RPS team proposes remediation programme to facilitate Cockburn residential development

Our Earth Sciences team based in the Subiaco office in Western Australia (WA) is currently finalising the remediation programme for a disused factory site in the city of Cockburn, WA. Following successful remediation, the site will be redeveloped as a residential community consisting of approximately 400 residential plots, a commercial centre and public open space.

The factory, which began operating as an abattoir and meat processing facility around 100 years ago, had an onsite wastewater treatment plant with unlined effluent disposal ponds – which now contain sediments identified as a minor ongoing groundwater contamination source. The ponds have been identified as an important feature for the local community, as they represent both the natural landscape and a useful legacy of the factory – which contributed much to the local economy during its long operation.

Remediation of the ponds via sediment excavation was initially considered but, during the development of the excavation plan, a risk of acid sulfate soils in the natural underlying sediments was identified. Removal of the shallow sediments would therefore require an extensive soil dewatering management plan, with considerable risk of heavy metal mobilization through the groundwater as a result of potential groundwater acidification.

In addition we identified that – although the sediments are contaminated – the ponds support a healthy aquatic life including longneck freshwater turtles, Koi and an abundance of other amphibians and invertebrates. Remediation of the ponds via excavation would require this aquatic life be captured and relocated to nearby wetlands. Heavy earth moving vehicles would then be required to dredge the ponds and remove the contaminated sediments – potentially destroying the surrounding vegetation which provides an ideal habitat for a diverse ecosystem.

Best practice for the management of acid sulfate soils recommends that where possible the soil should remain undisturbed. Given this – and as the benefits of removing the sediments appears to be outweighed by the negative impacts on the local ecosystem and potential risk of groundwater acidification – we produced an alternative remediation strategy which involves leaving the sediments undisturbed and allowing natural attenuation of the contaminants to occur. As the source of nutrient contamination – i.e. the factory – has been removed and a condition will most likely be placed on the land Title prohibiting the use of groundwater as a potable source or for irrigation purposes, the risk to human health is considered negligible. Natural attenuation will enable improvement of the water quality down gradient of the ponds, while preserving the integrity of the ponds – which are thriving natural wetlands.

This approach to the pond sediment contamination problem has produced a potential cost saving to the client of around $500,000 by avoiding earth works, haulage and disposal of contaminated materials and acid sulfate soil management during dewatering works.