The evolution of acid sulfate soils management in Western Australia

Managing acid sulfate soils alongside the boom in development

For the past 20 years, Perth’s population has grown steadily to reach 2.1 million with projections showing the upward trend will have our population hitting 3.5 million by 2050. Not surprisingly, alongside this growth is a boom in residential development and the need to secure more land.

With historic development occupying much of the easier to develop land, more recent developments have spread to low-lying areas, that are subject to waterlogging, and are on soils that don’t like to be disturbed.

These soils – called acid sulfate soils - are widespread in low-lying coastal areas of Western Australia. They’re typically waterlogged, largely oxygen free with tiny crystals of iron sulfide minerals. These soils are harmless when left alone and saturated, but if disturbed and exposed to oxygen, the soils can oxidise and release acid. This acid release can in turn mobilise metals into the groundwater and together cause contamination, resulting in damage to the environment, waterways, buildings, roads and other structures.

For two decades RPS has been helping to manage acid sulfate soils alongside the boom in development – and, thankfully, management approaches have improved markedly over this time.

In the beginning: soil management at Ellenbrook

Ellenbrook was one of the first developments, in the early 2000s, where acid sulfate soils were identified and became an issue requiring management. Located around 30 kilometres north-east of Perth, Ellenbrook is a ‘greenfield’ master-planned residential development – meaning it started from absolutely nothing and would grow to accommodate around 50,000 residents.

Not only did the area require homes, but it also needed all the supporting infrastructure (gas, electricity, wastewater, roads, etc.). That’s a lot of potential earthworks to lay the foundations of a brand-new suburb.

Issues were discovered as earth was moved, soil profiles exposed, and groundwater was seen to acidify. The need to protect local groundwater and regional drinking water became a key environmental concern – and so began the task of managing the disturbed soils. Not surprisingly, initial management processes were a learning curve, often requiring time-consuming investigations, and with treatment and remediation costly in order to mitigate risks that were occurring on a large scale. Uncovering issues, however, was a breakthrough as it meant they could be further studied, risks-communicated, and with learnings applied across industry.

Since the first years at Ellenbrook, there have been significant advances in how we investigate, assess, and manage acid sulfate soils, and the Ellenbrook development has gone from strength to strength, securing over 30 awards at state, national and international levels.

Thirty years of growth at Ellenbrook

Groundwater and site management

Increased knowledge and experience have led to huge improvements on development sites in Western Australia.

Better planning, engineering design and construction methods have improved project outcomes. Targeted investigations identify and avoid ‘high-risk’ soils altogether, with this detail informing overall project design and constructability. After years of trying to make some sites work, we are now in a place where we can better assess the costs of treatment and alternative approaches. This can include reducing trenching, using trenchless technology (such as directional drilling), and limiting areas exposed to dewatering by staging dewatering programs and reducing groundwater drawdown.

Managing soil has also taken substantial steps forward. Back in the day, large open service trenches were commonly dug out with much of the acid sulfate soil excavated sent off to landfill – in part because no one knew what to do with it. Now, we have the know-how to either treat soil onsite for re-use, minimise the disruption, and/or alter designs to better suit the development site.

There have also been technological advances for treating and improving groundwater – where soils may have previously caused contamination. Once very large treatment systems, the size of several small shipping containers, were used to treat the pumped de-water. These have been replaced by readily portable and automated lime-dosing units that work to adjust the acidity of pumped groundwater before it is returned to the shallow groundwater.

Tailoring reports for development sites

When acid sulfate soil management was in its early stages, technical reporting tended to be written for the benefit of the environmental regulator only. Reports demonstrated how scientific investigations were undertaken and the significance of results against regulatory guidelines. They were scientific investigations and written accordingly.

Over the years RPS has worked with numerous contractors across Western Australia, which has given us the experience and understanding to prepare reports that not only satisfy regulatory requirements but also make sense for the contractors implementing the on-site management.

Our refined approach means we focus on giving practical, clear advice about site limitations and restrictions as well as best practice mitigation measures. Our reports are tailored to meet the specific needs of the audience, including contractors, specialist builders, water authorities, architects, engineers, etc., so they are relevant, practical and easily understood. They are also written to provide flexibility, options, and to avoid costly delays together with clear statements on the environmental outcomes that should be achieved.

Improved co-ordination and research