Over time, even well-constructed and properly maintained boreholes experience declining performance. Screens become blocked, bacterial films colonise pump components, encrustation forms on casing walls, and physical deterioration of the borehole structure reduces hydraulic efficiency. Borehole rehabilitation is the process of restoring a deteriorated or failing well to acceptable operational performance — and in many cases, it is far more cost-effective than drilling a new borehole.
When is Rehabilitation Warranted?
Rehabilitation should be considered when one or more of the following is observed:
- Significant decline in specific capacity (yield per unit drawdown) compared to baseline measurements.
- Increasing turbidity or sand content in the pumped water.
- Deteriorating water quality — particularly rising iron, manganese, or bacterial counts.
- Visible corrosion, joint displacement, or collapse detected by CCTV inspection.
- Pump failure caused by abrasion from fine sediment entry.
- Extended period of non-use with no record of prior maintenance.
A thorough diagnosis precedes rehabilitation. CCTV inspection, pumping tests, and water quality analysis together define the nature and extent of the problem, which in turn determines the appropriate rehabilitation approach.
1. Mechanical Redevelopment
For boreholes where screen blockage or gravel pack compaction is the primary problem, mechanical redevelopment is the first-line technique. This involves surging, jetting, or air lifting — the same methods used in initial borehole development — applied more intensively to dislodge accumulated fines and restore hydraulic conductivity.
Mechanical redevelopment is low-cost and non-invasive. It is most effective when the screen and casing remain structurally intact and the problem is primarily accumulation rather than mineralogical encrustation.
2. Chemical Treatment
Where encrustation or biofouling is the dominant problem, chemical treatment is required to dissolve or disperse the blocking material.
Acid treatment (typically hydrochloric or sulfamic acid) dissolves carbonate, iron oxide, and manganese oxide encrustations that have formed on screen slots and in the gravel pack. The acid is introduced into the borehole in a calculated dose, allowed to react for a defined contact period, and then purged by pumping to waste.
Polyphosphate and dispersant treatments break up clay and silt bridges that have formed in fine-grained formations. They are gentler than acid treatment and suitable for formations or screen materials that would be damaged by acid.
Biocide treatment targets iron-related bacteria and other biofilm-forming organisms. Chlorine is the standard biocide for water wells, but in severe biofouling cases, more concentrated shock chlorination or specialist biocide products may be required.
Chemical treatments are most effective when followed immediately by mechanical redevelopment to remove the loosened or dissolved material before it can resettle.
3. High-Pressure Jetting
High-pressure jetting during rehabilitation uses the same principle as during initial development but at greater intensity and with more targeted application. Specialist jetting tools direct pressurised water at specific screen intervals identified as blocked by CCTV inspection, physically clearing slot obstructions and penetrating into the gravel pack.
When combined with simultaneous air lifting or pumping to remove dislodged material, high-pressure jetting can restore screen open area close to its original condition.
4. Liner Installation
Where CCTV inspection reveals structural damage to the casing — corrosion holes, joint failure, or partial collapse — the compromised section must be addressed to prevent contamination ingress and further deterioration. In many cases, a liner (a smaller diameter casing inserted inside the damaged section) can seal the defect without requiring full casing replacement.
Liner installation reduces the internal diameter of the borehole, which may affect pump sizing and yield. However, it is usually far less disruptive and expensive than casing replacement or redrilling.
5. Screen Replacement
If the screen is irreparably blocked or physically damaged — corroded through, collapsed, or encrusted beyond the reach of chemical and mechanical treatment — it may need to be replaced. This is a complex operation involving removal of the pump and rising main, extraction of the existing screen (where possible), and installation of a new screen assembly.
The feasibility of screen replacement depends heavily on the borehole construction and the depth and nature of the damage. In some cases, it is more cost-effective to seal the existing borehole and drill a new one adjacent to it.
6. Pump and Rising Main Replacement
In many rehabilitation scenarios, the borehole structure itself is sound but the pump and rising main have deteriorated to the point of failure. Pump replacement is a relatively straightforward operation that can restore full yield quickly, provided the borehole itself is performing adequately.
Before reinstalling a new pump following any rehabilitation work, the borehole should always be thoroughly redeveloped and disinfected to avoid contaminating the new equipment.
Post-Rehabilitation Assessment
Rehabilitation is complete only when the borehole has been tested and the results demonstrate improved performance. A pumping test conducted after rehabilitation, and compared against the original commissioning test data, quantifies the improvement achieved and establishes a new performance baseline. Water quality sampling confirms that chemical treatments have been fully flushed and that the borehole is bacteriologically safe before return to service.