Isotopic signatures and trace metals in geothermal springs and their environmental media within Soutpansberg

Abstract

Geothermal springs are natural geological phenomena that occur throughout the world. South Africa is endowed with several springs of this nature. Thirty-one percent of all geothermal springs in the country are found in Limpopo province. The springs are classified according to the residing mountain: Soutpansberg, Waterberg and Drakensberg. This study focused on the geothermal springs within the Soutpansberg region; that is, Mphephu, Siloam, Sagole and Tshipise. The study was aimed at elucidating on the isotopic signatures and trace metals concentrations from the geothermal springs to their environmental media in Soutpansberg region. This study also assessed the interconnectivity of the isotopic signatures within the ecosystem and evaluated the potential human health risks associated with trace metals from geothermal springs and surrounding soils in the study areas. Geothermal springs and boreholes were sampled for a period of twelve months (May 2016 – May, 2017) to accommodate two major seasons in the study areas. The surrounding soils were sampled vertically from a depth of 10 cm to 50 cm for trace metals and isotopic compositions. Three different plants were sampled at each of the study sites, namely, Amarula tree, Guava tree and Mango tree at Siloam; Acacia tree, Fig tree and Amarula tree at Mphephu; Amarula tree, Lowveld mangosteen and Leadwood tree at Sagole; Sausage tree, Amarula tree and Acacia tree at Tshipise. To achieve the objectives, the physicochemical, geochemical and isotopic compositions of the geothermal springs, boreholes, soils and vegetation were analysed using ion chromatography (IC) (Dionex Model DX 500), inductively coupled plasma-mass spectrometer (ICP-MS), HTP-Elemental analyzer, Liquid water isotope analyzer (LWIA-45-EP) and Liquid scintillation analyzer. The temperature, electrical conductivity (EC), pH and total dissolved solid (TDS) of the geothermal springs and boreholes samples were measeured in situ and in the laboratory. Trace metals analysed in geothermal springs, boreholes, soil and vegetation include Beryllium (Be), Chromium (Cr), Manganese (Mn), Cobalt (Co), Nickel (Ni), Copper (Cu), Arsenic (As), Selenium (Se), Cadmium (Cd), Antimony (Sb), Barium (Ba), Vanadium (V), Zinc (Zn), and Mercury (Hg). vii | Isotopic signatures and trace metals in geothermal springs and their environmental media within Soutpansberg Results obtained from this study in the studied geothermal springs and boreholes were classified according to their temperature as hot and scalding; except for tepid boreholes. This study has provided comprehensive physicochemical, geochemical and isotopic compositions of the geothermal springs within the Soutpansberg region (Siloam, Mphephu, Sagole and Tshipise). The local meteoric line (δD = 7.56δ18O + 10.64) was generated from rainwater in Vhembe district. This is a crucial component for depicting the source and flow path of the geothermal springs/boreholes; and could be used for future isotopic hydrological studies within the locality. Rain formation processes within Soutpansberg occurred under isotopic equilibrium conditions with minor evaporation effect during rainfall. The δD and δ18O values of the geothermal spring water/boreholes confirm that the waters are of meteoric origin, which implies that rainfall is the fundamental component of these groundwaters because they were derived from the infiltration of rainwater, with significant contribution of another type of water in the deeper part of the aquifer. Na-Cl and Na-HCO3 were established as the water types, which are typical of marine and deep groundwaters which are influenced by the ion - exchange process. The reservoir/aquifer temperature of these springs ranges between 95 – 185°C (Na-K geothermometer), which implies most of the waters are mature water (not native). Hence, geothermal springs water is a mixture of the rainwater and salt water. Radiocarbon values of the geothermal springs ranged from 2700 to 7350 BP, this implies that they are submodern and a mixture of submodern and modern waters. Tritium relative age also corroborates with radiocarbon age, that is the groundwaters were recharged before and after 1952. This gives an indication that the rainfall contributes to the geothermal springs recharge. Various radiocarbon correction models were employed and constrained by tritium relative age. Ingerson and Pearson, Eichinger and Fontes and Garnier correction models have been shown to be the most appropriate models for radiocarbon correction of groundwater in this semi-arid region. Although, geothermal springs water and boreholes are not fit for drinking due to high fluoride content, they could be used for the following: domestic uses (drinking exclusive) due to its softness, direct heating in refrigeration, green-housing, spa, therapeutic uses, aquaculture, sericulture, concrete curing, coal washing and power generation. In contrast with mentioned uses, viii | Isotopic signatures and trace metals in geothermal springs and their environmental media within Soutpansberg the studied geothermal springs are currently used for domestic purposes (drinking inclusive), limited irrigation and spa (swimming and relaxation). This is an eco-hydrological study that shows the interconnectivity of isotopic signatures among water (rainwater, geothermal springs and boreholes), soils and vegetation. The soil-water reflects the rainwater/geothermal springs water in isotopic composition, which is more depleted as a result of isotopic fractionation in soil. δD values of soil-water increase, whereas δ13C values in soil-water decrease with the soil depth at all sites. Two equations connecting δD and δ13C in soil-water were deduced per season for soil-water; δ13C = 0.0812δD - 10.657 in winter; δ13C = -0.0278δD - 21.945 for summer. δ13C in soil-water is induced by Crassulacean Acid Metabolism (CAM) (mixture of C3 and C4 photosynthetic cycles) with a stronger C4 trend, which corroborates with δ13C of the geothermal springs. From literature, Amarula and Acacia trees have been documented for isotopic compositions, while this study has given additional information on other plants including Lowveld, Leadwood, Sausage, Fig, Guava and Mango trees. These plants are categorised as C3, C4 and CAM plants. C3 plants include Amarula, Lowveld and Leadwood trees; C4 plants include Acacia and Sausage trees; and CAM plants include Fig, Guava and Mango trees. This study shows that with CAM soils, there is a possibility of having either C3, C4 or CAM vegetation. This finding has shown that the δD and δ13C isotopes in water, soil and vegetation are interrelated, which has been statistically justified. This study has shown the potential human health risks associated with trace metals concentrations from geothermal springs and their surrounding soils. From the geothermal spring’s water, it was found that As, Cr and Cd were the highest contributors to the cancer risk with children having a higher risk than adults. Whereas in soils, it was found that Cr, As and Co were the highest contributors to the cancer risk in the studied communities. Therefore, the cancer risk is high in the general population; that is 1 in 72-162 individuals in children and 1 in 7-107 individuals for adults. The ingestion route seems to be the major contributor to excess lifetime cancer risk followed by the dermal pathway. Therefore, proper monitoring and control measures to protect human health, particularly in children, should be implemented for safety. The study also explored the use of surrounding trees ix | Isotopic signatures and trace metals in geothermal springs and their environmental media within Soutpansberg for phytoremediation and found their uptake capacity to be high, thus, they could be used as bio-indicators to assess the level of contamination of trace metals in the soil. In conclusion, this study has eludicated on the isotopic signatures and trace metals concentrations from the geothermal springs and their surrounding soils and vegetation within Soutpansberg. This study has contributed towards the advancement and enhancement of the existing knowledge of the geothermal systems, such that water resource management could be applied successfully in the respective areas with similar characteristics for the benefit of the local communities and society at large. Hence, this study recommends that proper monitoring and control measures need to be put in place to protect human health, especially in children.