MODELING SALTWATER INTRUSION IN THE LOWER BURDEKIN DELTA, NORTH QUEENSLAND, AUSTRALIA

Overexploitation of coastal aquifers causes seawater intrusion and eventually salinisation of groundwater in many parts of the world. Artificial recharge can be used to maintain groundwater levels and subsequently control seawater intrusion. This technique, however, is often costly and ineffective in areas where excessive groundwater pumping occurs.

The Burdekin Delta is a major irrigation area situated in the dry tropics of North Queensland and provides a major water resource for the irrigation of sugarcane. The Delta is unique because it overlies shallow groundwater system and is close to the Great Barrier Reef. Water management practices include large recharge pits to assist with artificial replenishment of groundwater systems. More than 1800 groundwater pumps are used for irrigation purposes and excessive pumping has resulted in seawater intrusion near the coastline.

In this paper we describe the use of a variable density flow and solute transport model, SUTRA, to define the current and potential extent of saltwater intrusion in the Burdekin Delta aquifer under various pumping and recharge conditions. A 2-D vertical cross-section model has been developed for the area, which accounts for groundwater pumping and various artificial recharge schemes currently being used in the delta. The Burdekin Delta aquifer consists mainly of sand and clay with granitic bedrock. The model domain uses vertical cross sections along the direction of groundwater flow. A typical vertical cross section with a depth of 30 m on the left boundary, 45 m on the right boundary (facing seawater), and 5000 m in length was used. The initial conditions were based on the land use prior to agricultural development in the area when the seawater interface was in its assumed natural state. The results address the effects of seasonal variations in pumping and artificial and natural recharge rates on the dynamics of saltwater intrusion. The simulation has been carried out for a range of recharge, pumping rates and hydraulic conductivity values. For a typical run with average values for these parameters, the groundwater salinity (³10 % of seawater salinity) moves about 3 km in the aquifer in 30 years. The impacts of possible management scenarios on groundwater quality have also been investigated.