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

Marais, Leander Hugo (2001-03)

Thesis (MSc)--Stellenbosch University, 2001.

Thesis

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".

AFRIKAANSE OPSOMMING: Hierdie studie was onderneem met die doel. om grondwater eienskappe te ondersoek in geologies verskillende gekraakte / genate aquifere en by verskillende dieptes met die hulp van chemiese, isotopiese en 14C-datering resultate, om sodoende te toets vir 'n "tweede dieper liggende aqulfeer", met verskillende eienskappe. Doleriet gange en plate (Jura), kimberliet gange en pype (Kryt), sowel as OW lopende sinusvormige mega-verskuiwings en monoklienes van die Kaapse Plooi Gordel, Vorm die strukturele omgewings in die studie gebied. Die dominante litologie in die studie gebied is fluviaal gedeponeerde sandsteen en moddersteen van die Beaufort Groep. Die opvallendste water-tipe wat in die studie gebied waargeneem word is 'n grondwater wat geen dominante katione of anione toon nie, tweedens is daar 'n Na-S04 tipe grondwater wat gevolg word deur 'n Na-HC03 tipe water en daarna 'n Ca-S04 grondwater tipe. Die hoof oorsaak van saliniteit in the grondwater is die infiltrering van verdampte water na die grondwater-tafel, deur die verryking in 01BOen 02H, wat stadige infiltrasie van water na hewige reënval episodes voorstel. Die verskil van isotoop waardes by vlak en diepper watervlakke, steun die moontlikheid van die aanwesigheid van 'n "tweede dieper liggende aquifeer". Water met die lae 1BO-waardes (met diepte) dui op 'n opvangsgebied meer na die noordoostelike binneland, soos byvoorbeeld die Hex Rivier Berge en die Pramberge. Meeste van die grondwater monsters wat geneem is by 'n redelike diepte toon 'n 14C-datering waarde van ongeveer 200 jaar, alhoewel die water uit 'n evolutionere oogpunt nie so oud is nie en sodoende ondersteuning bied aan die teorie van die beweging van dieper liggende water en die bestaan van 'n "tweede dieper liggende aquifeer". Die chemiese karakter van grondwater word hoofsaaklik beheer deur die infiltrering van verdampte oppervlak water na die grondwater-tafel, die topografiese geaardheid van die opvangsgebied, geologiese invloede (soos die prosesse van presipitering, oplossing en ioon uitruiling), sowel as die infloed van die mens. Wisselvalligheid in die kwaliteit van grondwater word veroorsaak deur verskille in reënval, infiltrasie, evaporasie, topografie, grond tipe en diepte, plantegroei en die aktiwiteite van die mens. Verskille op mikro-vlak word veroorsaak deur die aard van die grondwater vloei deur die Karoo gesteentes, volgens die verskil in hidroliese geleiding tussen vloei in die matriks en vloei in die krake / nate. Daar is ook 'n verskil in die tydsbestek wat grondwater in die twee hoofstrukturele komponente deurbring en so die verskil in mineralisasie en saliniteit in die dinamiese grondwater veroorsaak. Die projek moet gesien word as die basis vir voortdurende studie om konkrete antwoorde te verseker vir die gebruik in grondwater bestuur projekte van die gekraakte / genate rots aquifere in die Karoo. Enslin (1950) konseptualiseer Karoo doleriet gange as volg: "die effek van indringing en verbrokkeling van sedimentêre gesteentes is dat die deurlaatbaarheid verhoog word en dat die kontak sone verander is na 'n aquifeer wat lê tussen die soliede gang en die versadigde, lae deurlaatbare wand-gesteentes".

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