Use of natural isotopes and groundwater quality for improved recharge and flow estimates in dolomitic aquifers

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.