Introduction
-
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.
-
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
Source of data
Classification of zones of suitability for manual drilling
Assessment of feasibility
-
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 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.
Class of feasibility | Description |
---|---|
Not feasible (NF) | Not feasible because of the presence of shallow hard rock, large depth to water, and a thick lateritic hard layer |
Feasible (F) | Manual drilling can be successfully done in this hydrogeological context |
Feasible with special techniques (FS) | Manual drilling can be done, but the presence of hard intercalated layers necessitates the use of special techniques at a certain depth (e.g. percussion) to break it, together with other common methods to drill in unconsolidated sediments |
Classification of the potential for exploitation
Type of pump | Description and usage |
---|---|
Hand pump | The most common pumping system for hand-drilled wells. Although the aquifer could be more productive, the expected yield from hand-operated pumps is approximately 0.2 L/s, with a maximum limit of 0.4 L/s in optimal conditions. They are suitable for the whole range of static water-level depth considered as feasible for manual drilling (maximum depth of static water level = 40 m) and they can provide water for groups of approximately 250 users for their domestic needs |
Solar pump | Solar-powered pumps can be installed in highly productive hand-drilled wells. They can supply water for a larger community (and eventually connect to small water distribution systems). Their yield depends on the power generated by solar panels (whose extension and power production can vary considerably); as a reference value, 1 L/s is considered as a reasonable yield achievable with a solar pump in a productive aquifer, providing water to 1,000 users |
Rope pump | 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 |
Class of potential | Description |
---|---|
NP: potential null | 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 (H
ex = 0) |
LP: low potential | 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 |
MP: moderate potential | 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) |
GP: good potential | 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) |
EP: excellent potential | 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) |
Assigning the final class of suitability
Feasibility | Potential for exploitation | Combination | Class of suitability |
---|---|---|---|
Not feasible | |||
NF | – | NF | Not suitable |
Feasible | |||
F | NP | F-NP | Not suitable |
LP | F-LP | Suitable with poor results | |
MP | F-MP | Suitable | |
GP | F-GP | Suitable | |
EP | F-EP | Suitable | |
Feasible with special techniques | |||
FS | NP | FS-NP | Not suitable |
LP | FS-LP | Suitable with poor results | |
MP | FS-MP | Suitable | |
GP | FS-GP | Suitable | |
EP | FS-EP | Suitable |
Data processing and interpretation
Texture class | Categories |
---|---|
Coarse texture | Sand, gravel |
Medium texture | Marl, silt |
Fine texture | Clay, vegetal layer, lignite, laterite, alterite |
Hard rock | Basalt, limestone, dolerite, gabbro, granite, sandstone, quartz, schist, silex |
Hard laterite | Lateritic crust |
Input table | Output table after data processing | ||||||
---|---|---|---|---|---|---|---|
Depth (m) | Texture | From (m) | To (m) | Coarse (%) | Medium (%) | Fine (%) | Hard (%) |
From 0 to 4 | Sand | 0 | 2 | 100 | 0 | 0 | 0 |
2 | 4 | 100 | 0 | 0 | 0 | ||
From 4 to 10 | Sandy clay | 4 | 6 | 30 | 0 | 70 | 0 |
6 | 8 | 30 | 0 | 70 | 0 | ||
8 | 10 | 30 | 0 | 70 | 0 |
-
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
Validation with measured K values from field tests
Results and discussion
Comparison of K values obtained from analysis of stratigraphic logs and pumping tests
Statistic |
K
Rupp’s method (assuming soil class Sa1) |
K
Rupp’s method (assuming soil class Lsa1) |
K
Original Bouwer and Rice method |
---|---|---|---|
Mean | 0.42 | 0.44 | 1.22 |
Median | 0.25 | 0.28 | 0.48 |
Characterizing the shallow aquifer and the suitability for manual drilling based on borehole-log positions
Classification of feasibility for manual drilling
Classification of suitability for manual drilling
Class of potential | Range of T
ex (m2/s) |
---|---|
NP: potential null | 0 |
LP: low potential | Between 0 and 3.10 × 10−5
|
MP: moderate potential | Between 3.10 × 10−5 and 5.10 × 10−5
|
GP: good potential | Between 5.10 × 10−5 and 1.3 × 10−3
|
EP: excellent potential | More than 1.3 × 10−3
|