Borehole Water Quality in The Gambia: Salinity, Iron and Testing

Yield is not the same as quality

When people discuss boreholes they talk about depth and yield. Both matter. Neither tells you whether the water is fit for what you intend to do with it.

Water that is perfectly good for construction, washing down and flushing may be unsuitable for drinking. Water that is fine for one crop may damage another. And the two most common problems in this country — salinity and iron — are both invisible in a bucket.

Salinity

Saline intrusion is a genuine constraint in parts of The Gambia, particularly nearer the coast and along stretches of the river where salt water penetrates inland. It varies by location and by depth, and it can vary between two boreholes that are not far apart.

For domestic use, salinity affects taste first and becomes unacceptable well before it becomes dangerous. For irrigation it is more serious: salt accumulates in the soil over repeated watering, and salt-sensitive crops will show stress long before you notice anything wrong with the water. Over several seasons, irrigating with marginal water can damage the soil itself.

Iron

Iron is common in groundwater across the region. It is not generally a health concern at the levels usually encountered, but it is a considerable nuisance: reddish-brown staining on sanitaryware and laundry, a metallic taste, and gradual deposition in pipework, pumps and irrigation emitters.

A useful indicator: water that comes out clear and then turns cloudy or orange after standing in a bucket for an hour is showing dissolved iron oxidising on contact with air. Iron is treatable — aeration and filtration are the usual approach — but the treatment needs sizing to the measured concentration rather than guessed at.

Bacteriological contamination

This is the one that actually makes people ill, and it is largely a construction and siting issue rather than a geological one.

Deep water in a confined aquifer is usually clean. It becomes contaminated when surface water finds a path down the outside of the casing, which is what a proper sanitary seal around the upper casing is there to prevent. Siting matters equally: adequate separation from septic tanks, soakaways, latrines and animal pens, and avoiding positions downgradient of them.

A shallow hand-dug well is far more exposed on both counts, which is the main reason a properly constructed borehole is safer, not simply the depth.

What to test, and when

• At completion, before the pump is finally sized and before you rely on the supply: salinity or electrical conductivity, iron, pH, and a bacteriological test.
• Before using the water for drinking, every time, regardless of how the borehole looks.
• Before planting a new crop under irrigation, if salinity was anywhere near marginal.
• Annually thereafter, and particularly at the end of the dry season when concentrations tend to be at their highest.
• Any time the water changes — taste, smell, colour, cloudiness or staining that was not there before.

Testing at the end of the dry season rather than shortly after the rains gives you the less flattering figure, which is the one worth planning against.

If the results are poor

There are usually options, and they are worth considering in order of cost.

1. Match the use to the water. Water unsuitable for drinking is often perfectly fine for washing, flushing, construction and salt-tolerant crops. Reserving the borehole for those uses may solve the problem at no cost.
2. Treat it. Iron responds well to aeration and filtration. Bacteriological contamination often traces to a defect in the sanitary seal or headworks that can be repaired.
3. Look at a different depth. Water quality can differ between aquifers on the same site, and in some cases a deeper or shallower target gives a materially better result.
4. Reposition. Where the problem is local contamination rather than regional salinity, moving the borehole away from the source can resolve it.
5. Supplement with rainwater harvesting. Roof catchment through the June-to-October rains, with adequate storage, is genuinely useful where groundwater is marginal.

Keeping a good borehole good

• Keep the wellhead sealed and the headworks sloped so surface water drains away from the casing rather than towards it.
• Do not allow animals, washing or fuel storage next to the wellhead.
• Investigate sand in the discharge early — it usually indicates a problem with the screen or the gravel pack and it will damage the pump.
• Watch for a gradual fall in yield. Slow decline is worth investigating before it becomes failure.
• Keep the borehole record: depth, casing details, screen positions and the original tested yield. Whoever works on it in ten years will need it.