What lies beneath? High resolution site characterisation tools used to develop LNAPL conceptual site models

The relationships between observing light non-aqueous phase liquid (LNAPL) in a well and then understanding the amount of LNAPL in the ground, the ability of the LNAPL to migrate, and the potential risks presented to receptors are often complex and mis-understood. Unfortunately, often the default assumptions to LNAPL observations are to overestimate the amount in the formation, its mobility, and the potential risk it presents. High resolution site characterisation (HRSC) tools are designed to collect environmental data at the appropriate scale where heterogeneities in the subsurface control the migration and distribution of contaminants. HRSC is not just tools to collect field data but also involves interpretation, strategies, methodologies, and management/field practices. HRSC was used to develop robust LNAPL conceptual site models (LCSMs) at three sites. Site 1 is an active service station site with multi-walled tanks and HDPE lines. It has a central tank farm. No active leak was identified but LNAPL was observed in a monitoring well during the bi-annual monitoring of Underground Petroleum Storage System (UPSS). Limited sub-surface information from the existing four monitoring wells was available. Site 2 was a 7,200 m² fuel depot decommissioned in 2003. The site had seven underground storage tanks (USTs), three aboveground storage tanks (ASTs), fill gantry, interceptor unit, pipe work, fuel bowsers, fill points, and there was an adjacent fuel depot on the cross-gradient boundary. Previous site assessment works had been undertaken with the installation of 16 monitoring wells. Site 3 was a release from a fuel pipeline. The area of investigation was 21,330 m2 which included a car park, recreational facilities, and a creek. There was no monitoring well network and no historical sub-surface investigations to inform the investigation. (author)