Horizontal variation in trace elements and soil characteristics at Siloam and Tshipise geothermal springs, Limpopo Province, South Africa

Abstract

The Limpopo Province is the location of 31% of South Africa’s geothermal springs. The springs at Siloam and Tshipise are among those springs which fall into the ‘scalding category’ with average temperatures of 67.7°C and 54.6°C, respectively. The aim of this study was to determine the horizontal variation in trace element concentrations and soil indicators in surface soil associated with the geothermal springs at Siloam and Tshipise. Results show that, in general, the trace element concentrations present in the soil decrease with horizontal distance from the springs. Water and soil samples were collected from May to July 2014, with the soil sampled at 0–15 cm depth at 5 m intervals in the horizontal distance from the geothermal spring. The physicochemical parameters of the water were determined as well as the trace elements. The soil samples were digested using microwave digestion and trace elements were determined using an inductively coupled plasmamass spectrometer (ICP-MS). Soil indicators – pH, soil organic matter (SOM) and soil water retention (SWR) – were also determined. The resulting data were subjected to both descriptive and factor analyses. Results showed only one factor determining the variation in geothermal water, attributed to the rock–water interaction in the deep aquifer (geology); two factors were obtained for variation in soil indicators and trace element concentrations, attributed to spring geology and the soil pedogenesis. There was a strong correlation between the trace elements and soil indicators at the 95% confidence level. There is generally a positive correlation between the trace elements and soil indicators if the soil pH, SOM and SWR are high; in these instances most of the trace elements in the soil will also be relatively high. A two-factors-without-replication ANOVA (p ˂ 0.05) showed that there is a significant difference in trace element concentrations at different distances.

Keywords: geothermal springs, trace metals, soil indicators, rock-water interactions, soil pedogenesis