The La Grange project was initiated by the WA government to stimulate the expansion of irrigated agriculture in the West Kimberley. The La Grange groundwater area, south of Broome was targeted because of the potential for rapid expansion of the area irrigated. The Broome Sandstone is the dominant fresh groundwater resource in the 36,000 km2 area, with an allocation limit of 50 GL/a. The objective of this study was to improve the understanding of the soils and groundwater at La Grange so that irrigated agriculture can profitably expand without impacting existing water users and wetlands or inducing seawater intrusion.
Field investigations included a soil survey, bore census, airborne electromagnetic (AEM) survey, drilling, test pumping and groundwater geochemical sampling. Groundwater head data was analysed to produce a contemporary watertable surface, which indicated that most coastal wetlands and larger inland wetlands are reliant on the Broome Sandstone. About 500 km2 of productive soils overlie areas where groundwater is shallow enough to be pumped economically.
The AEM survey provided spatial data on the base of the aquifer and an estimation of the location of the saltwater interface. The AEM was also used to strategically locate bores to provide a regional monitoring network for changes in groundwater level and early warning of movement of the SWI, as well as spatially distributed hydrochemistry sampling points.
Isotope analyses indicate that recharge (11.6 to <16.5 mm/yr) only occurs after intense rainfall events in the wet season. Three hydrochemical methods produced similar estimates. Groundwater in the Broome Sandstone ranges from ‘modern’ at the watertable to older than 20,000 years at its base in coastal areas. Predictions made by a regional groundwater model indicate that using a preferred area map produced during this project to guide abstraction will be significant in minimising impacts on sensitive wetlands and the SWI.