A Declining Borehole in the Sandveld: Deepen, Pipe or Ration?
Kgetleng is a village of 2,800 people on the edge of the sandveld. Its only borehole was drilled in 1998 to 90 m and once yielded 4.0 litres per second; the latest test gives 2.5 litres per second, and the water now contains about 1,150 mg per litre of dissolved solids - above the 1,000 mg/l guideline used for drinking water - rising by roughly 40 mg/l a year. The diesel pump breaks down about six times a year and fuel costs have doubled in three years. The nearest reliable surface water is a dam on a river 14 km away, with an estimated dependable yield of 3 litres per second for the village; the river itself flows only from January to April. A pipeline to the dam would cost about four times as much as deepening the existing borehole. Rainfall is 400 mm a year. Several pit latrines stand within 30 m of the borehole.
1. Explain why the borehole yield is falling and the salinity rising. Use the information given, not general statements.
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Yield falling
- Abstraction exceeds recharge. Rainfall is only 400 mm and most of it is lost to evaporation and transpiration, so recharge to a 90 m aquifer is slow. Pumping 2,800 people plus livestock from one borehole draws water out faster than it arrives, so the water level in the borehole and around it falls.
- Cone of depression. Continuous pumping creates a cone-shaped drawdown around the borehole. As the water level drops, the pump has to lift from deeper, and the borehole can no longer sustain 4.0 l/s.
- Pump age and breakdowns. Six failures a year is consistent with a pump working near its limit in declining water levels.
Salinity rising
- Concentration effect. With less fresh water in storage, dissolved salts from the surrounding rock and sand make up a larger share of what is pumped - the water becomes more mineralised as the aquifer is drawn down.
- Recycling of returns. Waste water and seepage from the village soak away and re-enter the ground, carrying salts back to the borehole.
- Contamination risk from latrines. Latrines within 30 m of the borehole point to nitrate and bacterial risk, especially as the water table is drawn down and flow paths shorten. This is a health issue separate from salinity and would show up as rising nitrate, not just dissolved solids.
Chain of causation to state: drought-prone rainfall → limited recharge → over-abstraction → falling water level and cone of depression → declining yield and rising salt concentration → more pumping hours and breakdowns → higher fuel cost per cubic metre.
2. Compare deepening the borehole with building the pipeline, using at least three criteria, and recommend a course of action.
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Comparing the options
| Criterion | Deepen borehole | Pipeline to dam |
|---|---|---|
| Cost | About one quarter of the pipeline | Four times higher capital cost, plus pumping 14 km |
| Water quantity | Extra depth may add yield, but the aquifer is the same one already over-used; gain is uncertain | Dam yield of 3 l/s is dependable and larger than present supply |
| Water quality | Salinity already at 1,150 mg/l and rising 40 mg/l a year; deeper water is often more mineralised, not less | Surface water is usually lower in dissolved solids but needs treatment for turbidity and bacteria |
| Reliability | Diesel pump fails six times a year; still one source | Needs a booster pump and power; a single pipeline is also a single point of failure |
| Speed | Weeks | A year or more of survey, funding and construction |
The critical reasoning point is that deepening treats a symptom. It buys depth, not recharge. If the aquifer is being mined, a deeper borehole simply extracts the same limited water from further down and may worsen salinity.
Recommended phased plan
- Immediate: repair and service the pump, meter abstraction, and start modest rationing plus leak repair - this buys time cheaply.
- Short term: commission a hydrogeological study and test nitrate and fluoride before spending on deepening; if water quality fails, deepening is wasted money.
- Medium term: secure funding for the pipeline as the permanent solution, with the borehole retained as the standby source.
- Throughout: relocate or line latrines, and protect the borehole head. A clean source is useless if it is contaminated 30 m away.
3. Suggest three demand-management measures and explain why each is worth doing even if the pipeline is eventually built.
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1. Metering and a rising block tariff. If households pay a low flat rate, there is no signal about scarcity. A tariff that rises after a basic free allowance usually cuts discretionary use - car washing, garden watering - by a noticeable share without denying anyone their basic need. It also generates revenue for maintenance.
2. Leak detection and repair, starting at the tank and standpipes. In many villages a quarter or more of pumped water is lost before it reaches a tap. Fixing that 'loses' nothing and reduces pumping hours, fuel and pump wear immediately. It is the cheapest water 'source' available.
3. Rainwater harvesting from roofs and the school, plus livestock watering away from the domestic point. With 400 mm of rain falling in a short season, roof catchment can take pressure off the borehole for several months, and separating cattle from the domestic supply reduces both demand and contamination.
Why do this even with a pipeline? Because a pipeline does not create water for free - every litre pumped 14 km costs fuel and maintenance, and the dam yield of 3 l/s is finite. Efficiency measures lower running costs for the life of the scheme, delay the need to expand it as the village grows, and keep the old borehole viable as an emergency source. Demand management also protects the aquifer, which remains the fallback if the pipeline ever fails.