climate." a chemical analysis of the water supply showed that it had an "extraordinary" amount of impurities, including excessive amounts of organic matter, nitrates and chlorides. this indicated that sea water and sewage had infiltrated the town's supply of drinking water. the findings led to the old spanish aqueduct and wells being closed down permanently as it was realised that they posed a severe hazard to public health. late 19th century improvements aguadores (water sellers) filling their barrels at a fountain in gunner's parade (now governor's parade) the former water catchments on the east side of the rock of gibraltar in 1992 another cholera outbreak in 1865, which killed several hundred people, prompted the establishment of a sanitary commission for gibraltar. ironically, the fragmented water provision system in gibraltar probably hindered the spread of the outbreak as it isolated the contaminated water supplies. inhabitants who drew their water from private wells or cisterns were far less likely to succumb to the disease than those who relied in public wells or imported spanish water. the commission set about trying to find an alternative supply of drinking water for the town and garrison. it sought to increase the amount of rainwater captured by passing gibraltar's first public health ordinance in 1869, which ordered that all new dwellings should have underground tanks. at the instigation of the commission, the british colonial office also commissioned the first geological survey of gibraltar, which was undertaken in 1877 by a.c. ramsey and j geikie. although it produced useful data, it failed to locate any groundwater or aquifers. borings were carried out on the isthmus between gibraltar and spain in an unsuccessful attempt to find potable groundwater, but led to an improved supply of brackish water useful for sanitary purposes. the commission instigated a system, which is still in use today, of pumping the salty water into a reservoir in the upper town from where it was distributed back down into the town for sanitary use. fresh attempts were made to sink boreholes in 1892 and 1943 but neither attempt was successful in finding drinking water. another technological solution was implemented in 1885 – obtaining fresh water from the sea by desalination. this had been proposed as far back as 1869, when it was envisaged that condensers would be used to produce 122 cubic metres (4,300 cu ft) of pure water per day from seawater. the idea was initially rejected due to the high cost but by the mid-1880s concern was growing that gibraltar's water supply would be inadequate in the event of a new siege. the war office had a change of heart and agreed to fund the project. the first purified water from the newly installed condensers was produced in march 1885. because gibraltar still had no distribution network for drinking water, it could only be delivered from barrels in donkey carts or by hand. despite the cost, the new water supply enabled the inhabitants of gibraltar, for the first time in their history, to rely on a source of pathogen-free water. the new system proved its worth only a few weeks after it was installed when cholera broke out in the town of la línea de la concepción just across the border with spain. 191 of la línea's 12,000 inhabitants died of the disease, but only 22 died in gibraltar – about five per cent of the death toll in the previous epidemic. the search for new supplies of water led engineers at the turn of the century to exploit the open expanses of the upper rock, where catchments were constructed by clearing vegetation and sealing fissures with cement or grout. this formed smooth expanses of rock, serving as runoff areas that could direct water into channels. such catchments were primarily used by the military to provide water to the otherwise hard-to-supply batteries and outposts that existed in numerous places around the upper rock. in 1903, the city's chief engineer came up with a plan to apply the same principle on a massive scale on the great gibraltar sand dune along the precipitous east side of the rock in the sandy bay area. the 10 acre slope, which has an incline of 35°, was covered with corrugated iron sheets attached to a timber framework. it used an average of 2,400 sheets per acre. in 1903, an area of 40,000 square metres (430,000 sq ft) was surfaced in this way, followed by a further 56,000 square metres (600,000 sq ft) between 1911–14 and another 40,000 square metres (430,000 sq ft) between 1958–61. the area of the catchments eventually reached 243,000 square metres (2,620,000 sq ft). the water ran off into channels at the foot of the catchments, about 100 metres (330 ft) above the sea, and flowed through a tunnel to reservoirs on the west side of the rock. despite their striking appearance, the catchments did not produce a great deal of water – about 60,000 cubic metres (2,100,000 cu ft) in a rainy year. new and more cost-effective methods of desalination made the catchments uneconomic and in 1991 the decision was made to discontinue them, as they were expensive to maintain. they were subsequently dismantled and the slope has since been returned to nature. water supply and sanitation today one of the huge reservoirs inside the rock of gibraltar that supplies the peninsula with water shortages of water between 1949 and 1986 led to the costly temporary expedient of importing water from the united kingdom, the netherlands or morocco. on a few occasions, newly commissioned oil tankers were employed to carry up to 36,000 cubic metres (1,300,000 cu ft) of water at a time, taking advantage of their maiden voyages to the middle east to carry the water in tanks that had not yet been contaminated with oil products. such an approach was unaffordable and since 1953, gibraltar has come to rely on the desalination of sea water, which now accounts for over 90% of the potable water supply. the running costs are high, however, as the energy required means that the cost of acquiring water through desalination is about three times higher than getting it from wells. a refuse incinerator commissioned in 1993 is used to contribute heat to help run the desalination system. the former systems of catchments and private cisterns have been discontinued. due to political tensions with spain over the disputed status of gibraltar, piping water from the spanish side of the border has never been seen as a viable option and gibraltar's water supply is entirely self-contained within the peninsula. the water supply was partly privatised in 1991 and is now maintained by aquagib, a subsidiary of the british water company northumbrian water, under licence to the government of gibraltar. there are presently two separate public water systems in gibraltar – one providing potable water and the other salt water for flushing toilets, firefighting, street cleaning and other sanitary purposes. the salt water system pumps nearly 4,000,000 cubic metres (140,000,000 cu ft) annually, about four times the volume of potable water. the potable water system uses twelve reservoirs excavated inside the rock of gibraltar. they are supplied by a combination of rainwater and water pumped up from a desalination plant at sea level. for the return journey, the water is pumped to the moorish castle reservoir in the upper part of the town before being distributed around the peninsula. the underground reservoirs were excavated in conjunction with the tunnels of gibraltar in a series of phases from the 1890s to the 1960s. the first five were constructed from 1898 to 1915, with another four completed between 1933–38. one more incomplete reservoir was temporarily used as a barracks and storehouse for 4th battalion the black watch during the second world war. after the war, two more reservoirs and auxiliary channels were constructed between 1958–61. the british garrison had its own separate water supply housed within the rock. the salt water system is divided into two sectors, serving the north and the south of the peninsula respectively, with two separate pumping stations at sea level. the north part of the system pumps water up from the north mole to the calpe tank and moorish castle reservoir on the north side of the rock. to the south, another pumping station located at gun wharf pumps water up to the europa road reservoir and a series of tanks and reservoirs at haynes' cave, spyglass, engineer road, queen's road and windmill hill.