Additional investigations were carried out by the Water and Rivers Commission to determine whether the Dianella contamination problem was an isolated occurrence or a more widespread contamination problem. Investigations were carried out around five pest control depots in the Perth metropolitan area in 1995 and 1996 (Appleyard et al., 1997). A number of pesticides were detected in groundwater near washdown areas, with concentration of some pesticides exceeding ANZECC criteria for drinking water and the protection of aquatic ecosystems. Persistent organochlorine pesticides such as dieldrin, which have not been used for many years, are still being detected in groundwater. The most widely detected pesticide in groundwater was diazinon. This pesticide is not normally considered to have a high mobility in water, but its mobility is being enhanced by washdown water being discarded to infiltration basins and soakwells.