ESTIMATING RECHARGE THROUGH AN ARID ZONE RIVER: A COMPARISON OF GROUNDWATER TRACER AND PHYSICAL TECHNIQUES, GREAT ARTESIAN BASIN, AUSTRALIA

The Great Artesian Basin (GAB) is Australia's largest groundwater resource extending over 1.7 million square kilometers or 22% of the continent. The southwest GAB is located in arid central Australia and contains the only reliable water source in the region. Obtaining rigorous estimates of groundwater recharge to GAB aquifers is critical in understanding the balance between the water requirements of environmental assets, such as the iconic mound springs, and the increasing demand for consumptive groundwater use. Estimating recharge in this environment is challenging due to the paucity of groundwater and surface water monitoring infrastructure, time series water level data and the irregularity of flood events. Environmental isotopes have been used to characterize focused recharge from the Finke River system to the GAB aquifer and to estimate recharge rates. Those estimates are then compared with recharge rates calculated from physical techniques.

Results indicate groundwater close to the Finke River has a depleted stable isotope signature (d¹⁸0 = -9.37 to -10.35‰) implying a rapid recharge mechanism. In contrast, groundwater distant to the Finke River (>25km) has an enriched stable isotope signature (d¹⁸0 = -7.83 to -6.09‰) suggesting longer residence time in the soil zone. Carbon-14 (¹⁴C) activities are elevated around the Finke River (78 - 101 pMC) and show a clear decline with distance from the river (3 – 13 pMC). Results confirm the operation of the Finke River as a recharge sink. Elevated ¹⁴C concentrations (>85 pMC) suggest a thermonuclear component and the presence of modern groundwater. Recharge rates were calculated from ¹⁴C derived groundwater velocities applying a method based on Vogel (1967). Groundwater level and stage height data were used in an analytical mound recession model (Hantush, 1967) to estimate recharge from a recent flood event. Annualized recharge rates from the physical methods (500 mm/year) compare favorably with the environmental isotope inferred recharge (240 – 900 mm/year).

Vogel, J, C., 1967: Investigation of groundwater flow with radiocarbon. - In: Isotopes in Hydrology, International Atomic Energy Agency, Vienna pp 355-369.

Hantush, M, S., 1967: Growth and decay of groundwater-mounds in response to uniform percolation. Water Resources Research. Vol 3, No 1, pp 227-234