INVESTIGATION OF SOIL AND GROUNDWATER ACIDITY, STIRLING

INVESTIGATION OF SOIL AND GROUNDWATER ACIDITY, STIRLING

Cover
Summary & Recommendations
Introduction
Background
Response to Problems
Methodology
Results
Conclusions
References

Attachments

INVESTIGATION OF SOIL AND GROUNDWATER ACIDITY, STIRLING

1.0 INTRODUCTION

Identification of Problem and Immediate Response

Future urban development in the Perth metropolitan area is likely to encroach into a number of existing and historical wetlands where sediments commonly contain iron sulfide minerals, principally pyrite. These minerals form under waterlogged conditions when there is no oxygen available to allow the sulfides to decompose. The sulfide-rich material is commonly known as potential acid sulfate soil (PASS) because it has the potential to oxidise to sulfuric acid on exposure to air. Exposing of the pyrite to air by excavation, drainage, stockpiling of peaty materials or lowering of groundwater table can cause the sulfide to form sulfuric acid, often releasing toxic metals from the soils including arsenic into groundwater. Until recently the presence of PASS in wetlands has not been fully investigated. Inappropriate management of these areas can produce immediate and a longer term impacts with substantial health, environmental and economic consequences such as:

polluting water supplies with aluminium, iron, arsenic and other metals, possibly causing significant health problems if untreated water is used for drinking;
corroding concrete and metal structures by acid water; and
causing death of aquatic organisms and riparian vegetation.

¹ Cook, F.J et al - Irrigation and drainage: Effects on acidity export from acid sulfate soils.

Under natural conditions, PASS may produce some acidity after drought periods as a result of oxidation of the exposed sulfides. In a natural system the main driving forces for water fluctuations are evapotranspiration and rainfall¹. Recent increases in groundwater abstraction for irrigation and excessive dewatering by urban developers has contributed to declining water table in the Stirling area. The situation is further compounded by the stockpiling of thousands of tonnes of pyritic materials that have allowed the rapid oxidation of the sulfides to form sulfuric acid. Surface water bodies in a development site were found to be acidic with pH values between 2 and 3. The council lakes within a recreational park located north west of the development site recorded a pH level of less than 3.2.

Several domestic bores next to the development site were impacted by acidic groundwater ranging from pH 2.6 and 4 at a depth of 12-15 metres below ground level.

The groundwater acidity screening conducted between January and February 2002 on more than 800 domestic bores, revealed more than 40 bores are affected by acidity between pH 2.6 and 5.5 down to a depth of 10 - 12 metres. More than 20 domestic bores were contaminated with arsenic and other metals. The arsenic concentrations exceeded the National Health and Medical Research Council recommended threshold of 7µg /L for drinking water.