An underground gasoline storage tank at a service station has leaked its content into the upper unconfined aquifer, resulting in the presence of NAPL (non-aqueous phase liquid). Gasoline NAPL’s constitute a long-term source for BTEX (benzene, toluene, ethylbenzene, xylenes) compounds.

 

Perth is situated on the Swan Coastal Plain (alluvial and aeolian), which is characterized by a lack of surface drainage due to the high permeability of the sediments.

 

As to the local hydrogeological setting of the site under study, 7-12 m of medium to fine sand dunes of the Quaternary Bassendean Sands overlies a thick clay aquitard. In the water table fluctuation zone, an iron-stained dark-brown sand layer with a thickness between 5-60 cm is found. The saturated thickness of the unconfined aquifer is ca 6 m and, depending on the season, the phreatic level can fluctuate up to 1.80 m. The local groundwater velocity is within the 100-170 m/yr range (Thierrin et al., 1995) and the flow is oriented in the southeast direction. 

 

Anaerobic sulphate-rich groundwater conditions predominate in the affected aquifer zones. As it turned out from Davis et al. (1999) research, sulphate seems to act as the major electron acceptor for the anaerobic biodegradation reactions.