A hydrogeochemical evaluation of groundwater in fractured rock aquifers using trace elements and stable isotopes at Loxton in the Central Karoo

[摘要] ENGLISH ABSTRACT: This study was conducted to assess groundwater characteristics of geologically different fracturerock aquifers, at different depths, by means of chemical, isotope and 14C-dating results and to testfor a deeper seated aquifer, with different characteristics.Jurassic dolerite dykes and sills, Cretaceous kimberlite fissures and pipes, as well as EWtrending sinusoidal megafolds, comprise the structural domains of the study area. Fluvialsandstone 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 inparticular. The second is a water type in which Na-S04 is dominant, followed by a Na-HC03dominated 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 pondedwater during excessive rainfall events.There is a fair difference in isotopic values between surface measurements and measurementstaken at depth, enforcing the possibility of a second deeper seated aquifer. The water with thelower 180 values, for samples at depth suggest that this water has a source further inland, fromrainfall on the range to the NE, the Hex River Mountain or Pramberge, which has greatly depleted180 values relative to SMOW.Most of the groundwater samples taken at depth indicated a 14C~dating of century age (±200years), 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 seconddeeper seated aquifer system.The chemical character of the groundwater is predominantly controlled by the infiltration ofevaporated surface and subsurface water, the topographical nature of the catchments, geologicalinfluences (i.e. the process of dissolution, precipitation and ion exchange) and the influence ofman.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 withinmatrix and fracture components of the groundwater flux. The residence times are often differentfor these two main components and give rise to the differences in mineralization and soluteproportion in passing groundwater.This project should be seen as a basis of continuing study to provide the concrete answersneeded 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 hasbeen increased and the contact zone has been changed into an aquifer lying between thesolid dyke and the saturated, low permeability country rock.

[发布机构] Stellenbosch University