Geohydraulic characterisation of fractured rock flow regimes

Abstract

Summary provided by author:

The following five different geohydraulic methods were applied in the regional case studies: well hydrograph recession analysis, hydraulic tests, groundwater tracer techniques, hydrochemical investigations and groundwater modelling. The thesis includes a description of the state-of-the-art for each method and suggestions for further improvement of the analysis techniques. The applicability of each method to describe the flow pattern was assessed separately for each investigation area. Theinvestigations resulted in the development of conceptual hydrogeological models for the fractured aquifers

The study focuses on the assessment of the hydraulic impact of dominant fault and dyke structures, the regional distribution of different aquifer systems and the quantification of groundwater flow parameters. As the aquifers in both studies are characterised by a low primary porosity of the host rock the groundwater flow is mainly restricted to fractures, which are only abundant along dykes, faults and contact zones. The role of intrusive dykes and faults as conduits of groundwater flow could be demonstrated for both sites. A highly permeable vertical dyke zone largely influences the groundwater flow pattern at Lindau. The ore dyke intersects the surrounding granite that has a much lower permeability. A drainage experiment was carried out in order to assess the hydraulic impact of the dyke on the regional groundwater flow. The groundwater system was artificially drained over a period of almost 14 months by opening a tapped borehole located in the observation tunnel. This borehole intersects the dyke some 80 m below the ground surface. The analysis of the hydraulic response at the observation wells was combined with the results of hydrochemical investigations and tracer tests. The results of the investigations were used to delineate the subterranean catchment of the dyke

In addition, a well hydrograph recession analysis was performed at Lindau. This analysis, originally developed for streamflow and subsequently adopted for the evaluation of spring discharge and groundwater levels in karst aquifers, was for the first time applied to groundwater level data from wells in granitic rock. The analysis proved to be an effective tool to obtain a regional distribution of the mean residence time and the permeability of the rock from groundwater level data

The presented study of the Tsumeb Aquifers included hydraulic testing and the development of a numerical groundwater flow model. It was an integral part of the comprehensive hydrogeological investigation program carried out in the framework of the Tsumeb Groundwater Study. The Tsumeb Aquifers proved to be one of the potential additional sources to meet the growing water demand of the Central Area of Namibia. The 3-D Tsumeb groundwater model has been established as a tool for the assessment of the groundwater resources and provided essential information for the establishment of environmentally sustainable groundwater management. As a result of this study, the regional groundwater potential of the area could be quantified for the first time. The groundwater model revealed the eminent importance of the dolomitic outcrops of the Otavi Mountainland for the replenishment of the groundwater resources