Abstract
The application of a model to several dolomitic aquifers in the RSA is presented and has successfully simulated the reappearance of 14C injected from nuclear tests in the discharge from springs. This is based on a new conceptual model, which accounts for the large variations of 14C in the groundwater still representing recently recharged water. The input of 14C is related to the recharge mechanism to yield low concentrations if the infiltration is direct; and higher concentrations if the recharge water interacts with biogenic CO2 generated in the soil zone. The model has produced estimates of the recharge parameters and their controls but requires an independent estimate of the average recharge e.g. the chloride mass balance method. A close match has been obtained with 14C measurements over the past three decades. A shallow component of the recharge mixes with a larger and older deep-water component. The model has provided the turn-over times of water in the system, which represent the storage capacity as multiples of the mean annual recharge. Quantitative estimates of the recharge of dolomitic aquifers could be derived from the bicarbonate concentrations of the spring waters. The model has also been used to simulate the response of the limited temporal tritium measurements and single determinations of CFC for some springs. This has revealed significant differences that are related to additional dilution of the tritium tracer in the unsaturated zone, in comparison to 14C and CFC.
Highlights
Background to the studyIn view of their importance as significant groundwater resources the dolomitic aquifers of South Africa have been the focus of studies over many years (Enslin and Kriel, 1967; Fleisher, 1981; Taylor, 1983; Bredenkamp et al, (1985); Foster, 1988; Vegter and Foster, 1990; Simonic, 1993; Bredenkamp et al, 1994; Veltman, 2003; Stephens et al.,2004) A variety of methods have been applied to improve the assessment of the recharge and flow characteristics of these aquifers
The successful simulation of the reappearance of the bomb 14C in dolomitic springs, based on the new tracer model programmed in Excel, has:
Provided evidence in support of the bi-modal recharge model controlling the input of 14C, it appears that reasonably good simulations could be obtained by using a simpler model
Summary
Background to the studyIn view of their importance as significant groundwater resources the dolomitic aquifers of South Africa have been the focus of studies over many years (Enslin and Kriel, 1967; Fleisher, 1981; Taylor, 1983; Bredenkamp et al, (1985); Foster, 1988; Vegter and Foster, 1990; Simonic, 1993; Bredenkamp et al, 1994; Veltman, 2003; Stephens et al.,2004) A variety of methods have been applied to improve the assessment of the recharge and flow characteristics of these aquifers. The average recharge could be derived from the spring flows if the recharge area of the springs can be delineated reliably, and the chloride content of the rainfall could be inferred from the best match between the simulated chloride concentrations and the measured chloride of the spring water.
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