combined inversion of electrical resistivity and transient...
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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-
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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).
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Figure 1. Location map for Al Quwy’yia urban area and the acquired TEM and VES data sets and profiles.
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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.
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Metwaly et al.: Mapping Groundwater Contamination using TEM and VES Techniques
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
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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.
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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-
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Figure 6. Successive resistivity slice maps with the longitudinal profiles along the study area.
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Metwaly et al.: Mapping Groundwater Contamination using TEM and VES Techniques
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.
References
Al-Ahmadi, M.E., 2011, Groundwater quality assessment in
Wadi Fayd, Western Saudi Arabia: Arab J Geosci. DOI
10.1007/s12517-011-0337-0.
Danielsen, J.E., Auken, E., Jørgensen, F., Søndergaard, V.,
and Sørensen, K.I., 2003, The application of the
transient electromagnetic method in hydrogeophysical
surveys: Journal of Applied Geophysics, 53, 181–198.
ElKholy, S.M., Sabry, T.I., and AlSalamah, I.S., 2009,
Environmental contamination risks due to leaking
underground fuel tanks (LUFT) of gas stations in
AlQassim Region, Saudi Arabia: Second International
Conference on Environmental and Computer Science
(DOI 10.1109/ICECS.2009.100), 3–8.
Ernstson, K., and Kirsch, R., 2006a, The transient electro-
magnetic methods: in Krisch, R. (ed.), Groundwater
Geophysics: A Tool for Hydrogeology, Berlin, Springer,
179–226.
Everett, M., and Meju, M., 2005, Near surface controlled-
source electromagnetic induction: Background and
recent advances: in Rubin, Y., and Hubbard, S. (eds.),
Hydrogeophysics, Springer, The Netherlands.
Food and Agriculture Organization of the United Nations
Rome, 2009, Groundwater management in Saudi
Arabia, 14 pp.
Hoetzl, H., 1995, Groundwater recharge in an arid karst area
(Saudi Arabia), Application of tracers in arid zone
hydrology: in Proceedings of the Vienna Symposium,
August 1994, IAHS Publ., 232, 195–207.
Inman, J.R., 1975, Resistivity inversion with ridge regression:
Geophysics, 40, 798–817.
Interpex, Ltd, 2008, IX1D v2 Instruction Manual: Interpex
Limited, USA, 1–67.
McNeill, J.D., 1994, Principles and application of time domain
electromagnetic techniques for resistivity sounding:
Technical Note TN-27, Geonics.
Meju, M.A., Fontesz, S.L., Oliveiraz, M.F.B., Limaz, J.P.R.,
Ulugergerli, E.U., and Carrasquilla, A.A., 1999, Re-
gional aquifer mapping using combined VES-TEM-
AMT/EMAP methods in the semiarid eastern margin of
Parnaiba Basin, Brazil: Geophysics, 64, 337–356.
Metwaly, M., Elawadi, I., Moustafa, S., and Al Arifi, N., 2012,
Tracing groundwater contaminated plumes using
TDEM and 2D ERT data sets at Al-Quwy’yia Area,
Central Saudi Arabia: 5th International Conference on
Water Resources and the Arid Environments (ICWRAE
5), 530–537.
TEM-RESearcher manual, 2007, Version 7: Applied Electro-
magnetic Research (AEMR), The Netherlands.
Young, M.E., Macumber, P.G., Watts, M.D., and Al-Toqy,
N., 2004, Electromagnetic detection of deep freshwater
lenses in a hyper-arid limestone terrain: Journal of
Applied Geophysics, 57, 43–61.
Journal of Environmental and Engineering Geophysics eego-19-01-05.3d 22/1/14 01:38:23 52
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