Geology - Ground and Surface Water Supply
by Freddie Gomez
Nuns' Well is acknowledged by consensus to be the only remaining construction for water storage in Gibraltar. There is no conclusive evidence however as to its origin, construction or purpose for its use. This early (supposed) cistern is supposedly infilled with water that drains away along the rock fractures of the Beefsteak Fault across from it. Recent research however has proven that this underground construction is mostly infilled by surface water (rain), street flushing and in particular sea spray in strong levanter weather.
The geological state of the rock, that is, the amount of fractures, fault zones and Karstic solution rock surfaces which provided natural strings and underground streams served for the construction of cisterns and aqueducts. Records of surveys indicate that most (if not all) of the early water supply was derived from fractures in the limestone, such as the copious supply from the Orillion Fault which flowed into the laguna area (hence the name Laguna). Others emanated from the Alameda Sands due to their inability to hold water.
By the 19th Century the supply of water to the garrison and town was derived mostly from springs under the Alameda Sands which bank the dockyard parameter walls. These springs are located on the low permeability shales which underlie the permeable limestone. The shales form a barrier to the percolation of ground water which consequently emerges as a spring line. Additional water was collected from naturally formed catchments (rocky outcrops cleared of vegetation and fissures and cracks sealed with cement. The whole area was then painted - several coats - with a mixture of lime and cement which once dried became impermeable). The flowing water ran into gullies and underground storage (cisterns). Water was also collected from roofs and underground cisterns.
The increasing demand for water from an ever growing population could not be met by the methods employed at the time for obtaining water, especially in the summer's long period of drought, for most of the wells' water became brackish through saline intrusion when large amounts of water were drawn.
In an effort to improve groundwater supply A.C. Ramsey and J Geikie were commissioned in 1876 by the Colonial Office to conduct the first geological survey of the Rock of Gibraltar. In 1877 Ramsey recommended a deep borehole on the Isthmus sands to penetrate an aquifer he suspected lay under the Quaternary Sands. However, Ramsey's inference of the rocks structural geology appears to be in error for the borehole he recommended, followed by another undertaken in 1892 and a later one in 1943, failed to locate any groundwater or aquifer.
In 1903 an entirely new and original concept was devised by the City Engineer of Gibraltar. His idea was to make use of the huge sand slope on the east side of the rock to construct water catchments. The slope covered an area of 10 acres with an average inclination of 1 inch to 1 foot which to all intents and purposes was the ideal place. Embedded in the wind-blown sands of the slope were big stone blocks which were blasted away to acquire a surface as evenly as possible. Timber piles of cross purling and rafters were driven into the sand slope, creating a wooden frame on which corrugated iron sheets held by drive-screws rested. An average of 2400 sheets per acre was used to cover the area of the catchments. A channel and a footpath were also constructed at the lower perimeter of the collection area to connect the tunnel cut in 1898/1900 to house reservoirs No's 1 to 4.
The years 1911 to 1914 saw the construction of a further reservoir with a 2,000,000 gallon capacity and the catchments area increased by a further 14 acres. From 1928 to 1945 more reservoirs were excavated increasing the capacity of storage of water by a further five million gallons..
At the same time as the east side of the Rock became a hive of activity, at its north end 15 shallow wells were sunk into a lens of fresh water that overlies the salt water beneath the porous Quaternary sediments of the Isthmus. The water abstracted from these wells is still today pumped to reservoirs within the rock. These wells exploit an extremely heterogeneous aquifer which has a rather higher permeability. Despite salinity increasing with depth, these wells produce 159 mega litres annually, accounting for 25% of Gibraltar's fresh water requirements.
Water is also abstracted from other boreholes driven horizontally as opposed to vertical at the North Face of the Rock, to shallow freshwater lens that lie beneath the whole area of the Rock, (at approximately sea-level). The Gibraltar Limestone bedrock has a very low porosity for water storage, so boreholes driven horizontally provide for the greatest continuity through a usable aquifer. In recent years these boreholes were put and positioned down by hydrologists of the British Geological Society, intersecting major fault zones within the Rock whose shattered rocks provide natural gullies for groundwater flow. All groundwater abstracted in this manner is derived from the rain that falls upon the Rock which has a fluctuating yearly average rainfall of 381mm to 1092mm, markedly lower than the United Kingdom. However the Rock has a higher rainfall than the adjacent lowland areas of Spain. The faulted and fractured karstic-solutioned limestone's with their high angle bedding planes that form the main mass of the Rock are capable of absorbing large amounts of rainfall, which at first sight give the impression of having potential for substantial groundwater storage. However, the natural water-table is significantly decreased by Gibraltar's network of tunnels, and there is contamination in the groundwater caused by sea-water spray blown onto the land, and breakage in underground main salt water pipes, this being evident in the areas close to the bay, the dockyard and commercial moles. Therefore the prospect of locating substantial additional reserves of usable groundwater remains rather low.
One of Gibraltar's most famous and impressive feature is the Levanter cloud. The prevailing wind direction is
westerly or easterly with directional frequencies about the same though easterly winds prevail during the summer months. The Levanter cloud forms when the moisture-laden east winds hits the precipitous eastern cliffs and are deflected upwards to condense over most of the northern end of the town.