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What is aquifer replenishment? How does it work?

Aquifers are natural stores of water in the ground – between soil particles. Water from these aquifers is extracted for use when water is pumped from groundwater bores. Aquifers are naturally replenished through infiltration of rainfall and stormwater in stormwater infiltration basins. Artificial aquifer replenishment involves the injection through bores, or engineered infiltration of water to an aquifer. Any type of water can be used for injection or infiltration, but the quality of water when extracted must be appropriate for its end use.

Given that wastewater is produced all year, it is a very reliable source of water, but its most appropriate use is for irrigation – that is only required in dry months. Therefore treated wastewater can be stored through aquifer replenishment in winter for summer extraction by groundwater bores. Surface drainage water produced in winter can also be recharged to an aquifer for storage for use in dry weather.

Technology exists to treat recycled water to almost any quality. Water must be treated to quite a high standard before aquifer replenishment takes place. This is particularly true if recycled water is to be injected using bores. If the water is not appropriately treated, the bore could clog up with solids or microbiological activity due to the nutrient content.

The highest possible level of treatment uses reverse osmosis technology. Reverse osmosis uses membranes to filter out all but the water molecules themselves. There is no addition of chemicals, but high energy usage. This is the technology being used in the Kwinana Water Reclamation Project and can be used for desalination of seawater. The cost and energy needed to treat water increases with the purity of water required. However, soil itself also acts to purify water as it infiltrates to the aquifer, or while it is stored there. Bacteria and viruses in recharged water are rapidly consumed by native microbes in the soil, thus cleaning the water as it passes through the ground.

Why replenish aquifers with treated wastewater?

Treated wastewater is a very reliable source of water – volumes vary little with the weather so it is always available! If used as an alternative source, the water is called ‘recycled water’. We are in a drying climate and groundwater levels are dropping. This may affect wetlands, the amount of water we have available in our homes and the viability of agricultural industries and heavy industries that provide us with food and other resources.

The Swan Coastal Plain is a suitable environment for aquifer replenishment due to our uniform sandy soils that act as a natural filter to purify water as it passes through. Australia has world class expertise in the field of aquifer replenishment and some of the leading researchers in this field are based in WA. Using dams and tanks to store recycled water and pipes to transfer it is expensive. Using aquifer replenishment, water can be safely stored in an aquifer and transferred to the end user practically and cost effectively.

Where does wastewater go now?

Currently almost all wastewater from the metropolitan area is treated at either the Woodman Point, Subiaco or Beenyup Wastewater Treatment Plants, and piped to the ocean through three ocean discharge pipelines – at Point Peron, Swanbourne and Ocean Reef.

Is aquifer replenishment with recycled water already used?

Yes. Aquifer replenishment occurs near several wastewater treatment plants in WA where treated wastewater is disposed of by infiltration from ponds. Two areas where this occurs are at the Kwinana Wastewater Treatment Plant and at the Gordon Road Wastewater Treatment Plant in Mandurah. In these areas nearby bores access groundwater that is derived partially from the infiltration basins. This water is used by industry and to irrigate grassed areas with no adverse effects. An aquifer replenishment scheme irrigating grassed areas also exists in Geraldton.

In South Australia aquifer replenishment is being trialled to increase storage for a recycled water irrigation scheme for horticultural production.

Aquifer replenishment with highly treated wastewater occurs in the USA where water is used for drinking after storage underground. The most well-known location for this is in Orange County, California.

Can we recycle water without aquifer replenishment?

Yes. Water can be recycled directly through a separate recycled water reticulation network to areas requiring irrigation (“purple pipes”). This is a more expensive method of recycling water because all the summer demand for irrigation must be satisfied at once and winter wastewater flows cannot be used.

Water recycling without the use of aquifer replenishment also happens extensively throughout country WA, where it is highly valued for the irrigation of grassed areas including playing fields and golf courses. Recycled water is also used to grow trees for timber and to irrigate a vineyard in WA. Similarly in South Australia there are extensive schemes where recycled water is used to grow food crops.

How long does the wastewater stay in the aquifer before it is used?

This depends on how the scheme is arranged, including which aquifer it is planned to recharge and where, when and for what purpose extraction is proposed. Recycled water could stay in the aquifer for as little as two months if its purpose is purely for lawn irrigation. Depending on the method of aquifer recharge, the Department of Health requirements for storage in the aquifer could be different. As infiltration through soil provides an effective filtering system, if recycled water is recharged by infiltration then the time required in the aquifer is likely to be less than if water is injected via bores into a saturated part of the aquifer.

How do you stop recycled water mixing with the groundwater we drink?

The groundwater we drink comes from two major sources. The first of these is the superficial groundwater aquifer known as the Gnangara Mound (north of the Swan River). This mound of fresh water is very large, stretching from the coast to Ellenbrook and from Gingin in the north down to the Swan River. The water forms a mound from which groundwater flows away from the centre. Drinking water bores are placed near the top of this mound. Other uses of this groundwater are only found downstream. Thus aquifer replenishment can be located such that it is guaranteed to have no impact on drinking water bores.

The deeper groundwater extracted for drinking water supplies is sourced from confined aquifers (pressurised artesian aquifers) that are below thick clay layers separating them from the superficial aquifer. Water in these confined aquifers (the Leederville and the Yarragadee aquifers) travels very slowly. Therefore the water we drink has been recharged some distance away many thousands of years earlier. If recycled water were to be used to replenish a confined aquifer it would increase pressure in the aquifer throughout the system allowing greater extraction of native water elsewhere. The water would take between 200 years and 1000 years to flow 1 km, and by careful selection of replenishment sites can be managed to avoid extraction for public drinking water.

What is the likely environmental impact? eg on the Swan River or the marine reef system?

Recycled water is likely to have significant concentrations of the nutrients nitrogen and phosphorus, even once treated to a very acceptable level for irrigation. This nutrient content would replace fertilisers currently used on irrigated turf and users of recycled water would be expected to reduce their fertiliser use accordingly. Thus most of the nutrient in the water would be captured by the irrigated turf. Any water recycling scheme would be designed to have negligible environmental impact on the Swan River and the marine environment and would require approval by Department of Environment.

Are there any health impacts in using this water?

No, there will be no health impacts of using recycled water. However depending on the treatment level, there may be management requirements to prevent health risks. For example, if water is treated to a standard suitable for irrigation, it may be recommended that groundwater not be used within a certain distance of the recharge zone without further treatment (in the order of 100-500 metres), and that groundwater not be drunk or used in swimming pools without further treatment. These recommendations would apply within some distance of the recharge zone that would be based on the time the water is stored in the aquifer and the equivalent distance the water travels.

Who regulates aquifer replenishment schemes?

The water allocation and environmental regulation areas of the Department of Environment regulate aquifer replenishment schemes, as does the Department of Health.