Evaporative discharge of groundwater from the margin of the Great Artesian Basin near Lake Eyre, South Australia

Author: Peter Horace Woods

Woods, Peter Horace, 1991 Evaporative discharge of groundwater from the margin of the Great Artesian Basin near Lake Eyre, South Australia, Flinders University, School of Chemistry, Physics and Earth Sciences

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Abstract

The Great Artesian Basin is Australia's largest groundwater basin and represents an important resource. Its overall water budget implies that about half the discharge is by bores, and of the remaining discharge only a small portion emerges in springs, the rest is assumed lost by leakage to the overlying water table, from where it evaporates. This leakage is assessed using a steady-state convection-diffusion model to estimate evaporation from vertical profiles of the concentration of chloride and deuterium in the unsaturated and saturated zones. The theory has been extended to account for shallow water tables, and a nomogram constructed that enables the estimation of evaporation from a plot of ln(c-cres) vs depth, as is used with an infinite bottom boundary condition here and in earlier work. An important parameter needed to make the calculation is the impedance factor; this is examined in detail using existing literature. To make the estimates of evaporation, isotopically representative water must be extracted from soil samples. Extraction methods have been extensively investigated. Azeotropic distillation with hydrocarbons is a robust and reliable technique. The use of toluene gives best results for water, and kerosene for non-gypseous porous materials, with accuracies of about 1.5 and 0.2% for [delta]D and [delta]18O respectively. Errors increase at low soil water contents (high matric suction). The use of hexane, with distillation time restricted to two to three hours, provides pore water from gypseous soils. The distillate is isotopically biased by about -3% and -1.1% for [delta]D and [delta]18O respectively, with an accuracy of about 2% and 0.3%. This analytical advance permits the extraction of both pore and crystal water from the same gypseous sample. An international intercomparison of techniques to determine the isotopic composition of soil water was conducted. This reveals a much greater scatter of results than the accuracy claimed by most workers. Comparisons of the absolute values of soil isotopic data between laboratories using different techniques should be made with caution. Twenty-four holes were drilled in a field program near Lake Eyre in South Australia. Evaporation estimates could be made from 11 holes, and range from about 0.5 to 4.5 mm yr-1. The average leakage is estimated to be between 2 and 4 mm yr-1 above the artesian parts of the basin margin in the field area, and amounts to five to ten times the combined discharge of springs in the area. The measurement show that leakage to the water table and subsequent evaporation can account for a large part of the water balance of the basin, and this conclusion is also supported by other isotopic and hydrochemical evidence. The natural leakage of artesian water from the basin margin gives a limit to the amount of the resource that can be safely withdrawn by users, in this case a large mining venture. The mine has an expected lifetime of several decades and careful management of the borefield is required to sustain the water supply in the long term.

Keywords: Great Artesian Basin,hydrogeology,evaporative discharge,deuterium,oxygen-18,azeotropic distillation
Subject: Earth Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 1991
School: School of Chemistry, Physics and Earth Sciences
Supervisor: Dr H H Veeh