Former Gasworks Demolition and Remediation: Tar, Cyanide and the Tanks on No Drawing

Every Australian town of any size had a gasworks until the 1960s or 1970s, and most of those sites have been through two or three uses since without anyone dealing with what is in the ground. Coal tar, spent oxide with cyanide, ammoniacal liquor, benzene and a gasholder base full of tar sludge do not go away. When the site is finally redeveloped, the remediation is the project and the demolition of whatever stands on it is the opening move.

This is how we approach former gasworks demolition and remediation, what will hurt you, and the mistakes that turn a redevelopment into a decade of liability.

What you are actually dealing with

• Coal tar in the ground as free product, dense and heavier than water, sitting in tar wells, under retort houses and pooled on the clay beneath the site. It moves slowly and it is still moving.
• Gasholder bases: large circular brick or concrete tanks, often several metres deep, full of water and tar sludge, sometimes built over and forgotten.
• Spent oxide, the purifier waste, containing cyanide complexes and sulphur, often spread across the site as fill or dumped in a corner.
• Ammoniacal liquor and phenols in the groundwater, and benzene, toluene and naphthalene vapours that make the site an occupational hygiene problem as well as an environmental one.
• Asbestos in whatever buildings remain and in the fill, and lead from the retort house and the paint.
• Underground structures everywhere: tar wells, liquor tanks, flues, pipes, retort foundations. The site plans, if they exist, are not complete.
• Neighbours. Gasworks were in the middle of town, so the site boundary is a street, a school or someone's back fence.

Step one: investigate properly

A detailed site investigation with enough boreholes, test pits and groundwater wells to find the tar, map the underground structures and define the plume. A hazardous materials survey of the standing buildings. A geophysical survey to find the gasholder bases and tanks that the drawings missed. On a gasworks the investigation stage saves its cost several times over, because every structure found in the ground during demolition stops the job.

Step two: agree the end state and the remediation strategy

Residential or school use means removing or treating the source material and validating to a strict standard. Commercial or industrial use may allow containment and capping. The strategy is agreed with the EPA and, in NSW, usually a site auditor, before anything is priced. It sets the excavation depths, the treatment or disposal route for each material and the validation plan.

Step three: demolish with the ground in mind

Buildings and slabs come down with the asbestos and lead dealt with first, but the slabs and hardstand are left in place until the remediation sequence calls for them to go, because they are capping the vapours and the tar. Demolition plant works to a plan that avoids driving tar deeper or breaking into unidentified tanks. Debris is kept separate from contaminated soil so clean concrete can be recycled.

Step four: the gasholder bases and tar wells

These are the hazardous part. Water is pumped out, treated and disposed of under trade waste or EPA approval. Tar sludge is removed by vacuum or excavator under vapour controls, with air monitoring for benzene and naphthalene at the work face and the boundary. The structure is then broken out and the ground beneath it excavated to the depth the investigation set. Odour management is a daily task, not an afterthought; neighbours complain about gasworks sites more than anything else we do.

Step five: excavate, classify, treat or dispose

Contaminated soil is excavated in a staged sequence, classified by zone, and either treated on site (bioremediation, stabilisation or thermal treatment where the volumes justify it) or taken to a facility licensed for the classification. Spent oxide and tar impacted material carry the highest classifications and the highest disposal cost, so separating them cleanly from lesser affected soil is where the money is saved. Groundwater is managed throughout with dewatering, treatment and discharge approvals.

Step six: validate and close out

Validation sampling of the excavation floor and walls, clean fill placement, groundwater monitoring, and the site audit report or validation report the planning consent requires. The documents are what let the land be sold or built on; the remediation is not finished until they are.

What not to do

• Do not break a gasholder base open without knowing what is in it and where the water will go.
• Do not lift the slabs early. They are capping the vapours and the odour.
• Do not mix tar impacted soil with the rest. The whole load takes the highest classification.
• Do not skip the geophysical survey. The drawings do not show the tanks that were built over.
• Do not start without an odour and vapour plan. The neighbours will stop the job before the EPA does.

Approvals and notifications

Planning consent with remediation conditions, EPA notification and in many cases a declared or regulated site process, site auditor engagement in NSW, dewatering and discharge approvals, licensed demolition and asbestos notifications, waste tracking for every regulated load, and a community notification plan. Councils will expect a remediation action plan before consent and a validation report after.

Who should be doing this

A contractor that runs soil and environmental remediation as a core service alongside demolition, with its own excavation plant, its own Class A asbestos crews, a working relationship with site auditors and the EPA, and experience managing odour and vapour next to houses. Perfect Contracting carries out contaminated site remediation and demolition across Australia from its Sydney and Brisbane bases. Read more about our soil and environmental remediation and bioremediation work, or send us the details.