 These occurred in the early 1990s, and were caused by failure of cement behind the conductor and the 9 5/8-inch casing.This was identified as a result of five groundwater monitoring boreholes installed at the Singleton Oil Field in 1993.The leak was from the well cellar (cement-lined cavity in which the well-head sits) via the preinstalled conductor and the 9 5/8-inch casing, both of which appear not to have been adequately cemented in-situ in at least one well.