Classifying zones of suitability for manual drilling using textural and hydraulic parameters of shallow aquifers: a case study in northwestern Senegal
Classifying zones of suitability for manual drilling using textural and hydraulic parameters of shallow aquifers: a case study in northwestern Senegal
F. Fabio Fussi 0 1 2 4
Letizia Fumagalli 0 1 2 4
Francesco Fava 0 1 2 4
Biagio Di Mauro 0 1 2 4
Cheik Hamidou Kane 0 1 2 4
Magatte Niang 0 1 2 4
Souleye Wade 0 1 2 4
Barry Hamidou 0 1 2 4
Roberto Colombo 0 1 2 4
Tullia Bonomi 0 1 2 4
0 University Cheik Anta Diop , Dakar , Senegal
1 University of Thies , Thies , Senegal
2 Department of Environmental Science, University Milano Bicocca , Piazza della Scienza 1, Milan , Italy
3 F. Fabio Fussi
4 SNAPE (Service Nationale de Points d'Eau) , Conakry , Guinea
A method is proposed that uses analysis of borehole stratigraphic logs for the characterization of shallow aquifers and for the assessment of areas suitable for manual drilling. The model is based on available borehole-log parameters: depth to hard rock, depth to water, thickness of laterite and hydraulic transmissivity of the shallow aquifer. The model is applied to a study area in northwestern Senegal. A dataset of boreholes logs has been processed using a software package (TANGAFRIC) developed during the research. After a manual procedure to assign a standard category describing the lithological characteristics, the next step is the automated extraction of different textural parameters and the estimation of hydraulic conductivity using reference values available in the literature. The hydraulic conductivity values estimated from stratigraphic data have been partially validated, by comparing them with measured values from a series of pumping tests carried out in large-diameter wells. The results show that this method is able to produce a reliable interpretation of the shallow hydrogeological context using information generally available in the region. The research contributes to improving the identification of areas where conditions are suitable for manual drilling. This is achieved by applying the described method, based on a structured and semi-quantitative approach, to classify the zones of suitability for given manual drilling techniques using data available in most African countries. Ultimately, this work will support proposed international programs aimed at promoting low-cost water supply in Africa and enhancing access to safe drinking water for the population.
Groundwater development; Unconsolidated sediments; Water supply; Drilling; Senegal
Introduction
Despite overall progress, 748 million people still had no
access to safe drinking water in 2012, 325 million (43%) of
whom were living in Sub-Saharan Africa
(WHO/UNICEF
2014)
. Groundwater has proven to be the most reliable
resource for meeting rural water demand in Sub-Saharan
Africa, and it can derive from different types of aquifers.
Unconsolidated sediments cover 22% of Sub-Saharan
Africa, and at least 60 million people in rural areas obtain
water from this type of aquifer
(MacDonald and Davies 2000)
.
Manual drilling refers to several drilling methods that rely
on human energy to construct a borehole (i.e. a narrow hole
bored in the ground using some kind of drilling tools) and
complete a water supply
(Danert 2015)
. These techniques
are well established in certain countries, e.g. Bangladesh,
Bolivia, India, Kenya, Niger, Nigeria and Madagascar. In
recent years, however, there has been an increased interest in
promoting manual drilling in regions where this technique is
rarely applied even though it could improve access to clean
water. UNICEF (United Nations International Children’s
Emergency Fund) has been involved in this process in
Africa since 2004
(Gaya 2004)
. Other organizations have
gathered experience in pilot projects to promote manual
drilling in different regions
(e.g. Forsyth et al. 2010)
; these
techniques are now becoming increasingly relevant in several
countries
(Danert 2009, 2015)
.
Although different techniques for manual drilling are
available
(Carter 2005; Danert 2015)
, they can be applied
only where shallow geological layers are relatively soft (i.e.
it is possible to drill using manually powered tools) and the
water table is not too deep (i.e. that exploitable water strikes
occur within a depth range achievable with manual drilling
techniques; 50 m can be a realistic value for the maximum
depth commonly reached by hand-drilled wells, although
there are examples that reach up to 100 m). Therefore, the
promotion of manual drilling to improve water supply
requires a preliminary identification of those zones with
suitable hydrogeological conditions. Such an approach was
initially tested in Chad using the water points’ database of the
national water authorities (Direction de l’Hydraulique) and a
simple procedure of visual interpretation of the variations in
static water level and the distribution of geological units
(Gaya et al. 2005). The Chad study was used for an intense
program of promotion of manual drilling by UNICEF and
the national water authority, which resulted in thousands of
new excellent hand-drilled wells in the country
(Danert
2015)
, with a network of private contractors trained in
high-quality construction techniques. Following the Chad
experience, UNICEF produced maps of suitable zones at
country level within Africa, applying a schematic model
with some modifications in 12 countries between 2008
and 2010
(Fussi 2011; Fussi et al. 2013)
. Suitable zones
for manual drilling were derived from the analysis of three
main parameters: geological suitability (depending on the
thickness and permeability of shallow layers), water depth
suitability (depending on static water level in the first
aquifer) and morphological suitability (based on the existence of
landforms that facilitate the accumulation of unconsolidated
sediments or erosion). This method was based on the
analysis of water point databases, geological maps and the
Shuttle Radar Topography Mission (SRTM) digital
elevation model, integrated with qualitative experience of local
hydrogeologists. Major relevance is given to the national
water point database, i.e. the centralized archive of water
point data generally held by the national water authority in
each country. This source of information contains a large
amount of data but generally presents several problems in
their quality (not up-to-date, duplicated records, incorrect
and incomplete information). These data were analysed only
through a simple qualitative visual interpretation (a robust
and quantitative procedure of elaboration was not
proposed). A similar approach derived from a cross analysis
of shallow geology (obtained from the simplification of
geological maps) and depth to water (assigning an estimated
range of this parameter to the category of landforms
obtained by processing the SRTM digital elevation model) was
used in Madagascar
(Salama 2008)
. A different method was
applied in Tigray, northern Ethiopia
(Huisman Foundation
2014)
. This approach is based on direct field survey
(observation of landscape characteristics, soil type and water level
in hand-dug wells) and discussion with local experts.
A method to investigate one of the relevant aspects to
consider for the feasibility of manual drilling (water depth) was
proposed in Niger
(Thomas et al. 2012)
. It is based on a
multivariate statistical procedure for the estimation of water depth
through an indirect set of information (remote sensing, terrain
modelling, thematic maps). Even though it does not propose a
complete method for the identification of suitable zones, this
research does suggest a method to characterize the shallow
hydrogeological context.
Regardless of the sources of information, all these methods
to determine the suitability of an area for manual drilling take
the same criteria into consideration: properties of the shallow
geological layers and water-level depth. The study in Tigray
had a detailed field survey and therefore is applicable only for
relatively small study areas; however, the other methods all
have similar critical aspects:
&
&
&
They depend on existing information (like thematic maps,
qualitative experience of local experts) that is often
incomplete and does not homogeneously cover the whole
country, leading to unreliable interpretation where this
information is limited.
They can support regional- and national-scale analysis,
but they are not applicable at local level, given the coarse
scale of the mapping products and low spatial density of
information.
They are largely based on qualitative perception and
assessment of experts, without a structured data analysis
procedure. This makes it difficult to replicate the study
and compare the results in different areas.
The main objective of this study is to propose a new
method to identify suitable zones for manual drilling,
b a s e d o n a s e m i - q u a n t i t a t i v e e s t i m a t i o n o f
hydrogeological data and a robust process of data
analysis. Specifically, the proposed approach aims at improving
existing methods by:
–
–
–
A more structured and quantitative procedure to analyse
existing geological datasets, especially those coming
from national water point databases and borehole
stratigraphic logs.
A systematic approach for borehole data processing, with
inclusion of information available only in hard copy and a
limited field survey for a general validation. Integrating
this information makes it easier to increase the level of
detail of the mapping products, compared with previous
maps at regional or country level.
A more objective, systematic and replicable workflow.
Materials and methods
Description of the study area
The study area is the Louga region in north-western Senegal,
between 14°70′ and 16°10′ north and 14°27′ to 16°50′ west
(Fig. 1), and covers 24,874 km2. It was selected in order to
target zones with different shallow hydrogeological features
and different suitability for manual drilling, as obtained from
the existing study in the framework of the UNICEF program
(Kane et al. 2013)
.
The total population of the study area is 880,482
inhabitants (more than 500,000 inhabitants in rural areas), with 57%
having access to safe drinking water and 17% having adequate
sanitation
(PEPAM 2015)
. Climatic conditions are
characteristic of the Sahelian regime, with rainfall approximately 350–
500 mm/year (with a decreasing trend moving to the north),
concentrated between June and October. Along the coastal
area, there is the effect of a humid wind brought by the
Azores Anticyclone. Morphology is mainly flat, with limited
undulation formed by sandy dunes. From the geological point
of view, the study area is situated in the Tertiary
SenegaloMauritanian sedimentary basin, constituted by interbedded
layers of limestone, sandstone, marl and clay, and elongated
in the N–S direction for 1,400 km from Mauritania to Guinea
Bissau. This sedimentary basement is covered by sands or
sandy clay. Moving eastward, there are different sandy
formations (with an increase in the presence of clay): the coastal
dunes, the Ogolian red dunes, and the Continental terminal.
Source of data
The method is based mainly on a detailed analysis of
previously existing data, most of it acquired free of charge. Direct
field data collection is limited and principally focused to
Fig. 1 Location of the study area,
the Louga region (delineated by a
red line) in Senegal
validate the interpretation. The main sources of data are
thematic maps and hydrogeological information obtained from
the national database of water points of Senegal. Geology,
soils, morphopedology and land-cover datasets have been
obtained from 1:500,000 maps published for the national plan for
land-use and development
(The Remote Sensing Institute
South Dakota State University 1986)
. Water points data were
obtained from the national inventory held by DGPRE
(Direction de la Gestion et de Planification des Ressources en
Eaux) in Senegal; this database is part of the geographic
information system of water resources
(République du Sénégal
2000)
from SGPRE (Service de Gestion et de Planification
des Ressources en Eau), and it is managed with the software
PROGRES
(ANTEA/BURGEAP 2007)
. Three categories of
data have been obtained from the water point database:
1. General inventory of water points (1,277 records in the
study area, including deep boreholes and hand-dug wells).
They cover all the study area, although there is a much
higher concentration on the western side. From this
dataset, it was possible to obtain the position and total
depth of the boreholes/wells and the depth to static water
level.
2. Inventory of piezometers (45 records in the study area,
almost completely concentrated in the coastal region,
not more than 50 km away from the sea). The piezometers
have the same information as the general inventory of
water points, plus the description of the main aquifer
and a few time-series of static water level observations.
3. Stratigraphic logs of boreholes (131 in the study area,
mainly concentrated on the western side). These logs have
the same information as the general inventory, plus the
lithological description and position of different layers
found during the drilling. No stratigraphic logs are
available for hand-dug wells.
Since the information of elevation stored in the national
water point database proved to be unreliable, it was neglected,
and the elevation was obtained from publicly available digital
terrain models (Aster Global Digital Elevation Model v.2,
30m resolution). The national water point database stores
information concerning mechanized boreholes, but limited
attention is paid to large-diameter shallow wells. Furthermore,
stratigraphic logs are generally not detailed for shallow layers,
and data about hydraulic parameters refer to deep, fractured
aquifers. Concerning the use of static water-level data to
estimate the depth to the shallow water table, two aspects must be
carefully considered:
1. Most of the data (from boreholes and piezometers) refer to
a deep fractured aquifer, whose piezometric level can
differ from the shallow water table in the case of confining
layers between the two aquifers (as in the central-eastern
side of the study area); here, the estimation of the depth of
water that can be exploited using manual drilling is
uncertain.
2. Static water level in the national water point inventory
was measured in different years and seasonal conditions.
The difference in water level could be the consequence of
temporal changes and may not be related to the
piezometric gradient of the water table. However, the extremely
limited information available from the temporal series of
static water level
(data from four piezometers in the
coastal zone, whose water level was measured in October 2011
and August 2014)
shows a change over time smaller than
30 cm.
Classification of zones of suitability for manual drilling
The methodology proposed in this report to classify suitable
zones for manual drilling is based on a structured and
semiquantitative analysis of available borehole log data. The
method can be applied in three steps: (1) assessment of feasibility,
(2) estimation of potential for exploitation, (3) final
classification of suitability.
Assessment of feasibility
Assessing the feasibility of manual drilling in a specific
location means evaluating whether the existing hydrogeological
conditions allow the completion of a hand-drilled well (with
the different techniques available). This assessment was
carried out by analysing two main parameters extracted from
borehole logs: the presence of hard layers and the depth to
the water level. The locations of hard stratigraphic layers can
be derived from the locations of hard rock (in this case the
layers generally represent the maximum possible depth of
manual drilling in that area) or compact laterite (the layers
are intercalated between unconsolidated sediments, and they
can be broken with special manual drilling techniques, if they
have limited thickness, and drilling can continue deeper).
Based on these considerations, the procedures to assess
feasibility for manual drilling can be schematized as a sequence of
three conditions, evaluated through Boolean operators (yes/
no), and schematized in Fig. 2:
&
&
Condition 1: depth to hard rock. In the assessment of
feasibility for manual drilling, hard rock is considered a
solid layer which has significant hardness and
compactness, and which is generally impossible to perforate using
drilling tools operated by human energy (drilled without
power obtained from mechanized machine). In terms of
the geological process, such hard rock layers could
correspond to the bottom of the weathered and unconsolidated
material covering the fresh compact rock, or the lower
limit of unconsolidated layers derived from the
depositional process on top of the basement rock. The presence
of hard rock at the ground surface, or shallower than 10 m,
means that manual drilling is not recommendable; even in
the case of a water table close to the ground surface, a
water-column depth of maximum 10 m leads to unreliable
water supply from the wells, especially during dry periods
(when the water table becomes deeper). Therefore, in this
situation, a complete and successful hand-drilled well is
considered not feasible.
Condition 2: depth to water. This parameter refers to the
minimum depth of water strikes that can be attained in
hand-drilled wells. In the case of the unconfined water
Fig. 2 Schematization of the procedure to assess feasibility of manual
drilling. F feasible, FS feasible with special techniques, NF not feasible
table, the piezometric level corresponds to the first water
strike, while in the case of confined aquifers, there are
different levels. As mentioned in section ‘Introduction’, a
reference value of 50 m was assumed as the maximum depth
commonly achievable using manual drilling; this seems to
be consistent with previous experience in Senegal. Given
this assumption, and considering that a few metres of water
column is needed to ensure a positive result, the limit of
40 m as a maximum water-level depth was kept as a
threshold for the feasibility of manual drilling.
Condition 3: presence of hard laterite. One common
weathering product in West African soils is laterite, which
is rich in aluminium and is sometimes present in very hard
layers and in some other situations as a reddish, clay-rich,
unconsolidated material. Hard lateritic crusts can be
perforated if their thickness is limited, but special techniques (e.g.
percussion) are required. Based on direct experience of
manual drilling experts, perforation is probably possible
for a hard lateritic crust thinner than 5 m, while if the layers
are thicker, manual drilling is considered not possible.
With this procedure, three classes of feasibility can be
distinguished (Table 1): not feasible (NF), feasible (F) and feasible
with special techniques (FS).
Classification of the potential for exploitation
After having identified where manual drilling is feasible (i.e.
classes F and FS, with the option of special techniques required),
the second step is the classification of the potential for
exploitation in these zones. This classification can give an
indication of the expected yield, availability of water during the dry
season (because of seasonal fluctuation of the water level), the
type of pump that can be installed and the size of the population
served. The potential yield of the well is related to
hydrogeological factors (i.e. the geometry and hydraulic
characteristics of the target aquifer) and engineering aspects (quality of
construction and performance of pumping system). In the
proposed methodology, the hydrogeological aspects have been
classified by extracting two parameters from the analysis of borehole
logs with the procedure described in section ‘Data processing
and interpretation: thickness and hydraulic conductivity (K) of
saturated layer. The potential for exploitation with manual
drilling is related to the hydraulic transmissivity in the
exploitable interval (Tex), which itself is calculated as shown in Eq. (1)
T ex ¼ Kex
H ex
ð1Þ
where Tex is the hydraulic transmissivity (m2/s) of the
exploitable layer by manual drilling (this means up to 50 m
deep). Hex is total thickness (m) of the saturated exploitable
layers, corresponding to the difference between static water
level and 50 m, if the upper limit of hard-rock-layer depth is
>50 m or the difference between static water level and the
upper limit of the hard-rock layers if <50 m. Kex is average
hydraulic conductivity (m/s) in the saturated exploitable layer.
The potential for exploitation is considered in relation to
the performance of different pumping systems available for
hand-drilled wells (Table 2). Five classes of potential for
exploitation are defined (Table 3). This generic approach can be
applied to different regions although threshold values of Tex
are related to site-specific conditions (e.g. local characteristics
of aquifers, depth of water table, seasonal fluctuations).
Assigning the final class of suitability
The final class of suitability derives from the combination of
feasibility and potential for exploitation. With this method, 11
possible combinations (Table 4) are defined, with three classes
of suitability: not suitable, suitable with poor results, suitable.
Data processing and interpretation
Borehole-log data were processed with the software
TANGAFRIC
(Fussi et al. 2014)
, specifically designed for
this purpose during this research. Four steps were followed:
1. Standardization and identification of common categories
(on the basis of the most common stratigraphic terms in
the datasets from Guinea and Senegal).
2. Assignment of standard categories to the description of
each layer of the stratigraphic logs by means of manual
codification by two local hydrogeologists, adapting the
procedures used in the software TANGRAM at the University
Milano Bicocca, Italy
(Bonomi 2009; Bonomi et al. 2014)
.
3. Extraction of textural composition of layers from the in
terpretation of the codes corresponding to the main texture
Types of pumps usually installed on hand-drilled wells and their expected yield
Type of pump Description and usage
A rope pump consists of a loose hanging rope that is lowered into a well and drawn up through a pipe that reaches the water. On the
rope, round disks or knots matching the diameter of the pipe are attached which pull the water to the surface. These pumps are
suitable and cheap systems to collect water from hand-drilled wells in the case of small communities (approximately 50 users).
They have a low yield (0.1 L/s as reference value), and they can be installed only where the static water level is not deeper than 15 m
component, secondary texture component, and texture
adjective.
Classification of the possible textural categories allowed
in the coding process in five classes (Table 5): three
classes discriminated on the basis of grain size (coarse,
medium and fine) for unconsolidated sediments, and two other
classes for hard layers (hard rock and hard laterite).
Textural classification of sediments is derived from
qualitative descriptions by drillers. However, a possible
indication of the dimensions of particles for each category can
be obtained from literature
(e.g. Fetter 1994)
.
The stratigraphic data were processed, and the percentage
of each texture class (using a weighted average procedure)
was extracted for a sequence of intervals with a regular step.
Table 6 shows an example of a log with Bsand^ between
depths 0 and 4 m and Bsandy clay^ between depths 4 and
10 m.
Hydraulic conductivity (K) depends on texture (in the case
of unconsolidated materials) or characteristics of fracturing (in
the case of rocks). Ideally, the hydraulic conductivity of
geological layers must be defined through direct measurements
on site-specific samples (in the laboratory), or in situ
measurements (e.g. pumping test). When this information is not
available, K can be estimated from a hydrogeological knowledge of
the region and published values of K measured in similar
contexts for the same type of layers, defining a relation
between the texture of sediments and K of superficial deposits
(MacDonald et al. 2012)
. The available local information on
shallow aquifers was limited. The data on existing pumping
test found in the study area carried out in boreholes from the
national database of DGPRE and SNAPE (Service Nationale
de Points d’Eau, Guinée) indicate K values referred to deep
fractured hard rock, while none of the pumping tests provide
hydraulic parameters referred to unconsolidated shallow
layers. Also, there are strong limitations on the availability
of hydrogeological studies.
Physically, manual drilling can be done, but the well will be dry, since the water level is deeper than the hard-rock
depth (i.e. the maximum depth achievable by manual drilling). Therefore, the porous aquifer is completely dry (Hex = 0)
Hand-drilled wells can be equipped with hand pumps, but expected yield is low (less than 0.2 L/s). After intense
pumping, or in the case of a decreasing water level, the well is likely to become dry
Hand-drilled wells can be equipped with hand pumps and provide a reliable water supply; pumping cannot be
continuous for a long time (for example, for 2 h or more, as is frequent during peak hours for water collection
in rural areas in Africa). Not suitable for intense utilization by large groups (i.e. less than 100 users)
Hand-drilled wells can provide a continuous water supply, with adequate yield using hand pumps. Suitable for
medium-sized communities (approximately 250 users, which is often a guideline value of maximum users for hand
pumps in water supply programs)
Hand-drilled wells can provide an excellent yield (higher than 0.5 L/s). The well can supply water for a continuous
utilization of hand pumps and can sometimes be equipped with solar pumps (providing water to a larger population)
Different methods are available to estimate hydraulic
conductivity from grain size of sediments. The following K values
for different texture classes (as defined in Table 5) were
obtained from literature
(Domenico and Schwartz 1998; Fetter
1994; Freeze and Cherry 1979; Neuzil 1994; Sheperd 1989)
and adopted:
K = 10−4 m/s for coarse material (corresponding only to
sand deposits in this region, as no gravel is present)
K = 10−5 m/s for medium texture material
K = 10−6 m/s for fine texture material
In the case of consolidated hard materials, K = 10−6 m/s
was assumed (considering the presence of unconsolidated
sediments filling the empty space of the hard layer). However,
when hard rock represents more than 50% of the components,
the layer is assumed to be the upper limit of the basement, and
manual drilling cannot be performed. With the distribution of
hydraulic conductivity lognormal, the standard practice was
followed of calculating the weighted standard mean of values
attributed to the individual lithology, both as percentages of
each stratigraphic level and as components of the stratigraphic
stretch analysed
(Sanchez-Vila et al. 1995)
. In this way, the K
value was estimated for each interval of 2 m, obtaining from
the weighted average of log[K] of each texture class multiplied
by its percentage, as shown in Eq.(2):
log½K interval ¼ log½K coarse
% coarse þ log½K medium
% medium þ log½K fine
% fine
þ log½K cons:
% cons:
ð2Þ
where Kinterval indicates the estimated hydraulic conductivity
of the interval composed by a mix of different textural classes.
At this point, it is possible to estimate the transmissivity in the
exploitable layer (Tex), multiplying the average K in the
saturated layer (Kex, between the static water level and the upper
limit of basement or the maximum possible depth of 50 m)
and its thickness (Hex).
The output table was processed obtaining the following
parameters for each borehole log: depth to hard rock, depth
to water, thickness of hard lateritic layers, average estimated
hydraulic conductivity and hydraulic transmissivity of
exploitable layer. The meaning of Bdepth to rock^ and Bdepth
to water^ was explained earlier. Since there is often no sharp
transition from unconsolidated layers to hard rock, it was
assumed that the depth to hard rock corresponds to the upper
limit of layers having more than >50% of textural component
in the output table classified as hard rock, while the depth to
water was approximated with the data of static water level.
Validation with measured K values from field tests
The estimated values of hydraulic parameters from the
interpretation of stratigraphic logs were compared with measured
parameters obtained from two field campaigns (May 2014 and
March 2015) within the study area in Senegal involving
pumping tests in large open wells, to obtain direct
measurements of hydraulic parameters for the shallow aquifer (the
expected target for manual drilling).
A total of 11 pumping tests were completed, covering
different geological units. They are mainly distributed in the
western and central part of the study area, as large-diameter
wells are extremely rare in the eastern sector.
Since both field campaigns occurred in the late dry season,
it was difficult to find wells with an adequate water column to
carry out pumping tests for an extended period. Thus, the
pumping phase was ideally undertaken for 1 h, but in several
cases it was interrupted after a shorter period because the
water column was too small to run the pump in a safe
condition. The recovery phase was monitored for 1–1.5 h, which
Input table
Depth (m)
suitable for the interpretation, given the geometry of the
system and the development of the test. In fact, this method was
designed for the interpretation of slug tests in fully or partially
penetrating wells, tapping unconfined aquifers.
K was estimated using Bouwer and Rice’s original method
(Bouwer and Rice 1976)
as well as the modified equation for
ln (Re/Rw) proposed by
Rupp et al. (2001)
for soil classes Sa1
(Sand) and Lsa1 (Loamy sand) to take into account the
influence of unsaturated hydraulic conductivity. Rw is the radius of
the well, and Re is the effective radius over which the
depression of static water level is dissipated (in Rupp’s method, this
parameter depends on soil texture). The following
assumptions were considered (Fig. 4):
1. Condition similar to fully penetrating wells (D = L).
Considering that there is generally a concrete slab at the
bottom of improved large-diameter wells in Senegal,
water flow is therefore only horizontal and the base of the
well can be considered an impermeable layer.
2. The well is fully screened, therefore filtration occurs along
the whole well surface between the water table and the
provided the most relevant information for the estimation of
hydraulic parameters.
Since the effect of storage capacity in large diameter wells
is not negligible during the pumping phase, the recovery data
are a better diagnostic of aquifer parameters than the
drawdown, particularly for short periods of pumping
(Barker and
Herbert 1989)
. The methods to interpret recovery data based
on the assumption of equilibrium between drawdown in the
well and depression in the water table
(e.g. Papadopulos and
Cooper 1967; Herbert and Kitching 1981; Barker and Herbert
1989; Herbert et al. 1992)
were not considered suitable.
Observing the linear shape of the drawdown curve (Fig. 3)
and the ratio between the volume of the well that was emptied
during the test and the total volume of water extracted, a Bslug
test^ approach for the interpretation was selected. Slug tests
are good for the estimation of aquifer properties in hand-dug
wells because they are commonly used in low-permeability
environments, take into consideration the storage of water in
the well, are easy to conduct in the field and are versatile
(Mace 1999)
. They consider the conditions of an
instantaneous water removal from the well and no contribution of
water from the aquifer during the pumping phase.
Amongst the different methods proposed
(e.g. Hvorslev
1951; Cooper et al. 1967; Bouwer and Rice 1976; Rupp
et al. 2001; Uribe et al. 2014)
, Bouwer and Rice’s methods
(and modifications by Rupp et al. 2001) seemed the most
Fig. 4 Geometry and symbols for partially penetrated wells in an
unconfined aquifer. From
Bouwer and Rice (1976)
. H = distance between
water table and bottom of the well, D = distance between water table and
bottom of the aquifer, L = length of screened section of the well,
Rw = radius of the well, Rc = internal radius of concrete rings
bottom of the well (H = L). Although the screened section
of the well (perforated concrete rings) is smaller, the
presence of gravel packing up to the water table facilitates
filtration even from the shallower part of the aquifer.
Results and discussion
Comparison of K values obtained from analysis
of stratigraphic logs and pumping tests
K values obtained from the set of pumping tests executed in
large-diameter wells were compared with the estimation of
hydraulic conductivity obtained from the interpretation of logs
from boreholes located closer than 7 km from the pump-tested
wells. This threshold distance was selected by calculating the
minimum distance required to have at least two stratigraphic
logs at an acceptable distance from the wells (i.e. a distance
where the stratigraphic information of the borehole logs can
be considered reasonably valid to represent the characteristics
of shallow geological layers at the well’s position). The radius
of 7 km can lead to a discrepancy in geological conditions
between the open well and surrounding stratigraphic logs,
especially in the presence of lateral variations (generally
limited in the study area) and undulated topography. However,
the low density of borehole logs (i.e. the large distance
between stratigraphic data) made it difficult to assume a smaller
threshold value and the researchers were obliged to accept this
source of error in the comparison of K values. The
interpretation considered the texture of those geological layers at the
same depth as the saturated interval in the well. As observed in
Table 7, the mean and median of the difference in the
estimation of K obtained through the interpretation of pumping tests
(direct measurement in the field) and processing of borehole
logs is limited. This difference has been estimated for only
seven pumping tests (out of the 11 that were executed), as
some of the tests gave results considered not reliable because
of the limited water column in the wells and difficulty in
properly measuring the change in water level.
The results providing the best fitting with K value obtained
from stratigraphic logs (giving the lowest mean and median
value of the difference) were obtained using Rupp’s method
and considering sandy soil; in 5 out of 7 cases the difference in
Log K between the two methods is smaller than 0.5. Two
points gave a higher value of difference in K value; one
possible justification can be the presence of variations in texture
of shallow geological layers between the open well (where the
pumping test was carried out) and the surrounding borehole.
Therefore, it can be considered that the K estimated during
the analysis of stratigraphic logs by using TANGAFRIC is
reasonably consistent with the results of pumping tests, and
Table 7 Estimation of mean and median of the difference between Log
K obtained through interpretation of pumping tests and borehole logs (7
samples). K expressed in m/s
Statistic K K K
Rupp’s method Rupp’s method Original Bouwer
(assuming soil class (assuming soil class and Rice method
Sa1) Lsa1)
Mean
Median
is possibly slightly underestimated. One also has to consider
that this method (comparing K values from pumping tests in
large wells with the surrounding borehole logs using texture
and K data of layers at the same depth as the static water level
of the pumping test) is based on the assumption that there is no
lateral modification of texture and K (only vertical
modification) in the area and that topography is flat; if these conditions
are not taken into account, they can introduce other relevant
factors that may make this comparison less correct. On the
basis of geological and morphological features of the study
area, it was considered reasonable to assume limited lateral
variability and thus confirm the validity of the method.
Characterizing the shallow aquifer and the suitability for manual drilling based on borehole-log positions
The method for the assessment of suitability for manual
drilling was applied to the study area, and the feasibility, potential
for exploitation and suitability were elaborated using maps.
The results of the semi-automated analysis of borehole logs
(and the comparison with direct field measurements) show the
distribution of the different hydrogeological parameters
considered and assign the appropriate class of suitability at
borehole-log positions. The distribution of the different
parameters to assess suitability in the study area is discussed.
Classification of feasibility for manual drilling
Depth to hard rock In most of the study area, the bedrock
(i.e. the upper limit of tertiary sedimentary rocks under the
unconsolidated deposits) is deeper than 10 m, therefore
satisfying the criteria defined in the feasibility model for manual
drilling (class F and class FS). In 52% of the cases, bedrock is
deeper than 50 m, therefore manual drilling can reach its
maximum depth without limitation because of geological
conditions. The unconsolidated shallow layer presents a small
thickness in many of the boreholes concentrated in the eastern
sector, although there is high variability in the same area (Fig.
5). As a consequence, there are limitations associated with the
exploitable water column in hand-drilled wells.
Depth to water This is the most important limiting factor for
the implementation of manual drilling in the study area. 45%
of the logs show a static water level deeper than 40 m (Fig. 6);
therefore, in this situation, manual drilling is considered NF
according to the limits defined in the proposed classification
model (Fig. 2, condition 2). The average value of water depth
is 35 m, with a standard deviation of 10.3 m. However,
estimating the depth of exploitable water from information on
static water level of deep boreholes can lead to an unreliable
interpretation in the case of boreholes that tap confined deep
aquifers. In this situation, hand-drilled wells can only reach
exploitable water that is much deeper than the estimated water
level, and in some cases this may be not feasible. This
situation is frequent in the central and eastern sector of the study
area, where the true depth of the shallow water table can be
estimated from data on static water levels in large-diameter
wells. Since this information is rarely registered in the national
database, direct field observation is suggested.
The most critical zones are located SE of Kebemer, in the
SW sector of the study area, where the depth of groundwater
makes manual drilling not feasible in several locations.
Further east, there are more sites where the water depth is
shallower than 25 m. However, two relevant aspects must be
considered in this zone: the limitation in drilling depth because
of the presence of hard rock can make it difficult to have a
sufficient water column in the well. Also, the extreme scarcity
of large-diameter wells can indicate the presence of confining
layers and possible errors in the estimation of the depth of the
shallow water table. The coastal strip has no borehole log
information, but the direct observation of large-diameter wells
in the field indicates that groundwater is shallow (in most of
the cases not deeper than 10 m).
Thickness of hard laterite Hard laterite is not frequently
found in the study area (Fig. 7). Over much of the area there
is generally no laterite; therefore, it does not represent a
limiting factor for the implementation of manual drilling (Fig. 2,
condition 3). The presence of thick hard lateritic layers can
represent an obstacle to feasibility (when laterite is thicker
than 5 m the condition is considered not feasible, class NF)
only in the south-eastern side of the study area, and in some
locations in the centre (around Linguere). Furthermore, in the
southern part of the region there are a number of logs showing
the presence of laterite with thickness less than 5 m; in this
situation, manual drilling is considered feasible, but
percussion techniques are required (class F-SP).
Assessment of feasibility for manual drilling After having
calculated the three key parameters (depth to hard rock, depth
to water, thickness of laterite), it is now possible to assign a
specific class of feasibility for each of the borehole logs
(Fig. 8), following the procedure explained in Fig. 2. In the
whole study area, 62% of the borehole logs show feasible
conditions for manual drilling. Along the western coastal strip
there are feasible conditions in almost the whole group of logs,
while in the Ferlo Valley the trend is mixed, with a
predominance of feasible conditions. Limitations in the feasibility of
manual drilling are present in the southern part (mainly
Fig. 7 Thickness of laterite layers in stratigraphic logs
because of the depth of the water level) and the NE sector (in
this case, the main constraint is the limited thickness of
unconsolidated layers).
It is important to underline that the presence of confined
aquifers and the lack of direct data on water levels in large
wells can lead to unreliable information concerning the depth
of the shallow water table in the eastern part of the study area
and impact the estimation of feasibility of manual drilling.
Classification of suitability for manual drilling
Hydraulic conductivity of exploitable saturated layer The
estimated hydraulic conductivity of the porous shallow
aquifer in the exploitable layer shows Log(K) values characteristic
of an intermediate texture class; considering that the medium
grain size fraction is limited, the K value can be attributed to a
mix of sandy deposits with an important clay fraction.
A v e r a g e L o g ( K ) i s − 4 . 8 3 ( c o r r e s p o n d i n g t o
K = 1.5 × 10−5 m/s), with a standard deviation of 0.4. The
lowest K values in the exploitable layer are found SE of
Kebemer and in the eastern sector of the study area. The
highest values are in the coastal zone. Furthermore, a few
boreholes show a high estimated K value in the exploitable
layer along the river valley NW of Linguere (Fig. 9). In
general, hydraulic conductivity in the first metres is higher than in
the deeper part of the porous aquifer: the average value of Log
(K) between 0 and 10 m deep is −4.4 (K = 4.10 × 10−5 m/s),
while between 20 and 30 m, it is −5.0 (K = 1.10 × 10−5 m/s); in
both cases standard deviation is 0.4 or 0.5. This is related to a
general trend (90% of the whole data set) of decreasing coarse
textural fraction in the deeper layers of the shallow porous
aquifer.
Thickness of exploitable saturated layer Comparing the
depth to hard rock and the depth to water, the thickness of
the saturated layer is obtained. For the classification of
suitability for manual drilling, only the layer at maximum depth of
50 m is considered exploitable. Furthermore, manual drilling
is not considered feasible when the exploitable layer is thinner
than 10 m. Thirty percent of the 131 boreholes do not have an
exploitable saturated layer, as the water level is deeper than the
depth to hard rock (or deeper than 50 m), while another 29%
of logs show an exploitable layer thinner than 10 m; therefore,
they are not considered to have suitable conditions for manual
drilling. Forty-one percent of boreholes have sufficient
thickness of saturated layer for the implementation of manual
drilling, although in reality the possible effect of confining layers
in the eastern part can lead to a different (and less favourable)
situation (Fig. 10).
Potential for exploitation with manual drilling The thresh
old values of Tex that discriminate the different classes of
potential (Table 3) on the basis of site-specific conditions are
assigned using an approximate relation between Tex and
expected drawdown with a hand pump (expected yield 0.2 L/s)
and solar pump (expected yield 1 L/s) using Dupuit’s equation
for steady flow. The drawdown is compared with the
sitespecific condition of the expected maximum water column
in the well (depending on the depth of groundwater) and
seasonal fluctuations. In this way, it is possible to estimate
the sustainability of a continuous and intense pumping.
Considering the hydrogeological context of the study area,
Tex limits of the five class of potential for manual drilling
the threshold values of Tex shown in Table 8 were assumed.
With this classification, it is possible to assess the distribution
of potential in the whole study area (Fig. 11).
Suitability for manual drilling On the basis of the different
hydrogeological parameters described previously, it has
been possible to classify each borehole log in terms of
feasibility and potential for manual drilling. The final
class of suitability for manual drilling can be obtained
from the combination of these two, as described in
Table 4. The results of classification of suitability at
borehole-log positions are shown in Fig. 12.
In those areas where manual drilling is considered
feasible, one can observe a good potential in the coastal area
and in the Ferlo valley (i.e. along the dry river between
Mboula and Linguere, in the centre of the study area).
However, in this zone, the assessment of suitability can
be not reliable, given the uncertainty about the real depth
to water level in the shallow porous aquifer and the
difference between direct field observations in
largediameter wells and static water level data from deep
boreholes in the national database, as previously explained.
On the other hand, the area eastward from Kebemer (i.e.
the region of Ndiob, Ndoyenne, Darou, in the SW sector
of the study area) has several places with low potential,
because of the double effect of the depth to water
(therefore a small water column in the well, if maximum depth
is 50 m) and a higher percentage of fine materials (with a
low value of K). In the case of drilling deeper than 50 m,
both estimated feasibility and potential would improve in
this region, while in the NE sector the limitations imposed
by the presence of shallow hard rock would not change
the estimation of suitability. The estimation of feasibility
depends on the limits of depth defined for manual drilling
based on the experience of drillers. In this research, a
maximum well depth of the wells at 50 m (and a
maximum depth to exploitable water at 40 m) was assumed.
However, 20% of the data shows a water depth between
40 and 50 m. The class of feasibility for these points
would change in the case of increasing the maximum
estimated depth of drilling by 10 m (this would be
considered feasible).
Conclusion
This research allowed for the delineation of suitable zones to
implement manual drilling in a region of northwestern
Senegal, improving (i.e. increasing the scale of detail and the
reliability of the interpretation by validation with real
stratigraphic and hydraulic data) the previous map of suitable zones
in this country
(Kane et al. 2013)
.
During the research, a specific software (TANGAFRIC)
and a structured method to carry out a semi-automated
analysis of stratigraphic information through organization,
codification and processing of existing borehole-log data have been
developed. This activity was completed on a sample of 173
borehole logs (through manual input of stratigraphic codes
carried out by technical staff in Senegal, followed by an
automated procedure of data elaboration) in Louga region, but the
process can be replicated with other data sets. In Senegal, the
national database contains a detailed description of 1,419
stratigraphic logs scattered throughout the whole country,
but many more are available in hard copy at the regional
branches of the national water authority. The update of
information concerning existing water points is still a difficult
aspect; in recent years, increasing attention has been given to
setting up an efficient system for water point mapping and
monitoring. The lack of up-to-date information strongly limits
the reliability of studies on the functionality of water supply
and access to safe water, while it is less relevant for
hydrogeological interpretation. The information stored in the
national database is generally collected at the moment of
construction of water points, but most of the hydrogeological data
(especially stratigraphic description) does not change over
time and the information is still correct.
In other countries in Africa, large databases of water points
are stored by the national institutions, with limited capacity to
analyse this information and exploit it to formulate
hydrogeological interpretation. Tools and methods proposed
in this research could provide a valid support to study not only
the shallow hydrogeology but also deeper fractured
formations, contributing to groundwater exploration and
management.
In the next few years, manual drilling is expected to spread
in several countries. Various international organizations and
funding agencies are committed to supporting this process—
for example, UNICEF and its partners PRACTICA
Foundation and RWSN (Rural Water Supply Network) are
running an important technical assistance program to create
a highly professional manual drilling sector in many countries
in Western and Central Africa (Central African Republic,
Togo, Mauritania, Mali, Ivory Coast, Guinea, Niger, Nigeria,
DRC) funded by DGIS (Directorate-General for International
Cooperation of Dutch Government), DFID (Department of
International Development of UK Government) and other
donors. It would be important for scientific research to provide
valid tools for effective planning and implementation of this
process, which could produce a highly positive impact on
access to safe water and the living conditions of the
population.
Acknowledgements This study has been carried out in the framework
of the project entitled BUse of remote sensing and terrain modelling to
identify suitable zones for manual drilling in Africa and support low cost
water supply^, and is financed by NERC (National Environment
Research Council, UK) in the framework of the program UPGRO
(Unlocking the Potential of Groundwater for the Poor). The study has
been carried out in two different study areas, in Senegal (whose results are
presented in this report) and Guinea. This research has been possible
thanks to the joint activities and positive collaboration of different
partners from Italy, Senegal and Guinea (University Milano Bicocca,
Université Cheik Anta Diop Dakar, University of Thies, SNAPE,
Direction de l’Hydraulique of Louga region). We also want to
acknowledge the support received from Richard Carter during the selection of the
interpretation model for the pumping test, Kerstin Danert and Sean Furey
(who went to all the effort of disseminating the results of this research),
Alan McDonald and Yann Chemin during the preliminary review of this
report.
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