The role of geological structures in the hydrogeology of the Upper Crocodile River Basin, South Africa

Abstract

The Upper Crocodile River Basin (UCRB) is made up of fractured crystalline rocks such as quartzites, granites, gabbro’s and Bushveld Igneous Complex rocks that are intruded by dykes of varying ages and orientations. A study of the geological structures on the role they play on the hydrogeology of the UCRB was undertaken. The use of ArcGIS for lineament analysis and magnetics data for the interpretation of lineaments in the study area revealed three main trends. A NNE-SSW, E-W and a NW-SE trend, where the NW-SE and NNE-SSW trends form conjugates of each other and the E-W lineaments forming extension release joints. The SW lineaments share their orientation with the southern Pilanesberg dyke swarm whereas the E-W have dykes that are doleritic in composition and are said to be post-Karoo intrusions. Rose diagrams revealed dominant orientations for these lineaments to be 135o, 015o and 085o for the SE, NNE-SSW and E-W, respectively. The drainage pattern is dominantly dendritic, which develops in low porosity rocks with the orientation of the streams following that of the lineaments. Additionally, dendritic pattern is caused by homogeneous material, and that is, geology of similar resistance. Trellis drainage patterns occur in two areas, one being subject to lineament control and the other affected by both the lithology and lineaments. A total of four springs were used to estimate recession coefficients. Three of these were on dolomites and one on quartzites. The dolomitic springs share a similar recession coefficient, suggesting that regardless of their respective locations the dolomites share similar physical properties. The quartzite spring has a higher recession coefficient as such the physical properties are better suited for groundwater circulation than in the dolomites. The Maloney’-Eye has a bigger catchment, which resulted in higher discharge. The smallest spring catchment is observed for the Elandsfontein-Eye, however, it has a similar discharge to that of the quartzite spring. The streams in the UCRB are predominantly gaining streams. However, one of the streams loses and gains depending on the seasons. The gain is facilitated by the fractures and the loss is possibly facilitated by the karsts on the dolomites. The isotopic composition of the groundwater in the study area shows a depletion of heavy isotopes of δD and δ18O. The groundwater samples plotted along Meteoric Water Lines (MWL) shows an evolution pattern of water recharged directly from rainfall or rapid recharge through the fractures. Some samples are extremely depleted, which can be attributed to (1) recharge from high altitudes, (2) low humidity during condensation and/or (3) recharge during a colder climate. The average residence time of the groundwater based on tritium data is around 23 years. Lineaments are very important for the hydrogeology of the UCRB on the basis that they affect the drainage patterns, stream orientation and play a fundamental role in the groundwater circulation. Although, the lineaments facilitate the groundwater flow, the residence time is still high on average and the recession coefficients are also high suggesting deep circulation. This dominance of lineaments and similarities in environmental isotopes shows that the possibility of compartmentalisation (on dolomites in particular) is very low