Journal of Environmental Protection, 2009, 1, 20-30
Published Online November 2009 (http://www.SciRP.org/journal/jep/).
Borehole Drying: A Review of the Situation in the Voltaian
Hydrogeological System in Ghana
John Apambilla AKUDAGO1, Larry Pax CHEGBELEH1, Makoto NISHIGAKI2,
Nukunu A NANEDO3, Anthony EWUSI4, Kwabena KANKAM-YEBOAH5
1Graduate School of Environmental Science, Okayama University, Okayama, Japan
2Faculty of Environmental Science, Okayama University, Okayama, Japan
3Research and development Manager, World Vision Ghana Rural Water Project, Tamale, Ghana
4Geological Engineering, University of Mines and Technology, Tarkwa, Ghana
5CSIR Water Research Institute, P.O. Box M.32, Accra, Ghana
Abstract
Groundwater development for potable water supply for rural people in Africa especially in Ghana has in-
creased significantly over the past decades. The area underlain by the Paleozoic sedimentary formation
(Voltaian System) of the country in particular, has experienced this tremendous change. Groundwater in the
study area is normally exploited through boreholes fitted mostly with hand pumps. Though the boreholes
exhibit variable yields, most of them have yields greater than 13.5 l/min. Research carried out in the area
suggests that there is modern and enough recharge, yet borehole drying is a problem especially those with
low or marginal yields. A thorough review of the groundwater exploitation in the area, aimed at explaining
the circumstances that might lead to these phenomena on the field, has been conducted. The review shows
that boreholes with drill yields of usually <20 l/min, especially those drilled in the wet season, constitute the
highest percentage of the dried boreholes. Other construction material such as the filter media may also in-
fluence the drying process.
Keywords: Borehole Sustainability, Community Water Supply, Ghana, Groundwater Recharge and Depletion,
Voltaian System
1. Introduction
The demand for potable water has been increasing since
the last three decades due to increasing world population
growth. The main sources of water to match these de-
mands include surface waters (from rivers, lakes, streams,
ponds) and groundwater. In arid to semi-arid environ-
ments, temperatures are usually high and this eventually
results in high evaporation rates. Some researchers have
reported that surface water bodies that provide water for
use especially for irrigation have experienced recent re-
duction in volume and quality [1,2]. The location and
occurrence of surface water make them very susceptible
to pollution. Supplying water from these sources espe-
cially for domestic purposes require treatment which
could be very expensive especially in small settlement
communities or towns. In poverty stricken environments,
people try to use these waters as they exist, resulting in
outbreaks of water borne diseases.
Groundwater which occurs below the surface in the
soil pores, fractures, fissures and other weak geological
features or zones is relatively protected from bacterio-
logical contamination and evaporation and can be used
for domestic and industrial water supply. The ease of
tapping the resource at very close point of need gives it
an added advantage over surface waters. These merits
have caused people to heavily rely on groundwater for
domestic, agricultural and industrial purposes.
Groundwater is usually exploited from drilled bore-
holes, hand-dug wells or spring sources. The latter is
usually scarce in many flat or high altitude areas except
where the groundwater table rises above the ground sur-
face. Water supply from this type of source is relatively
meagre. However, boreholes and hand-dug wells have

J. A. AKUDAGO ET AL. 21
become the most popular way of supplying groundwater
to people in small communities.
It has been reported that about 1.5 billion people
worldwide depend on groundwater daily [3]. In Africa,
groundwater has proved to be very useful especially for
rural water supply for the 47% of the people with access
to potable water [4–6]. In this regard, it has been re-
ported that about 250,000 boreholes have been con-
structed for use in Africa [7]. Though the purpose of
these initiatives is to afford sustainable water supply in
terms of quality and quantity, there are many reports of
abandonment of some of these facilities in many places
[6–11]. The reasons assigned to the abandonment include
the mechanical breakdown of hand pumps and lack of
water in the borehole. It has been reported that hand
pumps were observed to be nonfunctional in some com-
munities in Mali where some women preferred to fetch
water from shallow hand-dug wells and surface water
sources [6]. With availability of modern technology,
hand pumps have now been made simple and mainte-
nance could be carried out at the community level where
breakdown is no longer a major problem. However, lack
of water in the borehole could result from many sources
such as lowering of groundwater table and depletion of
aquifer storage, improper borehole design and defects
from construction, clogging of the filter media and the
slots of the screen pipes.
Lowering of groundwater levels and depletion of aq-
uifer storage could arise when there is excessive pump-
ing compared with recharge [12–15]. Excessive pumping
could cause intrusion of saline water into fresh aquifers
to maintain groundwater levels in coastal areas. On the
other hand, if there is improper design or defects in the
construction of boreholes the aquifer zones could be
sealed. There have been reported cases of situation in
some boreholes in Ghana up to 18 m due to broken
screen pipes [16]. Consequently, the aquifers are blocked
from transmitting water to the boreholes. As reported in
literature, water of turbidity greater than 30 NTU causes
rapid clogging [17] especially in cases where improper
filter media has been used to construct the borehole.
Boreholes fitted with hand pumps are the main sources
of potable water supply for rural communities in Ghana.
Though the effort for providing potable water is to miti-
gate high water related diseases in many rural areas es-
pecially in the drought prone northern and eastern parts
of the country, drying of some boreholes have been re-
ported in some geological formations [18,19]. One of
such formations is the Voltaian System which is believed
to be late Proterozoic to early Palaeozoic in age [20] and
underlain by consolidated sedimentary rocks. Recharge
studies conducted using various techniques such as iso-
topic [19,21] and numerical models have shown that
there is fresh and enough recharge to groundwater
[22,23]. However, a lot of marginal or low yielding
(usually far less than 100 l/min) boreholes dry up espe-
cially in the study area after 1- 4 years of usage or less.
In this paper, a review of the borehole drying situation
especially in the Voltaian System of Ghana has been
carried out to understand the possible causes of it. The
review also tends to explain the reasons why redevelop-
ment of some clogged boreholes have not been effective.
2. Study Area
The Voltaian System which cuts across many parts of
Ghana, and extends to the Republic of Togo, is the main
area where the country depends for its food production.
It lies between latitude 11oN and 6oN and Longitude 1oE
and 2oW. The topography of the area is gently undulating
in the southern part whereas the northern portion is fairly
flat. Close to the geological boundaries, there exist steep
hills up to about 450m high in the north and 200-300m
high in the south. All the hills trend in the north-east
south-west direction. The annual rainfall in the Voltaian
Basin ranges from 750 mm in the north to 1600 mm in
the south, with evapo-transpiration averaging around 890
mm [24].
Drainage in the study area is enhanced by the Black
and White Voltas and the Oti Rivers, and finally flows
into the Volta Basin. However, there are smaller rivers
which drain into these main rivers. The vegetation in the
Voltaian System is wooden savannah in the north and
moist deciduous forest in the south. The vegetation in the
northern part of the System consists of the savannah
plants mainly dense annual and perennial grass, bushes
and trees.
The study area is the region with the least percentage
of people having access to potable water supply and con-
tributes the highest cases of guinea worm infection and
other water borne diseases in Ghana.
3. Geology and Hydrogeology
The Voltaian System occupies about 40% of the entire
land area of Ghana (Figure 1) and it is thought to be
about 3000 - 4000m thick. It covers most of the northern
part of Ghana. In most of these places surface waters
flows are ephemeral, occurring only during the wet sea-
son. The System consists of inter-bedded rocks including
mudstones, sandstones, arkose, conglomerate, shale, and
some limestone. The rocks are flat lying or gently dip-
ping except near the eastern margin of the basin adjacent
to the contact with the Precambrian rocks where the
lower members of the System are gently folded [25].
They are generally consolidated and are not inherently
permeable. Possible exceptions, however, do exist in
areas where the jointed sandstones, arkoses and quartzite
upon weathering have produced permeable surficial ma-
terials. Again, the rocks have undergone some degree of

J. A. AKUDAGO ET AL. 22
Figure 1. Geological map of the study area (modified after Dapah-Siakwan and Gyau-Boakye, 2000).
tectonic activity and most aquifers are made up of frac-
tures. However, there exist unconsolidated systems dot-
ted in many parts of the basin where good aquifers have
been located.
Available records show that for most of the areas
maximum borehole depth is about 90m with an average
depth of 48.1m [26]. However, there is report of a few
boreholes exceeding 100m, even up to about 150 m deep
in the far eastern part of the System [19]. Generally, the
Voltaian has very poor groundwater potential although
some water supplies come from fractures in the argilla-
ceous or loose zones in the arenaceous members [27]. In
some portions of the southern part of the Voltaian basin,
the weathered or loose zones range from 4 to 20 m thick
where many villagers rely on for hand dug borehole de-
velopment [18,28]. Borehole yields range from 5 to 1200
l/min, static water levels from 1 to 20 m and water table
fluctuation averaging about 4m [21,29,30]. The esti-
mated transmissivities range from 0.3 to 270 m2/day [29].
Most of the aquifers are semi-confined to confined and
groundwater quality conforms to the World Health Or-
ganization standards. However, there have been few
cases of high arsenic and fluoride levels in some drilled
boreholes [7,23,31]. Groundwater salinity has also been
a major challenge especially in the south-eastern and
north-western parts of the basin.
Groundwater recharge varies from location to location,
depending on the infiltration capacity of the surface and
the permeability of adjacent geologic material shielding
the aquifer. Available literature indicates that groundwa-
ter recharge in the Voltaian ranges from 3.7-5% of an-
nual rainfall [22,32,33] and groundwater abstraction is
estimated to be less than 5% of the annual groundwater
recharge [22,23].
4. History of Groundwater Development in
the Voltaian
Drilling of boreholes in the Voltaian System began far
back in the 1940s [24]. However, the few holes which
were drilled did not show good results resulting in the
scaling down of groundwater development in the area. In
the 1960s, few boreholes were drilled in the south-east-
ern corner of the System to provide potable water to
communities which were displaced as a result of the

J. A. AKUDAGO ET AL.
23
Bottom plug
Filter media
2mm<Φ<4 mm
Backfill
Plain pipe
Screen pipe
Opening=1mm
Cement grout
Well faceBottom plug
Filter media
2mm<Φ<4 mm
Backfill
Plain pipe
Screen pipe
Opening=1mm
Cement grout
Well face
Figure 2(a). Sketch of borehole construction in Ghana.
Bottom plug
Filter media
2 (mm)<Φ<4 (mm)
Backfill
Plain pipe
Screen pipe
Opening=1mm
Clogged material in
high yield borehole
Cement grout
Well face
Clogged material in
low yield borehole
Bottom plug
Filter media
2 (mm)<Φ<4 (mm)
Backfill
Plain pipe
Screen pipe
Opening=1mm
Clogged material in
high yield borehole
Cement grout
Well face
Clogged material in
low yield borehole
Figure 2(b). Sketch of filter clogging behind the screen.
construction of the Volta Lake [19]. In the mid 1970s,
the Canadian International Development Agency (CIDA)
entered the north-eastern end of the System to provide
boreholes fitted with hand pumps. Based on the experi-
ences gained from those previous drilling, the develop-
ment of groundwater increased tremendously from the
beginning of the 1980s. Within the last two decades
many Non Governmental Organizations (NGOs), such as
World Vision International, Water Aid, Church of Christ,
European Union (EU), United Nations Children’s Fund
(UNICEF) and CIDA have been carrying out drilling
operations in the System. There have been tremendous
improvements in the drilling and development technol-
ogy, and a complete change of siting boreholes using the
traditional terrain evaluation to integrated terrain evalua-
tion with geophysical surveys. However, due to the com-
plex nature of the geology, the drilling success rates are
still low, estimated to be about 50% with some of the
already existing boreholes drying up [7,18,19,29,34].
5. Borehole Construction
Boreholes completed in the Voltaian Systems are lined
with PVC pipes of about 140mm diameter. Previously,
stainless steel was also used to construct the boreholes
but since the inception of Community Water and Sanita-
tion Division (CWSD) in 1994, PVC is the only type of
pipes used. In the aquifer zone, a screen is placed to al-
low inflow of groundwater into the borehole. The annu-
lar space (usually~50 mm) between the borehole face
and the PVC screen pipe is filled with filter media up to
a few meters above the screen height. Figure 2(a) shows
a schematic section of a lined borehole. Before a bore-
hole would be considered for construction, it must meet a

J. A. AKUDAGO ET AL. 24
minimum yield of 13.5 l/min [35]. However, under dif-
ficult conditions, yields of at least 5 l/min are considered.
6. Borehole Rehabilitation
Boreholes drilled in the study area are usually commu-
nity owned, handed over to them after the government or
NGO drilling project is completed. It is the usual practice
to train local personnel to maintain, repair and manage
the boreholes. However, after the boreholes are handed
over to beneficiary communities, there is a big issue of
monitoring, maintenance and rehabilitation. This might
pose a lot of threat to the sustainability of the boreholes
as the people living in those communities lack the tech-
nology and funds to carry out such technical work [16].
Clogging may then set in with time if the fine particles
within the filter material and the slots of the screens are
not dislodged regularly.
7. Clogging of Filters
Clogging is a phenomenon that leads to reduction in
available pores for fluid flow and resulting in reduction
in permeability [36–38]. The cause of clogging could be
chemical, biological or physical otherwise known as par-
ticle clogging. In the study area, there has neither been
any reported case of chemical precipitation leading to
chemical nor any case of biological clogging. For the
purpose of this paper, the latter is discussed. Generally,
particle and water inflows into boreholes are sieved
through filters which can accommodate the fines for
some time. If the fines are accumulated for a very long
time they tend to block the inter-granular pores within
the filter media. Although the inflow into the well may
not change, the clogged filter media block the ground-
water from flowing into the borehole. The end result may
be the drying of the borehole. The early days of clogging
in boreholes might not be easy to observe especially if
the pump installed has a capacity far less than the actual
borehole yield. In many cases, borehole cameras are in-
stalled to assess clogging. However, this may not show
clogging between the filter and the face of the borehole
as shown in Figure 2(b). Images from the cameras may
only show clogging between the screen holes and the
filter media. In this regard, numerical models are re-
quired to predict the clogging so that early remediation
techniques can be applied.
8. Filter Media for Borehole Construction
Field monitoring on some boreholes in the far north-
western and north-eastern corners of the Voltaian System
was conducted [13]. They monitored the yearly static
water levels in 9 boreholes in the north-western and 10 in
the north-eastern parts of the study area for 3 years. The
authors concluded that the yearly falling water levels in
the boreholes were as a result of over-use of groundwater.
However, their results, in addition to static water levels
from newly drilled boreholes during the monitoring pe-
riod, suggested that the drying of boreholes may not be
due to lowering of groundwater levels or over-use but
other uninvestigated factors such as clogging.
It was observed and reported in literature that irrespec-
tive of the geological formation and aquifer material
composition, the filter media used for borehole construc-
tion were the same in size [16].
As a part of efforts to determine the suitability of filter
media used for borehole construction, both mathematical
formulations and laboratory observations were made
[39,40]. The filter pores were critically examined under
the microscope to understand the distribution of the
pores within the filter media. The authors compared the
pore sizes of filters made from natural river sand and
modified soil with cement. The pores were examined by
compacting separately (filter media passed through
sieves between 2 and 4.7mm sizes) the two types of fil-
ters in cylindrical moulds. The samples were impreg-
nated with resin and thin sections taken for microscopic
observations (Figures 3(a) and 3(b)).
The results of the microscopic studies are shown in
Table 1. The observations showed that the modal pores
of the filter media from the river sand were tinny com-
pared with the average pore size. The size of the modal
pores of the modified filter was about three times the
modal pore size for fluid flow in the natural river sand
filter media.
(a) River sand filter

J. A. AKUDAGO ET AL.
25
communities located close to the Lake Volta, totaling
about 45.8% of the population, however, do not depend
on groundwater for domestic purposes. Though the area
under study covers about 40 % of the land area of Ghana,
the population density is very small. Figure 4 shows the
estimated number of people living in areas covered by
the Voltaian System [41,42]. It is estimated that about
1.83 million people would have been extracting ground-
water from the Voltaian as at 2008. On an average, do-
mestic water consumption is about 25 to 50 l/person/day
[4,7,22]. The total annual groundwater volume exploited
for domestic use could rise to 3.34x107 m3 in the future.
The groundwater recharge from rainfall has been inves-
tigated and ranges from 3.7 to 5% of annual rainfall.
Rainfall data on the study area for the years 2000 to 2005
varies from 769.5 to 1101.5 mm. Groundwater recharge
from rainfall in 2005 would have been 28.5 to 38.5 mm.
The total volume of groundwater stored in the System
will have been 2.96x109 to 4.00x109 m3. Despite the fact
that groundwater abstraction is less than 1%, the rate of
failure of boreholes is high.
(b) Modified soil filter
Figure 3. Thin section of filter media (2-4.7 mm grain size [40]). Borehole operation survey indicated that out of 492
boreholes, 13% of them had failed within the first 7 years
after construction [7]. Though 8.5% of the failure was
attributed to pump breakdown, the remaining 4.5% indi-
cated lack of water in the borehole. A relation was re-
ported between borehole drying, yield and the season
when it was drilled. Figures 5 and 6 show the relation-
ships between dried boreholes, yield and season of drilling.
9. Groundwater Coverage and Depletion
Groundwater tapped from aquifers of the Voltaian is the
main source of potable water supply for domestic pur-
poses for most communities within the area. It was esti-
mated that about 54.2% of the people depend on
groundwater [41]. Tamale municipality and many other
Table 1. Summary of results from microscopic study [40].
Filter type (between 2mm and 4.7mm particle size) River sand Modified soil
Average microscopic pore size (mm) 0.98 1.02
Numerically determined average pore size (mm) 1.04 1.04
Modal pore size (mm) 0.38 0.96
2.9
3
3.1
3.2
3.3
3.4
3.5
3.6
year
2000
year
2001
year
2002
year
2003
year
2004
year
2005
year
2006
year
2007
year
2008
Population (million)
Figure 4. Estimated population living in areas covered by the Voltaian System.

J. A. AKUDAGO ET AL. 26
0
100
200
300
400
500
600
Total number
of boreholes
No of dried
boreholes
Yield <20
l/min
Yield(20-40)
l/min
Yield (40-80)
l/min
Number of boreholes
Figure 5. Relation between yield and drying of boreholes.
0
5
10
15
20
25
30
35
40
45
Total number of dried
boreholes
Boreholes drilled in
wet season
Boreholes drilled in dry
season
Number of boreholes
Figure 6. Relation between borehole drying and drilling season (wet season: July to February, Dry season: March-June).
10. Discussions
Borehole drying is a worldwide issue that needs serious
attention. As reported in Mali, lack of confidence on the
sustainability of boreholes has made women to prefer sur-
face water [6]. Over 90% of boreholes in Mali were ob-
served to be nonfunctional after one year of completion [8].
Similar reports have been read from South Africa, Uganda,
Nigeria and many other African countries [9,16,43]. Bore-
hole drying is a global problem especially in Africa. Re-
gional aquifer heterogeneity might be one of the main
causes of the drying. It is reported that over half the annual
renewable groundwater supplies in Sub-Saharan Africa are
located within only Democratic Republic of Congo, Re-
public of Congo, Cameroon and Nigeria [44].
In Ghana, the study area has low population density
compared with other areas, and depends mainly on
groundwater exploited through boreholes. Groundwater
is extracted and used mainly for domestic purpose. Field
observation in the study area has shown that drying
boreholes are generally of low yields at the time of drill-
ing (Figure 5). Boreholes with yields of less than 20
l/min are usually the worst affected. The current research
has shown that about 63.4% of the boreholes found to be
dried up had drill yields less than 20 l/min.
Drilling programmes are usually time bound, and this
influences the rate and season that boreholes are com-
pleted. Yields estimated in the wet seasons are normally
not reflective of the actual aquifer condition since they
might have become saturated. Figure 6 shows that about
78% of the dried boreholes were drilled in the wet season
(from July-February). It is generally designed that a
borehole should serve about 250 people with daily water
need of 25 l/day [4,5]. This implies that a minimum of
8.7 l/min should be required as a standard for a newly
drilled borehole to be considered for construction and
use. However, to account for water loses and other un-
foreseen circumstances, a higher minimum yield has
been set nationwide to be 13.5 l/min [35].
Quantitative recharge studies conducted in the area
showed that total extraction is less than 5% of the esti-
mated recharge [22,23] which also supports the findings
from isotopic studies conducted earlier [19,21]. From
Figure 4, the highest water demand was estimated to be
3.34 x 107 m
3 in the year 2008. Assuming the recharge
from rainfall is 28.5 mm and only 30% of stored water is
available for use, about 8.88 x 108 m3 could be exploited.
In this regard, only 3.8% of stored water would have
been used, thus confirming the results of previous re-
searches which suggest that there is enough recharge.
Interestingly, new boreholes drilled closer to some dried

J. A. AKUDAGO ET AL. 27
boreholes have always shown static water levels equal to
the initial water levels of the dried boreholes, although
the existing water levels in the dried boreholes are far
below expectation. This further suggests that the drying
is not due to depletion of groundwater, as intimated by
workers such as [22,23,32,33]. However, availability of
enough groundwater reserve does not necessarily mean
boreholes cannot dry up. Aquifer heterogeneity can also
cause the drying. For hand pump, only 6 hour pumping
test is conducted which might not be enough to stretch
the aquifer to its limits. Besides this, the pumping tests
are carried out on single boreholes which are not usually
reliable. The aquifer properties such as storativity and
transmissivities might not be accurately estimated espe-
cially as most aquifers are located in fractured rock en-
vironments.
Borehole construction errors could also be a possible
source of the drying issues. When boreholes are con-
structed, they are normally constructed with plain and
screen pipes, and filter media walled around the screen
as shown in Figure 2(a). Defective screen and plain pipes
could lead to siltation that could block transmission. To
ensure good filtering, a filter classification proposed by
[45] is suitable for use. The classification requires that
D15/d85 < 4 should be satisfied, where D15 means 15% by
mass of the filter particles are finer than that size and
85% of the particles are finer than d85 particle size of the
base material. Although many researchers have opposed
the numerical value and particle size pertaining to the
filter use [46–48], the ratio serves as a good guide to
filter selection. However, it is highly impossible to ob-
tain all the fines during drilling especially in the case of
saturated aquifer. Consequently, in borehole construction,
a general classification based on certain filter size range
is usually prescribed by the local authorities controlling
borehole development. For example, filter size ranging
from 2-4 mm has been recommended for all boreholes
drilled in Ghana by the Community Water and Sanitation
Agency (CWSA). The performance of the filter depends
on its pore size and the type of fine sediments found in
the incoming groundwater at the filter media-borehole
interface.
As mentioned earlier, microscopic studies on similar
filter material from a natural river source prescribed by
CWSA showed that the average pore size was 0.98 mm
and the modal pore size available for fluid flow was
0.38 mm [40]. However, results from the modified filter
showed that there were nearly equal average and modal
pores of 1.02 mm and 0.96 mm, respectively for fluid
flow (Table 1). The smaller modal pore sizes can easily
get blocked by incoming dirt compared to the larger pore
size.
Due to many human activities such as farming and
charcoal burning, the land is exposed to erosion and,
hence, during recharge especially in the fractured aquifer
systems, fines enter the fractures. These sediments may
be trapped by the filter media upon entering the borehole
face. Although seemly conceptual, as groundwater enters
the borehole face the velocity increases, and the change
in velocity could cause transportation or deposition of
finer material or repositioning of particles of the filter
media. Since the filter media is usually of very small
radial thickness ranging from about 5 to 10cm or a little
more, particles are capable of being pushed through if
the force with which they travel is high. In this regard,
boreholes with higher yields are capable of providing
enough force than low yielding ones. It has been reported
that fluid velocity controls particle penetration into po-
rous media and that higher velocities mean farther dis-
tance of transportation and deposition [36,38]. In muddy
environments, the slurry can be pushed into the borehole
or stick around the space between the filter media and
the borehole screen. In such situations, it is easy to suck
the dirt into the borehole during pumping.
In relatively low yielding boreholes, the entrance force
of flowing slurry might not be too high thereby resulting
in the deposition of the mud and fines around the point of
entrance into the filter media. Continual deposition of the
fines leads to clogging and cementation of the filter pores.
As the deposition continues, the inflow of groundwater
into the borehole is almost blocked. In such case, the
water level in the borehole may fall below the general
groundwater level as water is drawn from the stored wa-
ter by the users without replenishment.
Clogging in low yielding boreholes might start from
the area between the well face and that of the filter to-
wards the well screen, whereas, those of high yielding
may start from between the screen and the filter back-
wards (Figure 2(b)). In this regard, it is very common to
find low yielding boreholes having been redeveloped but
no water comes out. The cleaning exercise might not be
effective to dislodge the clogged material in the far side
of the filter-well surface. On the other hand, because of
the proximity of the clogged material to the screen in
high yielding boreholes, early cleaning intervention such
as rehabilitation may be very effective. This may explain
why some low yielding boreholes tend to fail even if
attempts are made to clean them.
During pumping, the deposition of fine particles flow-
ing with groundwater into the filter media depends much
on the velocity with which they travel and filter pore size.
If the weights of the individual fine particles in the flow-
ing water are greater than the forces pushing them in
motion, the particles may suddenly come to rest and ob-
struct the movement of much lighter ones even if their
sizes are smaller than the pores in the filter media. If the
drag forces are higher and the particles are smaller, then,
they may be transported through pores of the filter with-
out deposition. Due to the clayey nature of the rocks in
the Voltaian System, the swelling of these particles are

J. A. AKUDAGO ET AL. 28
possible if the particles are not quickly pushed through
the pores of the filter media.
Generally around the borehole face of a low yielding
borehole, the velocity of incoming fines cannot be com-
pared with those of high yields. Assuming that all the
boreholes of variable yields are located in the same geo-
logic aquifer rich in clay particles, and the same fine par-
ticles are found in the inflowing water, then, the low
yielding boreholes will quickly get clogged. Filter media
clogging could therefore be contributing to the depletion
of the water in the boreholes as inflow is adversely af-
fected. Clogging in low or marginal yield boreholes are
easy to observe because the yield diminishes abruptly
unlike the high yielding ones whose may still match the
flow rates of the hand pump for a very long time. Fre-
quent monitoring of the boreholes is therefore very im-
portant so that beneficiary communities can be advised.
11. Conclusions and Recommendations
1) Borehole drying is a worldwide issue especially
in Sub-Sahara Africa.
2) Drying of marginal or low yielding boreholes in
clay rich sediment environments such as the
Voltaian System in Ghana affects borehole sus-
tainability and water supply.
3) Borehole drying has a relation with yield and
season in which drilling was completed.
4) The review suggests that filter clogging could
be a possible cause of the drying of the bore-
holes. Although many researches have been car-
ried out, not much has been done in relation to
clogging especially filter clogging.
5) The type of filter media used in Ghana has
smaller modal pore sizes. These sizes can easily
be blocked by clay and finer particles.
6) The authors recommend that further work be
done to measure the fines transported with
groundwater in the area and to select the appro-
priate size of filter media for the construction of
boreholes.
7) Marginal boreholes also need to be rehabilitated
regularly to help reduce or dislodge clays and
other particles from clogged filter media before
cementation.
8) Drilling projects should consider the dry season
as most appropriate period for drilling and
borehole yield estimation.
12. Acknowledgements
The authors wish to acknowledge the anonymous re-
viewer for his critics that have contributed to improving
this paper.
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