Groundwater extraction in the Werribee irrigation district and the potential for salt water intrusion

Abstract

A network of channels supplies diverted river water to the Werribce Irrigation District for irrigation of market gardens. Historically the surface water supplies have been supplemented by groundwater extraction from the shallow alluvial sediments and basalts that underlie the district. In 2002 the surface water deliveries were drastically reduced due to drought and associated low river flows. As a result the rate of groundwater extraction from the aquifer increased to meet the shortfall in water supply for irrigation. The combination of reduced recharge through irrigation accessions and the increased pumping stresses placed on the aquifer led to a dramatic draw down in groundwater heads. Fears of salt water intrusion led to the enforcement of restrictions on groundwater extraction. A numerical groundwater model was formulated and calibrated against measured groundwater responses across the aquifer. Modelling results suggest that safe levels of groundwater extraction are limited by the potential for saline water leakage from the Werribee River and by the potential for salt water intrusion from the ocean. A resource management policy has been developed that relies heavily on the numerical groundwater model to predict the potential saline water inflows under various extraction conditions.