A geological and hydrogeological study of the Shu Shu thermal springs, KwaZulu-Natal.

Abstract

The Shu Shu thermal springs are located in central KwaZulu-Natal in South Africa at an altitude of 250m above mean sea level at the bottom of the Tugela Valley. They have been investigated in an attempt to ascertain whether or not they possibly share a common origin with fifteen other springs which are located in a north-northwestern trending, 1000km long zone within the eastern interior of the country. They have also been studied to establish if they potentially represent a viable and sustainable geothermal energy resource that may be developed in the future. Isotope ratios confirm that the thermal springs are meteoric in origin, and are likely recharged within a 130km long band located to the west toward the Great South African Escarpment. These waters then descend vertically to a depth of approximately 1827m – 2153m, at which point the fractures along which they move close due to escarpment-associated confining pressure. It is a reduction in this self-same confining pressure along the coastal plain, which occurs due to the continual removal of overburden, which forces the thermal water to migrate toward the east along ever-more opening fractures. Due to a slightly elevated geothermal gradient of 3.1°C / 100m at depth, this groundwater reaches a temperature of approximately 75°C – 85°C as it traverses toward the Shu Shu thermal springs over a time period of > 61 years (at present). The geochemical signature of the Shu Shu thermal waters is derived through leaching from the basement rocks through which they pass, with elevated concentrations of Na, K, Ca, Mg, Fe, Al, Si, F, Sr and SO₄ detected. Once below the floor of the Tugela Valley, at a depth of approximately 990m, the thermal groundwater commences its ascent, likely along the west-southwest – east-northeast orientated thrusts and associated fractures of the Tugela Terrane of the Natal Metamorphic Province. However, the thermal waters abut against those brittle structures trending north-northwest – south-southeast, which are perpendicular to the axis of least principle compressive stress, and so are ultimately forced to rise within the Shu Shu thermal springs. Prior to surfacing, the temperatures of these waters drop to approximately 67°C as a result of natural, conductive cooling processes. However intermixing with shallow, cold groundwater, which is discernible through geochemical and isotopic variations, and atmospheric impacts, further cool the thermal waters to a surfacing temperature of approximately 50°C. Nevertheless, a binary cycle geothermal energy power plant remains a possibility. Although thermal efficiency and volume flow rate values are favourable, heat transfer values are low and require further investigation through exploratory drilling. Nevertheless, the establishment of a 400kW power plant, should it occur, will typically be sufficient to service 625 rural homes.