Drinking water quality shown through field and model research
Abu Dhabi desert dunes suitable for underground water storage
To provide the residents of the coastal desert city of Abu Dhabi with sufficient drinking water, infiltration basins have been built for the installation of an underground water storage reservoir. In a commission from the Environment Agency – Abu Dhabi (EAD), KWR and Dutch cooperative WaterFocus conduct field and model research into the changes in water quality during its storage and recovery. The results are reassuring.
Abu Dhabi produces its drinking water from desalinated seawater from the Arabian Gulf. Without an underground buffer, the desalination facilities can only supply the capital city of Abu Dhabi with water for a maximum of 3 days. That is an unacceptably short period in the view of dr. Mohamed Dawoud of EAD, because emergencies can always occur – for example, the water intake can be restricted by a massive algal bloom in the Gulf. “That is the reason why we need to install a safe, underground reservoir of drinking water which can, without requiring further treatment, meet our drinking water needs for at least 90 days.” After 5 years of preliminary studies and 5 years of construction, the project is almost ready: 3 underground infiltration basins, each surrounded by 105 recovery wells, will together infiltrate approximately 10 million m3 of desalinated seawater annually to build up the underground reservoir. But until now an important question about the installation remained unanswered: What happens to the quality of the drinking water after lengthy underground storage?
The field research involves sampling from 31 monitoring wells at a temperature of 50°C.
Quick, reliable prognosis
The natural, fresh-to-brackish groundwater below the extensive dune area is not of a quality suitable for consumption as drinking water. The fear was that the naturally high concentrations of, among others, chromate, arsenate, vanadate, sodium, sulphate and fluoride, would, during the years-long underground storage, be released and render the expensive, desalinated seawater undrinkable. Pieter Stuyfzand, professor at VU University Amsterdam and principal scientist at KWR, says that “thanks to our worldwide reputation in the field of artificial recharge, KWR, together with WaterFocus, were selected to conduct the field and model research into the quality changes in the infiltrated, desalinated seawater during storage and recovery.” The August 2014 field research involves sampling from 31 monitoring wells at a temperature of 50°C, and detailed analysis in the Netherlands. The subsequent model research must provide a solid prognosis of the quality of the water to be recovered over a period of 90 days, following 27 months of infiltration and 10 years’ storage. “A kind of worst-case scenario,” explains Stuyfzand. Not an easy assignment, in light of the underground’s complex mineralogy, the particular composition of the natural groundwater, and complications, such as the dispersal of the infiltrated water during storage and possible upconing of brackish water during recovery. Furthermore, the EAD wants a quick answer, because of increasing concern that the water quality problem will throw a spanner into the practically-finished project worth hundreds of millions of euros.
An adapted version of KWR’s Easy-Leacher model – which is based on chemical principles, field observations and expert rules – is employed to provide a quick prognosis of the evolution of water quality. The calculations show that only after 80 days of recovery – following 10 years’ storage and taking due account of dispersal – the concentrations of a few elements could rise above the national drinking water standards. This, the client considers an acceptable risk.
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