Abstract
A field-scale pumping test and uranin tracer test was carried out at Hansrivier Farm in Beaufort West to determine the hydraulic properties of the Hansrivier Dyke/Karoo sedimentary rock aquifer. Analysis of borehole images and pumping test results reveal that the fractured-rock aquifer is highly transmissive due to the influence of the Hansrivier Dyke; however its influence is concentrated on extremely limited conductive zones. Results of the tracer test show that chemical transport can take place via multiple flow paths with differing hydraulic properties, such as fracture aperture, connectivity and perme ability. Furthermore, diffusion of a tracer into fractures and rock matrix has a universal retardatory travel time due to the various flow paths. In order to interpret the tracer test data, the Single Fissure Dispersion Model (SFDM) was used for inverse modelling of the breakthrough curves. The resulting effective diffusion coefficients are comparable with labora tory-derived values, and are therefore considered to be representative of the investigated part of the aquifer.
Highlights
The fundamental characteristics of fractured rock aquifers lie in their extreme spatial variability of hydraulic conductivity and groundwater flow rate
This paper presents the methodology and results of a field scale forced-gradient tracer test on a fractured aquifer at Hansrivier Farm, Beaufort West, in order to determine the aquifer’s hydraulic properties, which are understood to be influenced by a dolerite dyke
Based on the camera logging inspection, it could be seen that multiple fracture zones occur at depths of 10 m and 17 m, which concurs with historical data from the National Groundwater Archives (NGA) of the Department of Water Affairs (DWA)
Summary
The fundamental characteristics of fractured rock aquifers lie in their extreme spatial variability of hydraulic conductivity and groundwater flow rate. Subsequent to the deposition of the Beaufort Group and Karoo Sediments, a number of dolerite dykes and sills intruded into the sediments during a period of extensive magmatic activity over the entire South African sub-continent during the break up of Gondwanaland (Chevalier et al, 2001) This resulted in a network of dolerite dykes, sills and even inclined sheets, which are present in the study area. Due to limited available data for the area south of Beaufort West, the current dataset is restricted mainly to the municipal well fields situated to the north of the town (Fig. 1) This reflects the areas most extensively explored over the past 20 years (Rose and Conrad, 2007). Pumping test data from Rose and Conrad (2007) indicate that boreholes drilled on the Hansrivier Farm (south east of Beaufort West) on a contact zone between the dolerite dyke and fractured Teekloof formation produced sustainable yields in excess of 10 l/s
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