HYDROLOGY OF THE COCOA CREEK MUDFLAT, QUEENSLAND, AUSTRALIA: DOES IT FIT THE CURRENT SABKHA MODELS?

Previous sabkha hydrologic models do not account for the observed hydrologic processes that control ground-water and brine movement. Our investigation of Cocoa Creek mudflat indicates that multiple models may be needed to account for different sabkha hydrologic environments. Lateral ground-water flow in Cocoa Creek mudflat is controlled by a low permeability Pleistocene high near Mount Cleveland. Mount Cleveland is connected to the mainland by a tombolo, and Cocoa Creek inlet flows southeast to northwest through the middle of the tombolo. Multilevel piezometer nests (3 - 9 m deep) show that the brine concentration of the ground water generally increases with depth below marginal areas of the mudflat to a maximum of 97 ppt and in the barren mudflats to a maximum of 183 ppt.

Salinity-corrected ground-water head data from the piezometer nests indicate that the vertical component of ground-water flow is downward within most of the mudflat, and lateral flow generally is towards Cocoa Creek. The downward flow indicates that surface water is recharging the mudflat. Recharge from the surface occurs during spring tides when high tides flood the mudflat areas between shellbed ridges. The downward flow of surface seawater into the mudflat indicates that the mudflat currently is accumulating salts from seawater by evaporation after flooding. The downward flow may be driven by density differences that occur when the salts left on the mudflat after surface waters have evaporated are redissolved by the next high tide. The combination of downward head and greater density water pushes the more saline water to the bottom of the mudflat.

Possible mechanisms that induce brine accumulation in the mudflats include evaporative pumping, seawater flooding, or upward discharging of regional ground-water brines (new Abu Dhabi sabkha model). Although information on regional ground water is unavailable for the Cocoa Creek mudflats, the geology of the area indicates that a low permeability Pleistocene layer exists below the Holocene sediments that restricts upward ground-water flow. In addition, the downward ground-water heads in the shallow aquifer indicate that contributions from deeper ground water are not significant. Seawater flooding is the best explanation for the accumulation of brines on the mudflat given the current data.