WHAT REALLY HAPPENS INSIDE A BOREHOLE? 0723763173

Groundwater moves through soil, sand, gravel, weathered rock and fractures in solid rock. When a borehole intersects a water-bearing formation, groundwater can enter the borehole and collect within its casing or open section. A suitably selected submersible pump can then lift the water to the surface for domestic, agricultural, commercial or industrial use.

However, not every borehole produces the same quantity of water. Some boreholes provide a reliable supply for many years, while others experience declining water levels, sediment production, pump failures or changes in water quality. These differences depend on geological conditions, groundwater availability, borehole construction, pumping rates and maintenance practices.

Understanding what happens inside a borehole is important before drilling a new water source, installing a pump, investigating a water-supply problem or planning long-term groundwater use.

This guide explains the processes that occur underground, how groundwater enters a borehole, what happens when pumping begins, why water levels change, how sediment and minerals affect performance, and how to identify common problems.

2. WHAT IS A BOREHOLE?

A borehole is a narrow, deep opening drilled into the ground to investigate geological formations or access underground resources such as groundwater.

A water-supply borehole is designed to intersect a formation capable of supplying water. The drilling depth may range from relatively shallow depths to several hundred metres, depending on the geology, groundwater conditions and intended application.

A borehole is not simply a container that fills with water from below. It is connected to the surrounding geological formations, which may transmit groundwater through pores, fractures, joints or other openings.

The quantity of water available depends on the characteristics of these formations and the conditions under which water is withdrawn.

A properly constructed borehole generally has several important functions:

1. It provides access to underground groundwater.

2. It protects the water source against structural collapse.

3. It allows groundwater to enter through designated intake sections.

4. It provides a passage for installing a pump and associated equipment.

5. It helps protect the groundwater against contamination.

6. It enables controlled abstraction of water for the intended use.

The quality of the drilling, casing, sealing, development and pump installation determines how effectively these functions are achieved.

3. WHAT IS FOUND INSIDE A BOREHOLE?

The components inside a borehole depend on the geology and construction design. A typical water-supply borehole may contain the following.

3.1 Borehole casing

Casing is a pipe installed inside the drilled opening to support unstable formations and protect the borehole.

It may be made from suitable PVC or steel, depending on the geological conditions, mechanical requirements and construction specifications.

In loose formations, casing helps prevent surrounding material from collapsing into the borehole. It can also help isolate shallow formations that should not communicate with the deeper water supply.

Casing does not generate groundwater. Its function is to provide structural support and help control water entry.

3.2 Water-bearing formations

Water-bearing formations are geological materials that store and transmit groundwater.

They may consist of sand, gravel, weathered rock or fractured bedrock. Their ability to supply water depends on permeability, thickness, connectivity and groundwater availability.

A borehole may intersect several formations, but not every formation contributes useful quantities of water.

3.3 Well screen or slotted casing

A screen or slotted casing allows groundwater to enter the borehole while restricting the movement of formation particles.

The screen openings must be selected to suit the formation and, where applicable, the gravel pack.

If the screen openings are unsuitable, the borehole may produce excessive sand or experience restricted water entry.

3.4 Gravel pack

Some boreholes constructed in unconsolidated formations use a gravel pack around the screen.

The gravel pack is designed to stabilise the formation around the intake and help control the movement of fine particles.

Its grading and installation must match the geological material and screen design. An unsuitable gravel pack can contribute to sediment production or reduced hydraulic performance.

Not every borehole requires a gravel pack. Its use depends on the construction method and geological conditions.

3.5 Submersible pump

A submersible pump is installed below the water level and pushes water towards the surface.

Its motor drives an impeller assembly that adds energy to the water. The pump must be selected according to the required flow rate, total dynamic head, borehole characteristics and available electrical supply.

A pump that is too powerful for the borehole may cause excessive drawdown or sediment production.

3.6 Rising main

The rising main is the pipe that carries water from the pump to the surface.

It must withstand the operating pressure, support the pump assembly where required and provide an appropriate flow path.

Pipe sizing, material selection, joint quality and installation depth affect system performance.

3.7 Electrical cable and protection equipment

The electrical cable supplies power to the pump motor. Protection equipment helps guard against operating conditions such as overload, loss of phase in applicable systems, undervoltage and dry running when suitable protection is installed.

Electrical faults can cause a borehole to stop delivering water even when groundwater remains available.

Electrical inspection and repairs should be undertaken by qualified personnel using appropriate isolation procedures.

3.8 Water column

The water column is the body of water occupying the borehole below the water surface.

Its level changes as groundwater enters the borehole and water is withdrawn.

The water column's height is important when determining pump submergence, available storage within the borehole and the relationship between static and pumping water levels.

However, the volume of water standing inside the casing is not the same as the total quantity of groundwater available from the aquifer.

3.9 Sediment at the bottom

Sand, drilling cuttings and other particles may settle at the bottom of a borehole.

Some residual material may be removed during borehole development. Persistent sediment accumulation can indicate inadequate development, unstable formations, unsuitable screen design or excessive pumping.

If sediment approaches the pump intake, it can increase the risk of abrasive wear and blockages.

4. WHAT HAPPENS TO RAINWATER BEFORE IT REACHES A BOREHOLE?

Groundwater commonly originates from rainfall and other forms of precipitation that infiltrate the ground and eventually replenish underground water-bearing formations.

When rain falls, some water flows over the land surface into streams, rivers, drainage channels and other low-lying areas. Some evaporates, while vegetation absorbs another portion.

The remaining water may infiltrate the soil and move downward.

The rate of infiltration depends on several factors:

• Soil texture and structure.

• Vegetation and land cover.

• Ground slope.

• Rainfall intensity and duration.

• Soil moisture before rainfall.

• Surface compaction.

• The permeability of underlying geological materials.

Water that enters the soil does not automatically become groundwater. Some remains in the root zone, some returns to the atmosphere through evaporation and plant transpiration, and some moves laterally towards streams or other drainage pathways.

Water that travels sufficiently deep may eventually reach the saturated zone.

In an unconfined groundwater system, the upper boundary of this saturated zone is called the water table.

Groundwater recharge can take place at different rates. In some locations, water reaches the aquifer relatively quickly. In others, recharge may be slow because the water must pass through thick or poorly permeable formations.

This explains why a borehole may not respond immediately to heavy rainfall and why groundwater levels can continue declining during prolonged dry periods.

5. WHAT IS AN AQUIFER?

An aquifer is a geological formation capable of storing and transmitting groundwater in quantities that may be useful for water supply.

Aquifers are essential because they provide the groundwater that enters a borehole.

There are several common groundwater-bearing environments.

5.1 Sand and gravel aquifers

Sand and gravel aquifers contain interconnected spaces between particles. Groundwater moves through these openings.

Well-sorted, permeable sand and gravel can transmit substantial quantities of water. Fine-grained or clay-rich formations may restrict movement.

Boreholes in loose formations require appropriate intake design to control sediment entry.

5.2 Fractured-rock aquifers

In hard-rock environments, groundwater often moves through fractures, joints and faults rather than through the solid rock itself.

A borehole that intersects a connected fracture system may produce useful quantities of water. Another borehole nearby may produce little water if it misses the productive fractures.

Fracture connectivity is therefore an important factor in hard-rock groundwater development.

5.3 Weathered-rock aquifers

Weathering gradually alters rock near the ground surface and can create openings that store and transmit water.

Some weathered zones provide useful groundwater storage, particularly where they connect to deeper fractures.

The thickness, composition and permeability of the weathered material determine its contribution to borehole yield.

5.4 Unconfined aquifers

An unconfined aquifer has a water table as its upper boundary.

The water level can rise when groundwater recharge increases and fall when water is withdrawn or recharge declines.

Boreholes tapping unconfined aquifers may respond to seasonal rainfall patterns, nearby pumping and changes in land use.

5.5 Confined aquifers

A confined aquifer is overlain by a relatively low-permeability layer that restricts vertical water movement.

Groundwater within the formation is under pressure. When a borehole penetrates the aquifer, water may rise inside the casing above the top of the water-bearing formation.

If the hydraulic head is sufficiently high, water may flow naturally at the surface.

The behaviour of a confined aquifer depends on its pressure conditions, recharge, hydraulic properties and groundwater abstraction.

6. WHAT HAPPENS WHEN DRILLING REACHES A WATER-BEARING FORMATION?

During drilling, the contractor encounters different soil and rock layers.

Some formations may be dry or contribute very little water. Others may release groundwater into the borehole.

When a water-bearing formation is encountered, water may enter through permeable material or fractures, depending on the local geology.

The amount of water entering depends on:

1. The permeability of the formation.

2. The thickness of the water-bearing interval.

3. The connectivity of pores or fractures.

4. The hydraulic gradient.

5. The groundwater level and pressure.

6. The effectiveness of the completed borehole intake.

A water strike is an important drilling observation, but it is not a guarantee of high sustainable yield.

A formation may produce an initial flow yet be unable to sustain the intended pumping rate. Conversely, a formation with modest initial inflow may perform adequately under a lower pumping rate.

The completed borehole should be properly developed and tested before the pump and operating schedule are finalised.

7. WHAT HAPPENS WHEN THE WATER LEVEL RISES INSIDE A NEW BOREHOLE?

After drilling penetrates a water-bearing formation, groundwater may flow into the borehole and cause the water level to rise.

The resulting level depends on the hydraulic head of the formation and the conditions surrounding the borehole.

In an unconfined aquifer, the water level generally reflects the local water table, although the borehole may require time to stabilise.

In a confined aquifer, water may rise above the top of the water-bearing formation because of pressure.

The depth from a defined reference point, usually the top of the casing, to the water surface is recorded as the water-level depth.

For example, if the water surface is 25 metres below the measuring point, the water-level depth is recorded as 25 metres.

The measurement should be taken after sufficient stabilisation and with due consideration of any recent pumping.

A water level of 25 metres does not mean the borehole is 25 metres deep. Borehole depth and water-level depth are separate measurements.

A borehole could be 100 metres deep while its water surface is only 25 metres below the casing reference point.

8. WHAT HAPPENS WHEN A SUBMERSIBLE PUMP STARTS?

When the pump starts, the motor drives the impellers, which add energy to the water and push it into the rising main.

Water begins travelling towards the surface, and the quantity stored inside the borehole decreases unless groundwater enters at the same rate.

The water level normally drops during pumping. This decline is called drawdown.

Drawdown is calculated as:

Where:

• \(s\) is drawdown in metres.

• \(d_p\) is pumping water-level depth.

• \(d_s\) is static water-level depth.

For example, suppose the static water level is 22 metres below the measuring point. During pumping, the level falls to 37 metres.

The drawdown is:

The measured drawdown is 15 metres.

This figure must be considered alongside the pumping rate and duration. A drawdown of 15 metres may be acceptable in one borehole and problematic in another.

If the pump withdraws water faster than the aquifer can replenish the borehole at that rate, the water level may continue falling.

If the pumping water level approaches the pump intake, the pump may experience inadequate submergence, reduced cooling or loss of performance. Suitable protection equipment should be installed to help prevent dry running.

9. WHAT HAPPENS WHEN THE PUMP STOPS?

When the pump stops, water is no longer being removed through the rising main.

Groundwater may continue moving towards the borehole, allowing the water level to rise again. This process is called recovery.

The recovery rate depends on the aquifer's hydraulic properties, the pumping rate, the duration of pumping and the surrounding groundwater conditions.

A rapidly recovering borehole may have favourable hydraulic characteristics, but recovery speed alone does not establish the sustainable yield.

Similarly, slow recovery does not automatically prove that a borehole is damaged.

To assess performance, a professional should record the pumping rate, pumping water level, duration of pumping and water-level recovery over time.

These observations can help establish whether the borehole can reliably supply the intended demand.

10. WHY DOES ONE BOREHOLE PRODUCE MORE WATER THAN ANOTHER?

Borehole yield varies because underground geological conditions are rarely identical from one location to another.

One borehole may intersect a highly permeable aquifer or a connected fracture system. Another may penetrate compact rock with few connected openings.

Important factors include:

Aquifer permeability: Determines how easily groundwater can move through the formation.

Aquifer thickness: Influences the available flow area and storage characteristics.

Fracture connectivity: Determines whether groundwater can move through connected openings in hard rock.

Groundwater recharge: Influences how water is replenished over time.

Borehole construction: Poor screen design, damaged casing or inadequate development can restrict water entry.

Pumping rate: Excessive abstraction can cause large drawdowns and reduce supply reliability.

Nearby boreholes: Other users drawing from the same connected aquifer may affect groundwater levels.

Drilling deeper does not automatically increase yield. The important consideration is whether the additional depth intersects a productive formation and whether that formation can sustain the intended abstraction rate.

11. HOW TO CHECK WHAT IS HAPPENING INSIDE AN EXISTING BOREHOLE

A reliable borehole assessment combines measurements, equipment inspection and construction records.

The main checks include:

1. Static water level: Measure the depth to water when the borehole is not pumping and the level has stabilised.

2. Pumping water level: Measure the depth to water while the pump operates at a known discharge rate.

3. Discharge rate: Measure the volume of water delivered over a defined period.

4. Recovery: Record the rise in water level after the pump stops.

5. Electrical condition: Check the supply voltage, motor current and protection equipment using appropriate professional procedures.

6. Water quality: Investigate changes in colour, turbidity, smell or other characteristics, and arrange laboratory analysis where necessary.

7. Borehole construction: Review the drilling log, casing, screen arrangement, total depth and previous pumping-test results.

Together, these measurements help distinguish between groundwater-supply limitations, pump faults, sediment problems and water-quality concerns.

Never enter a borehole to inspect it. Boreholes present serious confined-space, drowning, entrapment and electrical hazards. Internal inspections should use suitable professional equipment and safe procedures.

12. CONCLUSION

A borehole provides access to an underground groundwater system. Water enters through permeable formations or fractures, collects inside the borehole and can be pumped to the surface.

Its performance depends on geology, aquifer properties, groundwater availability, construction quality and the way the pump is operated.

Water levels change during pumping and recovery. Sediment and minerals may affect the borehole or its equipment, while poor construction can increase the risk of contamination and structural problems.

The most reliable approach to borehole management is to use accurate measurements, appropriate pump selection, professional construction, regular maintenance and water-quality testing.