Three-dimensional simulation of the home island freshwater lens: Preliminary results

Home Island is one of the two main inhabited islands of the Cocos (Keeling) Islands, situated about 2800 km north-west of Perth. The island has an average elevation of approximately 2 m above MSL and supports a population of about 450. The geology of the island consists of coral sediments overlying a volcanic seamount. The upper sediments are of primary importance from a hydrogeological viewpoint. These sediments consist of unconsolidated coral sands and other fragments and coral limestone. An unconformity between the younger (Holocene) unconsolidated sediments and older (Pleistocene) coral limestone was found at depths between 10 m to more than 17 m. The island has a long term average annual rainfall of 1950 mm. Water supply is based primarily on groundwater extraction from a local freshwater lens and supplementary rainwater is collected from roofs. Groundwater is currently extracted via seven galleries. A recent study based on a two dimensional model (using the SUTRA package), simulating impacts of various level of pumping up to 200 m³ day^-1 determined the sustainable yield as 140 m³ day^-1. This paper describes progress regarding the development of a three-dimensional model of the freshwater lens using the SALTFLOW package, by discretising the aquifer to about 90 000 nodes and 83 000 elements. This is followed by the steady-state calibration of the model, estimation of the parameters affecting the system and assessing the sustainable yield of the lens. Preliminary results of the three dimensional modelling suggest that the sustainable extraction rate is less than 200 m³ day^-1, which is consistent with the results of the two dimensional modelling. This research project is continuing towards simulation of tidal forces, transient calibration of the model over a 10 year period from January 1988 to December 1997, estimation of sustainable yield of freshwater lens and simulation of various management options.