Application of hydrochemistry and residence time constraints to distinguish groundwater systems in the Karoo Basin prior to shale-gas exploration

Swana, Kelley (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.


ENGLISH ABSTRACT: The possibility of shale-gas development in the semi-arid Karoo Basin, South Africa has created the need to develop a hydrochemical baseline for deep Karoo groundwater. However, it is uncertain how the process of hydraulic fracturing (fracking) will affect the surrounding groundwater and environments, particularly shallow groundwater sources. Therefore, it is important to have a good understanding of the characteristics of the deep groundwater and its connectivity to the shallower groundwater systems which are of utmost importance to the farmers and local communities in the region. As a result of the requirement for information regarding the deeper groundwater in the Karoo Basin, this project was initiated in order to create a baseline of the deeper groundwater and assess how it differentiates from the shallow groundwater. Nineteen groundwater samples were collected from 8 locations throughout the Karoo Basin. At each site a deep sample was collected from a warm spring or borehole, and a corresponding shallow site was collected from a nearby shallow borehole. Following an initial assessment of the groundwater samples, three groups were identified; deep, shallow and mixed. The shallow samples could be identified by temperatures less than 25°C, high alkalinities, the presence of Mg2+, NO3- and U, as well as higher δ2H and δ18O values. Warmer temperatures usually above 25°C, low alkalinities less than 80 mg.L-1 HCO3-, elevated Na+ and F- as well as lower δ2H and δ18O values were characteristic of the deep samples. The results of the mixed samples consistently feel between those of the deep and shallow samples, indicates that natural mixing occurs between the deep and shallow aquifer systems. This has important implications as upward migration of poor quality deep groundwater and contaminants resulting from fracking activities have been known to occur in the USA. Significant differences are observed between the deep and shallow samples, resulting from different controls on the groundwater chemistry. For example, the process of nitrification occurs in the shallow groundwater resulting in elevated NO3- concentrations, whereas the dissolution of fluorite results in elevated F- concentrations in the deep groundwater. Furthermore, increased contact time with the host lithologies results in increased Na concentrations in the deep groundwater. Residence times of the groundwater samples were calculated using four isotopic tracers, namely, tritiogenic helium, radiocarbon, helium-4 and chrlorine-36. Overall, the deep groundwater samples showed much longer residence times than shallow groundwater, with radiocarbon producing the most probable residence times. Pollution and contamination of shallow aquifers as a result of fracking is possible. Therefore, continuous baseline monitoring is essential before, during and after fracking activities in order to identify abnormalities that could be associated with fracking.

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