What is a Hydrogeological Survey? Why You Need One Before Drilling

Introduction: The Foundation of Successful Borehole Drilling

Drilling a borehole is one of the most effective ways to secure a reliable water supply in Uganda, whether for a rural community, an agricultural operation, a mining project, or an institutional facility. But drilling without first understanding the underground water conditions is like building a house without surveying the land — it may work, or it may result in an expensive failure.

A hydrogeological survey is the scientific investigation carried out before borehole drilling to assess the groundwater conditions beneath a site. Its purpose is straightforward: to determine where water is most likely to be found, how much water is available, what the water quality is likely to be, and where the best drilling location is. In Uganda, where groundwater is the primary source of clean water for millions of people and where aquifer conditions vary dramatically from one location to the next, a hydrogeological survey is not a luxury — it is a necessity.

This article explains what a hydrogeological survey involves, walks through the process step by step, describes what the final report contains, and explains the very real risks of skipping this critical step. For a broader guide to the borehole drilling process in Uganda, see our article on borehole drilling in Uganda.

What is a Hydrogeological Survey?

A hydrogeological survey is a systematic investigation of the subsurface water conditions at a specific site or within a defined area. It combines geological knowledge, geophysical measurement techniques, and hydrological analysis to build a picture of the underground water system — known as the aquifer — and to recommend optimal locations for borehole drilling.

Hydrogeology is the branch of geology that deals with the distribution and movement of groundwater within rocks and soil. Every successful borehole taps into an aquifer — a layer of permeable rock, sand, gravel, or fractured bedrock that stores and transmits water. The challenge is that aquifers are invisible from the surface, and their characteristics vary enormously depending on the local geology, rock type, fracture patterns, depth, and recharge conditions.

A hydrogeological survey answers the following critical questions:

Is groundwater present beneath this site? Not every location has accessible groundwater. Some areas may have impermeable rock formations that do not store or transmit water in usable quantities.
At what depth is the water table? Knowing the approximate depth to water is essential for planning the drilling programme, selecting the right drilling method, and estimating costs.
How much water is available? The yield — or flow rate — of a borehole depends on the aquifer's permeability, thickness, and recharge rate. The survey helps estimate the likely yield before drilling begins.
What is the water quality likely to be? Groundwater quality is influenced by the rock types it passes through. Some geological formations can produce water with high mineral content, elevated fluoride, iron, or other parameters that affect suitability for drinking or irrigation.
Where is the optimal drilling location? The survey identifies the specific point on the ground where drilling is most likely to intersect a productive aquifer zone, maximising the chances of a successful borehole.

The Hydrogeological Survey Process

A comprehensive hydrogeological survey follows a structured process that progresses from broad-scale assessment to site-specific investigation. Each step builds on the findings of the previous one, progressively narrowing down the optimal drilling location.

Step 1: Desk Study

The desk study is the starting point of every hydrogeological survey. Before any fieldwork begins, the hydrogeologist compiles and reviews all available existing data about the site and its surroundings. This includes:

Geological maps showing the rock types, geological structures, and formations present in the area
Existing borehole records from the Ministry of Water and Environment's national borehole database, including drilling logs, pump test results, water quality data, and borehole success/failure records from nearby locations
Topographic maps and satellite imagery to understand the terrain, drainage patterns, and surface features that influence groundwater recharge and flow
Climate and rainfall data to assess the annual recharge of the aquifer system
Previous hydrogeological reports from studies conducted in the same area or region
Land use and potential contamination sources — the proximity of latrines, waste disposal sites, fuel storage, agricultural chemical use, and other pollution sources that could affect groundwater quality

The desk study provides the hydrogeologist with a preliminary understanding of the groundwater conditions and helps identify the most promising areas for field investigation. In many parts of Uganda, the availability of existing data varies considerably, making the hydrogeologist's experience and interpretive skill particularly important.

Step 2: Field Reconnaissance

Following the desk study, the hydrogeologist visits the site to conduct a field reconnaissance. This involves walking the ground, observing the terrain, vegetation, drainage patterns, and rock outcrops, and gathering information that cannot be obtained from maps and reports alone.

During the reconnaissance, the hydrogeologist:

Examines rock outcrops and exposures to confirm the rock type and identify fracture systems, weathering profiles, and other geological features relevant to groundwater occurrence
Inspects existing water sources in the area, including springs, shallow wells, and existing boreholes, noting their condition, flow rates, and water quality observations
Interviews local community members about their knowledge of water availability, seasonal variations, and the performance of any existing water points
Assesses potential drilling sites in terms of access for drilling equipment, proximity to the intended users, and distance from contamination sources
Identifies any constraints such as overhead power lines, underground services, property boundaries, or environmentally sensitive areas that may limit where drilling can take place

The field reconnaissance provides ground-truth information that validates or modifies the interpretations made during the desk study and helps the hydrogeologist select the most appropriate locations for geophysical surveying.

Step 3: Geophysical Profiling

Geophysical surveying is the core technical component of a hydrogeological survey. It uses non-invasive measurement techniques to investigate the physical properties of the subsurface rock and soil layers, identifying zones where groundwater is most likely to be found.

The two most commonly used geophysical methods in hydrogeological surveys in Uganda are:

Vertical Electrical Sounding (VES)

Vertical Electrical Sounding measures the electrical resistivity of the ground at increasing depths. The technique works by passing an electrical current into the ground through metal electrodes and measuring the resistance encountered. Different rock types and soil layers have different resistivity values — water-saturated, fractured, or weathered rock typically has lower resistivity than dry, unfractured rock.

By measuring resistivity at progressively greater depths, the VES method produces a vertical profile of the subsurface, identifying:

The depth and thickness of different geological layers
The position of the water table
Zones of weathered or fractured rock likely to contain groundwater
The depth to fresh bedrock

VES data are plotted on sounding curves and interpreted by the hydrogeologist to determine the most promising depth intervals for drilling.

Horizontal Electrical Profiling (HEP)

Horizontal Electrical Profiling measures the lateral variation in resistivity across a survey line at a fixed depth. This technique is particularly useful for mapping the horizontal extent of aquifer zones, identifying fracture zones, and locating the optimal point along a traverse for borehole drilling.

HEP surveys are often conducted along multiple parallel lines to build a two-dimensional picture of the subsurface conditions across the site. The results are presented as resistivity profiles or contour maps that highlight anomalous zones of low resistivity — the areas most likely to yield groundwater.

Other Geophysical Methods

Depending on the site conditions and the complexity of the hydrogeological setting, additional geophysical methods may be employed:

Electromagnetic (EM) surveys — useful for rapid reconnaissance mapping of conductive zones associated with groundwater or weathered rock
Magnetic surveys — can identify geological structures such as dykes and faults that control groundwater flow and accumulation
Seismic refraction — measures the velocity of sound waves through the ground to determine the depth to bedrock and identify weathered zones

Step 4: Aquifer Analysis

Once the geophysical data have been collected and interpreted, the hydrogeologist integrates all available information — from the desk study, field reconnaissance, and geophysical surveys — to develop a comprehensive understanding of the aquifer system beneath the site.

The aquifer analysis considers:

Aquifer type — is the aquifer an unconfined (water table) aquifer, a confined (artesian) aquifer, or a semi-confined aquifer? The aquifer type affects the expected yield, water level behaviour, and borehole design
Aquifer geometry — the depth, thickness, and lateral extent of the aquifer zone
Aquifer productivity — an estimate of the likely borehole yield based on the aquifer characteristics, rock type, and fracture intensity
Recharge conditions — the mechanisms and rate at which the aquifer is replenished by rainfall infiltration, river seepage, or other sources
Groundwater flow direction — the direction in which groundwater moves through the aquifer, which is relevant for water quality and contamination risk assessment
Water quality expectations — based on the rock types and geological conditions, the hydrogeologist can anticipate likely water quality parameters and flag any potential concerns

Step 5: Site Selection and Recommendations

The final step of the survey is the preparation of site-specific recommendations for borehole drilling. Based on the integrated analysis, the hydrogeologist identifies:

The recommended drilling location — the specific point (marked with GPS coordinates) where the borehole should be drilled to maximise the probability of intersecting a productive aquifer zone
The recommended drilling depth — the target depth for the borehole, based on the geophysical interpretation and aquifer analysis
The expected yield — an estimate of the likely flow rate from the borehole, expressed in litres per hour or cubic metres per hour
Preliminary borehole design — recommendations for casing diameter, screen placement, and borehole construction based on the anticipated geological conditions
Alternative drilling locations — in case the primary site is not available or if the first borehole does not achieve the target yield

What Does a Hydrogeological Report Contain?

The hydrogeological report is the formal output of the survey and the document that guides all subsequent drilling and construction activities. A comprehensive report typically includes:

Introduction and scope — the objectives of the survey, the site location, and the intended use of the borehole
Desk study summary — a review of all existing data consulted, including geological maps, borehole records, and climate data
Field observations — descriptions of the terrain, geology, existing water sources, and site conditions observed during the reconnaissance
Geophysical survey results — raw data, interpreted profiles, and maps from the VES, HEP, and any other geophysical methods used
Hydrogeological interpretation — a synthesis of all data into a model of the aquifer system, including aquifer type, geometry, and expected productivity
Recommendations — specific drilling locations (with coordinates), target depths, expected yields, and preliminary borehole design specifications
Risk assessment — an evaluation of the risks associated with drilling at the recommended site, including the probability of encountering low-yielding zones, poor water quality, or difficult drilling conditions

This report serves as both a technical guide for the drilling contractor and a record of the scientific basis for the drilling decision. It should be prepared by a qualified hydrogeologist and retained for future reference.

The Risks of Skipping a Hydrogeological Survey

The temptation to skip the hydrogeological survey and proceed directly to drilling is understandable — it saves time and upfront cost. But the consequences of drilling without adequate investigation can be far more expensive than the survey itself.

Dry Boreholes

The most obvious risk is drilling a borehole that produces little or no water. In Uganda, borehole failure rates in areas where surveys are not conducted can be significant. A dry borehole represents a total loss of the drilling investment — which can amount to tens of millions of Ugandan shillings — and leaves the community or project without a water supply.

Low-Yielding Boreholes

Even if a borehole intersects some water, the yield may be insufficient for the intended purpose. A borehole producing one or two hundred litres per hour may be adequate for a small domestic supply but wholly inadequate for a school, health centre, irrigation scheme, or mining operation. Without a hydrogeological survey to estimate the likely yield, there is no way to match the borehole's capacity to the demand.

Poor Water Quality

Drilling in the wrong geological formation can produce water with unacceptable levels of naturally occurring contaminants. In parts of Uganda, groundwater from certain rock types contains elevated levels of fluoride, iron, manganese, or salinity that make it unsuitable for drinking without treatment. A hydrogeological survey assesses the geological conditions and flags potential water quality concerns before drilling, allowing for informed decisions about site selection and water treatment requirements.

Contaminated Water

Siting a borehole too close to a latrine, waste disposal site, fuel storage area, or agricultural chemical application zone can result in the borehole drawing in contaminated water. The hydrogeological survey includes an assessment of contamination risks and recommends appropriate setback distances to protect the water supply.

Wasted Resources

For organisations and communities operating on limited budgets, the cost of a failed or underperforming borehole is not just financial — it is the loss of an opportunity to secure a reliable water supply. The relatively modest cost of a hydrogeological survey is a sound investment that dramatically increases the probability of a successful outcome.

Hydrogeological Surveys for Different Applications

While the core methodology remains the same, the scope and emphasis of a hydrogeological survey may vary depending on the intended application.

Community Water Supply

For rural water supply boreholes in Uganda, hydrogeological surveys are typically required by the Ministry of Water and Environment and by most development agencies funding water projects. The survey ensures that the borehole is sited to maximise yield and water quality while maintaining safe distances from contamination sources.

Mining and Industrial Water Supply

Mining operations and industrial facilities often require large and reliable water supplies for processing, dust suppression, and domestic use. Hydrogeological surveys for mining applications may need to assess aquifer capacity over larger areas, evaluate the potential for sustained high-yield extraction, and consider the impact of groundwater abstraction on local water resources and the environment.

Agricultural Irrigation

Irrigation boreholes must provide sustained flow rates over extended pumping periods. The hydrogeological survey for irrigation applications places particular emphasis on aquifer storage, recharge rates, and long-term sustainability of groundwater extraction.

Environmental and Baseline Studies

Hydrogeological surveys are also conducted as part of environmental impact assessments for mining, infrastructure, and development projects. These surveys establish the baseline groundwater conditions against which future changes can be measured, ensuring that projects can monitor and manage their impact on water resources.

ALOM's Hydrogeological Survey Services

ALOM Mining & Geohydro Services provides comprehensive hydrogeological survey and groundwater development services across Uganda. Our team of experienced hydrogeologists and geophysicists delivers the full spectrum of survey activities, from desk studies and field reconnaissance through geophysical profiling to aquifer analysis and site selection.

Our hydrogeological survey capabilities include:

Desk studies and data compilation — thorough review of existing geological, hydrogeological, and borehole data
Field reconnaissance and geological mapping — ground-truth assessment of site conditions and geological features
Geophysical surveys — VES, HEP, electromagnetic, and magnetic methods using modern field equipment
Aquifer analysis and modelling — interpretation of subsurface conditions and estimation of aquifer productivity
Site selection and borehole design — GPS-located drilling recommendations with target depths and expected yields
Comprehensive reporting — detailed hydrogeological reports that meet the requirements of government agencies, development partners, and project financiers

We work with communities, NGOs, government agencies, mining companies, and private sector clients across all regions of Uganda, applying the same rigorous scientific methodology whether the project involves a single community borehole or a multi-well water supply for a mining operation.

Conclusion

A hydrogeological survey is the essential first step in any borehole drilling project. It provides the scientific evidence needed to select the right drilling location, target the right depth, and anticipate the likely yield and water quality. Without it, drilling is a gamble — and the stakes are too high to leave to chance.

In Uganda, where aquifer conditions are complex and variable, and where every failed borehole represents a significant loss of scarce resources, the value of professional hydrogeological investigation cannot be overstated. The relatively modest investment in a properly conducted survey pays for itself many times over through higher borehole success rates, better water quality outcomes, and more reliable long-term water supply.

Whether you are planning a community water supply borehole, an irrigation well, or a high-capacity water source for a mining or industrial operation, start with a hydrogeological survey. It is the foundation on which every successful borehole is built.

Frequently Asked Questions

How long does a hydrogeological survey take?

A typical survey for a single borehole site takes between three and seven days, depending on the complexity of the site and the number of geophysical stations required. Larger projects covering multiple sites or wider areas may take longer. The desk study and reporting phases add additional time to the overall process.

How much does a hydrogeological survey cost in Uganda?

Costs vary depending on the scope of the survey, the site location, and the number of proposed borehole sites. However, the cost of a survey is typically a small fraction — often 5% to 15% — of the total drilling cost. Given that the survey significantly reduces the risk of borehole failure, it represents excellent value for money.

Is a hydrogeological survey legally required before drilling in Uganda?

While the specific requirements depend on the funding source and regulatory authority, the Ministry of Water and Environment and most development agencies require a hydrogeological survey report before approving borehole drilling. Even where not strictly mandated, professional practice and prudent project management strongly recommend conducting a survey.

Can a hydrogeological survey guarantee a successful borehole?

No survey can provide a 100% guarantee, as subsurface conditions can only be fully confirmed by drilling. However, a well-conducted hydrogeological survey dramatically increases the probability of success — typically to 85% or higher — compared to drilling without investigation, which can have failure rates of 30% or more in some areas.

What is the difference between a hydrogeological survey and a geophysical survey?

A geophysical survey is one component of a broader hydrogeological survey. The geophysical survey measures the physical properties of the subsurface using instruments, while the hydrogeological survey encompasses the full investigation — desk study, field reconnaissance, geophysical profiling, aquifer analysis, and site selection — to produce a comprehensive assessment of groundwater conditions.

Does ALOM provide both survey and drilling services?

Yes. ALOM Mining & Geohydro Services provides end-to-end groundwater development services, including hydrogeological surveys, borehole drilling, pump testing, water quality analysis, and borehole equipping. This integrated approach ensures continuity from investigation through to completed water supply. Contact us to discuss your project.