Water supply choices in Dryland district
Dryland district of 18,000 people has 420 mm average rainfall, but annual totals have ranged from 180 mm to 720 mm. The main borehole is 120 m deep and yields 4 litres per second, but its water has 1,800 mg/L total dissolved solids; the national drinking-water limit is 1,000 mg/L. A 22 km pipeline from the river would cost £4.2 million, but the river falls below the intake for 40–70 days in dry years. Roof area averages 25 m² per household, and there are 300 suitable roofs. A rainwater harvesting programme could collect 70% of runoff from those roofs. Daily demand is 20 litres per person. The district has a £1.5 million budget. A reverse-osmosis unit for the borehole would cost £800,000 plus £60,000 a year to run; it would produce 35% reject brine. A hydrogeologist warns that pumping could lower the water table and that salinity may rise. The council must decide how to improve supply for the next five years.
1. Calculate daily demand and the potential annual rainwater harvest. Compare them.
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Daily demand = 18,000 people × 20 L = 360,000 L/day = 360 m³/day. Annual demand = 360 × 365 = 131,400 m³/year. Rainwater: 300 roofs × 25 m² = 7,500 m². At 420 mm = 0.42 m, gross rainfall on roofs = 7,500 × 0.42 = 3,150 m³/year. At 70% collection efficiency = 2,205 m³/year, or about 6 m³/day if spread evenly. That is only 2,205 ÷ 131,400 = 1.7% of annual demand. Even in a wet year of 720 mm, the harvest would be 7,500 × 0.72 × 0.7 = 3,780 m³/year, still under 3% of demand. Rainwater is therefore a supplement, not a supply; it can reduce pressure in the wet season but cannot be relied on in the dry season, when demand may be highest. The borehole at 4 L/s = 345.6 m³/day, which is 96% of daily demand if it runs continuously, but salinity is the limiting factor, not yield.
2. Recommend a five-year supply plan within £1.5m. Justify the main choice.
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Choose borehole treatment plus rainwater and demand management; the pipeline is unaffordable and unreliable. The reverse-osmosis unit costs £800,000, leaving £700,000 for a 300-roof rainwater programme, public taps, leakage repair and monitoring. RO running costs are £60,000/year, so the council must budget £300,000 over five years, leaving about £400,000 for capital works. The borehole yield of 345.6 m³/day covers 96% of demand at 20 L/person/day, so with 5–10% demand reduction (leak repair, tap restrictions) supply can match demand. The 35% brine reject must be managed: do not discharge to farmland or the river without testing; use evaporation ponds or deep-well disposal, and monitor soil salinity. Rainwater adds only about 2,205 m³/year, but it is useful for schools and gardens and reduces peak demand. The pipeline's £4.2m exceeds the budget and fails for 40–70 days in dry years, so it is not a reliable primary source. Conditions: monitor the water table and salinity monthly; if salinity rises above 1,800 mg/L or the water table falls, reduce pumping.
3. Explain one environmental risk of the chosen plan and how it should be monitored.
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Risk: over-abstraction and salinisation. Pumping 4 L/s continuously could lower the water table, and the hydrogeologist warns that salinity may rise as deeper, more mineral-rich water is drawn in. RO also produces 35% brine; if disposed of poorly, it can salinise soil and groundwater. Monitoring: install at least three observation boreholes around the wellfield and measure water level and total dissolved solids monthly; keep a pumping log and compare with rainfall. Set trigger levels: if the water table falls by more than 1 m below the pre-pumping baseline, or salinity exceeds 1,800 mg/L, reduce pumping and switch to rainwater/demand management. For brine, test soil electrical conductivity quarterly at the disposal site and sample local wells. Publish results so the community can see whether the supply is being mined rather than managed. The aim is to treat the borehole as a regulated resource, not an unlimited tap.