Innovative Groundwater Solutions
The Department of Agriculture and Food, Western Australia (DAFWA) is currently undertaking a Royalties for Regions funded project in the La Grange Groundwater Area south of Broome. The focus is on providing improved confidence about soil suitability and water availability for potential expansion of irrigated agriculture in the region. Groundwater in the Broome Sandstone Aquifer (BSA) is the target water resource, however before the La Grange project commenced little was known about the physical and chemical attributes of the aquifer. This report provides the results of estimated groundwater recharge and residence times using environmental tracer techniques.
Sampling of 12 existing bores in the Broome Sandstone Aquifer during October 2014 has shown that the major ion chemistry is similar to that found previously in the LaGrange area. The Na-Ca-Cl-HCO3 to Na-Cl compositions are consistent with a source of solutes from aerosols in rainfall of marine origin, followed by minimal water-rock interaction.
Measured carbon-14 activities have been converted into apparent groundwater ages. After using several of the most widely used corrections schemes, which account for the addition of ‘dead’ carbon to groundwater by carbonate mineral weathering, most samples appear to be modern. Considering potential errors in sampling and analysis, and assumptions required for the correction schemes, these ‘modern’ waters may be up to 500 years old. The fact that these groundwater samples are so young in a regional groundwater system reflects the shallow depth of the bores compared to the total thickness of the aquifer, as well as the dominance of episodic recharge mechanisms across different soils and landforms.
In contrast, waters sampled from Munro Springs and the nearby outstation bore are in the order of 3000-5000 years and 10 000-12 000 years old, respectively. A recent AEM survey has been interpreted to suggest that structural controls and/or local topographic effects, plus potential coincidence of palaeochannels, may mean this water has been sourced from deeper parts of the Broome Sandstone originating further east.
The full range of groundwater carbon-14 ages and the steady-state chloride mass balance suggest long-term average recharge rates are in the range 5-15 mm/y. This range should now be tested through the proposed numerical groundwater flow model(s) for the region.
Stable hydrogen and oxygen isotope compositions of groundwater samples are all relatively depleted in the heavy isotopes compared to the composition of bulk rainfall samples, particularly rainfall in the dry season. This data supports a conceptual model of recharge only occurring after very intense monsoonal rainfall events in the wet season. These results are also similar to previous work in the West Canning Basin, the Dampier Peninsula, and the Fitzroy River catchment.
Number of Pages
Broome Sandstone aquifer, La Grange area, hydrochemistry, groundwater recharge, irrigated agriculture
Harrington, GA 2015, Preliminary isotope assessment, La Grange Groundwater Area, report to Department of Agriculture and Food, Western Australia, Innovative Groundwater Solutions Pty Ltd, Middleton.
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