Townley, L.R., and Chapman, B.M. (1993), Hydrogeological and hydrogeochemical implications of flooding the Enterprise Pit, Confidential report to Energy Resources of Australia Ltd, CSIRO Minesite Rehabilitation Research Program, 37pp., July 1993. [Confidential]

One option for decommissioning the Enterprise Pit at Pine Creek is to flood the Pit by diverting Pine Creek into the Pit and controlling the water level within a predetermined range. In order to be between the levels of a diversion structure on Pine Creek and the receiving waters in Copperfield Creek, the Pit water level will need to be between about 1197 and 1199 m, relative to Mine Datum. Diverting 100% of the flow in Pine Creek will allow the Pit to fill in about 5 years. Without any diversion, the Pit may never fill. The best way to ensure that good water quality in the Pit is to flood the Pit as fast as possible, Conversely, a decision not to divert Pine Creek into the Pit would almost certainly result in very poor water quality in the Pit. If the Pit is flooded, we believe there will be minimal impact on water supply bores for the township of Pine Creek.

This Report reviews the hydrogeology of the region near the Enterprise Pit. A discussion of local hydrogeology addresses three different periods: before, during and after the flooding of the Pit. We discuss possible sources of contamination entering the Pit, including acid and metal ions generated from the walls of the Pit, waste rock deposited inside the Pit, the northern waste rock dump and Czarina Pit. We have performed water balance calculations to support earlier predictions by AGC Woodward-Clyde, and also as a basis for simplified solute balance calculations. The latter demonstrate the need for surface water throughflow in order to flush the Pit and maintain good water quality. The last Sections of this Report identify a number of hydrogeological and hydrogeochemical implications of flooding the Pit and contain recommendations for monitoring before, during and after the flooding. Given the limited experience in Australia with intentionally flooding final voids, monitoring at the Enterpise Pit may provide an opportunity to obtain data which will benefit the mining industry in the longer term.

A revised Version 1.2 of this report was released in August 1994, and is available for download.