Traditionally, Australia has constructed its water storages on the surface � but this can lead to the flooding of productive land or environmentally sensitive areas, losses from evaporation and leakage, and sometimes contamination by blue-green algae.

A team led by CSIRO scientist Dr Peter Dillon and partners from the Centre for Groundwater Studies have come up with a quintessentially Australian solution: store and treat quite large volumes of otherwise wasted water by injecting it into naturally-occurring aquifers underground � even aquifers which are too salty to be useful in their current state.

"Our research shows we can pump water into the aquifer during the wet season and take it out again during the dry season or whenever it is needed," Dr Dillon says. "There are several benefits to this.

"First, it is cheaper to store water underground than to spend tens of millions of dollars constructing a dam on the surface. The minimum economic storage volume is several orders of magnitude smaller than surface dams and you only need to store enough to match demand.

"Secondly, as aquifers are far more common that good dam sites, you can generally locate the water supply much closer to the community or industry that uses it."

The environmental impact of underground storage is also far lower, provided the storage aquifer has been carefully chosen to avoid leakage into other aquifers and pressures are managed correctly

"There are no losses to evaporation and, in a confined aquifer, few losses from leakage," Dr Dillon says.

"The water will also be healthier. Storing it underground cleanses the water of microbes and sediment, while shielding it from pollution and toxic algae.

In a notable breakthrough in 1993, the CSIRO/PIRSA team in Adelaide showed it is possible to store fresh water in a saline aquifer, creating a large "bubble" of fresh water which can be tapped at will. Low rates of flow restrict the mixing of the injected fresh water with the salty groundwater.

Dr Dillon cautions that underground dams will not have the capacity of the very large surface storages. Typically, a single injection well can store 200,000 cubic metres of water, instead of 20 million cubic metres.

"Sometimes aquifers are stacked like pancakes and you could have several of them in one area, supplying different qualities of water for different water uses in a city, for example landscape irrigation and drinking supplies. Furthermore, you can expand storage as the population grows or industrial water use expands."

Subterranean storage helps to disinfect the water of the bacteria, viruses and protozoa which normally inhabit surface dams. "We've seen very promising declines in the number of microbes in water stored underground," he says.

Dr Dillon attributes this to the changed environment the bugs face � absence of natural hosts, low nutrients, change in temperature and oxygen availability, as well as subterranean organisms which prey on them.

"This research, along with other work on subsurface contaminant attenuation, has led to us recently winning a research contract with the American Water Works Association Research Foundation to explore water quality improvements in aquifer storage and recovery for drinking water supplies. This project which will be lead by the Centre for Groundwater Studies, also involves researchers from Netherlands, France and USA."

One approach to reducing demand on water supply catchments such as the Murray-Darling Basin and reducing surface water pollution has been to harvest some of the stormwater which washes through Australia's cities. This could be stored underground and then brought up again to water parks, ovals and market gardens during the hot, dry season.

In this way an environmental threat to urban creeks, bays and estuaries can be diverted to create greener cities. Underground storage suits the nature of Australia, which has more variable runoff than other continents, by storing water when it is plentiful for the times it is scarce, says Dr Dillon.

CLW scientists believe underground storage can be used in a wide variety of ways:

• to harvest city stormwater runoff and save it to irrigate parks and gardens during the dry season
• to supplement drinking water supplies for communities whose natural supply becomes salty or is infested by toxic algae in summer
• to harvest urban sewage effluent in winter, for watering crops instead of polluting the sea
• to provide water to fast-growing suburbs and industry on the outer metropolitan fringe without building new dams or destroying local rivers
• to make undrinkable groundwater drinkable by shandying it with fresh water harvested on the surface
• to provide farmers and horticulturalists with a new way to store water without having to construct costly surface dams
• to use surplus water from inland floods to recharge Australia's natural artesian and fossil aquifers
• to save precious water in the WA Goldfields for use and re-use by the gold extraction industry

The knowledge generated by the underground dams can also earn big export dollars for Australia, Dr Dillon adds. The concepts are simple, but sustainable operations which protect groundwater quality, require a sound understanding of the processes and good management.

The Centre for Groundwater Studies has run two national workshops on water banking to disseminate information to people who can put it to use in water supply, catchment management, local government, planning, and consulting.

"We realise that people need greater confidence to implement new technologies and now CGS offers an advisory and consulting service to facilitate uptake of innovative water management techniques such as underground dams." says Dr Dillon.

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