Using Environmental Tracers to Characterize Groundwater Flow Mechanisms in the Fractured Crystalline and Karst Aquifers in Upper Crocodile River Basin, Johannesburg, South Africa

Khahliso Leketa,Tamiru Abiye

doi:10.3390/hydrology8010050

Abstract

Environmental isotope tracers were applied in the Upper Crocodile River Basin, Johannesburg, South Africa, to understand the groundwater recharge conditions, flow mechanisms and interactions between surface and subsurface water. Stable isotope analysis indicated that recharge into the fractured quartzite aquifer occurs through direct mechanisms. The high variability in the stable isotope signature of temporal samples from Albert Farm spring indicated the importance of multiple samples for groundwater characterization, and that using a single sample may be yielding biased conclusions. The observed inverse relationship between spring discharge and isotope signature indicated the traces of rainfall amount effect during recharge, thereby suggesting piston groundwater flow. It is deduced that a measured discharge value can be used in this relationship to calculate the isotopic signature, which resembles effective rainfall. In the shallow alluvial deposits that overlie the granitic bed-rock, piezometer levels and stable isotopes revealed an interaction between Montgomery stream and interflow, which regulates streamflow throughout the year. This suggests that caution should be taken where hydrograph separation is applied for baseflow estimates, because the stream flow that overlies such geology may include significant interflow. The hydrochemistry evolution was observed in a stream fed by karst springs. As pH rises due to CO2 degassing, CaCO3 precipitates, thereby forming travertine moulds. The values of saturation indices that were greater than zero in all samples indicated supersaturation by calcite and dolomite and hence precipitation. Through 14C analysis, groundwater flow rate in the karst aquifer was estimated as 11 km/year, suggesting deep circulation in karst structures.

Highlights

Groundwater plays a central role in socio-economic development of regions with semi-arid/arid climate [1,2]
The stable isotope analysis indicates that the fractured quartzite aquifer that feeds Albert Farm spring is recharged via direct recharge mechanisms
Similar to rainfall events in Johannesburg, which have multiple moisture sources, the variability of stable isotope signatures and d-excess in Albert Farm spring is a reflection of variations in rainfall moisture sources rather than the variations in recharge mechanisms

Summary

Introduction

Groundwater plays a central role in socio-economic development of regions with semi-arid/arid climate [1,2]. Its continual discharge through springs and river beds contributes to stream flow, and apart from the melting of snow in cold regions [3,4], groundwater discharge, called baseflow, is often the main reason why perennial streams are able to sustain flows in dry seasons [5,6]. Baseflow occurrence is quite uncommon in arid and semi-arid regions because of the difficulty of direct recharge [1,7,8]. Because baseflow occurrence is not often expected in arid and semi-arid regions, the existence of perennial springs and streams in this climatic setting calls for a need to assess recharge and flow mechanisms associated with them, in order to support decision making for sustainable groundwater management of the aquifer

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Conclusion