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Nelson Mandela University (2019)

Testing potential groundwater reservoir connectivity using isotopes in the south-central Karoo region

Dlakavu, Sinazo

Titre : Testing potential groundwater reservoir connectivity using isotopes in the south-central Karoo region

Auteur : Dlakavu, Sinazo

Université de soutenance : Nelson Mandela University

Grade : Masters in Geology/Geoscience 2019

Résumé
Documenting areas with natural groundwater reservior connectivity using isotopes before the anticipated shale gas development in the Karoo Basin of South Africa could contribute to developing legislative framework for hydraulic fracturing. In this study, groundwater dynamics (primarily connectivity between old/deep and low temperature modern/shallow (<25°C) groundwater) are studied using isotopes (14C, δ18O-δ2H, 3H, and δ13C) in conjunction with geochemical parameters (sampling depth (masl) and (mbgl), temperature (°C), Electrical Conductivity (EC) (μS/m) and sustainable yield (l/s)). Previous groundwater studies conducted in the Karoo Basin suggest that 14C (pMC) has the least overlap among isotope signature constraints between the different aquifer/groundwater groups. The outcomes of 14C analysis for this study suggest that one groundwater sample is an old/deep groundwater sample (<53 pMC), and that three groundwater samples are composed of shallow/young groundwater mixed with old/deep groundwater (<74 pMC). These four relatively older samples have calculated 14C ages ≥ 1700 years and were collected from boreholes that are north-east of the Cape Mountains ( 50 km). The position of these samples suggests that deep-seated faults within the Cape Mountains and the basement of the southern Karoo Basin act as preferential pathways for deep groundwater migration (as is consistent with existing deep geophysical data) to shallower depths (<60 mbgl) where it cools and/or mixes with low temperature groundwater (<25°C). Three more groundwater samples were found to match the relatively old calculated 14C ages (≥ 1700 years) of the groundwater samples collected close to the Cape Mountains even though they have modern 14C (pMC) concentrations (>74 pMC). This implies that adjusting the 14C content in Dissolved Inorganic Carbon (DIC) based on its dilution by 14C free carbonates is influential in controlling the mixed groundwater ages, and that relatively older groundwater occurs at shallow depths in other parts of the study area possibly due to linked fault systems between deep and shallow aquifers. δ18O-δ2H relationships for the sampled groundwater suggest that groundwater samples collected within the main drainage of the Great Fish River plot close to the Global Meteoric Water Line (GMWL) indicating that the recharge water to this groundwater does not experience significant evaporation. The average isotope composition of the recharging water for the all of the sampled groundwater is -7.90 ‰ and -46.72 ‰ for δ18O and δ2H, respectively. This result plots halfway between rainwater 18O-2H relationship lines for the Indian Ocean and the Atlantic Ocean. This suggests that the rainwater from which the sampled groundwater was derived from evolved from both the Indian and Atlantic Ocean waters

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