water in the estuary due

to tidal influence will affect the freshwater aquifer below this surface water

body. This has been reported by Carr (1969). This is one of the main

problems in Bangladesh (Rahman and Bhattacharya 2006). The major

estuarial rivers and flood plains are present here and seawater intrusion is the

main problem. The salinity in the groundwater occurred due to cyclones,

storm surges, high spring tide inundation and capillary action. The intrusion

of seawater is a time varying event which is less during the period of June to

October. During this period the discharge into the sea is high and displaces

the interface seaward in estuaries and flood plains. Starting from the month of

November the seawater intrudes inland.

17

The geochemical process due to cation exchange is more common

in the coastal aquifer and promotes the salinisation and freshening of

groundwater due to recharge and discharge (Desai et al 1979; Cerling et al.

1989; Appelo et al 1990; Pulido-Leboeuf 2004; Andersen et al 2005;

Stuyfzand 2008). Jiaguo et al (2012) studied the process of dissolution and

precipitation along with ion exchange processes in the groundwater, river

water and seawater in Dagu river basin, China. They reported that calcite is in

supersaturated condition when seawater percentage increased above 40%. The

lesser the pH of the groundwater more the reaction with the aquifer matrix.

Martinez and Bocanegra (2002) identified that cation exchange processes and

calcite equilibrium are important hydrogeochemical processes that control

groundwater composition in the Mar del Plata aquifer, Argentina. Montety et

al (2006) studied the process of exchange reaction which was dominated by

calcite and dolomite precipitation in the Rhone delta. The temporal changes

usually occur in the groundwater quality due to recharge and discharge of

groundwater (Scheytt 1997; Moller et al 2007). Elango (1992) studied the

hydrogeochemical nature of the multi-layered aquifer of North Chennai and

brought out the relation of groundwater recharge to flow mechanisms. Along

the flow direction the freshening of groundwater takes place easily and the ion

exchange process is direct in which removal of Na+ to groundwater takes

place in the Castell de Ferro coastal aquifer, Spain which was explained by

Pulido-Leboeuf (2004). The complete recovering of groundwater quality may

be possible by rainwater harvesting, reducing pumping and rearrangement of

points of extraction of groundwater Bhattacharya et al (2008).

Elango et al (2003) identified carbonate weathering, silicate

weathering and ion exchange processes are responsible for groundwater

chemical composition in a part of Kancheepuram District, Tamil Nadu, India.

Thus, knowledge of hydrogeochemical processes that control groundwater

chemical evolution could lead to improved understanding of

hydrogeochemical characteristics of an aquifer. It was also explained by Jalali

(2007).

18

Scott and Vanderwal (1992) explained that dissolution of carbonate

mineral will increase the calcium and bicarbonate in the groundwater.

Decomposition of organic matter will lead to increased ammonium, carbonic

acid and organic complexes. It results in oxygen depletion and de-

nitrification. Similarly the pollution created by human influences such as

agricultural activity, seepage from septic tanks will increase the concentration

of nitrate, sulphate and potassium in the groundwater.

The knowledge about ionic ratios of freshwater and seawater is

used to identify the source of evolution of groundwater quality and various

ions present in the groundwater. Andreasen and Fleck (1997) used bromide

and chloride ratio as an indicator to identify the influence of contamination

source whether from anthropogenic such as effluent from septic tanks, sewer

lines, road-deicing salt, storm water infiltration and domestic effluent or from

seawater in Chesapeake Bay. Recently Han et al (2011) used bromide and

chloride ratio as an indicator for seawater intrusion. The ionic ratio between

iodine and chloride shows 10-100 times than in seawater probably from the

other source such as organic matter and redox process (Kim et al 2006).

El-Kashouty and El-Sabbagh (2005) came out with the

hydrogeological regime and hydrogeochemistry of groundwater in Nile delta,

Egypt and delineated the limit of seawater intrusion. This study reveals that

EC, Br and Mg2+

were produced by seawater intrusion and Ca2+

, F and I were

produced by anthropogenic activity. They delineated the limit of seawater

intrusion and the influence of hydrogeologic setting, historical pumping rate

and recent rise in population.

Korfali and Jurdi (2010) said that most common indicator of

seawater intrusion is increasing of chloride concentration, hardness and

conductivity of groundwater. The ionic ratio between chloride, carbonate and

bicarbonate called Simpson’s ratio is widely used as an indicator of seawater

19

intrusion in to the freshwater aquifer. The ionic ratio between Na+ and Cl

- is

usually less than marine water in the seawater intruded area. Similarly

Na+/Ca

2+ and Mg

2+/Ca

2+ are also used as good indicators for seawater

intrusion.

Sola et al (2010) used fraction of exchange between seawater and

freshwater to confirm the mixing of seawater in coastal aquifer of Almeria,

SE Spain. The fraction of exchange has been calculated to avoid the

ambiguity with ion exchange between aquifer and groundwater. The

percentage of seawater mixing was also used by Mondal et al (2010) in their

case study on seawater impact in Sadras, Tamil Nadu, India.

Mohamed and Worden (2005) carried out geochemical analysis to

evaluate the controlling factor for seawater intrusion in the Sherwood

sandstone, Merseyside, United Kingdom. In this region, apart from pumping,

other hydrogeological settings like fault and fold play a major role in the

groundwater chemistry. This was correlated by the spatial variation of

concentration of ions.

Baharuddin et al (2012) explained the behavior of coastal aquifer in

Carey Island in Malaysia which has a limited recharge area. There are two

types of landforms in which the agricultural activities, especially oil palm

plantation taken place. The suitability of groundwater for oil palm cultivations

was studied in this aquifer which is present under complex hydrogeological

settings. The integration of geo-electric and geochemical data has been

demonstrated. They reported that this area is no longer useful for cultivation.

This is due to causes of seawater intrusion which is already pronounced in the

marine eroded area and irrigation.

Kim et al (2006) carried out hydrogeochemical analysis to identify

the sources of salinity in land reclamation in Yeonggwant, Korea. Two types

20

of groundwater are Na+-Cl

- and Ca

2+-Cl

- types. The influence of fertilizer and

cation exchange in the aquifer material resulted in later type of groundwater.

The study explained that the nitrate is derived from fertilizer through

agriculture activity. The Na+-Cl

- type of groundwater is due to up-coning of

connate saline water at greater depth. The salinity is also caused by redox

process motivated by organic matter being buried due to land reclamation.

Agrochemicals are the main sources of nitrogen and other organic and

inorganic contaminants in the groundwater regime. Several authors have

reported the presence of agrochemicals in soils (Muir and Baker 1978).

Vitousek (1977) suggested that in humid climates, the process of

evaporation and crystallization is replaced by evapo-transpiration. Further, he

explained that sulfate and chloride are controlled by precipitation and evapo-

transpiration; sodium, silica, calcium and magnesium are controlled by

mineral weathering, while nitrate and potassium are controlled by plant

uptake.

Tole (1997) reported that groundwater is polluted by seawater

intrusion up to 6 km in Kwale district, south east coast of Kenya. It is mainly

due to over pumping especially in one place where the deep borehole

influences the quality of adjacent region as well. Even though most of the

wells are shallow, they are susceptible to contamination from septic tanks. In

few places the groundwater is influenced bacterially (E-Coli).

Gimenez-Forcada et al (2010) recognize three principal

mechanisms that take place in the coastal hydrogeochemistry in Elba Island,

Tuscany. Here the salinisation is not only by seawater intrusion but also due

to over exploitation. The presence of Mg, Ca2+

-HCO3- and Ca

2+, Na

+-HCO3

-

facies indicates the groundwater derived from weathering process. Three

mechanisms have been recognized for the groundwater quality in this aquifer

such as recharge that controls the weathering mechanism, mixing of seawater

21

and secondary reactions including cation exchange. The process of silicate

weathering plays a major role in deciding the groundwater chemistry.

Lee and Song (2007a; 2007b) used various ionic ratio including

HCO3-/Cl

-, Na

+/Ca

2+, Ca

2+/Cl

-, Mg

2+/Cl

- and Ca

2+/SO4

2- which are the most

useful indicators to delineate seawater intrusion in the coastal aquifers of

Buan, Korea. In coastal area, the interface between seawater and freshwater is

always present at the normal condition. The increase of pumping might

increase the inland distance of seawater wedge. To determine the groundwater

salinity, whether by anthropogenic origin or natural phenomena, long term

monitoring of groundwater chemistry required.

Kim et al (2009) studied the freshwater and seawater interactions

based on rainfall and tidal effect. This study was carried out in the coastal

zone of Jeju island, which is composed of acicular basalt, hydroclastites and

sandy/muddy sediments. They obtained time series data of water level,

temperature and EC at various depths by using multi-depth monitoring setup.

The mixing process of freshwater and seawater is varied enormously due to

tidal affect, rainfall recharge and hydrogeologic properties of the aquifer.

Terzic et al (2008) studied the hydrogeochemical behavior during

rainy season and summer period. This study was carried out in Blato aquifer,

island of Korcula, Croatia. During the summer period, the pumping rate was

very high and resulted in seawater intrusion. The presence of nitrate due to

agricultural activity and other human induced pollution was washed out

during the period of rainy season.

From all the above cited reviews, it is clear that studying and

understanding the geochemistry of groundwater is of high importance.

Hydrogeochemical studies helps to understand the process and behavior of

aquifer. It also helps to understand importance of regular monitoring of

22

groundwater quality. The continuous monitoring of groundwater quality in

coastal region provides information of process relating freshwater to seawater.

It can be used to delineate the zone of seawater influence and to understand

the changes in groundwater quality with respect to recharge and discharge of

groundwater.

2.1.3 Groundwater Modelling

Numerical modelling is one of the widely used approaches, due to

usability of complex boundary and aquifer condition (Freeze and Cherry

1979). It is one of the most important developments in hydrogeology and

widely used since 1960’s. Groundwater flow and saltwater intrusion in coastal

aquifer can be understood by using data extracted from the field. The field

condition even though more complex, has been incorporable into the

conceptualized model to find a solution with maximum accuracy. The lacking

of data may results in to less accuracy in the result. The parameters varying

within the model area have been assigned the form of matrix. Presently

several types of models are available to solve the groundwater problem in

coastal aquifer. These includes Sutra (Voss, 1984) SEAWAT (Guo and

Bennett 1998), CODESA3D (Gambolati et al 1999), GEO-SWIM

(Aharmouch and Larabi 2004), FEFLOW (Diersch 2005) etc., (Larabi 2007).

The development of a model has its own trend to achieve more relevant

solutions that may apply to the real world. The development of a model to

predict the seawater interface was explained by Abd-Elhamid and Javadi

(2011). Their study reveals that various models considering various

mechanisms. Few models are using steady state flow condition (Bakker and

Schaars 2012) and few models are using transient flow condition

(Psarropoulou and Karatzas 2012). Starting from the development of the

various numerical codes such as finite difference, finite element, finite

volumes, boundary element methods etc., and numerous developments was

achieved to obtain better results. Alternatively, the error reduction

methodology development is also in progress to create better results. Recently

23

various codes are developed in order to avoid lacking of data (Smaoui et al

2012; Engelhardt et al 2013).

Initially, in most of the cases, seawater intrusion was studied by

using sharp interface between freshwater and saltwater. Similarly, in most of

the cases, Ghyben Herzberg balance between seawater and freshwater in

which the Dupuit assumptions which is outflow face along the coastal

boundary, were not considered. Shamir and Dagan (1971) used numerical

schemes to solve the linearized equations of the seawater intrusion with the

consideration of Dupuit assumption with sharp interface approach. Wilson

and Sa Da Costa (1982) used the node point in the moving boundary and the

influence of layer discontinuity in the freshwater and saltwater interface was

studied. They used one dimensional finite element approach for groundwater

flow in a two layer model.

The Henry problem (Henry 1964) is the benchmark to validate the

numerical models. The modified and improved Henry analytical solution was

presented by Simpson and Clement (2003) in which the isochars are smooth

and reach the upper boundary. Henry, Pinder and Cooper are pioneer of the

saltwater intrusion studies. They were the first to simulate the saltwater

intrusion by using immiscible fluids (Diersch and Kolditz 2005).

There are many procedures such as direct differentiation, local and

global projection etc., to compute the Darcy flux, mass flux and heat flux.

Bues and Oltean (1999) developed a code which is able to converse cell-wise

mass distribution. This code is hybrid between mixed Finite Element Model

(FEM) and discontinuous FEM. This code was tested with benchmarks such

as Elder’s problem (Elder 1967) and Henry’s problem (Henry 1964). The

comparison of the results derived from this model with the results from semi-

analytical solution holds good match and proves the suitability of this

approach in seawater intrusion.

24

Huyakorn et al (1987) developed a three dimensional finite element

model for saltwater intrusion. It was based on the groundwater level and

concentration of the groundwater instead of pressure and density. The

saltwater intrusion in a confined aquifer was analyzed by using this three

dimensional finite element model. The result hold a good fit with Henry

analytical solution and the results obtained by Segol and Pinder (1976).

The development of models between one-dimensional, two-

dimensional and three-dimensional are interrelated under various stages of

approaches. The behavior of aquifer and flow mechanisms can be better

understood by using numerical three dimensional models (Cheng and Everett

2009). In 1992, USGS, along with NYCDEP, developed a three-dimensional

four-layer model of Brooklyn Queen’s aquifer system to find out the place of

yielding well (Kontis 1999). In the work of Kontis, a two-dimensional model

was developed to study the fluctuation of transition zone between freshwater and

saltwater under different pumping and recharge rates in the same aquifer

system. The motivation of this two dimensional study was to identify the

stability of transition zone in order to use as stationary boundary in three

dimensional model.

The effects of heterogeneity have been numerically studied by Fan

et al (1997). Their work demonstrates the influence of heterogeneity on flow

pattern. This is also explained by Revelli and Ridolfi (2000). The

management of groundwater resources was done by many authors with the

help of numerical modelling. The groundwater management studies were

done by evaluating the stress period on the aquifer system in coastal plain of

North Carolina, USA Reynolds and Spruill (1995). Fernando and Gerardo

(1999) used groundwater flow modelling to examine the impact of

urbanization of Aguascalientes, Mexico. They also recommended the place

for a new well field for safe yield. Kihm et al (2007) developed a model to

study the effect of groundwater pumping in the groundwater flow mechanism,

25

groundwater management and deformation of landforms in mountain plains,

Gyeonggi-Do, Republic of Korea.

Lee and Cheng (1974) used finite element approach and identified

the saltwater intrusion under steady state condition in two-dimensional coastal

aquifer. This result was compared with Henry analytical solution (Henry

1964) and the result held a good fit with it. Jakovovic et al (2011) simulated

the upconing of saltwater using numerical techniques. The work of Werner et

al (2009), a laboratory experiment of saltwater upconing was simulated

numerically, recently, with the use of FEFLOW. The groundwater level

depended flux was achieved in this numerical simulation.

Karahanoglu and Doyuran (2003) used finite element approach to

evaluate the behavior of coastal aquifer against seawater intrusion under the

process of cement quarrying located in south of Koaceli peninsular region. It

was a future-prediction model developed to understand groundwater

management for further excavation below sea level. The distance of 170 m

inland from the coastal boundary was predicted as a safe place for further

excavation, whereas regions along the coastal region were predicted to be

unsafe against seawater intrusion.

The effects of groundwater pumping were studied by Don et al

(2006). Here, the present pumping rate resulted in seawater intrusion along

the coastal plain of Kyushu Island of Japan. (Beatrice et al (2007) simulated

the aquifer groundwater system numerically to understand the behavior of

saltwater intrusion due to natural and human activities. The study area is

Ravenna coastal aquifer and there are multiple sources for salinity present in

this unconfined aquifer. This area has limited recharge, increased industrial

usage of groundwater, water level below mean sea level in most of the area,

recharge of surface water in to the groundwater etc., The simulation of such a

critical problem was done numerically and the results indicate the salinity will

be increased in future and can be diminished only by reducing pumping and

26

by land reclamation. Similarly, Magri et al (2009) discussed the interactions

between shallow and deep aquifer along the coastal region, the dissolution of

salt and the role of geologic features on the groundwater flow system.

Lenkopane et al (2009) used groundwater modelling technique to identify the

effect of tidal fluctuation on the estuary water and into the aquifer beneath

this estuary.

Kidmose et al (2013) used modelling technique to analyse climatic

impact on behavior of aquifer in Silkeborg, Denmark. The possibilities to

error in model structures were discussed and the reason for this was analysed

by Engelhardt et al 2013.

2.1.4 Work carried out in this area

Elango et al (1992) studied the groundwater quality in south

Chennai region and reported that the Na+ and Cl

- are the dominant ions and in

a few locations the groundwater was not suitable even for irrigation usage.

The source of contamination was brine water in estuaries and liquid and solid

wastes. It indicates that urbanization was comparatively very less than the

present and agriculture was more during the 1990’s.

Gnanasundar and Elango (1998) have carried out VES survey using

Schlumberger configuration and groundwater samples were collected. The

physical and chemical parameters were analyzed. The isoresistivity map was

prepared and the concentrations of chloride, electrical conductivity were

correlated. The influence due to sea, presence of clay towards canal boundary,

presence of poor quality water in the canal and effect of backwater in

Muttukadu were described. On the coastal boundary, the influence of sea was

identified at different depths in different locations. The influence of sea

towards eastern boundary was identified by using VES measurements. The

influence of the sea on the coastal boundary varied from 200m to 300m.

27

Gnanasundar and Elango (1999) VES survey has been carried out

in seventy locations. Water level and EC was also measured in wells near

VES locations. The apparent resistivity and thickness has been calculated for

each layer and the earth resistivity has been calculated by using value of EC.

The findings are earth resistivity, modified earth resistivity, water resistivity,

formation factor, longitudinal resistivity and transverse resistivity. By using

the same data Senthilkumar et al (2001) determined the layer parameters

including true resistivity and thickness. Formation factor was calculated from

the resistivity data by using Archie’s law and the empirical relationship

between hydraulic conductivity and formation factor, hydraulic conductivity

and modified aquifer resistivity and transmissivity and transverse resistance.

The aquifer parameters were estimated using the measured resistivity data.

Gnanasundar and Elango (2000) carried out simulation of the water

level regionally and identified the recharge characteristics and pumping

pattern by groundwater modelling. Mohan and Pramada (2005) developed a

numerical model to study the optimal pumping and behavior of this aquifer by

using SEAWAT.

Palanivelu et al (2006) conducted the groundwater quality

measurement to identify the effects of tsunami that occurred in 2004. The

southern part of this area (which is the northern part in this present study) is

influenced and it may be due to over pumping. The values of Total Dissolved

Solid recorded over time concluded that there was no major impact of the

Tsunami on water quality in this aquifer. It was due to short period of

transgression during the tsunami. The groundwater quality has deteriorated

due to deficiency of sufficient rainfall to prevent seawater intrusion.

Kuzhali et al (2009) report that, before 1950s, this area was

undisturbed by human activities and from 1950s to 1985 industrialization and

settlements began from the northern part of the area. During 1985 to 1997,

28

there was a steep increase in settlements and law of restrictions on

construction was applicable due to Coastal Regulation Zone (CRZ)

regulations. In the period of 1997 to 2006, there was a rapid increase in the

number of industries and huge economy was generated and deterioration of

environment resources ensured.

Annapoorania et al (2012) made an attempt to find groundwater

quality and status of seawater intrusion in the Chennai city by using inter-

elemental correlation analysis and statistical analysis. The groundwater is

influenced by seawater towards coastal region and towards the margin of

Adyar river, Covum river and Bay of Bengal. The influence of seawater into

the groundwater in this area was reported by Bhuvana and Ramesh (2012).

The maximum distance of influence of seawater from the coastal boundary is

up to 950 m during premonsoon and up to 550 m during post monsoon. The

influence is high in the northern part of this area.

An integration of groundwater chemistry using long-term

groundwater monitoring data, along with geo-electrical characteristics of the

aquifer and identification of behavior of aquifer under various hydrological

stress is in need. Hence, this study was carried out in this region as this

aquifer is expected to be very sensitive to changes in hydrological stress.