The characterisation of fractured Karoo Aquifers near Beaufort West

Abstract

A research project was undertaken to delineate aquifers in Beaufort West. Recent studies in the Karoo Basin utilised geochemistry indicators to differentiate between shallow and deep groundwater. The shale gas exploration studies in particular focused more on regional scale assessments. The opportunity existed to apply these methods on a local well field scale. The current local scale studies have been mainly limited to consultants’ reports with resource management limited to basic well field scale monitoring and interpretation. The problem with the current local scale approach is that well fields are operated on a “sustainable yield” level based on available fracture storage. Lateral inflows and outflows are not considered. The field work conducted at Beaufort West municipal well fields presented an opportunity to utilise available data and information to spatially characterise the groundwater, as well as to delineate a) flow directions, and b) potential compartments in the shallow aquifer. A better understanding of the flow dynamics is essential to improve the overall resource management and subsequent decision making. Geohydrological data was collected through several phases of fieldwork conducted for the Beaufort West Municipality. All existing data and information were collated into a comprehensive database. Maps were produced to spatially identify trends in the respective datasets. A good correlation was observed between borehole yields and transmissivity. High yielding boreholes are present in the municipal well field areas, although there is no clear relation between the borehole yields and the lithology, rather to the occurrence of secondary flow paths in the different formations. The degree of fracturing seems to dictate the successfulness of boreholes rather than the lithology. A good correlation was also observed between recharge and EC. The areas with lower ECs generally have either higher recharge rates or more localised recharge or a combination of the two. The inverse is observed for poorer quality groundwater where the recharge is likely to be less and more delayed within the unsaturated zone. A groundwater level contour grid was compiled over the study area to assist with the delineation of groundwater flow paths and groundwater compartments, originating from dolerite intrusions. Eight groundwater compartments were identified. A conceptual model in the form of a N – S section through the Town compartment was compiled to illustrate the groundwater flow regime in the compartment. Based on the interpretation of the isotope data, shallow and deeper groundwater flow systems were identified. The shallow groundwater system is linked to meteoric water, whilst the deeper groundwater system appears to be linked to connate water. The Beaufort West Spring, a fracture guided artesian spring related to the contact of the Teekloof formation and the Town dyke, is linked to the confined deeper groundwater system, whilst the bulk of the municipal abstraction is from the shallow system.