Between1972 and 2015, about 150 gigalitres of groundwater was extracted from artesian bores tapping the Gilbert River aquifer (located near Weipa Cape York Peninsular, Queensland, Australia). This extraction resulted in a cone of depression extending up to 80 kilometres from the borefield as defined by the one metre drawdown contour. There are proposals to establish addition borefields and an assessment of the likely impacts was required.
The specific objectives of the assessment were to:
The average annual rainfall over the recharge beds increases in a northerly direction from 1140 mm/year (near Coen) to 1950mm/year (near Heathlands).
In previous groundwater studies a constant annual recharge of 35 mm/year was applied over the recharge beds. A feature of this study area is recharge emerges as baseflow to streams. Stream measurements suggest recharge is influenced by antecedent dry conditions and to consecutive rainfall days. In order to replicate this aspect, daily estimates of recharge were derived for 12 rainfall locations using the SplashMulti software. It was found recharge is more episodic and has higher spatial and temporal variation then previously considered.
In order to account for the spatial and temporal variations in projected borefield extraction, recharge, and stream base flow, a transient regional groundwater flow model (Modflow_USG) at the day scale was required and was successfully calibrated to the groundwater level and stream flow data.
Scenario modelling indicates that after 40 years of additional demand, the cone of depression is likely to extend up to 120 kilometres from the proposed borefields that after 100 years from the cessation of pumping, there is near full recover.