Drilling a borehole creates a hole in the ground — but it does not automatically create a functioning well. Before a borehole can be put into productive use, it must be developed. Borehole development is the process of removing fine particles, drilling debris, and damaged formation material from the immediate vicinity of the borehole wall and screen. Done properly, it increases hydraulic conductivity, improves water clarity, and maximises the long-term yield of the well.

Why Development Matters

During drilling, the borehole wall becomes clogged with drill cuttings, mud filtrate, and disturbed formation material. In rotary mud drilling, a filter cake forms against the formation that actively reduces water inflow. Even in air-drilled boreholes, fine particles migrate into the gravel pack and screen slots during construction. If these materials are not removed, the borehole will deliver less water than the aquifer is capable of providing, and it will deteriorate faster over its operational life.

Development works by reversing or alternating the flow of water across the borehole screen, dislodging fine particles and carrying them to the surface for removal.

1. Surging

Surging is one of the oldest and most effective development methods. It involves rapidly moving a tight-fitting piston or surge block up and down inside the borehole casing. The back-and-forth motion creates alternating pressure surges that push and pull water through the screen and gravel pack, loosening fine material and drawing it into the borehole where it can be bailed or pumped out.

Surge blocks can be solid (closed-face) or valved (open-face). Valved surge blocks allow water to pass through on the downstroke, intensifying the surging action on the upstroke. The process is repeated across different depth intervals, progressively working from the bottom of the screen upward.

Surging is particularly effective in unconsolidated formations such as alluvial sands and gravels. It is low-cost, requires no specialised equipment beyond the surge block and a cable or drill string, and can be highly effective when carried out systematically with regular bailing between surge cycles.

2. Jetting

Jetting uses high-velocity water directed through a nozzle to break up bridging and compaction around the borehole screen. The jetting tool is lowered into the borehole and rotated while water is pumped at high pressure through nozzles aimed horizontally at the screen perforations and surrounding gravel pack.

The hydraulic force disrupts fine particle bridges that have formed across screen slots and penetrates further into the formation than surging alone. Jetting is often combined with simultaneous pumping from the borehole, so that dislodged material is immediately removed rather than allowed to resettle.

This method is especially useful in boreholes with fine-grained formations where surging may not be aggressive enough, and in rehabilitation of older boreholes where screen blockage is a primary problem. The equipment requirements are more substantial — a high-pressure pump, rotary jetting tool, and swivel assembly — but the results in problem formations are often superior.

3. Air Lifting

Air lifting introduces compressed air directly into the borehole below the water table. The air mixes with water, creating a buoyant air-water mixture that rises rapidly to the surface, drawing formation water behind it and generating a vigorous flow across the screen and into the borehole.

The surge created by intermittent air injection — pumping air in pulses rather than continuously — is particularly effective at dislodging fine material. Air lifting can be combined with a swabbing tool or with jetting for a more aggressive development programme.

Air lifting has a major practical advantage: it does not require submersible pump equipment in the hole during development, making it simple to observe and control. It is widely used in air-drilled boreholes where a compressor is already on-site from the drilling operation, making it a cost-efficient first development step.

Combining Methods for Best Results

In practice, the most effective borehole development programmes combine two or more of these methods. A typical sequence might begin with air lifting to clear the bulk of drilling debris, progress to surging to break up compacted zones, and finish with jetting to clear stubborn screen blockages. Between each stage, the borehole is pumped to remove mobilised fines, and the turbidity of the discharged water is monitored. Development is considered complete when the water runs clear and the yield stabilises.

The investment in thorough borehole development directly determines the operational performance and lifespan of the well. Skipping or shortcutting this phase is a common and costly mistake.

A borehole that underperforms its potential is a frustrating and costly outcome. Whether caused by poor construction, inadequate development, or suboptimal design, low yield can often be improved — sometimes dramatically — through targeted optimisation techniques. Understanding these methods allows project managers and hydrogeologists to extract maximum performance from an existing asset before resorting to the expense of drilling a new well.

Understanding Yield Limitations

Before applying any optimisation technique, it is essential to diagnose the root cause of poor yield. The two primary categories are:

• Aquifer limitations — the formation genuinely cannot supply more water, due to low permeability, poor recharge, or over-abstraction in the area.
• Borehole inefficiency — the aquifer has greater capacity than the borehole is accessing, due to screen blockage, poor development, inadequate penetration, or skin damage around the borehole wall.

Distinguishing between these two causes requires careful analysis of pumping test data, step drawdown tests, and specific capacity measurements. Optimisation techniques are effective primarily in the second category.

1. Enhanced Development

The most straightforward yield optimisation measure is completing or intensifying the borehole development process. Many boreholes are under-developed — either because the initial development programme was too short, or because fine material has continued to migrate into the screen zone after commissioning.

Repeating or extending development using surging, jetting, or air lifting can remove residual formation damage and improve hydraulic connectivity between the aquifer and the borehole. In many cases, a single day of aggressive redevelopment can increase specific capacity by 20–50%.

2. Acidisation

In limestone, dolomite, and other carbonate formations, weak acid (typically hydrochloric acid at 10–15% concentration) can be injected into the borehole under pressure. The acid dissolves carbonate minerals along fractures and in the pore matrix near the borehole wall, enlarging flow pathways and removing calcite encrustation that may be blocking natural fractures.

Acidisation is a standard technique in oil and gas well stimulation and is increasingly applied in water well engineering where geology permits. It requires careful handling of hazardous chemicals, neutralisation and disposal of spent acid, and thorough post-treatment flushing before the borehole is returned to service.

3. Hydrofracturing (Hydraulic Fracturing)

Hydrofracturing involves injecting water at high pressure into a sealed section of the borehole to create or extend fractures in hard rock formations. The process uses packer equipment to isolate a target zone, then pressurises that zone beyond the fracture threshold of the rock, propagating new fractures outward into the aquifer.

This technique is most effective in crystalline basement rocks — granites, gneisses, and quartzites — where groundwater is stored in fractures rather than pores. Hydrofracturing does not create water where none exists, but it can dramatically improve connectivity between the borehole and existing water-bearing fractures. Success rates vary, but yield improvements of two to ten times are documented in suitable geological settings.

4. Screen Replacement or Extension

Where yield limitations are linked to inadequate screen length, blocked screen slots, or a screen positioned in the wrong zone, mechanical intervention may be necessary. In some designs, the screen can be extended downward to penetrate a deeper productive zone, or replaced with a screen of larger open area or more appropriate slot size for the formation.

This is an intrusive and relatively expensive intervention but can be justified where the aquifer potential clearly exceeds what the current construction allows.

5. Pump Optimisation

Yield is not solely a function of the aquifer and borehole — the pump selection and drawdown management also play a role. Operating a pump at a rate that exceeds the safe yield causes excessive drawdown, air entrainment, and pump damage. Conversely, a correctly sized pump that maintains drawdown within the optimal range maximises sustained yield without stressing the system.

Variable speed drives (VSDs) allow pump output to be matched dynamically to aquifer response, improving overall efficiency and reducing wear. In boreholes with moderate yields, step-pumping regimes — cycling between pumping and rest periods — can deliver more total water over a day than continuous pumping at a rate the aquifer cannot sustain.

Long-Term Perspective

Yield optimisation is most successful when combined with ongoing monitoring. Regular measurement of rest water levels, pumping water levels, and specific capacity provides early warning of declining performance and allows corrective action before problems become severe. The best-performing boreholes are those that are actively managed, not simply installed and forgotten.

Groundwater drawn from a well-constructed borehole is generally of good microbiological quality. However, the process of drilling, development, and installation inevitably introduces bacteria, organic material, and surface contamination into the borehole. Before any water is used for human consumption, the borehole must be thoroughly disinfected. Disinfection is not a substitute for good construction — it is a final safeguard that ensures the borehole is bacteriologically safe at the point of commissioning.

When Disinfection is Required

Disinfection is mandatory in the following circumstances:

• After initial construction — before any first use of the borehole.
• After any maintenance or repair work — whenever tools, pumps, or materials have been introduced into the borehole.
• After rehabilitation — following redevelopment or screen replacement.
• After a period of non-use — boreholes left idle for extended periods can develop bacterial populations.
• After suspected contamination — if a positive coliform test or other microbiological indicator is detected.

Disinfection Agent: Chlorine

Chlorine is the standard disinfection agent for boreholes, applied either as:

• Sodium hypochlorite (liquid bleach) — typically 5–12% available chlorine, widely available and easy to handle.
• Calcium hypochlorite (granular or tablet form) — typically 65–70% available chlorine, more concentrated and easier to store and transport.

The target is to achieve a free residual chlorine concentration of at least 50 mg/L (50 ppm) throughout the borehole water column, which is sufficient to kill the vast majority of waterborne pathogens including E. coli, Salmonella, and enteric viruses.

Step-by-Step Disinfection Procedure

Step 1: Calculate the Volume of Water in the Borehole

Determine the volume of standing water in the borehole using the casing diameter and the depth from the water table to the bottom of the casing. The formula is:

Volume (litres) = π × (radius in metres)² × water depth (metres) × 1000

Step 2: Calculate the Required Chlorine Dose

To achieve 50 ppm free chlorine, calculate the mass of active chlorine needed based on the water volume. Account for the concentration of the chlorine product being used.

For example: achieving 50 ppm in 500 litres requires 25 grams of active chlorine. If using 70% calcium hypochlorite, this equates to approximately 36 grams of product.

Step 3: Prepare and Introduce the Chlorine Solution

Dissolve the calculated quantity of chlorine in a bucket of clean water before introducing it into the borehole. Pour or pump the solution down the full depth of the borehole, ensuring distribution throughout the water column. Lower the solution along the borehole walls to treat the casing surfaces.

Step 4: Circulate and Agitate

Surge the borehole using the pump or a bailer to circulate the chlorinated water throughout the water column and into the gravel pack and screen zone. The chlorine solution must make contact with all surfaces.

Step 5: Contact Time

Allow the chlorine solution to remain in contact with the borehole for a minimum of 12 hours, and preferably 24 hours. Do not pump or use the borehole during this period.

Step 6: Flush the Borehole

After the contact period, pump the borehole to waste until the chlorine smell dissipates and a field test confirms residual chlorine has dropped to below 0.5 mg/L. Do not discharge the chlorinated water into watercourses or in quantities that could damage vegetation.

Step 7: Bacteriological Testing

Following flushing, collect a water sample for laboratory bacteriological analysis. The borehole should not be brought into service until test results confirm the absence of total coliforms and E. coli.

Special Considerations

In boreholes with confirmed iron or manganese bacteria (Gallionella or Leptothrix species), standard chlorine disinfection may need to be preceded by a shock treatment with higher concentrations. Biofouling of this type is resistant to normal doses.

Where a pump is installed, the pump and rising main must be removed or treated in situ to ensure all equipment surfaces are disinfected along with the borehole.

Documentation

A disinfection record should be maintained for every borehole, noting the date, chlorine product and dose used, contact time, flushing time, and the results of post-disinfection bacteriological testing. This record forms part of the borehole completion documentation and is essential for ongoing compliance and quality assurance.

When the drilling rig packs up and leaves a site, it takes with it the only opportunity to directly observe the subsurface. What remains is whatever was recorded during the drilling process. A thorough completion report and borehole log are the permanent record of that observation — the foundational documents for every decision that will be made about the borehole for the rest of its life. Their importance cannot be overstated, yet they are frequently neglected or poorly executed.

What is a Borehole Completion Report?

A borehole completion report is a comprehensive technical document prepared at the end of the drilling and construction process. It compiles all information gathered during site investigation, drilling, construction, development, and testing into a single permanent record. It serves as:

• A technical reference for the borehole owner and operator.
• A legal document for regulatory submission and licence compliance.
• A baseline dataset for future maintenance, rehabilitation, and monitoring.
• A contribution to regional groundwater knowledge.

National regulations in many countries require submission of borehole completion reports to the relevant water authority. Even where it is not legally mandated, producing a complete report is considered best practice.

Contents of a Completion Report

A well-structured borehole completion report should contain the following elements:

Project Information

• Client name and contact details
• Site location (coordinates, address, land parcel reference)
• Project purpose and intended use
• Drilling contractor name and licence number
• Supervising hydrogeologist or engineer

Site Description

• Surface elevation and topographic context
• Land use and potential contamination sources in the vicinity
• Access and site conditions at the time of drilling

Drilling Information

• Drilling method and rig type
• Drilling start and completion dates
• Total depth drilled
• Drilling fluid type and volumes used
• Bit sizes used at each stage

Borehole Construction Details

• Casing diameter, material, and depths
• Screen type, slot size, and installed depths
• Gravel pack specification and placement depths
• Grouting and sealing details
• Wellhead construction and protection measures

Formation Log (Lithological Log)

• Detailed description of rock or soil samples collected at regular intervals during drilling
• Formation tops and depths
• Notes on colour, texture, grain size, hardness, fracturing, and water strikes

Geophysical Logs (if conducted)

• Natural gamma, resistivity, caliper, or other downhole geophysical logs
• Interpreted formation boundaries and aquifer zones

Development Summary

• Methods used, duration, and observations
• Volume of water pumped to waste
• Turbidity measurements over time

Pumping Test Results

• Step drawdown test data and analysis
• Constant rate test data, drawdown curves, and recovery
• Recommended sustainable yield
• Transmissivity and storage coefficient estimates

Water Quality Results

• Physical parameters (colour, turbidity, odour)
• Chemical analysis results
• Bacteriological test results
• Any treatment recommendations

Disinfection Record

• Date, method, chlorine dose, contact time, and post-disinfection test results

The Lithological Log

The lithological (or geological) log is the heart of the borehole record. It is a graphical and written column showing the sequence of formations encountered from surface to total depth. Prepared by examining drill cuttings (chips of rock and soil brought to the surface by the drilling fluid) at regular intervals — typically every metre — it documents:

• Rock type and formation name (where known)
• Colour and weathering state
• Grain size and sorting (for sedimentary materials)
• Fracture frequency and orientation (for hard rock)
• Depth of water strikes and estimated yield at each strike
• Any drilling observations (rate of penetration changes, loss of circulation, colour changes in return water)

The lithological log is most accurate when cuttings are examined and described by a trained geologist on-site in real time. Post-drilling reconstruction from memory or incomplete notes produces unreliable records.

Borehole Construction Diagram

Alongside the lithological log, the completion report should include a scaled construction diagram showing the full depth of the borehole with all casing, screen, gravel pack, and grouting zones clearly indicated. This drawing is indispensable for future maintenance and rehabilitation work — knowing exactly where the screen is, how deep the pump can be set, and where grout seals are located saves time and prevents costly mistakes.

Filing and Archiving

Completion reports should be provided to the client in hard copy and digital format. Copies should be submitted to the relevant water authority and retained by the drilling contractor. Digital archiving in a georeferenced database greatly increases the long-term value of the data, allowing regional hydrogeological analysis as the dataset grows over time.

A completion report is only valuable if it can be found and used when needed. Proper filing and archiving is as important as the quality of the data it contains.