Application of site specific adsorption isotherms for deriving soil remediation goals protective of groundwater – Toowoomba gasworks case study

Soil remediation goals for contaminated sites are typically based on exposure pathways related to human health (e.g. vapour intrusion, direct contact, ingestion etc.). However, in areas where potable quality aquifers are present, soil remediation to levels that are protective of the local groundwater resource, or groundwater-dependent ecosystem, may be required. This particular pathway is not defined by existing Australian guidance due to the uncertainties and complexities associated primarily with this pathway. One of the key sources of uncertainty is the chemical specific soil-to-water partitioning coefficient (KD), which are derived differently for organics and inorganics and can range widely in the published literature. The level of conservatism necessary to address this uncertainty can often lead to remediation targets which are more stringent than they need be; and consequently and unnecessarily increasing remediation timeframes and costs. The Toowoomba Gasworks site overlies the Toowoomba Basalt, a potable aquifer, which is used for urban water supply. The site soils were contaminated with ammonia, benzene, petroleum hydrocarbons, and naphthalene. Soil cleanup targets protective of the underlying basalt aquifer were required as part of the remediation works. In order to minimise uncertainty and develop realistic remediation goals, GHD in conjunction with ALS laboratories developed a soil column leaching method designed to rapidly produce high quality data for constructing adsorption isotherms. The isotherms were used as a basis for deriving site specific KD values, which were then used to produce site specific remediation goals. The application of site specific adsorption isotherms significantly assisted in timely and effectively remediating the site. (author)