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.
