Identification of the Saline Zone in a Coastal Aquifer Using Electrical Tomography Data and...

Identification of the Saline Zone in a Coastal AquiferUsing Electrical Tomography Data and Simulation

Maria A. Koukadaki & George P. Karatzas &Maria P. Papadopoulou & Antonis Vafidis

Received: 5 December 2005 /Accepted: 11 December 2006 / Published online: 26 January 2007# Springer Science + Business Media B.V. 2007

Abstract A novel approach that borrows methods commonly used in environmentalgeophysics was developed for obtaining the estimates of the aquifer parameters.Specifically, estimates of hydraulic conductivity were obtained from field measurementsof the electrical resistivity while accounting for the karsticity of the geological formations inthe area of study. Geophysically determined hydraulic conductivity estimates wereintroduced to a 3-D groundwater numerical simulator (Princeton Transport Code – PTC)to compute the hydraulic heads distribution of the area of interest. The calibration of thenumerical model was obtained matching the hydraulic-heads predicted by the simulatorwith the hydraulic-heads measured at specific well locations. Simulated hydraulic-headswere used with the Chyben-Herzberg equation to approximate the position of the sharpfreshwater/saltwater interface of the base of the water supply aquifer. The existence of thefaults impacts the groundwater flow and the distribution of the freshwater/saltwaterinterface.

Key words geophysical methods . coastal aquifers . groundwater flow simulation .

salinization . karst

1 Introduction

Knowledge of hydraulic conductivity and transmissvity is essential for the determination ofnatural water flow through an aquifer. Although these characteristics are mainly deducedfrom well logs and pumping test analysis, attempts have been made to employ geophysical

Water Resour Manage (2007) 21:1881–1898DOI 10.1007/s11269-006-9135-y

M. A. Koukadaki :G. P. Karatzas (*) :M. P. PapadopoulouDepartment of Environmental Engineering, Technical University of Crete,Polytechnioupolis 73100, Chania, Greecee-mail: [email protected]

A. VafidisDepartment of Mineral Resources Engineering, Technical University of Crete,Polytechnioupolis 73100, Chania, Greece

methods in order to reduce the amount of the necessary hydrogeological observations andits associated cost.

Louis et al. (1982) applied geophysical measurements in order to determine the aquifer’sparameters of Mornos River Valley in central Greece. The Mornos river valley was selectedas a test area in order to provide the adequate information on the aquifer hydrodynamiccharacteristics. Correlation tests were performed between electrical parameters measured bysurface electrical soundings and aquifer characteristics obtained from a certain number ofboreholes. Measurements of groundwater resistivity in monitoring boreholes led to thedivision of the investigated area in zones where the water quality remained constant. Thehydraulic conductivity was calculated from an empirical relationship, based on a series ofmeasurements of the formation factor F, while the aquifer thickness was calculated from theinterpreted resistivity soundings that led to the construction of a well controlledtransmissivity map. Consequently, the good agreement between aquifer hydraulicconductivities calculated from the interpreted resistivity soundings and those deduced frompumping test analysis proves the essential contribution of geophysical methods in thedetermination of the aquifer’ parameters.

Balia et al. (2003) applied a combination of geophysical techniques in order to show thefundamental role of geophysical survey techniques in the environmental study of coastalareas. This approach was applied at the Muravera plain in southern Sardinia, Italy, which isaffected by severe water salination. Moreover, three boreholes were drilled in order toprovide a better understanding of the geological and hydro-geological interpretation ofgeophysical data. The procession of the electrical soundings data confirmed the presence ofa wide, electrically low-resistivity zone which is associated with saltwater intrusionphenomenon. Specifically, the interpretation of the electrical data showed that the phreaticaquifer was affected by saltwater intrusion due to overexploitation and recent openedartificial fish farming channels while the salination of the confined aquifer was related tomorphological vicissitudes that occurred in the past. Also, the combined interpretation ofseismic, gravity and well data resulted in a representative geological characterization of themodeled area.

In this paper a new methodology is proposed based mainly on geophysical measurementthat could be used when no pumping tests and no well-log information is available, in orderto characterize the hydro-geology of a coastal aquifer. Although the hydro-geologicalinformation is important for the calibration of the geophysical data, it is expensive andtime-consuming. For this reason, the method of electrical resistivity has been used here,which is straight-forward and provides immediate results for the mapping of geologicalformations. The innovation in this work is that the hydraulic conductivity of the aquifer isdetermined from the electrical resistivity measurements and from empirical relationshipsbetween porosity and permeability of the geological formations. The task was quitedifficult, since there were no observation boreholes or core samples, and the whole workdepended on continuous trial and error tests. Though this work should be considered a testsurvey, it nonetheless provided reliable constraints for the values of hydraulic conductivity,or at least a sound analysis for further investigation.

2 The Method of Electrical Resistivity

Electrical methods are used in geophysics for the description of the subsoil geologicalstructure and it is based on the study of the electrical resistivity distribution in thesubsurface (geo-electrical structure). The parameter measured is either the electrical voltage

1882 M.A. Koukadaki, et al.

or the electrical resistivity of the subsoil. It is well known that each geological formationhas its own electrical resistivity. Consequently, the determination of subsoil’s electricalcharacteristics can assist in the specification of an unknown geological formation. The mostcommon electrical method is the method of electrical resistivity. In this method, an electriccurrent is introduced directly into the ground through electrodes and the electrical resistanceR is obtained from the voltage measurements by applying Ohm’s law. The electricalresistivity ρ is an electrical property of rocks and is calculated from:

r ¼ RA

L; ð1Þ

where

R the resistance of a cylindrical conductor (Ω),A the cross-section area of the conductor (m2),L the length of the conductor (m).

Electrical tomography is an electrical resistivity method which is based on the study of bothvertical and horizontal variations of the electrical resistivity in the subsurface. In this particularmethod, an array of electrodes is transferred along each electrical profile and measurements ofthe apparent electrical resistivity are taken for every set of electrodes. The values of apparentresistivity collected at all positions are used to create a subsurface pseudo-section. The realresistivity values are obtained by processing the measured values of apparent resistivity withinversion programs (Loke and Barker 1994).

3 Estimation of the Hydraulic Conductivity Based on Geophysics

3.1 General Concept

In the past, Heigold et al. (1979), Louis et al. (1982), Kelly and Reiter (1984), Niwas andSinghal (1984), Mazac et al. (1987) and Nunes (1998) have addressed the contribution ofenvironmental geophysics in estimating geological features of hydrogeological interest atdifferent sites. Different geophysical methods have been applied depending on the specificcharacteristics of the site. Well logs and pumping tests were also used in the abovementioned studies to determine, primarily, the porosity, permeability and hydraulicconductivity of the geological formations. Although these techniques are very useful andaccurate, they are expensive and usually applied at late stages of the study. However, thequestion is how the hydro-geological model should be constructed when the only source ofinformation about the subsurface is based on geophysics.

The relation between the hydraulic conductivity K and the permeability k of a geologicalformation is given by (Hubbert 1940):

K ¼ kgm; ð2Þ

where

K the hydraulic conductivity (m/sec),k the permeability (m2),+ the specific weight of the fluid (N/m2),μ the viscosity of the fluid (kg/m*sec).

Identification of the saline zone 1883

The values of + and μ, for a given fluid temperature, can be easily found in literature. Onthe contrary, permeability is usually measured from specimens in the laboratory. Anempirical expression for the permeability k in terms of the porosity 8 is given by Archie’slaw, (Archie 1942):

k ¼ a2 � 8b2 ; ð3Þwhere

8 the porosity (%),a2, b2 co-efficients that depend on the rock type (dimensionless).

Based on a series of experiments using samples of different composition and porosity,Archie (1942) introduced a relation that correlates the porosity 8 and the formation factor Fof a geological stratum:

F ¼ a1

8m) 8 ¼ m

ffiffiffiffia

F

r; ð4Þ

where m is the cementation factor which varies between 1 (for loose rocks) and 3 (forcompacted rocks), and α is the tortuosity factor which depends on the geometry of thepores. The formation factor F can be also calculated from the electrical resistivities of theformation (Archie 1942):

F ¼ R0

Rw; ð5Þ

where

R0 the saturated formation electrical resistivity (Ωm),Rw the formation water resistivity (Ωm).

By combining Eqs. 2–5, the hydraulic conductivity K can be expressed as:

K ¼ a2

ffiffiffiffiffiffiffiffiffiαRw

R0

m

r� �b2 γm; ð6Þ

The computation of the hydraulic conductivity K from Eq. 6 requires the estimation of a2,b2, m and α. Resistivities R0 and Rw can be obtained from field measurements. Thetortuosity factor α takes a value greater or equal to one for carbonates and formations withfracture type porosity (Katsube and Humet 1987; Salem 2001). Typical values of thecementation factor m that may vary depending on the rock type are presented in Table 1.

The constant parameters a2 and b2 in Eq. 3 are calculated from a series of laboratorymeasurements on various rock types, knowing a priori the values of k and ϕ. In this work,

Rock type Cementation factor m

Slightly consolidated sand 1.4–1.5Clean limestone 1.6–1.8Dolomitic limestone 1.8–2.0Crystal limestone 2.2–2.4

Table 1 Typical values ofcementation factor m (Kwader1985)


the parameters a2 and b2 were empirically specified, in order to have a first estimation ofthe hydraulic conductivity K since permeability and porosity logs will be performed in thefuture to confirm the estimated parameters values. Dewan (1983) has proposed a series oflaboratory derived semi-logarithic curves between porosity and permeability for variousrock types. In this paper the geological formations mainly present in the area of interest aremarly and Tripolis limestones, so the semi-logarithic curves of “chalky limestones” and“intercrystalline limestones”, were chosen for the empirical expression of a2 and b2 (Fig. 1).

By logarthming the Eq. 3 and based on the porosity-permeability curves of Fig. 1 thefollowing equations for the marly and Tripolis limestones formations were computed:

– Marly limestones: y ¼ 7:8356x� 10:319– Tripolis limestones: y ¼ 3:5618x� 11:197

A first estimation of the hydraulic conductivity K for the two main formations present inthe area of study is presented in Table 2.

Fig. 1 Laboratory derived semi-logarithic curves between porosi-ty and permeability for variousrock types (Dewan 1983)

Marly limestone Tripolis limestone

m 1.8 2.0α2 4.8*10−11 6.35*10−12

b2 7.8356 3.5618Rw (Ωm) 2.7 2.7R0 (Ωm) 50 80+ (N/m3) 9794.4 9794.4μ (kg/m*sec) 0.001083 0.001083K (m/sec) 1.32*10−9 1.37*10−7

Table 2 Parameters used tocompute the hydraulic conduc-tivity values for the two maingeological formations of thearea of study (without consider-ing any karstification)


3.2 The Problem of Secondary Permeability in Karstified Limestone

The expression for the hydraulic conductivity K in Eq. 6 does not consider vugs and/orfractures. This means that the values for K in Table 2 are invalid, as the area of study isdeeply karstified. Furthermore, pumping tests that were performed in the vicinity of themodeled area (Klidopoulou 2003), showed values in the range of 10−4 m/sec, which is quitehigher than the values estimated in Table 2. Consequently, the problem of karstification hasled to a false estimation for the values of a2, b2, m, α and K. The answer to this problemcomes from the study of the oil industry.

Aguilera (1998) and Byrnes and Bhattacharya (2004) have worked thoroughly in thisfield, since karstified limestones play an important role in the oil industry. In this case, therelation between porosity and permeability k (Eq. 3) was accordingly modified. Vugs andfractures are characteristic features of karstification and have a major impact on the value ofthe cementation factor m. According to Aguilera (1998) and Al-Hanai et al. (1999), vugsincrease the value of m, while fractures decrease it. Typical values of m for fractures andvugs are presented in Table 3. A value of 2 for parameter m is commonly used for inter-crystalline limestones, but cannot be used in the case of karstification. The decision thatshould be made is whether fractures or vugs play the most important role on secondaryporosity. According to Aguilera (1998) only 1–2% of the porosity is attributed to fracturesand the rest to vugs. Based on this conclusion, a value of m=2.3 was selected in this work.

Typical values for the dimensionless coefficients a2 and b2 were presented in the work ofBernabe et al. (2003), where mathematical relations between porosity and permeability fordifferent processes and materials were proposed. Based on this work, Eq. 3 can be modifiedinto the following form:

k � 8b2 ; ð7Þwhere b2 is an integer number and a2 is a unit value. According to Bernabe et al. (2003), inthe case of limestone dissolution, b2 takes values near 20. This value of b2 was estimatedfrom laboratory specimens, knowing permeability and porosity. However, the estimation of avalue near 20 is not complete baseless, if the initial values for b2 (Table 2) in a non-karstifiedenvironment are considered.

In this work, the final selection of the coefficient b2 was made based on the work ofKlidopoulou (2003), where typical values of the hydraulic conductivity were presented fora field site close to the area of interest. Specifically, a value equal to 22 was chosen for themarly limestone and a value equal to 17 for the Tripolis limestone, so as the hydraulicconductivities were similar to the ones that were presented in the work of Klidopoulou(2003). The values for parameters a, m, a2 and b2 used in this paper are shown in Table 4.

The validity of the dimensionless parameter b2 that has being used for the determinationof the hydraulic conductivity K was also confirmed by Pape et al. (2000), who applieddifferent mathematical relations between porosity φ and permeability k to different rock

Pore type Cementation factor m

Intercrystalline/intergranular 2.0Fractures 1.4Vugs 2.3Moldic >3

Table 3 Typical values of m forvarious pore systems (Choquetteand Pray 1970; Aguilera 1998;Al-Hanai et al. 1999)


types, based on the porosity of the formation. These relations were obtained experimentally, notonly in the laboratory but also in a larger scale, and conclude fractured limestones. The valuesof the hydraulic conductivity that resulted from the study of Pape et al. (2000) are quite similarto the values that were calculated based on the theoretical approach presented in this paper.

4 A Field Application of Geophysically Determined Hydraulic Conductivities

4.1 Overview of the Site and Saltwater Intrusion Issues

Salinity is a serious problem in many Greek coastal areas. The saltwater intrusionphenomenon becomes more severe during summer due to extensive over-pumping. Thearea of interest is located east of the city of Heraklio in Crete, Greece (Fig. 2). Inparticular, saltwater intrusion appears to place in the coastal aquifer of the industrial zone ofthe city and immediate action should be taken in order to control the problem.

Marly limestone Tripolis limestone

a 1 1m 2.3 2.3a2 1 1b2 22 17

Table 4 Values of dimensionlessparameters used for the estima-tion of hydraulic conductivity(considering karstification)

Fig. 2 Location of the area of interest (http://www.travelinfo.gr)


http://www.travelinfo.gr

The first objective of this application is to map the geology and salinity of the area usingresistivity measurements. The industrial zone lies on karstified limestone where severalfaults need to be delineated. The second objective is to simulate the groundwater flow usinga 3-D model, based on a first estimate of the hydraulic conductivity from geophysics, asdiscussed earlier. The third objective is to estimate the location of the freshwater/saltwaterinterface at the aquifer base using Ghyben-Herzberg relations.

4.2 Geology of the Field Site

The field site extends about 4,500 m south of the coastline. The following geologicalformations are present in the area of study:

(a) Bioclastic limestones of Neogene (St. Barbara formation): these are mainly marlylimestones with some intercessions of gypsum.

(b) Carbonate formations of Tripolis zone: this unit includes limestones, dolomiticlimestones and dolomites, which occupy the tectonic base of the area.

(c) Alluvial deposits and sea sands of Quaternary are also present along the east and westcoastline.

The three main geological formations in the area of interest are coloured differently andare presented on the simulated saltwater intrusion front along with the domain outline of thestudy area and three calibration boreholes (Fig. 3).

Three electrical tomography lines (T1, T2, T3) with a total length of 1,310 m were usedfor the delineation of the salinity zone. Geological information obtained from observation

4

3

Karstified Limestone

Marls

Alluvial Deposits

1

2

1km

Marly Limestone

Mediterranean Sea

N

Industrial Zone

Well locations

Fig. 3 Geological representationof the area of study (afterPapadopoulou et al. 2005)


wells at the industrial zone of Heraklio revealed the intrusion of saltwater in the subsurface.More specifically, saltwater was detected at observation wells B, C and D. According towater samples, the electrical conductivities σ were 3,846, 2,941 and 5,128 μS/m at a depth of85, 80 and 85 m, respectively. The electrical conductivity in saline water varies from 1,600 to4,800 μS/m. The maximum depth of the observation wells was about 110 m from the surface.

4.3 Interpretation of the Electrical Tomography Lines

Three electrical lines used for the delineation of the saltwater intrusion front werelocated at the Heraklio industrial zone. The Wenner-Schlumberger electrode array, whichoffers reliable results in subsurface mapping, was used for the electrical measurements.The electrical resistivity measurements were performed with the instrument StingR1. Theinversion program Res2DinV ver3.2 (Loke and Barker 1994) was used to process theapparent resistivity values.

For line T1, the maximum penetration depth was about 100 m, while for T2 and T3 themaximum depths were 131 and 101 m, respectively. Line T2 had the biggest penetrationdepth, since it was the longest. Electrical line T1 (Fig. 4) had a total length of 120 m andwas located at the northern part of the study area with direction NNW towards SSE.

Regarding the geoelectrical model, the following observations were made:

(1) Lateral variations in electrical resistivity were observed along T1. The increasedvalues towards east represent more consolidated limestones. Suspisions for theexistence of relict seawater are invalid, as the electrical resistivities values are lowerthan 1300 Ωm. The reduced values to the west are due to a subsurface water streamzone, which appears on the geological map of the area (Fig. 5).

(2) A drastic reduction in electrical resistivity (10–50 Ωm) was observed at the absoluteelevation of the mean sea level. This reduction can be attributed to the penetration of

Fig. 4 Geoelectrical section of electrical line T1


saltwater into the subsoil. Saltwater lowers electrical resistivity. This observation isalso confirmed from information obtained from observation well C inside theindustrial zone (Fig. 5).

Electrical line T2 (Fig. 6) had a length of 900 m and the measurements were performedfrom south to north. This geoelectrical section revealed lateral variations in electricalresistivity, with values in the range of 100–500 Ωm in the horizontal distance between 160

Well A

Well D

Well B

Well C

Water Treatment Plant

Mediterrenean Sea Fault zone 1

Fault zone 2

Water stream

Line T1

Line T3

Line T2

Fig. 5 Overlay of the three geoelectical sections on the geological map of the field site. (The two thickdotted lines represent the fault zones, while the continuous one an existing subsurface water stream and fourobservation wells)

Fig. 6 Electrical line T2, the intersection of lines T2 and T3 is also shown (after Papadopoulou et al. 2005)


and 560 m. These values represent marly limestones of Neogene. Two zones of reducedelectrical resistivities (10–50 Ωm) are also illustrated in Fig. 6. These are due to the faultzone 2 (Fig. 5). Higher resistivities (>1300 Ωm), represent the Tripolis limestones. Thesevariations in the resistivity pattern are not common in areas where marly and Tripolislimestones predominate. Although the values in these areas should be higher than 100 Ωm,extremely low values of electrical resistivity are scattered in this geoelectrical line.

Electrical line T3 (Fig. 7) had a length of 290 m from east towards west and wasperpendicular to line T2. A low electrical resistivity zone appears at the intersection of linesT2 and T3, while higher values of electrical resisivity predominate towards the east. Thesevalues (>1300 Ωm) correspond to Tripolis limestones and are in agreement with line T2.

5 Estimation of the Hydraulic Conductivity for the Field Case

The electrical tomography survey revealed fault zones and the partial extent of thefreshwater/saltwater interface. The simulation of the groundwater flow in the area ofinterest requires values for hydro-geological parameters, such as the hydraulic conductivityand effective porosity of the formations. The hydraulic conductivity was evaluated fromEq. 6: K ¼ a2

ffiffiffiffiffiffiαRwR0

m

q� �b2 γm

The values for parameters α, m, α2 and b2 in the above equation are presented in Table 4.The validity of b2 was confirmed by the work of Pape et al. (2000). Although the electricalresistivity of water Rw is spatially variable, it was considered equal to 2.7 Ωm, which is theaverage value of the electrical conductivity calculated from water samples taken at 17°C inthree different well locations (σ is the reciprocal of electrical resistivity ρ).

The value of R0 was determined from the observations of line T1, which revealedsaltwater in the subsurface. On the contrary, no indication of saltwater intrusion was noticedin lines T2 and T3. Line T1, in which the salinity zone was delineated inside the marlylimestone, was divided into two sections based on the values of the electrical resistivity ofthe water saturated geological formation. In the first section, a value of R01=30 Ωm waschosen for depths between 60 and 100 m, where salinity was relatively high, while a value

Fig. 7 Electrical line T3, the intersection of lines T2 and T3 is also shown


of R02=40 Ωm was considered for shallow depths (0–60 m) where salinity was relativelylow. This approach results in a better representation of the hydraulic conductivity of themarly limestone in the groundwater simulation model and, consequently, yields morereliable results regarding the groundwater flow. For the karstified Tripolis limestone, avalue of R0=80 Ωm was considered. The hydraulic conductivity of rocks is stronglyaffected by karstification. The values of parameters +, μ, Rw, R01 and R02, as well as thecalculated values of the hydraulic conductivity, are shown in Table 5.

6 Identification of the Saline Zone: Approximation of Spatially DistributedHydraulic Heads

A three-dimensional finite element-finite difference model for groundwater flow based onthe Princeton Transport Code (PTC) (Babu and Pinder 1984; Babu et al. 1997) was used tomodel the hydraulic heads distribution over the entire domain of interest. PTC uses thefollowing partial differential equation to represent groundwater flow, described byhydraulic head h:

@

@xKxx

@h

@x

� �þ @

@yKyy

@h

@y

� �þ @

@zKzz

@h

@z

� �� S

@h

@t

þXr

i¼1

Qid x� xið Þd y� yið Þd z� zið Þ

¼ 0

ð8Þ

where:

h hydraulic head (L),Kxx, Kyy, Kzz the hydraulic conductivity in the x, y and z direction (LT−1),S the specific storage coefficient (L−1),Qi the source/sink term at location i (L3T−1),δ( ) the Dirac delta function.

For the solution of the fully three-dimensional equations, PTC employs a uniquesplitting algorithm which reduces significantly the computational burden. The algorithminvolves discretizing the domain vertically into approximately parallel horizontal layers. Afinite element discretization is used within each layer allowing for the accuraterepresentation of irregular domains (Pinder and Gray 1977). The layers are connected

Marly limestones Tripolis limestones

+ (N/m3) 9794.4 9794.4μ (kg/m*sec) 0.001083 0.001083Rw (Ω m) 2.7 2.7R01 (Ω m) 30 80R02 (Ω m) 40 80K1 (m/sec) 8.98*10−4 (77.6 m/day) 1.2*10−4 (10.5 m/day)K2 (m/sec) 5.73*10−5 (4.95 m/day) 1.2*10−4 (10.5 m/day)

Table 5 Hydraulic con-ductivity K for Marly andfor Tripolis limestones forthe field site in the indus-trial zone of Heraklio,Greece


vertically by a finite difference discretization. The hybrid coupling of the finite element andfinite difference methods enables the application of the splitting procedure. PTC was usedin combination with Argus One (Olivares 2000; Pinder 2002), a geographic informationmodeling (GIM) environment used for introducing the data into the simulator.

The area of study was treated as an unconfined aquifer with a thickness of 30 m at thecoastline. The aquifer was composed of two layers as illustrated in Fig. 8. The bottom layerwas 25 m thick at the coastline and contained both marly and Tripolis limestones, while thetop layer was 5 m thick at the coastline and up to 110 m thick inland. The top layercontained marly limestones, Tripolis limestones and alluvial deposits. This two-layerscenario was considered in order to accurately represent horizontal and vertical variations inhydraulic conductivity.

Fig. 8 Conceptual cross-section of the coastal aquifer in the area of study

Fig. 9 The imposed boundaryconditions of the model


The model was discretized into 1,042 nodes and 1,961 triangular elements. The flowboundary conditions (bc) chosen during the calibration were: (a) constant hydraulic head(1st type bc) value of 30 m (datum assumed at the bottom of the aquifer) was imposedalong the shoreline, (b) 1st type boundary condition with decreasing value of hydraulichead from the sea towards inland was imposed at the eastern (along the fault) andsoutheastern boundaries, (c) 2nd type boundary conditions were applied at the welllocations and at the internal fault, (d) 3rd type leakage condition was applied at parts of theweastern and southern boundaries of the model allowing limited inflow of freshwater andfinally (e) no-flux conditions were applied elsewhere. (Fig. 9)

Ghyben (1888) and Herzberg (1901) examined the nature of the saltwater-freshwaterinterface in coastal aquifers under natural conditions. In the sharp interface approach,saltwater and freshwater are treated as two immiscible fluids. Assuming hydrostaticconditions, the weight of a unit column of fresh water (hf) (extending from the water table

Fig. 10 The general concept ofthe sharp interface between salt-water and freshwater

Fig. 11 The simulated hydraulicheads distribution (top layer)-Saltwater zone extent


to the interface) was balanced by a unit column of saltwater (J) (from the sea level to theinterface) they were able to calculate the interface between the fresh and the salt water asfollows (Bear 1979): (Fig. 10)

x ¼ rfrs � rf

hf � 40hf ; ð9Þ

where

J the interface depth below sea level (m),hf the hydraulic head of freshwater above sea level (m),ρf the density of freshwater (1 gr/cm3),ρs the density of saltwater (1.025 gr/cm3).

Two key assumptions are imposed in Eq. 9 which are that: (a) there is a sharp interfacebetween saltwater and freshwater and (b) seawater is considered static. Due to theseassumptions Eq. 9 over-predicts the inland extend of saltwater intrusion (Kohout 1960). Itfollows that the Ghyben-Herzberg equation sometimes is useful to water managers orresearchers seeking a rough estimate of the maximum extent of saltwater. For thickness ofsaline aquifer equal to J=30 m, Eq. 9 yields hf=0.75 m. This value corresponds to ahydraulic head of 30.75 m in the simulation model since a depth of 30 m below the sealevel was considered at the coastline. The simulated hydraulic heads field of the area ofstudy is presented in Fig. 11.

In Table 6 the simulated values and the field measurements at well locations 1, 2, 3 and4 are appeared to be in good agreement. According to the model calibration, the coastalwell 1 is inside the saltwater zone which is in accordance with the observed data sincesaline water is pumped at that location. At well location 2, the simulated hydraulic head isoverestimated, because the pumping activity within the industrial zone still occurs resultingin lowering locally the water table below the sharp interface. The simulated value ofhydraulic head at well 3 shows that freshwater inflow from the southeastern boundaryoccurs resulting in the enhancement of the groundwater resources and improvement of thegroundwater quality since the water table is almost 10 m above the mean sea level. Finally,the observed hydraulic head at well location 4 was only used as an indicator for thecalibration of the nearest model boundary since it is located outside of the domain of study.

In Fig. 11 the line corresponding to a hydraulic head value of 30.75 m reveals a saltwaterintrusion zone that extends at some locations more than 2,000 m from the coastline into theaquifer. No-pumping rates are applied in the simulation model, as the nearby wells havestopped pumping. The simulation results also indicate that the main penetration of saltwatertakes place at the east border of the area of interest which coincides with a fault startingfrom the inland and ending at the sea. The role of this fault has been verified from in-situgeological observations (Fault zone 1 in Fig. 5). The high values of the hydraulic heads in

Well IDnumber

Observed hydraulic head(m) (Calibration points)

Simulated hydraulic head(m) (Calibration points)

1 27 272 27 323 39 404 66 61

Table 6 Observed and simulatedhydraulic head values at the 4calibration points


the south are attributed to a flow of fresh water coming from the area outside the model.The fault that was mapped inside the modeled area from electrical line T2 (Fig. 4) seems toabsorb all the water coming from south.

In Fig. 12 the simulated hydraulic heads distribution for the case where the eastern faultis not taken into consideration is shown. The saltwater intrusion front (30.75 m) has beenconstrained in a narrow area of just 200 m into the aquifer, and this is not in agreement withfield measurements at well location 1 where high chloride ion concentrations and lowerthan 30.75 m value of hydraulic head are observed.

7 Conclusions

By combining the electrical tomography data for the geological formations of the area withmathematical relations for the hydraulic conductivity and permeability, new relationshipswere developed for the estimation of the hydraulic conductivity. The problem wascomplicated since the area of interest is characterized by karst fractures and vugs whereturbulent flow conditions may occur. The mathematical relations were modified to accountfor karst, so more reliable estimates of hydraulic conductivity will be obtained fromelectrical tomography data in karst terrains.

The computation of hydraulic conductivity from geophysical information was a difficulttask, since no well logs and no pumping tests were available. Nevertheless, porosity andpermeability logs along with pumping tests should be performed in the future, so aspermeability, porosity and hydraulic conductivity are accurately defined. Core sampleswould also be of great value in the determination of the constant parameters of the rocks. Inthis paper, a first estimation of hydraulic conductivities based on empirically equations and

Fig. 12 Calibrated hydraulicheads distribution without con-sidering the fault along the east-ern model boundary


on continuous trial and error tests was made, matching the theoretical values and the valuesestimated from pumping tests in an area close to the field site. Although, the values for Kare basically based on theoretical approaches, they offer quite reliable results for the 3Dsimulation model.

A 3-D groundwater simulation model was also developed using hydraulic conductivitiesestimates with electrical tomography data. Hydraulic head distributions were predicted withthe simulation model which matches reasonably well measured hydraulic heads collectedfrom pumping well locations. Simulated hydraulic heads were used with Ghyben-Herzbergrelation to estimate a maximum extent of the saltwater intrusion zone. The main intrusion ofsaltwater was observed from the coastline along a fault hypothesized to coincide with theeast border of the model. The interface between the fresh and the saltwater was about2,000 m away from the coast, revealing a large saline area. Though this work should beconsidered a test survey, it nonetheless provided reliable constraints for the values ofhydraulic conductivity, or at least a sound analysis for further investigation.

Acknowledgements The present work is part of the research project “Aquifer Protection from SaltwaterIntrusion using Artificial Recharge from Treated Industrial Wastewaters and Development of Tools andTechnologies for Sustainable Management of Sludge from Waste Water Treatment Plants” (SMILES) – EPAN2003–2006, funded by the Greek General Secretariat for Research and Technology. The authors will like to thankthe two anonymous reviewers for the insightful comments that have considerally improved the manuscript.

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