Category Archives: Raeli Hydro Insights

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.

 

 

 

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.

 

 

 

Water level is the single most informative measurement that can be made in a borehole. It integrates the behaviour of the aquifer, the performance of the borehole, and the effects of pumping into one continuous, measurable parameter. A well-maintained water level dataset is the foundation of informed borehole management — enabling early detection of problems, supporting abstraction licence compliance, and building the long-term understanding of aquifer behaviour that underpins sustainable groundwater use.

What Water Level Data Tells Us

Rest Water Level (Static Level)

The rest water level — measured when the pump has been off for a sufficient period for the borehole to fully recover — reflects the ambient pressure head of the aquifer at that location. Changes in rest level over time reveal:

• Seasonal recharge patterns: Rising levels in wet seasons, falling levels in dry seasons.
• Long-term aquifer trends: Persistent decline may indicate over-abstraction or reduced recharge from climate or land-use change.
• Regional pumping effects: Nearby high-volume abstraction can depress local water levels.

Pumping Water Level (Dynamic Level)

The pumping water level — measured during active pumping — combined with the rest water level gives the drawdown: the depth the water level is depressed below rest during pumping. Drawdown analysis reveals:

• Borehole efficiency: A widening drawdown at a constant pumping rate indicates increasing resistance in the borehole or pump system.
• Aquifer transmissivity: The rate and shape of drawdown development during a pumping test characterises aquifer hydraulic properties.
• Safe yield limits: The maximum drawdown before the pump intake is exposed or the yield becomes unstable defines the operational limits of the borehole.

Manual Water Level Measurement

The simplest and most widely used method for measuring water level is the electric contact dipper (also called a water level meter or e-dipper). This is a graduated cable with a probe at the end that completes an electrical circuit and triggers an audible or visual signal when it contacts water. The depth reading is taken from the cable graduation at the top of the casing.

Manual measurement is low-cost, reliable, and requires no power. It is the standard method for periodic monitoring visits. For accurate and comparable results:

• Always measure from the same reference point (typically the top of the casing).
• Allow sufficient rest time before measuring static levels (typically 4–24 hours after pumping stops, depending on the aquifer).
• Record the date, time, and pumping status alongside each measurement.

Continuous Data Logging

For boreholes where detailed, high-resolution water level data is required — operational wells, aquifer monitoring points, or research boreholes — continuous data loggers are installed. These devices measure and record water level at defined intervals (typically every 15–60 minutes) without human intervention, providing a complete record of aquifer behaviour over time.

Types of Data Loggers

Pressure transducer loggers are the most common type. A sealed pressure sensor is suspended below the water surface at a fixed depth. It measures the pressure exerted by the column of water above it, which is converted to water depth. Vented loggers compensate for atmospheric pressure changes automatically; non-vented loggers require barometric correction during data processing.

Float-operated loggers use a float and pulley to mechanically track the water surface. They are reliable and easy to understand but less accurate in deep or narrow boreholes and more susceptible to mechanical failure.

Shaft encoder loggers are used with a float and counterweight system and record float position electronically. They are well-suited to large-diameter wells with significant water level variation.

Data Storage and Retrieval

Most loggers store data internally on flash memory and are downloaded periodically via a data cable or Bluetooth connection to a laptop or field device. Increasingly, loggers are equipped with GSM or satellite telemetry that transmits data in real time to a remote server or cloud platform, enabling continuous remote monitoring without site visits.

Data Management and Analysis

Raw water level data is only useful when it is properly managed and analysed. Best practice includes:

• Consistent naming and units: All data files should be named with the borehole identifier, date, and measurement units clearly recorded.
• Barometric correction: Non-vented logger data must be corrected for atmospheric pressure variation before analysis.
• Plotting time series: Graphing water level against time reveals seasonal patterns, event responses, and long-term trends that are invisible in tables of numbers.
• Specific capacity calculation: Dividing yield by drawdown at regular intervals tracks borehole efficiency over time.
• Anomaly investigation: Sudden step changes, unexplained fluctuations, or persistent declining trends should trigger field investigation.

Integration with Abstraction Compliance

In regulated environments, water level monitoring is often a condition of the abstraction licence. Operators must demonstrate that abstraction is not exceeding sustainable limits, and water level data provides the evidence. Long-term datasets submitted to regulatory authorities also contribute to the regional groundwater monitoring networks that underpin basin-scale water resource management.

Building the Long-Term Record

The true value of water level monitoring increases with time. A single year of data shows seasonal behaviour; a decade of data reveals long-term trends and aquifer response to drought; multiple decades provide the statistical basis for climate adaptation planning. Starting a monitoring programme — even a simple manual measurement schedule — immediately after commissioning is one of the highest-value investments a borehole owner can make.

 

 

 

A borehole that produced abundant water at commissioning may, years later, struggle to meet demand. Yield decline is one of the most common and consequential problems affecting operational boreholes — and one of the most frequently misdiagnosed. Understanding whether declining yield is a borehole problem, an aquifer problem, or a combination of both is essential for determining the appropriate response and for making sound long-term water resource decisions.

Two Distinct Causes of Yield Decline

1. Borehole Deterioration

Borehole-related yield decline occurs when the hydraulic connection between the aquifer and the borehole is diminished — even though the aquifer itself retains its original capacity. Causes include:

• Screen blockage from incrustation, biofouling, or fine sediment accumulation.
• Gravel pack compaction or clogging reducing radial flow to the screen.
• Formation skin damage — a zone of reduced permeability around the borehole wall resulting from drilling damage that was never fully removed during development.
• Pump wear — a deteriorated pump produces less head and flow even if the aquifer is fully capable.

Borehole-related decline is characterised by increasing drawdown at the same pumping rate, declining specific capacity, and stable or rising rest water levels. It is potentially reversible through rehabilitation.

2. Aquifer Depletion

Aquifer-related yield decline occurs when the groundwater resource itself is diminishing. This may result from:

• Over-abstraction — pumping consistently exceeding aquifer recharge rates.
• Regional groundwater depletion — widespread intensive pumping by many users collectively exceeding basin-scale recharge.
• Reduced recharge — changes in rainfall, land use, or surface hydrology reducing the amount of water entering the aquifer.
• Seasonal stress — normal seasonal water table fluctuation in unconfined aquifers, which may be more pronounced during drought years.

Aquifer depletion is characterised by declining rest water levels — the water table or piezometric surface is falling even before pumping begins. This is a more serious condition than borehole deterioration, as it may not be reversible on the timescale of the borehole’s operational life.

Monitoring for Yield Decline

Specific Capacity Tracking

Specific capacity (pumping rate divided by drawdown) is the most sensitive indicator of borehole performance. It is calculated from regular measurements of pumping rate and pumping water level at a consistent, defined time after pump start. A declining specific capacity trend — even if yield in absolute terms appears acceptable — is an early warning of borehole deterioration.

Specific capacity should be calculated and recorded at least quarterly, plotted over time, and compared to the baseline established during the commissioning pumping test.

Rest Water Level Trends

Regular measurement of the rest water level (before pumping begins) tracks the aquifer’s long-term response to abstraction and climate. Persistent, multi-year decline in rest levels — particularly if observed in neighbouring monitoring boreholes — points to aquifer-scale depletion rather than a problem specific to the individual borehole.

Regional water level data from government monitoring networks, where available, provides essential context for interpreting trends in a single borehole.

Annual Pumping Tests

Conducting a formal pumping test — ideally a step drawdown test and a constant rate test — on an annual or biennial basis allows direct comparison with the original commissioning test. Changes in aquifer parameters (transmissivity, storage coefficient) estimated from successive tests can reveal whether aquifer properties are changing, which would indicate structural depletion rather than just a seasonal or borehole-related effect.

Distinguishing Borehole from Aquifer Problems

The diagnostic question — is this a borehole problem or an aquifer problem? — is answered by comparing two key measurements:

1. If rest water levels are stable but specific capacity is declining: The problem is in the borehole (screen, gravel pack, or pump). Rehabilitation is the appropriate response.
2. If rest water levels are declining and specific capacity is also declining: Both the borehole and the aquifer are under stress. Rehabilitation may improve borehole efficiency, but the underlying water resource issue must also be addressed.
3. If rest water levels are declining but specific capacity is stable: The aquifer is depleting but the borehole itself is functioning normally. Abstraction reduction, alternative source development, or artificial recharge may be needed.

Responding to Aquifer Depletion

Where aquifer depletion is confirmed, the range of responses spans from operational adjustments to policy-level interventions:

• Reducing abstraction rate to bring it within the estimated sustainable yield of the aquifer.
• Adjusting pumping schedules — pumping during off-peak hours and allowing extended rest periods can improve recovery.
• Deepening the borehole to access a deeper, more productive aquifer horizon — though this requires drilling and regulatory approval.
• Artificial recharge schemes — directing surface water or treated wastewater to recharge basins or injection wells to supplement natural recharge.
• Demand management — reducing consumption through efficiency measures to bring demand within the sustainable supply.

In basins where multiple users share an aquifer, coordinated abstraction management through a water user association or regulatory authority may be the only effective long-term solution.

The Value of Early Intervention

Both borehole deterioration and aquifer depletion are progressive processes. Early detection — made possible only by consistent monitoring — allows intervention before problems become irreversible. A borehole that is rehabilitated at 75% of its original specific capacity is far easier and cheaper to restore than one allowed to reach 25%. An aquifer managed proactively within its sustainable yield provides reliable supply indefinitely; one allowed to deplete may take decades or longer to recover.

 

 

 

Borehole drilling projects sit at the intersection of technical complexity, financial risk, and operational urgency. They involve significant upfront investment, uncertain subsurface conditions, specialised contractors, and outcomes that directly affect communities or businesses depending on water supply. Managing these projects well requires both technical competence and sound project management discipline — from the earliest planning stages through to commissioning and handover.

The True Cost of a Borehole

A common mistake in borehole project planning is equating the cost of drilling with the total cost of the project. Drilling is typically only 40–60% of the total investment required to deliver a functioning water supply. The full cost picture includes:

• Pre-drilling investigations: Hydrogeological survey, geophysical survey, and site selection studies.
• Permitting and licensing: Application fees, environmental assessment costs, and legal advisory fees.
• Drilling and construction: Rig mobilisation, drilling, casing, screen, gravel pack, grouting, and wellhead construction.
• Borehole development and testing: Development, pumping tests, and water quality analysis.
• Pump and equipment supply and installation: Submersible pump, rising main, cables, control panel, and surface discharge infrastructure.
• Civil works: Headworks, storage tank, reticulation pipework, concrete apron, and fencing.
• Commissioning and disinfection: Final water quality testing, disinfection, and performance verification.
• Project management and supervision: Hydrogeologist or engineer fees for supervision and reporting.
• Contingency: Reserve for unforeseen conditions or cost overruns (typically 10–20% of total budget).

Budgeting for the full scope from the outset prevents the frustrating — and unfortunately common — scenario where a borehole is drilled successfully but cannot be put into service because there is no budget for a pump.

Key Cost Drivers

Several factors have the greatest influence on borehole drilling costs:

Depth: Cost typically scales with depth, as more materials, time, and rig capability are required. Deeper boreholes in hard rock formations are disproportionately expensive.

Geology: Soft sedimentary formations drill quickly and cheaply; hard crystalline basement rocks (granite, gneiss) require more time and wear through more drill bits.

Diameter: Larger diameter boreholes accommodate larger pumps and higher yields but require more casing and screen material and wider borehole cutting, increasing cost.

Location and access: Remote sites with difficult road access increase rig mobilisation costs. Water for drilling must be transported to site if not available locally.

Hydrogeological risk: In areas of uncertain groundwater occurrence, there is a risk of drilling an unproductive or low-yield borehole — a cost that must be considered in the overall project economics.

Project Phases and Management Approach

A well-structured borehole project moves through clearly defined phases:

Phase 1 – Feasibility and Investigation: Hydrogeological and geophysical assessment, site selection, and preliminary yield and cost estimation. Output: a feasibility report and go/no-go decision.

Phase 2 – Design and Procurement: Preparation of borehole design specifications, tender documents, and drilling contract. Evaluation of contractor bids and contract award. Output: signed contract and mobilisation plan.

Phase 3 – Drilling and Construction: Active drilling, with daily supervision and reporting. Adaptive decision-making based on real-time geological observations. Output: completed borehole, development, and pumping test.

Phase 4 – Equipping and Commissioning: Pump installation, civil works, disinfection, and water quality testing. Output: operational borehole delivering water to the required standard.

Phase 5 – Handover and O&M Transition: Completion documentation, operator training, and establishment of the maintenance programme. Output: borehole formally handed over to the operator with a functioning maintenance plan.

Managing Cost Overruns

Cost overruns in borehole drilling projects most commonly arise from:

• Greater-than-anticipated depth required to reach a productive aquifer.
• Unexpected hard rock or lost circulation zones requiring additional materials and time.
• Multiple dry or low-yield holes in areas of complex hydrogeology.
• Changes in scope or design during drilling.

Mitigating these risks requires thorough pre-drilling investigation, a clearly defined drilling contract that allocates risk appropriately between client and contractor, and a contingency budget that reflects the actual risk level of the specific project. Day-rate contracts transfer geological risk to the client; fixed-price contracts transfer it to the contractor (who will typically price in a higher risk premium).

Monitoring Project Performance

Throughout the project, progress should be tracked against the original programme and budget. Key performance indicators include:

• Metres drilled per day vs. planned rate
• Cost per metre drilled vs. budget
• Milestone completion dates vs. planned dates
• Water quality results vs. required standards

Formal project reporting — daily drilling reports, weekly progress summaries, and milestone completion certificates — creates the accountability and documentation needed to manage contractor performance and protect the client’s interests.