Environmental Service & Design Pty Ltd were contracted to clean up and rehabilitate the silica tip at Stowport after the operator of the tip had a major slumping incident resulting in silica contaminating the catchment. Several properties had their dams contaminated and silica found its way to the Emu River.
What initially seemed a straight forward tip rehabilitation quickly became a complex project requiring reverse engineering and a major public consultation process. The historical uncontrolled nature of the land prior to use as a silica site, posed a significant risk and environmental managment issue. The outcome of the project was not just the rehabilitation of the site, but also a public education process involving silica health issues and the regulation of uncontrolled tip sites in the area. The rehabilitation created a working relationship between the consultants and Environment Division that resulted in what was learnt from the project being incorporated into permit conditions for a new silica tip lifting the standard. The new tip site was designed to enable recovery of silica in the future and not allowing issues that occurred at the old site develop. Currently both silica producers in Tasmania use the new secure site developed.
The rehabilitation plan was engineered in consultation with a large community group, local government and specialist consultants. The design had to be innovative to produce a stable site that prevented the silica from slumping and ensuring that large trees did not uncover silica in the future. The thixotropic nature of the silica made for a complex solution.
The EPN requirements were included into the rehabilitation plan. The civil engineer developed a plan to contour the silica into a stable configuration. Using reverse engineering, he looked at where the original tip design had failed and designed a new toe further down the valley. He needed to put a drainage blanket at the base to collect water and allow it to drain to a sample tank for checking leachate flows and quality. He developed a tell-tail overflow that overflows only if the drainage blanket blocks with fine silica, which could indicate a potential embankment failure due to excessive pressure at the embankment toe.
The site was then clay capped and diversion drains established at the boundaries. Topsoil was spread and native grasses planted. Large trees will be removed from the site as should they fall they would expose the silica and allow water infiltration.
Monitoring of the site has continued since completion of the civil works and the annual reporting to the stakeholders and DPHAE has confirmed that contamination levels in the water has dropped away quickly and is now approaching background levels. Physical monitoring quarterly has confirmed that there is no silica escaping the site and the ecosystem is recovering.
Consultation allowed the total cost of the rehabilitation to be kept below $400,000; by working with the community the material removed from the Stowport Hill Road Upgrade could be used as capping material. The coordination saved Burnie City Council significant costs in disposal and the rehabilitation project costs in purchasing and transporting capping material.