Background and Objective
The former Bacchus Marsh gasworks (1889 – 1973) is located within a wide alluvial plain of the Werribee River. Shallow groundwater on the alluvial plain occurs in an upper alluvial aquifer and is highly utilised for irrigation and rationed through the auctioning of water shares. The presence of coal tar NAPL (and associated dissolved phase groundwater impacts) emanating from former gasholders and tar and liquor disposal wells into the upper alluvial aquifer impinges further on upper alluvial aquifer utilisation. This presentation focusses on remediation of two coal tar NAPL plumes that was conducted in-situ attempting to meet Victorian EPA and client sustainability goals.
After extensive review of possible sustainable remediation options, surfactant enhanced aquifer remediation (SEAR) was selected to address the coal tar NAPL in the aquifer at the site. The surfactant selected was a non-ionic surfactant engineered to dissolve long-chain petroleum hydrocarbons. The surfactant was used in a variety of push-pull and recirculation methods to enhance coal tar recovery. The majority of coal tar NAPL was slightly denser than water (1.03 SG), had low viscosity at 15 degrees Celsius and rested within the poorly sorted clayey, sandy gravel lenses of the upper alluvial aquifer. After initial push-pull applications of SEAR, recirculation was established between the injection well and an average of three extraction wells. Approximately 7 different injection wells were utilised in each of the two coal tar NAPL plumes.
The mobilised NAPL and emulsified hydrocarbons were removed using multiphase extraction (MPE) technologies. Wastewater was treated on-site and reinjected under permit from the regulatory agency to the extent that was practicable. Reinjection of the water was conducted in a manner to optimise the surfactant plume behaviour while it was being extracted.
SEAR remedial efforts are ongoing and have resulted in the extraction of coal tar LNAPL and DNAPL, while extracting dissolved contaminants (benzene, naphthalene, ammonia, cyanide) on to activated carbon and resin filters. Most extracted groundwater was returned to maintain/restore aquifer capacity, thereby meeting the EPA’s and the client’s sustainability goals.
SEAR is a viable method of groundwater remediation to reduce recalcitrant hydrocarbon mass and decrease overall plume lifespan in unconsolidated aquifers.