Water Demand, Nitrate and Reuse in a Coastal Irrigation District

Nitrate and salinity look like two separate problems but share a cause: the aquifer is being asked to do more than it can.

Nitrate comes from the surface. 210 hectares of irrigated vegetables means fertiliser and manure applications, and limestone is highly permeable with thin soils, so nitrate moves quickly down through the unsaturated zone into the water table. The rise from 38 to 49 mg/L over seven years is roughly 1.6 mg/L a year - slow enough to look safe, fast enough to breach the 50 mg/L limit within a year or two. Nitrate is also slow to reverse: once in the aquifer, it takes years to flush, so a breach is not fixed by reducing fertiliser next season.

Salinity comes from the coast. Over-abstraction of 4,100 m³ a day beyond licensed yield lowers the freshwater head, allowing seawater to intrude into the limestone. 1,900 µS/cm is well above normal freshwater and indicates saline mixing.

The link is over-abstraction. Pumping beyond yield draws down the water table, which simultaneously pulls in seawater laterally and increases the capture of nitrate-rich recharge from agricultural land. The 2016–2023 salinity and nitrate records are therefore both symptoms of a demand-supply gap of about 6,500 m³ a day in August, not of a fertiliser problem alone.

Comparing like with like over a 20-year life:

• RO expansion: €14,000,000 ÷ (6,500 m³ × 365 × 20) = €14,000,000 ÷ 47,450,000 m³ ≈ €0.30/m³ of capital cost. It adds new water but consumes energy and produces brine.
• Reuse pipeline: €6,000,000 ÷ (7,500 m³ × 365 × 20) = €6,000,000 ÷ 54,750,000 m³ ≈ €0.11/m³, and it delivers more water than the RO expansion, because the 7,500 m³ is already being produced and discharged.

On capital cost alone the pipeline is clearly better value and it closes the 6,500 m³ gap on its own. It also removes a discharge to the sea.

But reuse does not solve the whole problem. It supplies farmers, not households or hotels, and it only reduces mains demand if farmers actually switch. The 0.35 against 0.90 €/m³ price difference gives a strong incentive, but the pipeline must reach the fields and the water must meet reuse quality standards for vegetables.

A sensible package is: pipeline first (the cheapest water, and it cuts agricultural abstraction), a smaller RO top-up for potable supply in peak August, and the tariff as a demand-management signal rather than as the main tool. Sequencing matters: build the pipeline, monitor whether aquifer salinity falls, and only then decide how much RO is really needed.

For: hotels and irrigated agriculture generate the peak summer demand that drives over-abstraction; a summer-only rise targets the season when the aquifer is under stress. Households already pay for a basic need, so protecting them is defensible on equity grounds. Hotels can pass part of the cost to guests and have the capital to invest in low-flow fittings, pools covers and greywater reuse, so the price signal can actually change behaviour.

Against: an 18% rise is blunt. A hotel that has already installed water-saving equipment pays the same rate as one that has not, so the incentive is weak unless the tariff is banded by consumption. Hotels are also a major employer and foreign-exchange earner; a cost increase during a competitive season may be absorbed by cutting staff hours rather than by saving water. Farmers are hit twice: a summer price rise and the 0.90 €/m³ mains rate, which is why the reuse pipeline is essential - without it the tariff simply transfers income from growers.

A better design is a rising block tariff for non-household users in July–September, with published consumption thresholds, plus a rebate for verified efficiency investment. That keeps the revenue signal but rewards the behaviour the utility actually wants. The tariff should be judged by how much abstraction it removes, not by how much revenue it raises.