Doctoral Degrees (Institute for Groundwater Studies (IGS))

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Browsing Doctoral Degrees (Institute for Groundwater Studies (IGS)) by Author "Gebrekristos, Robel Amine"

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    Site characterisation methodologies for DNAPLs in fractured South African aquifers
    (University of the Free State, 2007-05) Gebrekristos, Robel Amine; Usher, B. H.
    English: The objective of this research is to characterise fractured aquifers contaminated with DNAPLs in South Africa. The site characterisation approach is conducted in two case studies by integrating a range of site assessment techniques. It started with the noninvasive methodologies that provide rapid and cost-effective ways for screening. After developing the initial conceptual model, the investigation proceeded to invasive techniques that provide more information on DNAPLs and fractures on a local scale. This research indicated that fracture “hunting” is extremely difficult with the current technology, making DNAPL characterization very challenging. It also indicated that no single technique provides an unequivocal indication of fracture position and orientation, but that best results were obtained when several invasive techniques were used in a toolbox approach. These tools were most appropriate when integrated with hydraulic testing methods. The site characterisation was done in a continuous and iterative process, and each phase of investigation was used to refine the conceptual model of the site. The non-invasive techniques applied included: • General assessment of Test Site 1 by site inspection, interviewing employees, documentation studies and aerial photo analysis. • Hydrocensus • Direct observing for DNAPL contamination using UV light and Sudan IV shake tests. • Soil gas surveys by using various models of PIDs to delineate the organic vapour plumes. • Surface and airborne geophysics for outlining major structures that may play an important role for DNAPL migration pathways. This did not yield useful results at Test Site 1 due to noise interference from industrial activities and infrastructure. The non-invasive techniques were not applicable for direct evaluation of DNAPLs migrating in fractured aquifers; however, they were essential in understanding DNAPL-release zones and vapour plumes. They were also found to be relatively cost and time efficient. The invasive techniques applied include: • Test pits (excavations on the top 3 m soil) • Drilling using augur, percussion and coring methods. (An outside-in approach was followed during the drilling to avoid unnecessary DNAPL mobilisation). • Several different borehole geophysical and geochemical logging methods. Results from this diverse range of activities were integrated to construct conceptual models of the preferential fracture pathways of the two test sites. Riemann (2002) found that early time drawdown data could be used to estimate the Tvalue of the fracture zone with the Cooper-Jacop 2 method. However, in this research, the method did not yield results that were consistent with other observations. The tracer experiment was extended to evaluate the effective matrix diffusion coefficient of the mudstone in the Campus Test Site. The values at depths of 27 and 36 m were estimated as 3.4 x 10-6 and 7.8 x 10-6 m2/day, respectively. This means that the rate of dissolved mass disappearance from the fracture to the matrix is relatively high compared to other geologic formations. An in situ method of effective porosity estimation using tracer tests was employed instead of the traditionally measurements on rock/soil samples from the aquifer. In this way, uncertainty in porosity estimation due to the inherent alteration during sampling was eliminated. Column experiments were conducted to evaluate the accuracy of PITT for residual DNAPL saturation estimation. The saturation was estimated with an accuracy of 95%. However, the experiment was conducted in unconsolidated, homogenous sand and the experiment may not be accurate as such when applied to heterogeneous aquifer systems usually encountered in South Africa. With the current technologies available, PITT is believed to have limited applicability in fractured rock aquifers. Finally, based on the findings, a methodology has been proposed for the characterisation strategy of DNAPL contaminated sites in fractured South African aquifers