Two of the most common and insidious processes that reduce borehole performance over time are incrustation and biofouling. Both are natural phenomena driven by the chemistry and biology of groundwater, and both are manageable — but only if understood and addressed systematically. Left unchecked, they can reduce yield, degrade water quality, and ultimately cause irreversible blockage of screens and gravel packs.
Incrustation: Mineral Build-Up
Incrustation is the deposition of mineral solids on borehole screens, casing walls, pump components, and rising mains. It occurs when changes in pressure, temperature, or chemical equilibrium cause dissolved minerals to precipitate out of solution.
Iron and manganese incrustation is the most common type in groundwater systems. Groundwater often contains dissolved iron (Fe²⁺) and manganese (Mn²⁺) in anaerobic conditions. When this water is pumped and comes into contact with oxygen — at the screen face, in the pump, or in the distribution system — the iron and manganese oxidise and precipitate as reddish-brown or black solids. These deposits coat screen slots and gravel packs, progressively restricting flow.
Carbonate incrustation (calcium and magnesium carbonate) forms when hard water experiences a drop in CO₂ partial pressure, driving carbonate precipitation. This is most pronounced near the pump intake and in rising mains, where pressure changes are greatest.
Silica scaling is less common but can occur in certain geochemical environments, producing hard, glassy deposits that are particularly resistant to treatment.
Biofouling: Biological Clogging
Biofouling is the growth of microbial communities — primarily bacteria — within the borehole, screen, and gravel pack. While all groundwater contains bacteria, certain species thrive in the conditions created by pumping and can cause significant operational problems.
Iron-related bacteria (IRB) — principally Gallionella ferruginea and Leptothrix ochracea — oxidise dissolved iron to form gelatinous iron hydroxide sheaths as a metabolic byproduct. These sheaths accumulate rapidly, forming thick, slimy deposits that trap other particles and clog screens far faster than purely chemical iron precipitation would.
Sulphate-reducing bacteria (SRB) thrive in anaerobic conditions and produce hydrogen sulphide as a metabolic byproduct — responsible for the rotten egg smell occasionally encountered in borehole water. They also produce corrosive conditions that accelerate metal casing and screen deterioration.
Slime-forming bacteria produce extracellular polymeric substances (EPS) that form sticky biofilms on surfaces. These films trap fine particles, further compounding clogging.
Recognising the Problem
Signs of incrustation and biofouling include:
- Declining specific capacity without corresponding change in regional water levels
- Red, brown, or black discolouration of pumped water
- Slime or sediment deposits in storage tanks and distribution pipework
- Sulphurous odour in the water
- Rapid pump wear due to abrasive particles
- CCTV inspection revealing coated or partially blocked screen slots
Prevention and Control Strategies
Regular Redevelopment
The most effective prevention strategy is regular mechanical redevelopment — surging, jetting, or air lifting — before accumulations become severe. Redevelopment at 1–2 year intervals (depending on the severity of the chemistry and biology of the specific aquifer) dislodges material before it has time to consolidate into hardened deposits.
Shock Chlorination for Biofouling
Periodic shock chlorination — introducing a high-concentration chlorine dose and allowing extended contact time — is effective in controlling IRB and other biofilm-forming organisms. The recommended approach for active biofouling uses concentrations of 200–500 mg/L free chlorine, held for 12–24 hours, followed by vigorous mechanical development and thorough flushing.
A single treatment is rarely sufficient. Biofouling management typically requires repeated treatment cycles until bacterial populations are suppressed, followed by a regular maintenance chlorination schedule (typically annually or biannually).
Acid Treatment for Chemical Incrustation
Hydrochloric acid (HCl) at 5–15% concentration is effective in dissolving iron oxide, manganese oxide, and carbonate deposits. The acid is introduced in a targeted dose calculated to the volume of the treated zone, allowed to react, and then purged completely. In heavily incrustated boreholes, multiple acid treatment cycles may be required.
Acid treatment must be followed immediately by mechanical development to remove dissolved material and by thorough flushing before return to service. Careful handling, neutralisation, and disposal of spent acid are mandatory.
Polyphosphate Sequestration
In boreholes with moderate iron concentrations where incrustation is a persistent maintenance issue, continuous or periodic dosing of polyphosphate can sequester dissolved iron and manganese, keeping them in solution and preventing precipitation. This does not treat existing deposits but helps prevent new ones forming. It must be used cautiously in drinking water applications, as polyphosphate doses must remain within acceptable limits.
Monitoring for Early Detection
The most cost-effective approach to managing incrustation and biofouling is detecting them early. Quarterly measurement of specific capacity (yield per unit drawdown) and regular turbidity monitoring will reveal the onset of clogging before it becomes severe. When specific capacity falls more than 25% below the baseline established at commissioning, investigation and treatment should begin promptly.
