Geohydrological characteristics of the Msikaba, Dwyka and Ecca groups in the Lisikisiki area, Eastern Cape

[摘要] English: The study focused on the three major geological units, namely the Natal Group Sandstone(NGS or also called Msikaba Sandstone), the Dwyka Formation and the Ecca Group and wasaimed at investigation the geohydrological properties of these units and their potential to producesustainable groundwater supply. The methodology included Landsat lineament mapping, aerialphotography mapping, geological mapping, geophysical exploration, drilling and testing.The investigation can be considered successful as various high yielding boreholes were drilled,some of which constituted new records for the Eastern Cape. Although the entire approach can becommented, it was the geophysical exploration that proofed conclusive in the Msikaba Sandstonewhere the high yields were obtained. Especially the EM-34 produced excellent anomalies wheredolerite dykes were targeted and the fracturing on the sides of the dykes were very well defined.Recharge was calculated using the GIS / GRID based approach where a percentage rechargebased on lithological properties was used. The percentage recharge seemed high when comparedto the Chloride method where the two scenarios, coastal and inland, were considered. When thecoastal scenario was used, the percentage recharge was in the same order of magnitude (10 -12% of rainfall), but if the inland scenario was used the percentage recharge was much lower(<5%). If the pumping test results are considered, it seems that recharge mostly occurs along thefractured zone associated with the dykes on which the boreholes were drilled and not from the hostrock in the case of the Msikaba Sandstone. No monitoring boreholes were available to test thistheory in the Dwyka and Ecca, but it should be same in the case of the Dwyka. The Eccaproduced low yields and poor water quality and hence it is possible that the overlying doleritesheets have a negative influence on the recharge from rainfall. The fact that dolerite sheets wereintersected at shallow depth (30m) without yielding groundwater, substantiate this.The water quality was acceptable in both the Msikaba and Dwyka, but mostly poor to unacceptablein the Ecca. In terms of ranking the three geological units according to their envisagedgroundwater potential, they would rank as follows:o Msikaba Sandstone Ranked firsto Dwyka Ranked secondo Ecca Ranked thirdMain targets for drilling would include:o Regional dolerite dykeso Faults and larger lineamentso Geological contacts

[发布机构] University of the Free State