Department of Earth Sciences

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Browsing Department of Earth Sciences by Subject "Aquifers -- South Africa -- Loxton"

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    A hydrogeochemical evaluation of groundwater in fractured rock aquifers using trace elements and stable isotopes at Loxton in the Central Karoo
    (Stellenbosch : Stellenbosch University, 2001-03) Marais, Leander Hugo; Greeff, G. J.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: This study was conducted to assess groundwater characteristics of geologically different fracture rock aquifers, at different depths, by means of chemical, isotope and 14C-dating results and to test for a "deeper seated aquifer", with different characteristics. Jurassic dolerite dykes and sills, Cretaceous kimberlite fissures and pipes, as well as EW trending sinusoidal megafolds, comprise the structural domains of the study area. Fluvial sandstone and mudstone of the Beaufort Group are the dominant lithology of the study area. The main water type found in the area is a water type not dominated by any anions or cations in particular. The second is a water type in which Na-S04 is dominant, followed by a Na-HC03 dominated water and to a lesser extend a Ca-S04 type water. The main cause of groundwater salinity is the infiltration of evaporated water to the subsurface, suggested by the isotopic enrichment of 0180 and 02H, indicating very slow recharge from ponded water during excessive rainfall events. There is a fair difference in isotopic values between surface measurements and measurements taken at depth, enforcing the possibility of a "second deeper seated aquifer". The water with the lower 180 values, for samples at depth suggest that this water has a source further inland, from rainfall on the range to the NE, the Hex River Mountain or Pramberge, which has greatly depleted 180 values relative to SMOW. Most of the groundwater samples taken at depth indicated a 14C~dating of century age (±200 years), although in an evolutionary sequence the water is not such an old (evolved) water type, lending support to the theory about the migration of deeper seated water and thus a "second deeper seated aquifer system". The chemical character of the groundwater is predominantly controlled by the infiltration of evaporated surface and subsurface water, the topographical nature of the catchments, geological influences (i.e. the process of dissolution, precipitation and ion exchange) and the influence of man. Variability in water quality is caused by differences in rainfall, recharge, evaporation, topography, soil type and thickness, vegetation cover and antropogenic activities. Micro-scale differences occur due to the nature of groundwater flow in Karoo rocks, namely the resulting variations within matrix and fracture components of the groundwater flux. The residence times are often different for these two main components and give rise to the differences in mineralization and solute proportion in passing groundwater. This project should be seen as a basis of continuing study to provide the concrete answers needed to manage groundwater projects in the fractured rock aquifers of the Karoo. Enslin (1950) expresses the classical hydrological conceptualisation of Karoo dolerite dykes - lithe effect of induration and crushing of the sedimentary rock is that the permeability has been increased and the contact zone has been changed into an aquifer lying between the solid dyke and the saturated, low permeability country rock".