Water is essential for human settlements. Since prehistoric times, water availability has played a key role in determining the origin and fate of entire civilizations [1
]. In today’s industrial societies, drinking and irrigation water supply is often taken for granted. This is because water infrastructures are solidly engineered and there are enough resources to build and maintain them. As a result, failures are rare and the population does not need to worry about taps running dry. However, access to water remains an issue in many regions across the world. In developing countries, where a productive well not only means hydration but also food security, hygiene, health, and, arguably, a better chance for education, a significant share of the population does not yet have access to improved water sources [2
]. Millions of people live more than one kilometer away from the nearest faucet, and walk several hours each day to provide water for such ordinary needs as drinking or cooking [3
]. More often than not, fetching water is a task for women and children—a task that is carried out at the expense of education and leisure, and often leads to chronic injuries and to missing out on opportunities for personal development [4
In areas subject to the West African Monsoon, rainfall is erratic and concentrated almost exclusively in the wet season. People are often forced to rely on the unreliable to meet their domestic needs and grow low-value crops. Within this context, groundwater resources can play a key role in challenging poverty by providing a stable supply of water [5
]. Aquifers allow users to deliver for themselves, their crops, and livestock during long dry spells. This is partly because of the large quantities of water that are naturally stored in aquifer systems and partly because groundwater can be accessed with relative ease. Nevertheless, improved water sources such as mechanized boreholes may be extremely expensive for rural communities. In the Sahel region, a mechanized borehole costs several thousand USD [7
]. Bearing in mind that a large share of the population makes under 1 USD/day, this sum implies that drilling mechanized boreholes is beyond the means of most people. Devising cost-effective methods to access groundwater resources and build local infrastructures is thus crucial to ensure food security and protect human livelihood [8
Manual drilling is a discipline in its own right [10
]. It is conceptually different from digging and presents a series of specific features that set it apart from mechanized drilling. Hence, its potential and competitiveness need to be appraised in terms of relative strengths and weaknesses. For instance, a clear conceptual distinction exists between hand drilling and excavation (Table 1
). Manually drilled boreholes are not made by means of pick and shovel, but by replicating mechanical drilling methods by hand. As a result, the holes are small in diameter (typically 2 to 4 inches) and can be cased and equipped with gravel packs, submersible pumps, and sanitary seals, much like mechanized boreholes. Moreover, manual boreholes are deeper on average than excavated wells. This presents several additional advantages. Perhaps the main one is that the technique is not limited to the point when the water accumulated at the bottom of the hole makes it too difficult to keep digging by hand. In consequence, manual boreholes often exceed 30 or 40 m, while dug wells rarely reach 20. The possibility of drilling several meters below the water table implies that users are unlikely to run out of water during the dry season. Furthermore, deeper ground waters are better protected from contamination. This poses an added benefit in relation to excavated wells, where pollution is generally an issue [11
]. Finally, manual drilling is safer during the construction stage because the drilling crew works outside the hole and needs not worry about collapses.
Manual drilling is also different from mechanized drilling. For practical purposes, manual drilling is slower and more labor-intensive. In addition, its potential is limited by geological factors. However, when conditions allow, manual drilling provides a highly cost-effective alternative to access groundwater [12
]. For instance, in the geographical context where this research was carried out, a fully equipped manual borehole, including the pumping device, may be up to 95% cheaper than a fully equipped mechanized borehole of the same depth.
Manual drilling falls under the umbrella of appropriate technologies. This means that materials can be obtained locally and users can be trained to drill and maintain their own boreholes. As a result, there is often no need to rely on external technical support or specific spare parts. These are some additional reasons why manual drilling was included within the Millennium Development Goal Good Practices by the United Nations Development Group. Efforts have subsequently been made to raise the profile of manual drilling among stakeholders [17
]. Manual drilling also has the potential to contribute to several the Sustainable Development Goals, including Goal number 6, “ensure access to water and sanitation for all”.
1.2. Methodological Precedents, Research Objectives, and Novelties
It is well known that geographic information systems (GIS) are designed to analyze the interrelation between different layers of spatially distributed data. GIS is the tool of choice when integrating information from various sources to tackle complex problems. Geospatial information has proven useful to explore the link between water and poverty [18
]. In the case of groundwater resources, GIS has frequently been applied to delineate groundwater potential areas [20
], study the spatial distribution of aquifer recharge [25
], or assess the vulnerability of aquifer systems to pollution [28
The literature on manual drilling potential is comparatively scarce, despite the fact that most hand drilling techniques have been known for a long time [31
]. Sludging, for instance, is considered a traditional technique in countries like India or Bangladesh [12
], while percussion was already being used by the Chinese several millennia ago [32
]. Hand drilling is largely a forgotten art in the West, where it has been replaced by automated methods of all kinds. However, it is fair to say that manual drilling has experienced a revolution in the last two decades, partially fueled by international relief projects. Global estimates as to the total number of hand-drilled boreholes are hard to come by, but some telling figures exist. For instance, it is known that over 8 million hand boreholes have been drilled in Bangladesh, to go with several thousand in Nigeria, Niger, Madagascar, and Chad [12
]. Moreover, manual drilling is more or less widespread in countries like Bolivia, Nicaragua, or Senegal, and has become a flourishing business in rural areas of Bangladesh, Niger, and Sudan [33
Methodological precedents on mapping the feasibility of manual borehole drilling are limited. National-scale maps for drilling manual boreholes have been developed for different African countries in recent years [34
]. Other authors narrow down the geographical focus, thus attaining a higher spatial resolution in the results [37
]. Geographical and geological variables have been demonstrated to constrain the efficiency of manual drilling [39
]. For instance, progress may be reasonably fast in softer formations such as clay or sand, where it is possible to drill several meters per hour. Conversely, drilling rates in hardened layers may slow to just a few centimeters per day. The hardness factor is closely linked to the depth to the water table, which in turn translates into time, cost, and the potential to encounter unexpected difficulties. This implies that lowlands tend to be more suitable for manual boreholes than highlands. Other aspects, including accessibility, water quality, slope, or the aquifer’s hydrodynamic parameters also need to be taken into account, as they influence both the drilling process and the usability of the borehole [42
The key hypothesis behind this paper is that the above factors can be combined meaningfully into a geographic database in order to decide which areas of a given region are suited to manual drilling. The method is illustrated through its application to a bedrock aquifer in southern Mali (Figure 1
) and the outcomes are calibrated against geophysical data. Thus, the GIS database is best described as a decision support system that may allow us to make optimal decisions as to where to drill a manual borehole. By definition, a decision support system is a computer-based information system that can be used to underpin business or organizational decision-making [43
]. Decision support systems have the ability to integrate diverse aspects of the planning process, and may take into account the spatial dimension. As such, they have been broadly used in the management of natural resources, including water [44
], fire [46
], or wind [48
]. Decision support systems may be used to produce scenarios or forecasts [49
] and may encompass a large number of qualitative or quantitative variables.
The novelty of this approach is three-fold. In the first place, it is argued that the traditional focus on drilling feasibility may be broadened as a means to enhance the practical value of the outcomes. Feasibility, or potential, provides a qualitative depiction of reality (i.e., “high”, “medium”, “low”). It is also a relative measure, for it depends of user-dependent factors such as experience. Hence, feasibility maps find their appropriate context in larger geographical scales. By narrowing down the geographical focus, we are able to express the outcomes in terms of drilling cost and time, thus providing univocal outputs for non-experts. The results are immediately useful for decision-makers such as cooperation entities, incipient drilling businesses, or donors.
Secondly, maps in this manuscript are referred to a specific percussion-based drilling technique [50
]. Technique-specific maps are perceived to be a welcome methodological addition because each manual drilling type presents advantages and disadvantages of its own [12
]. For instance, methods such as hand augering are largely inappropriate when drilling through consolidated rocks such as laterite or sandstone, whereas percussion-based techniques such as the one at hand may prove suitable. This means that drilling in the same geographical area can be unfeasible for a given technique but feasible using a different one. Since a general feasibility map is partially unable to capture this reality, it is contended that technique-specific approaches are likely to render more practical outcomes. The choice of drilling method is due to the fact that the authors have direct experience with it, having drilled about 20 manual boreholes in geologically similar areas of Mali over the last two years. This means that there is sufficient direct knowledge to estimate relevant parameters such as drilling rates, wages, or material costs.
Finally, for the purpose of expressing the results in terms of time and cost, mapping should take into account casing diameters. A four-inch diameter borehole takes significantly longer to drill than a two- or three-inch one. Furthermore, the casing diameter constrains the choice of pumps. Most standard hand pumps such as the India-Mark types are designed for four-inch boreholes (three-inch models also exist, but are difficult to find in some countries). Thus, a narrower diameter implies the need to fit a powered pump (three-inch powered pumps can also be difficult to find) or to manufacture homemade pumps with local materials. This in turn affects the durability and cost of equipping the borehole.