Can domestic bores help save our drinking water?

Dr Smith will present results from a study of Perth’s groundwater levels at a public seminar today (June 8) at CSIRO’s Floreat Laboratories in Perth.

One solution to the problem of dwindling water supply was to increase the number of garden bores in Perth’s suburbs, but given Perth’s drying climate there is a doubt as to whether this is sustainable.

“Perth people know all about domestic bores because about a third of all gardens use groundwater from the shallow sandy aquifer,” Dr Smith says.  “However, millions of litres of drinking water are still being used to keep lawns and gardens green.”

Dr Smith says Perth’s groundwater supply is facing some serious threats, and that good data is essential to understand the resource and manage it wisely.

“More than 80 per cent of all the water used in Perth comes from groundwater,” he says.

“Less than 20 per cent now comes from the hills catchment, which historically was Perth’s main water supply. Around 45 per cent of the total water supply is treated to drinking water quality but it’s not all used for drinking.

“Encouraging more garden bores is one way to preserve precious scheme water for drinking uses. However, we need to know how many bores the aquifer can support without unacceptable impacts on groundwater dependent ecosystems and other uses such as public open space. About 16 000 rebates have been issued for drilling bores since February 2003, equivalent to around 14 bores per day.”

Dr Smith says the biggest threats to Perth’s groundwater are seawater incursion into the aquifer, the loss of valuable wetlands, and soil and groundwater acidification.

“It’s a careful balancing act,” he says. “We can’t draw so much out that it is replaced by seawater at the coast, or wetlands dry up, or that acid sulphate soils are exposed to the air. Natural replenishment of groundwater by seasonal rainfall and by directing runoff from roofs and roads into the aquifer can prevent this happening, but only if the balance is maintained.”

Dr Smith’s team examined more than 500 monitoring bore records in the greater Perth region. It concluded that there were some suburbs where increasing the number of bores could lead to less drinking water being used on domestic gardens.

The study also identified suburbs where increased extraction would not be viable if the current rate of water table decline continued. In these areas, more bores would only be possible if they, and the declining levels, were offset by more efficient use of the water by current bore owners.

The research team used data from the past ten years, and looked at what they termed freshwater ‘thickness’ at 543 sites in the Superficial Aquifer.

“We found that the level was stable beneath only 14 per cent of the study area,” Dr Smith says. “It was falling beneath more than 40 per cent of the area, and rising at only one per cent.

“Importantly, we couldn’t assess almost half of the greater Perth region because of a lack of historical water level data.

“It’s essential that we fill in the information gaps, and a project managed by the Swan Catchment Council is starting to address this deficiency.”

The drier climate is likely to be a significant factor in causing the decline in levels in the past ten years, something that is also happening in the Gnangara Mound, north of Perth. Increased urban infill, which increases recharge in most areas, is likely to have mitigated the impact of a drier climate. A new study is planned to identify opportunities to direct more stormwater into the aquifer to rectify the falling levels. 

The research supporting the report, Opportunity for Additional Self Supply of Groundwater from the Superficial Aquifer beneath Metropolitan Perth, was carried out by CSIRO, the Water Corporation, and the Department of Water, through the Water for a Healthy Country Flagship.