Malta and Water: Visit to Valletta’s Underground Cisterns beneath the Archbishop’s Palace
On the evening of 23rd June, as a follow up to the previous evening’s talk entitled ‘Malta and Water: Irrigating a Semi-Arid Landscape’, Dr. Keith Buhagiar took FAA members on a guided tour of the underground cisterns beneath the Archbishop’s Palace in Valletta. Everyone assembled in the garden of the Archbishop’s Palace where Dr. Buhagiar first gave an introduction for those who were unable to attend the previous evening’s talk.
Dr. Buhagiar first talked about mean-sea-level water aquifers present in the Globigerina Limestone which were discovered in the 1870 to 1880 period, which have been in use ever since. Over-extraction of water from these aquifers has resulted in their vastly decreased vertical thickness down to the current 1.0 to 1.5 metres, and the water becoming salty by contamination from the underlying denser saline water. Today, domestic water is a mix of around 40% aquifer water and 60% water from reverse osmosis plants, with water quality conforming to EU standards. Dr. Buhagiar commented that if we stopped water extraction today, it would take 40 to 50 years to reconstitute the mean sea level water aquifers. He stated that water in Malta is a fragile resource, and that if water production from the reverse osmosis plants should stop, say, as a result of a wide-spread power cut or threat from an oil slick, the lack of water supply would be disastrous for Maltese industrial and domestic needs.
On the topic of rainwater wastage, Dr. Buhagiar stated that construction and road surfacing act as impermeable membranes, thus rainwater run-off from these areas is not absorbed by the ground and stored. In areas where rainwater is absorbed, it takes a long time to seep through the Globigerina Limestone rock to the aquifer. In comparison, water from perched aquifers in Upper Coralline Limestone permeable rock remains of good quality.
Dr. Buhagiar mentioned that the source of water that accumulates under the Archbishop’s Palace is unknown, but discussed the following possible theories. Aquifer accumulation could be due to the Lower Globigerina Limestone overlying a localised impermeable marly deposit; leakage via fissures (unlikely); leakage finding its way here from existing reservoirs in Valletta still capturing water; or water coming from a sizeable aquifer in Hamrun / Santa Venera via fissures and leakage finding its way to Valletta. He added that as most of the Sciberras peninsula has been built on, there is much less exposed rock to absorb rainwater. Dr. Buhagiar also mentioned that an old 17th century map showed a fontana, which pre-dates the building development of Valletta by the Knights. The Archbishop’s Palace was built in 1622 and had the only fountain on private (church) property in Valletta. Other water supplies serving the Valletta public were via the Wignacourt aqueduct and pipeline from Rabat.
Dr. Buhagiar then led the group underground to view two water cisterns excavated in the rock, and galleries within Lower Globigerina Limestone rock with near-vertical fissures, through which water percolates from outside the Palace until captured in a cistern or aquifer. Also seen cut into the rock was a water channel with fresh running water.
At the end of the tour, Timothy Alden thanked Dr. Buhagiar for his interesting talk and guided tour of the underground cisterns, which was well received by all those FAA members present.