Gibraltar has no rivers, no lakes and no usable groundwater. Every drop of drinking water in the territory was seawater until it was forced through a reverse osmosis membrane, and that is not a simplification. The Environmental Agency states it flatly: all of the potable water here is obtained by desalinating seawater. Then there is the second half, which catches people out. When you flush a toilet in Gibraltar, what goes down the pan is also seawater, arriving through a completely separate set of mains that never touches the drinking supply. Two public water systems, in a territory of 37,936 people. It is one of the strangest pieces of municipal engineering in Europe and almost nobody writes about it. This is the question people actually ask, so here is the answer first. The tap water is safe, and by the standard international measure it is better than the water in four countries out of five. The [Environmental Agency](https://www.environmental-agency.gi/potable-water/) puts Gibraltar at five blue water droplets, the top classification on the World Health Organization based scale, and notes that only 20 per cent of countries reach it. Public drinking water is controlled by the Public Health (Potable Water) Regulations 1994 and the Public Health Act, which between them transpose European Council Directive 98/83/EC, and around 48 microbiological and chemical parameters have to be monitored. The sampling regime is more thorough than the rules demand. On Gibraltar's distribution volume the minimum is 16 Check monitoring samples and 3 Audit samples a year. The Agency says it more than doubles the Check figure and takes 4 Audit samples, and that is before AquaGib's own routine operational testing on top. AquaGib re-mineralises the water after desalination, which is what stops it tasting of nothing, and chlorinates it twice: once as it leaves the plant and again on transfer between reservoirs. The Environmental Health Department runs independent checks after that. One useful detail for anyone used to a two tier system elsewhere: the network delivers a single quality of water throughout. There is no separate lower grade supply for washing. What comes out of the shower is what comes out of the kitchen tap. The saltwater system is the part that surprises visitors. It is a genuine second public utility, with its own pumping stations and its own reservoirs, supplying toilets, fire hydrants and street cleaning. It runs in two sectors. The northern one pumps from North Mole up to the Calpe Tank and the Moorish Castle reservoir. The southern one pumps from Gun Wharf to the Europa Road reservoir and on to tanks at Haynes' Cave, Spyglass, Engineer Road, Queen's Road and Windmill Hill. | Feature | Potable network | Saltwater network | | Source | Seawater, desalinated by reverse osmosis | Seawater, pumped from the sea | | Volume a year | 1.7 million tonnes | Close to 4,000,000 cubic metres | | What it serves | Drinking, cooking, washing, showers | Toilets, fire hydrants, street cleaning | | Treatment | Re-mineralised and chlorinated twice | Not supplied to drinking standard | | Pumped from | Desalination plants to the reservoirs | North Mole and Gun Wharf | | Regulated under | Public Health (Potable Water) Regulations 1994 | Outside the potable water rules | The volumes are not trivial. The [record of Gibraltar's water system](https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in_Gibraltar) puts the saltwater network at close to 4,000,000 cubic metres pumped a year. Set that against the 1.7 million tonnes of drinking water distributed annually and Gibraltar moves more than twice as much seawater for sanitation as it does fresh water for everything else. The logic is obvious once you see it. Desalinating a cubic metre of water costs energy. Flushing a toilet does not require drinking water. So the Rock simply never fresh waters its sewers. Before desalination could carry the load, Gibraltar did something extraordinary to the east face of the Rock. It paved it. The Chief Engineer's plan of 1903 took a ten acre stretch of the great sand slope, inclined at 35 degrees, and surfaced it with corrugated iron sheets laid on a timber frame at roughly 2,400 sheets per acre. The point was to turn a sand dune into a rain catchment feeding reservoirs cut into the limestone. It grew in phases. The documented ones are around 40,000 square metres in 1903, another 56,000 between 1911 and 1914, and a further 40,000 between 1958 and 1961. Those three come to 136,000, while the catchments ended up covering about 243,000 square metres in all, so the record of the work in between is plainly incomplete. At full extent that is roughly 34 football pitches of corrugated iron bolted to the side of a mountain, visible from Spain, and for most of the twentieth century it was simply part of the view. The water went into reservoirs excavated inside the Rock itself. Eleven were completed in three bursts: five between 1898 and 1915, four between 1933