❓ Question regarding the impact of water table decline on the Gnangara Mound, specifically concerning pH levels, acid sulphate soils, and potential impacts on potable water. The Minister acknowledges the issue and outlines existing frameworks and planned assessments.
AnsweredQoN 394Legislative Assembly
QuestionView source ↗
(b) is it correct that the 6 metre drawdown of the water table on the Gnangara Mound has resulted in a dramatic drop in pH to as low as pH 3.5 at the water table; and (i) if so, what is the impact of this 100 fold increase in acidity (2 x pH units) in the water table of Perth’s largest single water resource for potable domestic supply; (c) how extensive is the presence of acid sulphate soils and potential acid sulphate soils on the Gnangara Mound; and (d) is the Minister aware of the problems associated with acidified groundwater; and (i) if so, what action is being taken to rectify these problems?
(d) is the Minister aware of the problems associated with acidified groundwater; and (i) if so, what action is being taken to rectify these problems?
(a) Yes. Based on limited historical drilling data, overseas experience and geochemical information, it is likely that the pH of groundwater in the water table on the Gnangara Mound would have been in the range of 5 to 6 before European settlement. (b) There has not been a 6 metre "drawdown" on the Gnangara Mound. The level of the superficial watertable has declined by up to 6 metres over the past 25 years. This is predominantly due to climate change, and not "drawdown" from abstraction. However, the general decline in the water table has been a significant factor in causing the pH of shallow groundwater to drop on the Gnangara Mound. (i) None. The pH of potable water supplies have not been affected as public water supplies are generally sourced from the base of the superficial aquifer or from the deeper confined aquifers. Any increase in acidity from acid sulphate soils is generally associated with shallow water tables. Public water supply treatment processes control variations in pH from different water sources. (c) Acid sulphate soil materials are likely to underlie most of the wetlands on the Gnangara Mound. (d) I am aware that the Minister for the Environment released the 'Proposed State Framework for Managing Acid Sulphate Soils' in June 2004. (e) Proposals to further assess the lakes and wetlands on Gnangara and Jandakot Mounds are being prepared by the Department of Environment.
(d) is the Minister aware of the problems associated with acidified groundwater; and (i) if so, what action is being taken to rectify these problems?
(a) Yes. Based on limited historical drilling data, overseas experience and geochemical information, it is likely that the pH of groundwater in the water table on the Gnangara Mound would have been in the range of 5 to 6 before European settlement. (b) There has not been a 6 metre "drawdown" on the Gnangara Mound. The level of the superficial watertable has declined by up to 6 metres over the past 25 years. This is predominantly due to climate change, and not "drawdown" from abstraction. However, the general decline in the water table has been a significant factor in causing the pH of shallow groundwater to drop on the Gnangara Mound. (i) None. The pH of potable water supplies have not been affected as public water supplies are generally sourced from the base of the superficial aquifer or from the deeper confined aquifers. Any increase in acidity from acid sulphate soils is generally associated with shallow water tables. Public water supply treatment processes control variations in pH from different water sources. (c) Acid sulphate soil materials are likely to underlie most of the wetlands on the Gnangara Mound. (d) I am aware that the Minister for the Environment released the 'Proposed State Framework for Managing Acid Sulphate Soils' in June 2004. (e) Proposals to further assess the lakes and wetlands on Gnangara and Jandakot Mounds are being prepared by the Department of Environment.
AnswerView source ↗
Answered
12 October 2005
Responded by
Minister for Water Resources
Response time
41 days
(a) Yes. Based on limited historical drilling data, overseas experience and geochemical information, it is likely that the pH of groundwater in the water table on the Gnangara Mound would have been in the range of 5 to 6 before European settlement. (b) There has not been a 6 metre "drawdown" on the Gnangara Mound. The level of the superficial watertable has declined by up to 6 metres over the past 25 years. This is predominantly due to climate change, and not "drawdown" from abstraction. However, the general decline in the water table has been a significant factor in causing the pH of shallow groundwater to drop on the Gnangara Mound. (i) None. The pH of potable water supplies have not been affected as public water supplies are generally sourced from the base of the superficial aquifer or from the deeper confined aquifers. Any increase in acidity from acid sulphate soils is generally associated with shallow water tables. Public water supply treatment processes control variations in pH from different water sources. (c) Acid sulphate soil materials are likely to underlie most of the wetlands on the Gnangara Mound. (d) I am aware that the Minister for the Environment released the 'Proposed State Framework for Managing Acid Sulphate Soils' in June 2004. (e) Proposals to further assess the lakes and wetlands on Gnangara and Jandakot Mounds are being prepared by the Department of Environment.
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