How emergency aquifers would work

Government project would store 90-day supply of millions of litres of drinking water in vast natural tanks in case desalination plants failed.

The Government said last week that increasing the emergency water supply would be a priority, easing some anxieties over the country's reliance on desalination plants and its vulnerability to sea pollution such as algal blooms or a major oil spill.

Officials at the International Symposium on Managed Aquifer Recharge detailed the Government's strategy of pumping millions of litres of desalinated water into underground aquifers - geological formations that have existed for thousands of years -tripling the emergency supply to last at least 90 days.

Those involved with the project explain how the water will be stored and cared for in the buried pockets.

Why it makes sense

The aquifers are the best option because they are less expensive, better for the environment and less exposed to potential security threats than a vast installation of above-ground tanks would be, said Mohamed Dawoud, the water resource department manager for Abu Dhabi's Environment Agency.

Surface ponds and lakes lose water to evaporation and are susceptible to micro-organisms and pollution.

Water tanks are about 10 times more costly, according to those involved with the project. But the underground system is a viable option only if most of the costly water originally deposited can be retrieved - and immediately.

As long as the area surrounding the aquifer is protected against pollutants, the system will be efficient "because a high percentage of the recharged water can be pumped out again, and it is expected to be of the same high quality," said Mohsen Sherif, a professor of water resources and the chairman of the civil and environmental engineering department at UAE University.

Storing water underground is best to protect it against bacteria, said Hassan Fath, a professor of practice and water and environmental engineering at Masdar Institute.

"It's very serious and important to consider how people will suffer if, for any reason, the country's water desalination plan is blocked for even more than a few days," he said. "Like any well, this is going to be well-preserved and well-sealed. It is really the best solution."

How it works

The excess fresh water produced by the UAE's desalination plants during the wetter winter months will be banked in the underground layers.

As a pipe injects the drinking water 60 to 900 metres deep into the ground, the water balloons around the end of the pipe, swelling up against the ground and existing pools of murky groundwater.

The water that creates an outer layer mixes with the native, salty water and becomes a buffer zone protecting the emergency supply, much like a tank would, said Rolf Herrmann, a hydrogeologist and the technical manager for Schlumberger Water Services. The private company has been contracted to construct mazes of pipes, monitoring wells and pumping stations in Liwa, Al Schweib and a third location yet to be determined.

What gets lost

At least about one-tenth of the water will be lost, Mr Herrmann said. Most of that makes up the protective wall around the drinking water.

Sometimes, the fresh water will separate and rise above the saline water, which is much more dense. As the desalinated water rises over time, it creates a top layer much like oil with water. This poses a problem in keeping the fresh water at the same depth as the pipe when it needs to be drawn back out.

The existing water may also slowly shift, pushing the fresh water away from the pipe.

"It's impossible to know when this water is needed and where the fresh water line will be at that time, whether it's in two years, five years or 10 years," Mr Herrmann said.

"The challenge is constantly adjusting the best way to optimise recovery of the fresh water without drawing too much of the saline mixture."

When to check it

The water will be pumped and circulated regularly, and will require constant monitoring and readjustments until it needs to be sucked up.

"If you leave the water to sit alone for 10 years, waiting for a disaster, the pipes will rust and who knows if it will work when the water is needed right away," Mr Herrmann said.

"The system is much like a car in that it needs to be kept running every so often to keep it alive."

Pails of water can regularly be retrieved for sampling, measurement and analysis from monitoring wells. "When you put millions of dollars worth of water into the ground, you want to check if it's still there," he said.

Where the water goes

The aquifer recharge areas are chosen based on depth, mineralogy, salinity, space, confining layers of the ground, future planning of the area, an environmental impact study and accessibility. Officials will also consider how close the aquifers are to infrastructure such as power lines and whether there are nearby pollutants, such as nitrogen from fertiliser run-off.

"If you fail in finding the right location, the project itself will become a failure," Mr Herrmann said.

Who

Abu Dhabi's Environment Agency and the Ministry of Environment and Water are in charge of the project. They will appoint a technical committee in the next few weeks to oversee the aquifers.