Development of a conceptual geohydrological model for a fractured rock aquifer in the Karoo, near Sutherland, South Africa

Mahed, Gaathier

doi:10.2113/gssajg.119.1.33

Cited by 4 publications

(5 citation statements)

References 21 publications

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“…The stable hydrogen isotopic composition in water (δ 2 H–H 2 O) vs. the stable oxygen isotopic composition in water (δ 18 O–H 2 O) of springs (squares) and boreholes (circles) from the Karoo Basin. The bold line denotes the global meteoric water line (GMWL) modeled after Craig () and the dashed line denotes the local meteoric water for the Karoo basin with slope approximately 6.2 (Miller et al ; Mahed ). The Karoo Basin samples show evidence of evaporation consistent with recharge in an arid environment as opposed to the addition of brines.…”

Section: Resultsmentioning

confidence: 99%

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Pre‐drill Groundwater Geochemistry in the Karoo Basin, South Africa

et al. 2018

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Enhanced production of unconventional hydrocarbons in the United States has driven interest in natural gas development globally, but simultaneously raised concerns regarding water quantity and quality impacts associated with hydrocarbon extraction. We conducted a pre-development assessment of groundwater geochemistry in the critically water-restricted Karoo Basin, South Africa. Twenty-two springs and groundwater samples were analyzed for major dissolved ions, trace elements, water stable isotopes, strontium and boron isotopes, hydrocarbons and helium composition. The data revealed three end-members: a deep, saline groundwater with a sodium-chloride composition, an old, deep freshwater with a sodium-bicarbonate-chloride composition and a shallow, calcium-bicarbonate freshwater. In a few cases, we identified direct mixing of the deep saline water and shallow groundwater. Stable water isotopes indicate that the shallow groundwater was controlled by evaporation in arid conditions, while the saline waters were diluted by apparently fossil meteoric water originated under wetter climatic conditions. These geochemical and isotopic data, in combination with elevated helium levels, suggest that exogenous fluids are the source of the saline groundwater and originated from remnant seawater prior to dilution by old meteoric water combined with further modification by water-rock interactions. Samples with elevated methane concentrations (>14 ccSTP/kg) were strongly associated with the sodium-chloride water located near dolerite intrusions, which likely provide a preferential pathway for vertical migration of deeply sourced hydrocarbon-rich saline waters to the surface. This pre-drill evaluation indicates that the natural migration of methane- and salt-rich waters provides a source of geogenic contamination to shallow aquifers prior to shale gas development in the Karoo Basin.

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Section: Resultsmentioning

confidence: 99%

“…The study also included other nearby drinking‐water or agricultural wells for comparison with a total of n = 22 samples. Samples included 2 deep boreholes (depth ranging between 1400 and 4200 m), 9 springs/artesian wells, and 11 groundwater samples from shallow boreholes (Mahed ; Swana ).…”

Section: Study Area and Methodologymentioning

confidence: 99%

Pre‐drill Groundwater Geochemistry in the Karoo Basin, South Africa

et al. 2018

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“…By comparison, heterogeneous, localized groundwater recharge occurs primarily during seasonal flooding events (Le Maltre et al ). Most of this localized recharge occurs within fractured top‐of‐rock aquifers and fractured areas near dolerite dike intrusions, which dominate actively produced groundwater flow systems in the area (Mahed ). The actively produced aquifers include Permian‐age sandstones (e.g., Waterford Fm.)…”

Section: Geological and Study Backgroundmentioning

confidence: 99%

Hydrocarbon‐Rich Groundwater above Shale‐Gas Formations: A Karoo Basin Case Study

et al. 2018

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Horizontal drilling and hydraulic fracturing have enhanced unconventional hydrocarbon recovery but raised environmental concerns related to water quality. Because most basins targeted for shale-gas development in the USA have histories of both active and legacy petroleum extraction, confusion about the hydrogeological context of naturally occurring methane in shallow aquifers overlying shales remains. The Karoo Basin, located in South Africa, provides a near-pristine setting to evaluate these processes, without a history of conventional or unconventional energy extraction. We conducted a comprehensive pre-industrial evaluation of water quality and gas geochemistry in 22 groundwater samples across the Karoo Basin, including dissolved ions, water isotopes, hydrocarbon molecular and isotopic composition, and noble gases. Methane-rich samples were associated with high-salinity, NaCl-type groundwater and elevated levels of ethane, He, and other noble gases produced by radioactive decay. This endmember displayed less negative δ C-CH and evidence of mixing between thermogenic natural gases and hydrogenotrophic methane. Atmospheric noble gases in the methane-rich samples record a history of fractionation during gas-phase migration from source rocks to shallow aquifers. Conversely, methane-poor samples have a paucity of ethane and He, near saturation levels of atmospheric noble gases, and more negative δ C-CH ; methane in these samples is biogenic and produced by a mixture of hydrogenotrophic and acetoclastic sources. These geochemical observations are consistent with other basins targeted for unconventional energy extraction in the USA and contribute to a growing data base of naturally occurring methane in shallow aquifers globally, which provide a framework for evaluating environmental concerns related to unconventional energy development (e.g., stray gas).

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“…The supply and distribution of potable water are one of the major challenges currently facing the communities across the studied region, as illustrated by the onset of the 2015 drought in Cape Town. By the end of the dry season in May 2017, the drought was declared the city's worst in a century [25][26][27]. Then in February 2018 and October 2019, many other towns across the region were declared drought disaster areas following the several years of precipitation well below average rainfall; e.g., Port Elizabeth received 42% below the average amount of 271 mm between January and July in 2018 [28].…”

Section: Linking Geology and Biology Through Hydrocensus-clean Water mentioning

confidence: 99%

Earth Stewardship Science—Transdisciplinary Contributions to Quantifying Natural and Cultural Heritage of Southernmost Africa

et al. 2020

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Evaluating anthropogenic changes to natural systems demand greater quantification through innovative transdisciplinary research focused on adaptation and mitigation across a wide range of thematic sciences. Southernmost Africa is a unique field laboratory to conduct such research linked to earth stewardship, with ‘earth’ as in our Commons. One main focus of the AEON’s Earth Stewardship Science Research Institute (ESSRI) is to quantify the region’s natural and cultural heritage at various scales across land and its flanking oceans, as well as its time-scales ranging from the early Phanerozoic (some 540 million years) to the evolution of the Anthropocene (changes) following the emergence of the first human-culture on the planet some 200 thousand years ago. Here we illustrate the value of this linked research through a number of examples, including: (i) geological field mapping with the aid of drone, satellite and geophysical methods, and geochemical fingerprinting; (ii) regional ground and surface water interaction studies; (iii) monitoring soil erosion, mine tailing dam stability and farming practices linked to food security and development; (iv) ecosystem services through specific biodiversity changes based on spatial logging of marine (oysters and whales) and terrestrial (termites, frogs and monkeys) animals. We find that the history of this margin is highly episodic and complex by, for example, the successful application of ambient noise and groundwater monitoring to assess human-impacted ecosystems. This is also being explored with local Khoisan representatives and rural communities through Citizen Science. Our goal is to publicly share and disseminate the scientific and cultural data, through initiatives like the Africa Alive Corridor 10: ‘Homo Sapiens’ that embraces storytelling along the entire southern coast. It is envisioned that this approach will begin to develop the requisite integrated technological and societal practices that can contribute toward the needs of an ever-evolving and changing global ‘village’.

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Development of a conceptual geohydrological model for a fractured rock aquifer in the Karoo, near Sutherland, South Africa

Cited by 4 publications

References 21 publications

Pre‐drill Groundwater Geochemistry in the Karoo Basin, South Africa

Pre‐drill Groundwater Geochemistry in the Karoo Basin, South Africa

Hydrocarbon‐Rich Groundwater above Shale‐Gas Formations: A Karoo Basin Case Study

Earth Stewardship Science—Transdisciplinary Contributions to Quantifying Natural and Cultural Heritage of Southernmost Africa

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