Combined Inversion of Electrical Resistivity and Transient...

Combined Inversion of Electrical Resistivity and Transient Electromagnetic Soundings for MappingGroundwater Contamination Plumes in Al Quwy’yia Area, Saudi Arabia

Mohamed Metwaly1,2,3, Eslam Elawadi4, Sayed S.R. Moustafa2 and Nasser Al-Arifi51Archaeology Department, College of Tourism and Archaeology, King Saud University, Riyadh 12372-7524,

Saudi Arabia2National Research Institute of Astronomy and Geophysics (NRIAG), Cairo 11421, Egypt

3Prince Sultan Bin Salman Chair for Developing National Human Resources in Tourism and Archaeology,

King Saud University, Riyadh 12372-7524, Saudi Arabia4Nuclear Materials Authority (NMA), Cairo, El Katameya, P.O. Box 530, Egypt

5Geology and Geophysics Department, College of Sciences, King Saud University, Riyadh 2455, Saudi Arabia

ABSTRACT

Time-domain electromagnetic (TEM) soundings and vertical electrical soundings (VES)

were conducted to assess the subsurface groundwater contamination in Al Quwy’yia area

through tracing the conductivity changes caused by leachate contamination of groundwater.

Most of the contamination sources are coming from the incomplete sanitary system and theillegal dumping of wastewater along the lowland area outside the town. Contamination of the

groundwater poses a major threat because most of the local inhabitants rely on ground water to

supply up to 60% of their water needs for various life activities. TEM and VES data sets have

been acquired along four longitudinal profiles to cover the most urban area of Al Quwy’yia. The

combined inversion of VES and TEM data sets increase the resolving certainty of the subsurface

resistivity models. The basement rock has the highest resistivity values, whereas the

uncontaminated limestone has moderate resistivity response in comparison with the

contaminated zones, which have a lower resistivity response. The constructed cross-sectionsand resistivity slice maps along the area provide valuable information about the shallow seepage

from the septic tanks, as well as the deep infiltration from the dump site at the southern part of

the study area.

Introduction

Misuse of groundwater resources is one of several

problems in arid countries where the water budget is

very low. The central part of Saudi Arabia is considered

one of the most arid regions in the Middle East, where

the average annual precipitation does not reach 100 mm/

yr (FAO, 2009). The average evaporation rate is very

high, reaching 3,000 mm/yr. Seasonal precipitation is the

main source for recharging the shallow aquifers across

the central region of Saudi Arabia. Al Quwy’yia area is

one of the most important cities in the central region of

Saudi area. It is located about 160 km west of Riyadh

(the capital of Saudi Arabia), at the contact between the

Arabian shield and Arabian shelf along the highway

between Riyadh and Taif (Fig. 1). It has been given

much attention in the past decades as the Saudi

government considers this area an extension for the city

of Riyadh. Thus, there has been comprehensive devel-

opment in all sectors, with increases in population and

living standards. Along with the increase in development

is an increase in the rate of groundwater consumption,

which exceeds the rate of recharge for both the shallow

and deep aquifers. Moreover, there are many sources of

direct or indirect groundwater contamination within the

area. The two most noticeable sources of contamination

are the incomplete sanitary system along the entire

urban area and the illegal dumping of wastewater in the

lowland area outside of town (Metwaly et al., 2012).

These types of groundwater contamination have delete-

rious effects on the residents’ health, especially where

they are relying on ground water as the main source for

domestic activities.

Little is known about groundwater pollution in the

central region of Saudi Arabia. However, recently

Metwaly et al. (2012) conducted the first research trial

utilizing transient and 2-D electrical resistivity tomogra-

phy for characterizing the signature of the contaminated

groundwater plumes close to the wastewater dump site in

the Al Quwy’yia area. Other studies regarding ground-

Journal of Environmental and Engineering Geophysics eego-19-01-05.3d 22/1/14 01:36:43 45

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JEEG, March 2014, Volume 19, Issue 1, pp. 45–52 DOI: 10.2113/JEEG19.1.45

water contamination and subsurface pollution have been

carried out along areas focused on hydrocarbon contam-

ination from surface and subsurface gasoline tanks(ElKholy et al., 2009; Al-Ahmadi, 2011).

Surface geophysical measurements, in particular

electrical resistivity soundings and time-domain electro-

magnetic soundings have been carried out in the Al

Quwy’yia area for the purpose of tracing the subsurface

contamination plumes and identifing the affected and

unaffected subsurface zones. The two techniques are

considered to be efficient, inexpensive, and fast forexploring the groundwater conditions and tracing the

pollutions plumes in arid areas (Meju et al., 1999;

Danielsen et al., 2003; Young et al., 2004).

Geological Setting

The study area is located directly along the eastern

boundary of the Arabian shield as part of a large plainextending north-south over several hundred kilometers;

between the Tuwaiq Mountains in the east and the

basement complex in the west (Fig. 2). This plain dips

from the west (950 m a.s.l.) to the east (632 m a.s.l.),

with many dry wadis running generally from west to

east. The west and south-west corner of the study area is

comprised of crystalline rocks, with dark rounded hillsdissected by white sandy wadis and separated by large

sand dune areas. Locally, most of the Al Quwy’yia area

is covered by Quaternary deposits, which are in the

form of alluvium and thin sand sheets as the result of

basement and limestone erosion processes (Fig. 2). The

thickness of such deposits ranges from a few meters to

tens of meters. The calcareous units of Khuff formation

underlie the Quaternary deposits, and they outcrop on ahilly belt about 20-km wide at the eastern side of the

basement. Toward the west and to the south of Al

Quwy’yia, the Khuff Formation directly overlies the

basement complex, as indicated in well Qap2-1 located

at the eastern side of the Al Quwy’yia area (Fig. 2). The

Khuff Formation comprises various types of limestone

and dolomite, shale and siltstone, sandstone and marl.

Most of the ground water, which is coming fromseasonal rainy events, is accumulated and stored in the

shallow aquifer of the Khuff Formation (Hoetzl, 1995).


Figure 1. Location map for Al Quwy’yia urban area and the acquired TEM and VES data sets and profiles.

46

Journal of Environmental and Engineering Geophysics


Figure 2. General geological map for Al Quwy’yia environs showing the location of the study area and the lithological

description of well Qap2-1.

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Metwaly et al.: Mapping Groundwater Contamination using TEM and VES Techniques

Geophysical Methods and Data Acquisition

Two geophysical techniques were used to explore

the subsurface lithological succession and contaminated

groundwater zones: transient electromagnetic (TEM)

sounding and vertical electrical sounding (VES). It was

essential to cover the entire planned study area with the

TEM stations, as the technique is considered fast and

convenient, provided a safe distance (hundreds of

meters) from any infrastructure is maintained. However,

many VES measurements were conducted close to and

between the TEM stations. The VES models were used

to constrain the TEM inversion, resulting in enhanced

combined inversion models.

Transient Electromagnetic Sounding

The TEM method is sensitive to conductive bodies

in the subsurface, with a depth of investigation ranging

from a few meters to hundreds of meters. However, the

TEM method is poor for resolving resistive formations

and is susceptible to interference from anthropogenic

noise (Everett and Meju, 2005; Ernstson and Kirsch,

2006a).

The TEM data were acquired along the study area

(Fig. 1) using a TEM-Fast 48 system from AEMR Ltd

with a 50 m by 50 m coincident loop. Measurement time

ranged from 4 ms to 16 ms, with 48 time windows and a

base frequency range from 3.2 kHz to 11 Hz, respec-

tively. The turn off and turn on time varied depending

on the type of measurement, with a maximum value of

90 ms. The data sets were acquired using two different

injecting currents (1 and 4 amperes), with the highest

quality data set used for processing and analysis. The

data processing provided an estimation and presentation

of 1-D models for the conductivity distribution with

depth along the measured stations in the study area.

TEM-RESearcher, a Windows integrated software

system, was used to process and invert the TEM data

(TEM-RESearcher manual, 2007). Results are presented

as electrical resistivity models versus depth.

Vertical Electrical Soundings

The VES data were acquired with a SYSCAL R2

system by IRIS using a Schlumberger electrode array.

The current electrode separation (AB) started with 1 m

and extended to 600 m at successive logarithmic steps.

The injected current (preset times) has maximum values

of 2,000 mA during a 500 ms pulse duration. The

number of stacks was set to a maximum of five,

depending on the data quality values. The electrode

coupling with the ground was checked before carrying

out the measurement for each of the four electrodes at

each measurement. The software IX1D (Interpex, Ltd,

2008) was used to process the VES data sets and obtain

the subsurface resistivity model that was used as an

initial model during the processing of the TEM data.

Comparison between VES and TEM Data

The initial model for TEM measurements is the

output of a nearby VES station. The VES model consists

of three layers, which is in agreement with the

subsurface lithological model (Fig. 2). The inversion

program incorporates the Inman ridge regression

routine (Inman, 1975) to achieve a best fit to the

observed data. The best fit can be achieved either

automatically through the software or manually by

adjusting the layer thickness and resistivity values.

Figure 3 shows the VES and TEM measurements

acquired close to well Qap2. The lithological descrip-

tions in the well do not have any information about the

groundwater contamination. The inverted VES and

TEM resistivity models are in relatively good agreement,

particularly at depths greater than 40 m. The mismatch

between the two models at shallower depths (,40 m) is

a result of greater resolution of shallower structures

obtained from the dense suite of VES measurements

at small electrode separations (Fig. 3(a)). In contrast,

the large loop TEM system is designed for acquiring

information at greater depths, and thus is less accurate

at resolving small changes in resistivity at shallow

depths. Therefore, the VES model shows greater detail

at shallower depths than the TEM model (Fig. 3(b)).

Neither the VES or TEM resistivity model clearly

identifies the accurate depth of the first weathered layer.

This is because of the arid conditions at the surface,

which extend to the top of the underlying limestone

layer (Figs. 3(b)–(c)). However, both models show a

high resistivity response for the first weathered layer.

The resistivity values decrease in the limestone layer,

which could be intercalated with shales, marls, and

argillaceous materials. The two models clearly define the

bottom of the limestone at 60-m depth. Below that

depth, the resistivity values increase in both models

because of the presence of a massive (dry) limestone

layer (Fig. 3(c)). The TEM model exhibits a higher

resistivity value relative to the VES model (Fig. 3(b)).

For studying the responses from different envi-

ronments along the study area, three sets of VES and

TEM measurements were acquired (Fig. 4). The selected

sites include one far away from any contamination

source (Fig. 4(a)); a second site close to the dump site

(Fig. 4(b)), which is considered as one of the main

sources of subsurface contamination; and a third site

close to the basement outcrop (Fig. 4(c)). The VES

measurements were conducted close to the TEM

stations to improve the consistency of the resulting

resistivity models and decrease the uncertainty in both


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Figure 3. Comparison of two VES (13) and TEM (53) data sets (a) and their resultant inversion models (b) with thenearby well (Qap2) (c) (refer to Fig. 1 for location).

Figure 4. The different responses of the VES and TEM data sets measured at three different locations along the studyarea. Upper plots are the VES and TEM field data; lower plots are the resultant resistivity models.

49


models. In general, the three data examples show three

layers, which are comparable with the subsurface geology

within the area (Fig. 3(c)). The first layer in the threeexamples is the weathered zone with variable resistivity

and thicknesses (Fig. 4). The resistivity values then

decrease because of the presence of a contaminated

limestone layer caused by infiltration from surface septic

tanks. The thickness of the second layer ranges between

25 m and 38 m. The third layer shows variable resistivity,

depending on the location of the acquired data set. The

resistivity of this layer exhibits relatively high values($200 Ohm-m) opposite the massive uncontaminated

limestone (Fig. 4(a)), with the value increasing to

.350 Ohm-m in response to the basement complex

(Fig. 4(c)). In contrast, the modeled resistivity from both

the VES and TEM measurements in the zone close to the

dump site (Fig. 4(b)) has low values (,5 Ohm-m). As

most of the low resistivity zones are away from any

infrastructure, the low resistivity values are only causedby the limestone rock contaminated with wastewater

from both the dump pond and septic tanks.

TEM Data Analysis

The acquired TEM data were arranged along fourprofiles to cover the urban as well as the dump site area

(Fig. 1). The four profiles extended from the southwest

towards the northeast, following the topographic relief.

To get acceptable resistivity models from the analysis of

the TEM sets along the profiles, the same strategy of

combined inversion between the available VES and TEM

was applied. At many stations, only TEM measurements

were acquired; however, the VES model was used as aninitial model (Fig. 5). At the stations where both VES and

TEM measurements were acquired, the combined inver-

sion model was extracted using the shallow information

from the VES model and the deeper information from the

TEM model (Fig. 5). This was done because the accuracy

of the resistivity model is greater at shallower depths,

where there are sufficient data points from the short

spacing offsets, whereas the accuracy of the TEMresistivity model is greater at deeper depths, i.e., at late

time measurements (McNeill, 1994; Meju et al., 1999). A


Figure 5. Examples of the processed TEM and VES data sets along one profile showing the output combined models.

(a) VES and TEM field data; (b) resistivity models; (c) constructed resistivity cross section.

50


set of measured and processed TEM and VES data is

presented in Fig. 5(a). These data were used for

constructing the profiles (Fig. 5(b)). Because the 1-D

model presentation is inadequate to describe the subsur-

face features, it is essential to present the data in a 2-D

manner to laterally and vertically trace the important

subsurface features. The resistivity data were arranged

for each measured TEM station considering the elevation

and the distance to the next station. Then a kriging

gridding routine was used for interpolating the resistivity

and depth values between the sparse stations. The result is

a 2-D cross-section representing the subsurface resistivity

distribution (Fig. 5(c)). The blue colors define the low

resistivity values, which represent the contaminated

zones, whereas the yellow and reds represent the

unaffected limestone and basement rocks, respectively.

To delineate the possible subsurface contamina-

tion zones and locate pathways of leachate plumes, sets

of resistivity slice maps as well as four longitudinal

profiles were constructed (Fig. 6). The resistivity slice

maps were extracted by using the resistivity distributions

with depth along the measured TEM data sets at

different elevations, starting from the shallowest loca-

tion (820 m a.s.l.) and ending at 720 m a.s.l. A

geostatistical gridding (kriging) method was applied to

smoothly interpolate the measured data, and a linear

color scale was used to visualize the limited resistivity

range (1–550 Ohm-m). Analyzing the resistivity distri-

bution from top to bottom along the study area reveals

important information:

1) At shallow depths (820 m a.s.l.), the low resistivity

character is dominant in the central area of the

region. This is caused by the leaching effect of

sewage water from the septic tanks across the

urban area, as well as seepage of surface wastewa-


Figure 6. Successive resistivity slice maps with the longitudinal profiles along the study area.

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ter from the dump pond at the southern part(Fig. 6(a)).

2) At depths 800 m a.s.l. to 780 m a.s.l., the low

resistivity zone becomes smaller and migrates toward

the eastern and the southern directions. The uncon-

taminated limestone and basement appears in the

central and southwestern parts (Figs. 6(b)–c)).

3) At greater depths (from 760 m a.s.l. to 720 m a.s.l.)

(Figs. 6(d)–(f)), the low resistivity zones becomedominant in the southeastern part only, where the

dump site is still active. Therefore, at depth 720 m

a.s.l., the only source of subsurface contamination is

coming from the wastewater dump site (Fig. 6(f)).

4) The basement complex can be traced based on the

high resistivity character at the southwestern parts

of the study area and extends to the eastern and

northern parts at depth (Figs. 6(c)–(g)).

Conclusion

Al Quwy’yia area is an important city in the

central region of Saudi Arabia that is located at the

contact between the Arabian shield and Arabian shelf.

The region is experiencing rapid development in all

sectors, many of which produce sources of groundwater

contamination. The two most noticeable sources are the

incomplete sanitary system along the whole urban area,

and the illegal dumping of wastewater in the lowlandarea outside the city. Surface electrical resistivity

soundings and time-domain electromagnetic measure-

ments were carried out along Al Quwy’yia for the

purpose of tracing subsurface contamination plumes

and identifying the affected and unaffected subsurface

zones. The models obtained from the VES data

inversions were used as initial input models to the

TEM data inversions. Through analyses of the VES andTEM data sets, we were able to distinguish contami-

nated and uncontaminated zones within the study

area. The contaminated zones exhibit a relatively low

resistivity in comparison with the uncontaminated

zones. The processed TEM data were arranged along

four longitudinal profiles and different resistivity slice

maps. The shallow subsurface zones (,800-m depth) are

contaminated primarily from infiltration of septic tanks.At greater depths, the contaminated zones decrease and

shift toward the southeastern part of the study area,

where the dominant source of contamination is the

dump site. Also, the uncontaminated limestone and

basement rocks have been defined at different depths.

Acknowledgment

This work was supported financially by the National

Plan for Science, Technology and Innovation (NPST) pro-

gram, King Saud University, Saudi Arabia (Project No. 09-

ENV836-02). The authors appreciate the editorial assistance

provided by the JEEG editor.

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