Nygren - Et Al (2016) - Groundwater Occurence & Quality in Bulawayo Province (Uppsala University)
Nygren - Et Al (2016) - Groundwater Occurence & Quality in Bulawayo Province (Uppsala University)
Nygren - Et Al (2016) - Groundwater Occurence & Quality in Bulawayo Province (Uppsala University)
augusti 2016
Edvard Nordenskjöld
Erik Östblom
Project Work in Aquatic Engineering, 15c
Supervisors: Constant Chuma, Mervyn Gumbo and Fritjof Fagerlund
June 2016
2
Acknowledgements
We want to thank Constant Chuma, Mervyn Gumbo and the Applied Physics Department at the
National University of Science and Technology, NUST, with the field work and all the help before
and during our stay in Bulawayo, Roger Herbert for letting us borrow a conductivity meter, Fritjof
Fagerlund and Constant Chuma for helping us as supervisors for the entire project and we would like
to thank ISP, ATE and SIDA for making this a possibility for us and many other project applicants.
3
Table of Contents
1. Introduction .................................................................................................................................... 6
2. Background ISP Project ..................................................................................................................... 6
3. Purpose and objective ........................................................................................................................ 7
4. Theory ............................................................................................................................................... 7
4.1 Theory of contaminant transport processes ................................................................................. 7
4.2 Theory of groundwater quality .................................................................................................... 8
4.3 Theory of infiltration ................................................................................................................... 9
4.4 Theory of Resistivity surveys .................................................................................................... 10
4.5 Theory of geomagnetism ........................................................................................................... 12
5. Method ............................................................................................................................................. 13
5.1 Site descriptions ........................................................................................................................ 14
5.1.1 Harry Allen Golf Course..................................................................................................... 14
5.1.2 Barbour fields ..................................................................................................................... 14
5.2 Geomagnetism method .............................................................................................................. 14
5.3 Electrical Resistivity Survey methods ....................................................................................... 15
5.3.1 Vertical Electrical Sounding ................................................................................................... 15
5.3.2 Electrical Resistivity Tomography ......................................................................................... 15
5.4 Infiltration method ..................................................................................................................... 17
5.5 Sampling water .......................................................................................................................... 17
6. Results ......................................................................................................................................... 17
6.1 Harry Allen Golf Course Results ............................................................................................... 17
6.1.1 Geomagnetism Results ....................................................................................................... 18
6.1.2 Vertical Electrical Sounding Results .................................................................................. 19
6.1.2 Electrical Resistivity Tomography Results ............................................................................. 20
6.1.3 Infiltration Results .............................................................................................................. 22
6.2 Barbour Fields ........................................................................................................................... 22
6.2.1 Geomagnetism results ......................................................................................................... 23
6.2.2 Vertical Electrical Sounding Results .................................................................................... 23
6.2.3 Electrical Resistivity Tomography Results ......................................................................... 24
6.2.4 Infiltration Results .............................................................................................................. 26
6.3 Sampling Results ........................................................................................................................ 26
7.Discussion......................................................................................................................................... 30
7.1 Harry Allen Golf Course ............................................................................................................. 30
7.1.1 Vertical Electrical Sounding ................................................................................................ 30
7.1.2 Electrical Resistivity Tomography ....................................................................................... 30
4
7.1.3 Infiltration ........................................................................................................................... 30
7.1.4 Compilation of Harry Allen Golf Course .............................................................................. 31
7.2 Barbour Fields ............................................................................................................................ 32
7.2.1 Vertical Electrical Sounding ................................................................................................ 32
7.2.2 Electrical Resistivity Tomography ....................................................................................... 32
7.2.3 Infiltration ........................................................................................................................... 32
7.2.4 Compilation of Barbour Fields ............................................................................................ 33
7.3 Comparison of the field sites ..................................................................................................... 33
7.4 Geomagnetism ........................................................................................................................... 34
7.5 Sampling .................................................................................................................................... 34
7.6 Broader context in Bulawayo province ...................................................................................... 35
8. Conclusion ....................................................................................................................................... 35
9.References ........................................................................................................................................ 36
Appendix ............................................................................................................................................. 38
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1. Introduction
This study has been conducted in the Zimbabwe’s second biggest city, Bulawayo, situated in
the mid-west part of the country. The population is over 1 million people. Bulawayo
metropolitan is underlain by a rather complex crystalline geology, mostly based on a granite
and greenstone belt. The precipitation, 550 mm per year, is unevenly distributed, mostly
occurring during the rainy period, November – February. Maximum and minimum mean
annual temperatures for the study are 25.8 °C and 12.7 °C, respectively (Chuma et al., 2013).
Bulawayo is situated on the Zimbabwean central plateau, at about 1400 m altitude, that
stretches from the southwest northwards with altitudes between 1.0-1.6 km (Wikipedia, 2016;
Wikipedia, 2007). The major part of the city is underlain by the Matsheumhlope aquifer and
the rest underlain by a low yielding granitic aquifer. These two aquifer systems form the
Bulawayo Urban aquifer system (Hlatywayo et al., 2013).
These conditions make droughts recurring, drinking water security a considerable problem,
and a field in which more studies is needed. The International Science Program (ISP),
established by Uppsala University, supports projects in developing countries such as
Zimbabwe in the fields of chemistry, physics and mathematics. In Bulawayo, the National
University of Technology (NUST) has raised funding from ISP in 2013 in order to create a
knowledge base of the groundwater potential in the Bulawayo Urban aquifer, including
surveys mainly covering the geology of the area and its impact on groundwater potential. ISP
is funded by the Swedish Development Cooperation Agency (SIDA), Uppsala University and
since 2012 also from Stockholm University.
The presence of groundwater has been difficult to map. However, Chuma et al from NUST
have in 2013 mapped the geology and groundwater occurrence of Bulawayo metropolitan
area. Further studies in the field are in progress and this Minor Field study (MFS) also
financed by ISP, is one of them. The sites included in this MFS project were situated in the
Bulawayo Metropolitan area, including groundwater sampling in private and public
boreholes, electrical resistivity tomography (ERT), vertical resistivity survey (VES),
infiltration and geomagnetic measurements at Harry Allen Golf Club (HAGC) and Barbour
fields (BF).
There are mainly three different methods to use when it comes to examining the groundwater
resources; geological, geophysical and geohydrological methods (Domenico & Schwartz,
1997). In this study the emphasis has been on geophysical methods in order to evaluate
ground water resources and their characteristics.
6
The project is split into different parts to achieve the objective. The methods that are used are
vertical electrical sounding (VES), electrical resistivity tomography (ERT), seismic refraction
tomography, geomagnetic techniques, chemical and microbiological analysis, infiltration
tests, two dimensional (2D) vadose zone transport modelling, and borehole drilling and
testing (Hlatywayo et al., 2013).
4. Theory
4.1 Theory of contaminant transport processes
The groundwater table is dynamic and depends on many parameters. In an open aquifer, the
groundwater table approximately follows the topography and its proximity to the ground
depends a lot on the period of the year and the volume of uptake for drinking water. Pump
tests are a way of examining in which direction the groundwater flows in an aquifer and how
!!
much water can be extracted per time unit. Groundwater flow Q ( !
) can be expressed using
Darcy’s law (Equation 1) for most granular material, and even fractured rock when expressed
as an equivalent porous medium (Domenico & Schwartz, 1997).
!!
𝑄 = 𝐾𝐴
!"
(1)
Where K (ms-1) is the hydraulic conductivity, A the cross sectional area (m2) and dh/dL the
hydraulic gradient (-). The hydraulic head (or groundwater potential) h (m) is the sum of the
pressure head ψ and the elevation head z.
ℎ = ψ + z (2)
For laminar flow, the water flow increases linearly with an increasing groundwater potential.
When it comes to water flow in fractured rock, Darcy’s law can be applied only when the
fractures are expressed as an equivalent porous medium. Otherwise, the water flow in the
fractures has to be calculated in discrete models (Domenico & Schwartz, 1997).
The way contaminants are transported and spread in a media is rather complex and depends
on many immobilization and chemical processes, such as sorption, advection and dispersion.
How water flows is important, as the water flow is the mode of transport for the
contaminants. The direction of water flow is from high to low hydraulic head h (m). The flow
7
direction of water with its velocity can be used, together with geophysical information about
the contaminant, to calculate the contaminant’s flow direction (Domenico & Schwartz, 1997).
TDS could be directly derived from the measured EC. As the name suggests, TDS refers to
all dissolved minerals in the water, measured in mg/L or ppm, and gives an approximated
concentration of them. The dissolved solids are either negatively or positively charged ions
(Harter, 2003). The respective charges of the anions and the cations should in total always
equal each other. When there are lots of charged particles in water, the EC will get a high
value that in turn can be transformed to a TDS value, which increases proportionally to the
EC. This relationship can be expressed as in equation 3 (Connellan, 2013):
!!"
TDS = 1000 × ECw µS/cm × 640!"/!" (3)
8
In comparison to Harter’s (2003) 500 mg/L TDS quality limit, an alternative classification of
limit values for TDS comes from World Health Organization’s (WHO) (Fawell et al., 2016).
There the TDS quality limit is 900 mg/L, or “fair” (Table 2).
Table 2. Classification of irrigation water based on total dissolved solids, TDS (Fawell et al., 2016)
According to Harter (2003) a TDS in water above 500 mg/L is not suitable for drinking. The
majority of the dissolved constituents are inorganic. Normally organic material is added from
top soil horizons. The inorganic constituents are mineral compounds and nutrients (Harter,
2013). Salinity is a measure of the accumulation of salt in water and soils. It includes
hundreds of different ions. However, chloride (Cl-), sodium (Na+), nitrate (NO3-), calcium
(Ca2+), magnesium (Mg2+), bicarbonate (HCO3-) and sulfate (SO42-) are the most common.
They can accumulate for reasons such as precipitation, rock weathering or anthropogenic
activity (California Protection Agency, 2010).
9
To calculate the hydraulic conductivity, the shape factor of the measuring well is needed. The
shape factor of a measuring well depends of the ratio of the height of water in the well H (m)
to the well radius R (m) as well as soil texture, although the latter not to a large degree. This
means that empirical formulas curve-fitted to numerically calculated data of the shape factor
(from the H/R ratio) can be used to describe the well-shape factor for soils with different
textures. In this study, for both structured loams and clays, the following formula describing
the shape factor curve produced the best, almost perfect, fit:
! !.!"#
!
𝐶= !.!!"!!.!"#! !
(4)
The empirical formula can describe the shape factor without introducing additional errors in
the calculation of the hydraulic conductivity (Zhang et al., 1998).
The macroscopic capillary length parameter α* is also needed to calculate the hydraulic
conductivity and is determined according to the texture-structure category. Soils that are both
fine textured (clayey or silty) and unstructured, as in our measurements, have a capillary
length value of 0,04 cm-1. The formula used to calculate Kfs for the one-head GP method
(combined reservoir or inner reservoir) where Q is the steady-state rate of fall of water
(cm3min-1) in the borehole calculated from the measurements:
!! !×!×!"×!"
𝐾!" ( !! ) = ! (5)
!!! ! !!! ! !!! ∗
!
Where d is the head of water established in the borehole (cm), r the borehole radius (cm) and
!! !"#
the conversion factors are 10 !"
and 60 !!
(Chuma, 2015).
11
study of horizontal or near-horizontal interfaces. The current and potential electrodes are
maintained at the same relative spacing and the whole spread is progressively expanded about
a fixed central point. Consequently, readings are taken as the current reaches progressively
greater depths. The technique is extensively used in geotechnical surveys to determine
overburden thickness and also in hydrogeology to define horizontal zones of porous strata.
(Kearey et al., 2002)
We used two array configurations in our studies, the Wenner and the Schlumberger arrays. In
the Wenner configuration, current and potential electrodes are maintained at an equal spacing
a. During VES measurements, the spacing a is gradually increased about a fixed central point.
With the Wenner configuration all four electrodes need to be moved between successive
readings. This labour is partially overcome by the use of the Schlumberger configuration in
which the inner, potential electrodes have a spacing 2l which is a small proportion, no more
than one fifth, of that of the outer, current electrodes (2L). In VES surveys the potential
electrodes remain fixed and the current electrodes are expanded symmetrically about the
centre of the spread. With very large values of L it may, however, be necessary to increase l
also in order to maintain a measurable potential. (Kearey et al., 2002)
In areas with limited open space for a long survey line, the conventional Wenner array has a
disadvantage in that there is a large reduction in horizontal coverage when the electrode
spacing is increased in order to achieve a deeper depth of investigation. For example, in order
to increase the depth of investigation by two times, the electrode spacing a has to be
increased to 2a. In this case, the total length of the array is increased from 3a to 6a. At the
same time, the width of the pseudosection decreases by 3a with each level of measurement.
(Geotomo, 2010)
4.5 Theory of geomagnetism
Magnetic surveys are done to investigate anomalies in the Earth’s magnetic field, which gives
information about the subsurface geology. The anomalies are caused by underlying rocks
with magnetic features (Kearey et al., 2002). The rocks are magnetized (equation 10) due to
being in a magnetic field (Milsom, 2003).
M=kH (10)
Where M is the magnetization (A/m), k is the susceptibility and H is the magnetic field
strength (T) (Milsom, 2003).
Because the rocks being magnetized, they will also have a magnetic field that is be added to
the Earth's magnetic field and gives rise to the anomalies (Kearey et al., 2002).
The new magnetic field (equation 11) then can be reduced to equation 12 (Kearey et al.,
2002).
𝐵 = 𝜇! 𝐻 + 𝜇! 𝐽! (11)
𝐵 = 𝜇𝑅𝜇! 𝐻 (12)
where B is the magnetic field (T), µ0 is the permeability of vacuum (N/A2), µR is a
dimensionless constant expressing relative magnetic permeability, H is the magnetizing force
(N), and J is the induced magnetization (A/m). (Kearey et al., 2002)
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Rock forming minerals generally have a very low magnetic sensitivity which stems from the
small part of the minerals that is magnetic. The two geochemical groups that give the mineral
a magnetic character are iron-titanium-oxygen and iron oxide. The three most important
magnetic minerals are magnetite, pyrrhotite and maghemite (Milsom, 2003). The most
common magnetic mineral is magnetite (Kearey et al., 2002). Hematite will only cause small
anomalies which means that iron ores may not show large anomalies (Milsom, 2003).
Basic igneous rocks have a strong magnetic character because magnetite is a large part of the
rock; however, the magnetite content decreases with increasing acidity. The magnetic
character of metamorphic rocks also changes. If the pressure of oxygen is too low, the
magnetite will be resorbed and the iron and oxygen will be distributed to other minerals as
the metamorphism increases. If the pressure is high, magnetite can be formed. Sedimentary
rocks are only magnetic if it contains a large amount of magnetite. When anomalies of the
magnetic field are found in sedimentary areas there is usually an underlying layer of igneous
or metamorphic rocks or there are intrusions in the sediment (Kearey et al., 2002).
The magnetic content of rocks varies a lot and there are overlaps between different
lithologies, which makes it hard to distinguish the lithology from magnetic surveys alone
(Kearey et al., 2002).
Anomalies of the magnetic field can for example be caused by folded or truncated sills or
lava flows, dykes, metamorphic basement rocks, massive intrusions or magnetite ore bodies.
The anomalies vary from tens of nT to many thousands nT, from the low magnetic field of a
metamorphic basement to the high magnetic field of magnetite ores (Kearey et al., 2002). In
populated areas, there are often pieces of iron and steel on or in the ground which will affect
the magnetic field. The effect of iron or steel material on the magnetic field varies due to
several reasons, for instance whether or not it is buried and the size of the material (Milsom,
2003).
The Earth’s magnetic field also varies during the day, whereas it is nearly stable during the
night. The Earth's magnetic field decreases from sunrise until about 11am and increases from
about 11am until 4pm (Milsom, 2003).
5. Method
This section covers the methods used in this study on the different locations. The order in
which the methods were performed was logically divided depending on the information
gleaned from them. The procedure started with geomagnetic measurements, followed by ERT
and VES and finally infiltration tests. As the magnetometer shows the variation of the
magnetic field at the site, which is interpreted to locate groundwater, the ERT, VES and
infiltration measurements are tested for further information. As a complement to this work,
the geological map of Bulawayo was used to get a sense of where the faults were situated,
especially so at Barbour fields. This is of importance as the VES and ERT lines should align
perpendicularly to the cracks and faults to obtain the highest resolution.
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5.1 Site descriptions
5.1.1 Harry Allen Golf Course
Harry Allen Golf Course (HAGC) is situated opposite to NUST and consists of a total area of
approximately 1 km2 (Google Earth). HAGC stretches with its 18 holes through a part of the
Matsheumhlope aquifer, whose components are of interest to this study. It was chosen to
conduct the first measurements for the geophysical methods, Geomagnetics, VES, ERT, and
infiltration tests. HAGC’s proximity to NUST and its open fields made it a good first site to
apply all methods that were going to be incorporated in the research study, in order to assess
the state of the aquifer.
5.1.2 Barbour fields
The second site to be investigated using the geophysical methods was a former dumping site
in the high-density suburb Barbour Fields (BF), in the western part of Bulawayo. The
circumstances of how long the dumping has been going on and to what extent are not known.
However, a public borehole situated at the site showed an electrical conductivity (EC),
salinity, and TDS too high for human consumption. The site was therefore interesting in
terms of water and contaminant flow. Attempts were made to examine where the
groundwater was located, as well as depth to bedrock and hydraulic conductivity.
5.2 Geomagnetism method
Two GSM-19 Overhauser magnetometers, produced by GEM systems, were used to measure
the magnetic field and to record the time and date when the measurements were conducted.
The GSM-19 consists of three different devices; a GPS, a sensor and a control unit (Figure 1).
One of the GSM-19 was used as a base station and it measured the magnetic field from the
background to compensate for it. It was stationary and therefore the coordinate was recorded
by the GPS in a single measurement. The magnetic field, time, and date were stored every 0.5
s for both magnetometers in synchronicity. The other GSM-19 was used to measure the
magnetic field of the sampling site. The magnetic field of the sampling sites was measured by
walking with the sensor on the back and a GPS tracker on top (Figure 1).
Figure 1. The devices included in the GSM-19. 1-GPS, 2-sensor, 3-control unit. The direction of the sensor
was always perpendicular to the direction of movement and parallel to the sensor at the base station.
14
Before the measurement of the magnetic field started, boundary locations were taken with the
GPS at points where the sampling area started and ended. The boundary locations and
specific distances between the lines, 25 m at HAGC and 20 m at BF, were used to calculate
the number of lines needed. Furthermore, the GPS provided the user with the direction and
location of the parallel lines. The measurements with the GSM-19 were done walking in a
straight line, from one side of the sampling site to the other.
The collected data was thereafter downloaded and processed in SURFER into maps that
depicted the magnetic field of the sampling site.
5.3 Electrical Resistivity Survey methods
The MINISTING single-channel earth resistivity/Induced Polarization (IP) meter from
Advanced GeoSciences Inc (AGI) measured electrical resistivity in these surveys. Stainless
steel electrodes and multi-core cables available in the department of applied physics at NUST
were used as well. These surveys include VES that gives higher resolution in deeper layers
vertically downwards from a point, and ERT that gives higher resolution horizontally in more
shallow layers (4.4 Theory of Resistivity surveys). The resolution corresponds directly to the
number of measurement (data) points (4.4 Theory of Resistivity surveys). The sites that had
shallower impermeable bedrocks could be reached using only ERT and might therefore not
need VES.
5.3.1 Vertical Electrical Sounding
The VES lines were done using the Schlumberger array (4.4 Theory of Resistivity surveys). The
distances between the potential and the current electrodes could be chosen according depth
penetration. The four steel electrodes were hammered at the correct positions and the multi-
core cables were connected to them, with the measuring instrument in the middle. In non-
conductive dry ground, such as BF, salt water was applied to the electrodes in order to obtain
good readings. This was repeated for each measurement, although after the first one, only two
electrodes needed to be moved.
Three VES lines were located at HAGC in a cohesive line, each 300 m long, spanning 900 m
in total (Figure 2). Near the end points of the cohesive VES lines, two ERT lines were
conducted. Two VES lines were measured at the Barbour fields, one 300 m long running
parallel to the road demarcating the dump site, and the other 260 m long, approximately
perpendicular to the first ERT line made at the site (Figure 3). Notes were recorded in the field
as to correct for calculated standard deviations surpassing 2%, so that it could be dismissed in
the model. The software program RES1dINV was used, using the least-square method to
transform the measured apparent resistivity to an image of the modelled resistivity. This is an
inverse resistivity model and the image extended vertically down, successively, from the first
point, in one dimension.
5.3.2 Electrical Resistivity Tomography
The ERT lines were made using the Wenner array (4.4 Theory of Resistivity surveys). The steel
electrodes were hammered to the ground every 5 m, for the full length of the desired ERT
line. The direction of the line was given by a tape measure. The cables were connected along
the electrode line, with the users moving back and forth, taking measurements from 5 m
between the electrodes all the way to 55 m.
15
Figure 2. ERT and VES lines as well as Auger holes at HAGC . The yellow lines illustrate the three ERT
lines, the green lines show the VES lines while the pink dots depict the auger hole point measurements.
The numbers indicate in which order the respective lines and Auger hole measurments were conducted.
Three ERT lines were located at the golf course, each 250 m long, two of them located near
the end points of the cohesive VES line (Figure 2). Three ERT lines were made at the Barbour
fields. They were 200 m, 160 m and 150 m long. The two longest lines were approximately
perpendicular to each other, extending between the borders of the dumpsite, the wall and the
road (Figure 3). The third and shortest ERT line ran approximately parallel to the road. The
software program RES2dINV was used, with the same inverse modelling used with the VES.
The modelled image extended horizontally at depth corresponding to the distance between
the electrodes. All these depths put together resulted in the tomographic image.
Figure 3. ERT and VES lines as well as Auger holes at BF. The yellow lines illustrate the three ERT lines,
the green lines show the VES lines while the pink dots depict the auger hole point measurements. The
16
numbers indicate in which order the respective lines and Auger hole measurments were conducted. No
results were obtained for the measurements made at auger hole 3, and so those results are not illustrated
elsewhere in this report.
6. Results
6.1 Harry Allen Golf Course Results
The method procedure at the golf course started with the geomagnetic method, followed by
VES, ERT and lastly infiltration rate tests. The results of the geomagnetic method indicated
17
suitable places for the VES and ERT lines, whereas the auger holes were drilled at the end of
the ERT lines where the soil types were observed to be different.
6.1.1 Geomagnetism Results
The results of the geomagnetic method were depicted using the software SURFER (Figure 4).
The lowest value observed was 29450 nT and the highest was 30450 nT. Some of the features
of the map are unlikely to represent the true geology of the area, but instead objects of higher
magnetism that affects the magnetometer (4.5 Theory of geomagnetism ). This is likely the
case, for example, of the orange dots in the lower right corner of the map, perhaps indicating
a surface steel pipe observed at HAGC. The geomagnetic method was used as a first indicator
of fractures and lineaments at HAGC, interpreted from great contrasts in magnetism, as for
example in the upper right corner of the map between the numbers. This area indicates a
variability of 50 nT over a longer line, and could be interesting to cut across with an ERT line
to obtain a higher resolution of resistivity. More areas were identified with similar
characteristics (Figure 4).
Figure 4. The geomagnetic method applied at the HAGC with image created in the software program
SURFER. The south and easting coordinates are in WCS 1984, the scale bar determined in meters and
the magnetism depicted in nT. Image by Mervyn Gumbo, Research Fellow, National University of
Science and Technology, Bulawayo.
18
6.1.2 Vertical Electrical Sounding Results
The results from three VES lines are illustrated together with a table of the resistivity, height,
depth and altitude of the modelled layers.
Vertical Electrical Sounding 1
The resistivities of the first measurements depict a low resistivity that then gradually
increased with each measurement. The model calculated that three layers had the best fit. The
layers had the resistivity of 38.3, 19.4 and 170 Ωm. The total depth to the third layer was 4.07
m (Figure 5).
Figure 5. The VES1 results from HAGC. The dots are measurements, the black line is the line between
the measurements, the red line is the model and the blue line corresponds to the layers that the model
calculated. The table at the lower right corner shows the resistivity (ρ, Ωm), height (h, m), depth (d, m)
and altitude (Alt, m). Resistivity (Ωm) is at the x-axis and depth (m) on the y-axis.
19
Figure 6. The VES2 results from HAGC. The dots are measurements, the black line is the line between
the measurements, the red line is the model and the blue line corresponds to the layers that the model
calculated. The table at the lower right corner shows the resistivity (ρ, Ωm), height (h, m), depth (d, m)
and altitude (Alt, m). Resistivity (Ωm) is at the x-axis and depth (m) on the y-axis.
Figure 7. The VES3 results from HAGC. The dots are measurements, the black line is the line between
the measurements, the red line is the model and the blue line corresponds to the layers that the model
calculated. The table at the lower right corner shows the resistivity (ρ, Ωm), height (h, m), depth (d, m)
and altitude (Alt, m). Resistivity (Ωm) is at the x-axis and depth (m) on the y-axis.
20
resistivity. The results in this section correspond to the inverse model resistivity. Generally,
the resistivity was low at the surface and increased with depth.
Electrical Resistivity Tomography 1
There was one section with comparatively lower resistivity at greater depths, at
approximately 85 m horizontally. At approximately 200 m horizontally there is also a section
with comparatively lower resistivity at greater depths (at approximately 21.5-26.2 m depth)
(Figure 8).
Figure 8. The ERT1 results from HAGC. The x-axis is horizontal length (m), the y-axis is depth (m) and
the resistivity values are displayed according to color. The darker blue is the lowest resistivity value (Ωm)
while dark red represents the highest. The top image is the measured apparent resistivity, the middle one
is the calculated apparent resistivity and the lower image is the resistivity given by inverse modelling.
Figure 9. The ERT2 results from HAGC. The x-axis is horizontal length (m), the y-axis is depth (m) and
the resistivity values are displayed according to color. The darker blue is the lowest resistivity value (Ωm)
while dark red represents the highest. The top image is the measured apparent resistivity, the middle one
is the calculated apparent resistivity and the lower image is the resistivity given by inverse modelling.
21
Electrical Resistivity Tomography 3
The resistivity at the third location was lowest at the surface but increased with depth. The
largest resistivity could be seen at around 80 and 160 m horizontally at the depth 31.3 m
(Figure 10).
Figure 10. The ERT3 results from HAGC. The x-axis is horizontal length (m), the y-axis is depth (m) and
the resistivity values are displayed according to color. The darker blue is the lowest resistivity value (Ωm)
while dark red represents the highest. The top image is the measured apparent resistivity, the middle one
is the calculated apparent resistivity and the lower image is the resistivity given by inverse modelling.
22
6.2.1 Geomagnetism results
The lowest value observed was 28200 nT whereas the highest was 31400 nT. The map
displayed a large, curved area with higher magnetism, some areas with extremely high
values, and a more homogeneous magnetism in the rest. However, even the more
homogeneous area exhibited more variations than those seen at the golf course. Interesting
areas with higher variations in magnetism were more likely to be seen if the location of the
dumpsite was also taken into account. Notice that the variations between the colours are 200
nT at Barbour Fields, compared to only 50 nT at HAGC. It was difficult to identify
potentially interesting areas for the ERT lines on this map (Figure 11). This is why the
geology map of Bulawayo from the Zimbabwe Geological Series was used (Turner, 1991).
Figure 11. The geomagnetic method applied at Barbour Fields with the image created in SURFER. The
south and easting coordinates are in WCS 1984 and the magnetism depicted in nT. Image created by
Mervyn Gumbo, Research Fellow, National University of Science and Technology, Bulawayo.
23
Figure 12. The VES1 results from BF. The dots are measurements, the black line is the line between the
measurements, the red line is the model and the blue line corresponds to the layers that the model
calculated. The table at the lower right corner shows the resistivity (ρ, Ωm), height (h, m), depth (d, m)
and altitude (Alt, m). Resistivity (Ωm) is at the x-axis and depth (m) on the y-axis.
Figure 13. The VES2 results from BF. The dots are measurements, the black line is the line between the
measurements, the red line is the model and the blue line corresponds to the layers that the model
calculated. The table at the lower right corner shows the resistivity (ρ, Ωm), height (h, m), depth (d, m)
and altitude (Alt, m). Resistivity (Ωm) is at the x-axis and depth (m) on the y-axis.
24
Electrical Resistivity Tomography 1
At approximately 35 m horizontally, a higher resistivity was seen at the surface and a lower
resistivity at about 8.5-12.5 m depth. At about 80 m horizontally, a section with low
resistivity was seen through the entire depth. In general, the right part of the ERT line showed
a lower resistivity than the left side. The lowest resistivity was seen in the two sections
described above. The largest resistivity was seen at the depth 21.5-26.2 m between the
horizontal length 40 and 80 m (Figure 14).
Figure 14. The ERT1 results from BF. The x-axis is horizontal length (m), the y-axis is depth (m) and the
resistivity values are displayed according to color. The darker blue is the lowest resistivity value (Ωm)
while dark red represents the highest. The top image is the measured apparent resistivity, the middle one
is the calculated apparent resistivity and the lower image is the resistivity given by inverse modelling.
Figure 15. The ERT2 results from BF. The x-axis is horizontal length (m), the y-axis is depth (m) and the
resistivity values are displayed according to color. The darker blue is the lowest resistivity value (Ωm)
while dark red represents the highest. The top image is the measured apparent resistivity, the middle one
is the calculated apparent resistivity and the lower image is the resistivity given by inverse modelling.
25
Electrical Resistivity Tomography 3
The lowest resistivity was at the surface and it gradually increased with depth. The higher
values were measured at the depth of 26.2 m (Figure 16).
Figure 16. The ERT3 results from BF. The x-axis is horizontal length (m), the y-axis is depth (m) and the
resistivity values are displayed according to color. The darker blue is the lowest resistivity value (Ωm)
while dark red represents the highest. The top image is the measured apparent resistivity, the middle one
is the calculated apparent resistivity and the lower image is the resistivity given by inverse modelling.
26
Figure 17. The borehole distribution within the Bulawayo province, with an emphasis on the metropolitan
area. There is one triangle for every borehole, 120 in total, with the border outlining the Bulawayo
province. The highest sampling density can be seen in the south-eastern parts of the area, where private
bore holes are common. In the western area the sampling density is low, where private bore holes are few
and public bore holes are common.
The groundwater electrical conductivity variation is described in (Figure 18). Areas shown in
blue and green (Figure 18) corresponded to the range of good drinking water quality,
according to Table 1. The blue to light blue areas dominated in the southern and eastern parts
of Bulawayo, with a field running through to the west. The green colour corresponds to water
with values ranging from 750-2250 µS/cm, where 2250 µS/cm is the limit for drinkable water
(Table 1, Balachandar et al., 2010). Conductivity values shown in orange and red are over
2250 µS/cm and according to this simple definition, not suitable for drinking. The water that
has the highest conductivity, dominates in the central and western part of the study area
(Figure 18).
Table 5 displays examples of two sites with values indicating water not suitable for drinking,
and two sites with two nearby boreholes indicating large discrepancies.
27
Table 5. Examples of EC and TDS at different locations with either very large discrepancies between
nearby boreholes, as in Hillside, or contaminated sites, BF and Pelandaba
Figure 18. The conductivity variation within the Bulawayo province, with an emphasis on the
metropolitan area. The conductivity was measured within the range of 0-6400 µS/cm. The lowest values
are depicted as dark blue changing via light blue, green, yellow to red, which is the highest. The scale is
measured in km.
The TDS and Salinity maps showed a clear connection to each other following the
Conductivity map in shape and appearance. The limit values for TDS were set with a quality
limit of 500 mg/L (Harter, 2003) indicated with the same principle of colours as for the
conductivity. The most obvious difference compared to Conductivity was the northern part of
the map, which showed a bigger contaminated field, with an additional orange colour area
(Figure 19).
28
Figure 19. The TDS distribution within the Bulawayo province, with an emphasis on the metropolitan
area. TDS was measured within the range of 0-4200 mg/L. The scale is measured in km.
The salinity map had even more similarities with the Conductivity map than the TDS. The
limit for drinking water was set to 1 g/L with a maximum of 2 g/L, or ppt. The salinity
showed a more continuous belt of blue stretching between the two contaminated spots in the
north-west (Figure 20).
Figure 20. The salinity distribution within the Bulawayo province, with an emphasis on the metropolitan
area. The salinity was measured within the range of 0-2 g/L. The scale is measured in km.
29
7.Discussion
7.1 Harry Allen Golf Course
7.1.1 Vertical Electrical Sounding
The first VES line (Figure 5) indicates that the bedrock is at the depth of 4.07 m. The red line
has a 45o angle in the last layer and there may be water at this level. There is a decrease in
resistivity from layer one to two that could also imply that there is water between the depths
between 2.15-4.07 m.
The second VES line (Figure 6) had a larger difference in resistivity from layer one to two
that, as before, could mean there is water in this level. This also agrees with the angle the red
line has, 45o, which indicates that there is water. The last layer of the second VES is probably
the bedrock due to the large resistivity and the depth of 69.7 m.
Figure 7 depicted the third VES line and compared to the other two at HAGC the basement
rock is closer to the surface at 18.4 m. There are also four layers of rock. The second layer
has a very low resistivity, 4.8 Ωm, and is very thin, only 0.900-1.104 m. The presence of
metal pipes is suspected here that would be used for watering the golf course, which affects
the measurements. However it could also be something else, which is why it is interesting to
further investigate this part of the course. The 45o angle is achieved in the bedrock.
7.1.2 Electrical Resistivity Tomography
From the results of ERT 1 there may be water at the depth of around 29m, due to the lower
resistivity (the light green part), the area around the light green also has lower resistivity. This
means that there may be water there too but it may just be a crack with a different rock. The
small area with the largest resistivity is probably the base rock. In ERT 2 there is very high
resistivity close to the surface of Earth. As mentioned above, the high resistivity areas are
probably the basement rock, but there are lower resistivity areas at the left and right end of
Figure 9. It is possible that there is water in these areas. However, it seems that the rock layers
are gradually increasing the resistivity. ERT 3 showed a gradually increasing resistivity and
the basement rock seems to exist at the depth of 31 m. There may not be water in this area
because there is no abnormality in the resistivity values.
7.1.3 Infiltration
The results of saturated hydraulic conductivity from HAGC can be compared to the
descriptions of the soil types of the auger holes in order to obtain further information about
the soil layers (Appendix). It was observed that for all the auger holes at HAGC larger
gravel/rocks were mixed with finer particles. Clay was the finer material in the case of
HAGC 2-3. In the case of HAGC 4-5 loam was the slightly coarser material (more mixed-in
silt as opposed to clay), but still fine. Higher hydraulic conductivity usually means larger
pores, either resulting from larger particle sizes in general or a more homogenous soil with a
lack of mixture of different particle sizes (4. Theory). It seems clear that the 50 cm and the
150 cm layers had a significantly lower hydraulic conductivity, about half that of the 100 cm
layer. Two of the three trials for the 50 cm layer demonstrated a higher hydraulic
conductivity, and the third trial was probably the outlier. More trials would be needed to
30
confirm this but, nonetheless, the tests indicate higher gravel content in the 100 cm layer, or
alternatively a lower degree of mixing that would cause its higher porosity, permeability and
hydraulic conductivity. For HAGC 2 it seemed that the gravel/clay mixture was fairly
constant for all the soil layers. At HAGC 3 the hydraulic conductivity was highest for the
upper 50 cm depth layer, significantly so when compared to the deeper layers. The 100 cm
layer had the lowest hydraulic conductivity because of the slight increase for the 150 cm
layer. It consists of closer hydraulic conductivity values between the layers, but for one of the
layers, the permeability is twice that of the other layers. For this experiment too, the cause
could be different degrees of mixing or a higher content of coarser particles. For HAGC 4 it
was observed that the gravel percentage increased at depths below 50 cm (Appendix). It can
be concluded, from the results, that the gravel part decreased again below 100 cm. This result
resembled that of HAGC 3, with a significantly higher permeability for one layer, in this case
the 100 cm layer, but with a smaller difference, like for HAGC 2. This depends on which of
the two experiments for the 100 cm layer is considered more likely, although the factor
between it and the lower permeability is larger than about 2, for the other experiments. For
HAGC 5 the observations were of finer particles that were comparatively less so than for
HAGC 2-4, i.e. sandy loam (Appendix). Gravel was mixed into a further degree in deeper
layers, which could have the effect of decreasing the pore size since the sandy loam could be
situated in between the coarse gravel (Appendix). This would then explain the decreasing
permeability for HAGC 5. However, the way it decreased depends on which of the trials for
the 50 cm layer best describes the reality of the upper soil layer. If it is the lower value, the 50
and 100 cm layers are very similar in permeability and it decreases drastically to the lower
150 cm layer. If it is the higher value that best describes the upper soil layer, then the
decrease in permeability is drastic between all the layers, with factors of about 2.5 and 5 from
the upper to the lower layers. More experiments are needed to determine which experiment
best describes the upper soil layer, but in any case, a decrease in permeability with depth
looks certain for HAGC 5.
7.1.4 Compilation of Harry Allen Golf Course
The VES measurements indicated that there might be water stored between the first and
second layer for line 1 and 2. VES line 3 had a second layer with low resistivity, although
pipes could have disturbed the measurements. The most significant ERT line was the first, as
it clearly showed a deep lying part with low resistivity, and thus possibly a water storage. The
other two ERT lines had mainly a resistivity that increased with depth. Comparing the VES to
the ERT lines, the ERT lines give more information about the deeper lying water whereas
VES lines mainly depicted water close to surface. VES measures the resistivity in points (one
dimensional) and ERT in two dimensions, making the ERT results more accurate when there
are fewer VES points. Choosing ERT and VES lines cutting the area with a high magnetic
field variation could have been important to get these results. The auger holes represent
certain soil layers through observation. The hydraulic conductivity measured from the
infiltration tests should be compared to the resistivities for the ERT lines. The resulting
resistivity could be compared with the hydraulic conductivity at the auger hole. When the
values for resistivity are linked to mineral content, it could be possible to predict the
hydraulic conductivity from the mineral content.
31
7.2 Barbour Fields
7.2.1 Vertical Electrical Sounding
Figure 12 depicted the first VES line at BF and showed that there are three different layers
with a total depth of 13.7 m. The bedrock should be the layer at the bottom, which is at a
depth of 13.7 m. Where the red line is at 45o there may be water. The 45o angle in Figure 12
can be seen at different levels which make it hard to draw any real conclusion and is therefore
interesting to investigate further.
The next VES line (Figure 13) illustrated an unusual line with a resistivity of 266 Ωm (from
the surface to a depth of 0.9 m) in the first layer changing to 2.93 Ωm (from 0.9 to 31.5 m) in
the second layer and finally to 1176 Ωm (from 31.5 m) in the last layer. The line might have a
45o angle at the last layer but due to the large resistivity there should not be any water at this
depth. However, it would be interesting to see the ERT line there to study the layers further
and to deduce the reason for the large difference in resistivity.
7.2.3 Infiltration
The same comparison was done with the results from Barbour Fields. The observations were
made that all the material was mixed up at the dump site so the hydraulic conductivity was
probably highly variable. At BF 1, the fine ash observed at 50-100 cm could have the effect
of clogging up the shallower layers, thereby slowing its infiltration rate (Appendix). For BF 1,
the hydraulic conductivity at 100 cm could be higher than the fine ash would indicate because
of the sewer tank and the sewage below with its extremely high permeability. The dump site
at Barbour Fields is known to be an artificial soil environment and it is highly probable that a
hole was dug surrounding BF 1 to dump the sewage below 100 cm (Appendix). The hydraulic
conductivity measurements at BF 1 illustrated a low value for the upper layer compared to
the corresponding measurements at HAGC. The hydraulic conductivity for the 100 cm layer
at BF 1 was higher than 2 of the corresponding layers at HAGC, but lower than 3, and
32
markedly higher than the upper layer at BF 1. Both of these measurements could be explained
by the observations made concerning the clogging up of the upper layers, thereby slowing the
infiltration rate there. Even the markedly higher conductivity at the 100 cm layer could be
caused by the sewage below. These measurements do not prove the explanations given for
the observations, but they are not disproven either and remain plausible. At BF 2 a hole dug
and later filled up is also strongly suspected of surrounding the auger hole. But the 50-100 cm
layer was coarser and less likely to form aggregates that decrease pore size at BF 2, thereby
indicating a higher porosity at BF 2 than BF 1 (Appendix). That was probably why BF 2 was
too conductive to measure already at 100 cm, and also why the permeability was higher at 50
cm (although a quite normal value for HA); there was less clogging up from the deeper soil
layer at 50-100 cm. Again, the observations remain plausible explanations, but are not
definitely proven to cause the measurement results.
7.2.4 Compilation of Barbour Fields
The ERT and infiltration measurements corresponded very well, both showing the effects of
an artificially altered environment. The ERT results showed this through extremely large
contrasts in resistivity that probably did not reflect any original geology of the area. Objects
of high resistivity in combination with high porosity areas with low resistivity were probably
measured due to modifications. The infiltration illustrated this through low infiltration at the
surface where fine ash was observed, and extremely high infiltration reflecting extremely
high porosity further down. The two auger hole results likely do reflect the ERT results, at
least the ERT 1 line at BF, considering their close proximity (Figure 3). The VES
measurements did not in any significant way depict an artificial soil environment since it only
provided a one-dimensional illustration of resistivity as they are point measurements. The
geomagnetism results did give an indication of an artificial soil environment with large
contrasts in the magnetic field, but not at the level of the ERT lines with greater measurement
depths. Its purpose was to indicate potentially rewarding position of ERT lines, which was
instead made with the geological map of Bulawayo province (Turner, 1991).
33
ERT results in that man has not significantly modified HAGC while BF has been dug and
covered with soil, which is the reason the hydraulic conductivity at larger depths cannot be
measured.
7.4 Geomagnetism
The main purpose of the geomagnetic method was as a preliminary analysis to determine
useful locations of VES and ERT lines. It was not meant to distinguish lithology from
magnetic surveys, not least since the magnetic content in rocks overlaps between different
lithologies (4.5 Theory of geomagnetism). The geomagnetic method was rather used to discover
large contrasts in magnetism between nearby areas that could signify cracks, faults, or
fractures in the ground. Such high-contrast areas are interesting to both cut across and run
parallel with ERT lines. The perpendicular line would reveal if the magnetic contrast
translates to large differences in electrical resistivity as well, which would indicate
information about the soil structure, especially larger features such as faults and fractures. A
parallel ERT line to the high-contrast magnetic line would further display the differences
within the area, which the perpendicular ERT line might obscure with its larger contrasts
between areas. However, it is important when choosing locations for ERT lines to remember
that the high-contrast areas, or anomalies, can have different causes, like pieces of iron and
steel in populated areas. The anomaly from a magnetic object would vary depending on the
size of the object and the degree of burial, if it is buried (4.5 Theory of geomagnetism). In the
case of HAGC, a steel pipe could be seen during the geomagnetic surveys, but if it hadn’t, the
regularity of the unusually high anomaly should alert the user to the cause, and not cause him
to perform ERT lines there.
7.5 Sampling
The great similarity between the TDS, electrical conductivity and salinity maps, especially
the two latter ones, were expressed in the results section. The charged ions in the water are
what cause its conductivity, while the TDS contain non-conductive materials as well (4.2
Theory of groundwater quality). This could explain why the TDS map exhibits a darker orange
colour than the other maps for the northern part of Bulawayo province. The map displaying
the location of the measurement boreholes gives an indication of the certitude of the three
maps discussed above, as determined by the density of the measurements. The eastern and
western areas were very well sampled, while part of the southern and north-western areas
were more sparsely surveyed, further away from the city centre (Figure 17). As the results
from the groundwater sampling indicated, the range from lowest to highest value for the
parameters EC, TDS and salinity is broad. Most remarkable is that the range can differ so
much between two close locations. The peak value for all the quality parameters is at, as
could be suspected, the BF public borehole. It is situated just near a former dumping site that
is still leaking contaminants rich in inorganic contaminants. Another not as obvious example
is in Hillside, where two private boreholes (Id 65 and 66, Table 5) 300 m apart also differed
strongly. They had a TDS of 780 and 1440 mg/L, EC of 990 and 1830 µS/cm and a salinity
of 0,3 and 0,8 g/L, respectively. Those two very close boreholes showed a difference in
quality that is somewhat strange and might be due to different plumes of contaminants. The
34
two boreholes could be near the border of two plumes resulting in almost twice as high
contamination at the sides.
7.6 Broader context in Bulawayo province
In a broader context of groundwater occurrence and quality in Bulawayo province, HAGC
could represent a more natural soil environment whereas Barbour Fields could represent a
significantly more artificial and polluted soil environment. These locations could be assumed
to be relatively closely related to other such environments in Bulawayo province. Both types
of environments are important in this respect, the first to locate potential boreholes for
drinking water and the second to investigate the spread of contaminant transport likely to
occur at contaminated sites. The measurements needed in order to find high-yielding
borehole locations depend on the required certainty, budget and time limits of the researchers
involved. More measurements are needed if more certainty is required that water will be
found when boreholes are drilled. Generally, boreholes are drilled for one or several
households in a private regime without any geophysical measurements made beforehand.
They are needed in order to analyse groundwater flow direction, speed of transport, water
volume and water table of an area. This is done in order to analyse such things as sustainable
yield and water quality that depends on the geology and water quantity of the area. To locate
high-yielding boreholes throughout the whole year at sufficient depths, a relatively sure
method is the potential fractures identified by low-resistivity areas of the ERT measurements.
In contrast to this, artificial soil environments are not investigated in order to locate drinking
water at clearly contaminated sites; at least not before decontamination measures are applied,
but rather to investigate the speed of contaminant transport. This was not done for BF in this
study since groundwater modelling is needed from the infiltration results in order to assess
the contaminant transport of chemicals moving with the water through advection (4.1 Theory
of contaminant transport processes). At the artificial and contaminated soil environment of BF,
discussions and results were also illustrated, as well as the more natural environment of
HAGC, from the perspective of finding good locations for boreholes. However, at an
artificial site quite expensive and time-consuming decontamination methods are needed in
order to render the water from these sites drinkable, putting them at a disadvantage compared
to more natural areas in terms of yielding drinking water from boreholes.
8. Conclusion
The ERT and hydraulic conductivity results confirm that the Barbour Fields is a landfill,
where garbage has been deposited. The ERT and VES results at HAGC indicate groundwater
occurrence, while the infiltration results at HAGC display differing tendencies of hydraulic
conductivity with depth. The methods need to be analysed in regard to what contributes to the
most useful results, and the two areas could be said to represent a natural and an artificial
environment, like other areas throughout the Bulawayo province.
The sampling maps depicted large parts of Bulawayo province being under the quality limits
for water in terms of TDS and EC. These maps, and the risk assessments they represent, are
greatly affected by a few boreholes displaying values much larger than the quality limits.
Nearby locations in the sampling maps could depict very contrasting values, probably
because of contamination plumes.
35
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37
Appendix
Table 6. Observations made about the soil types and layers surrounding the auger holes
The following section covers the linear regressions pf the cumulative infiltrated volume over
time, which, together with the infiltration template (Chuma, 2015), was the basis for the
calculation of the saturated hydraulic conductivity. These linear regressions are illustrated by
Figure 21, Figure 22, Figure 23, Figure 24, Figure 25, and Figure 26.
38
Figure 22. Linear regression at the 3rd auger hole, HAGC.
39
Figure 24. Linear regression at the 5th auger hole, HAGC.
40
Figure 26. Linear regression at the 2nd auger hole, BF.
41