Drought-year water balance on a Mediterranean island
On a Mediterranean island, annual water demand is 45 hm3. Public supply accounts for 25 hm3, agriculture 12 hm3 and tourism 8 hm3. Natural recharge averages 18 hm3/yr but falls to 11 hm3 in drought years. Reverse osmosis produces 20 hm3, but at high energy cost and with brine discharge. Distribution leakage is 25% of public supply. Agricultural abstraction is largely unmetered, and coastal boreholes show chloride rising from 180 to 420 mg/L in five years. A drought year is forecast. The utility proposes a new 4 hm3/yr reverse-osmosis plant; the regulator prefers demand management. Other options include a five-year leak reduction programme saving 6 hm3/yr, and 5 hm3/yr of treated wastewater for agriculture. The government must choose a package that balances cost, energy, food production and groundwater protection.
1. Construct a water balance for a drought year and quantify the supply-demand gap. Show your assumptions.
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Drought-year supply: recharge 11 hm3 plus reverse osmosis 20 hm3 equals 31 hm3 before leakage. Public supply demand is 25 hm3, and leakage is 25% of that, or 6.25 hm3; delivered public supply is therefore 18.75 hm3. Total delivered supply is 11 + 20 − 6.25 = 24.75 hm3. Demand is 45 hm3, so the gap is 20.25 hm3. If leak reduction saves 6 hm3/yr, delivered supply becomes 30.75 hm3 and the gap falls to 14.25 hm3. Adding the new 4 hm3 plant reduces the gap to 10.25 hm3. Wastewater reuse of 5 hm3 for agriculture does not increase public supply, but it substitutes for 5 hm3 of agricultural abstraction; if counted against total demand, it closes 5 hm3 of the gap, leaving 5.25 hm3 with all three measures. Assumptions: recharge is treated as available supply, environmental flows are not reserved, and leakage savings are realised within the year, which is optimistic. The key conclusion is that the gap is large and no single option closes it.
2. Compare the four options using at least four criteria, and recommend a package. Explain why new supply alone is risky.
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Criteria should include volume, cost, energy, environmental risk, speed and equity. Leak reduction delivers 6 hm3, has moderate cost and low energy, reduces energy use and salinisation, but takes five years and needs skilled labour. New reverse osmosis delivers 4 hm3, has high capital and energy costs, produces brine and greenhouse gases, and is fast, but is vulnerable to energy prices and membrane supply. Wastewater reuse delivers 5 hm3, has moderate cost, needs tertiary treatment and storage, and requires public acceptance for food crops; done well, it also reduces nutrient discharge, but it must meet irrigation standards. Tariffs and demand management could reduce tourism and domestic demand, but are regressive unless block tariffs are designed carefully. I would recommend leak reduction first, reuse for agriculture second, and a smaller or temporary reverse-osmosis plant only if a residual gap remains. New supply alone is risky because it locks in energy and brine costs, may encourage continued over-abstraction, and does not address the coastal chloride trend. A portfolio is more resilient and spreads risk across sources.
3. Explain the chloride trend and its implications for the abstraction licence.
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Chloride rising from 180 to 420 mg/L in five years indicates seawater intrusion or up-coning. A freshwater lens floats on seawater; if abstraction exceeds recharge, the interface rises and chloride increases. At 420 mg/L, the water is above the 250 mg/L drinking-water guideline, so the boreholes are unsuitable for potable supply without treatment. The trend, not just the level, is the warning: a 2.3-fold rise in five years suggests over-abstraction. The abstraction licence should be reviewed to require metering, cap total abstraction below recharge, impose seasonal limits, and establish a monitoring network with chloride triggers, for example 250 mg/L. If the licence is not reduced, the aquifer may become irreversibly saline and require desalination, shifting costs to the utility. The regulator should link licences to recharge and to the reuse scheme, so that recycled water substitutes for groundwater. Transition support for farmers may be needed. The precautionary principle applies because saline intrusion is hard to reverse.