ABSTRACT
The Yanchep Caves, Western Australia, Perth, contain a unique ecological system that is dependent on groundwater for its survival. Due to declining groundwater levels in the Gnangara Groundwater Mound in the same area since 1969, mainly from reduced rainfall, increased use of groundwater for public and private water abstraction and pine plantations, the groundwater levels under the caves have declined thus increasing the stress on the dependent cave fauna. Despite the artificial sprinkling system for protecting the fauna, the water levels have continued to decline since monitoring began in 1991. It has become obvious that a more robust and generous provision of water is needed. To evaluate the amount of groundwater required for each of the caves and to determine the effects of pumping groundwater from the superficial aquifer, a three-dimensional groundwater flow model was constructed using Visual MODFLOW Pro version 4.1 coupled with MODFLOW SURFACT. The models estimate that a total discharge rate of up to 3.6 Gl per annum would maintain water in ponds in each priority cave in both summer and winter until 2015. The effects of pumping on groundwater dependent environments have been evaluated. The cone of depression resulting from pumping of these bores stabilises in two years and indicates that there may be a 0.5 m drawdown within approximately 1 km of the boreholes with no impact on the lake system towards the east after the second year of operation.
4.1 INTRODUCTION
