Estimating the hazard presented by radon gas in the western part of the Western Cape Province.

Radom is a radioactive noble gas produced by the decay of unstable isotopes of uranium and thorium. It decays rapidly, having a half-life of only 3.82 days, which, since it can be inhaled, makes it a major cause of Human exposure to radiation, being considered the second largest cause of lung cancer after smoking by the World Health Organisation. Radon is generally encountered by Humans as a result of living on granite rock with high concentrations of naturally occurring nuclides, which therefore tends to produce radon gas. This gas can build up inside dwellings raising the risk to those who dwell in them. The geology of the western part of the Western Cape Province of South Africa is dominated by the so-called Cape Granite Suite, which stretches from Saldanha Bay in the northwest through the town of Paarl to Franschhoek in the southeast. Radon emissions from the Cape Granite Suite have never been investigated, so it is unclear what threat this presents to residents of the area.

In a paper published in the South African Journal of Science on 30 July 2019, Jacques Bezuidenhout of the Faculty of Military Science at Stellenbosch University examines uranium concentrations in the Cape Granite Suite, and their relationship to indoor radon concentrations.

Bezuidenhout chose the Baviaansberg granite outcrop for the initial part of the study, as it is relatively undisturbed by human activities and lies on the same granite complex as the town of Paarl, making comparison possible. This was mapped with ArcMap software into measurement squares that were 48.6 m east-west and 47.6 m north-south. Measurements were taken at the centre of these squares using a 7.62 cm x 7.62 cm sodium iodide scintillation crystal coupled to a scintiSPEC Multi-Channel Analyser and connected to a Trimble Yuma Rugged tablet computer with a Global Positioning System to record the exact location and the level of gamma ray emissions. Measurements were taken during the dry summer months to prevent any effect of moisture on gamma radiation.

The NaI(Tl) scintillation detector with a 7.62 x 7.62 cm crystal coupled to the scintiSPEC Multi-Channel Analyser and connected to the Trimble Yuma Rugged tablet computer via a USB connection. Bezuidenhout (2019).

Radon potential in the study area was estimated by linking the indoor radon concentrations to the measured uranium concentrations in the soil on Baviaansberg. Since the relationship between indoor radon and uranium concentrations depends on the characteristic of the geographical locations, the Tusham Ring Complex and the Himachal Pradesh region of India were consequently selected to determine these relationships because of the geographical similarity of these regions to the study area.

The extracted relationships between indoor radon concentrations and uranium concentrations from the Himachal Pradesh and Tusham Ring Complex were compared with radon build-up calculations that were done on the radon exhalation rates of the Tusham Ring Complex. The radon exhalation rate and the uranium concentrations of rock and soil were measured in the Tusham Ring Complex by Singh et al. (2008). 

The measured radon exhalation rates were used to estimate indoor radon concentrations, using a cuboid volume (3 m x 3 m x 2 m) as standard for the estimation, and these estimated indoor radon concentrations were then plotted as a function of the uranium concentrations. This relationship compared reasonably well with the one that was extracted from the Himachal Pradesh measurements. The uranium concentrations and indoor radon potentials were interpolated and mapped using ArcGIS and QGIS software, and the results for the study area were compared to indoor radon measurements taken in Paarl.

The topography of Baviaansberg were overlaid on a Google Earth image. It was evident that the steeper slopes are mainly confined to the southern side of the hill, most likely as a result of the dominant southerly onshore winds of the region. These steeper slopes are also characterised by exposed granite outcrops which are observable as white spots on the Google Earth image. Smaller granite outcrops are also visible on the northern, less steep slopes on the hill.

Google Earth image of Baviaansberg, with the topography map overlaid. The study area is indicated by a blue square. Bezuidenhout (2019).

High concentrations of uranium were found to be mainly confined to the steeper slopes, but some elevated concentrations also occur on the nearby flatter areas. High radon potential coincided mainly with the steepest slopes of the hill. This high radon potential also corresponds to the areas within the vicinity of exposed granite outcrops. If there were buildings in these areas and they were occupied similarly to those in Himachal Pradesh and Tusham in India, it is estimated that the indoor radon might exceed 400 Bq/m³, compared to a maximum level of 300 Bq/m³ 

The areas of high radon potential, however, also consistently extend beyond the granite outcrops into the less steep foot of the hill. The summit of Baviaansberg and the surrounding area demonstrate no elevated radon potential. Earlier studies have shown that some slopes of the hills in the area consist of granitic soils, most probably due to weathering of the parent rock and subsequent aeolian and alluvial disposition in its proximity. This geology would explain the high estimated radon potential below the granite outcrop in the study area. The radon potential was directly related to the uranium concentrations, irrespective of soil type. It should, however, be noted that finer grained soil would more readily support radon escape because of its larger exposed active areas.

Heat map of the estimated radon potential overlaid on a Google Earth image of Baviaansberg. Bezuidenhout (2019).

Granite outcrops found in Paarl are similar to those found in the study area and the granite in Paarl and Baviaansberg originates from the same geological unit. Results of the measurements that were performed in the granite rich areas of Paarl show that in about 6% of the houses in Paarl, indoor radon concentrations exceed 300 Bq/m³. This finding correlates well with the estimated radon potential on Baviaansberg, which predicted that 5.7% of the points would exceed 300 Bq/m³. It is therefore reasonable to conclude that similar levels of radon potential would occur in other towns in the area and that a possible radon hazard does exist. This conclusion, however, needs to be confirmed with more indoor radon measurements, which are planned for various towns in the region. The method that is described in this study for the estimation of radon potential can be used elsewhere to rapidly and remotely identify areas subjected to possible radon hazards.

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Assessing the potential for low-enthalpy geothermal energy generation in South Africa.

South Africa is the largest producer of greenhouse gasses on the African continent, and one of the largest producers worldwide. This is largely due to the country's dependence on coal to produce energy, with over 80% of electricity in South Africa being produced from coal burning power stations. Aware of the problems associated with greenhouse gas production, the nation has set itself the ambitious target of generating 40% of its energy needs from renewable sources, such as wind, solar, or hydroelectric plants, by 2030. This target is made harder by rising demand for electricity, with a rising population and many populations denied electricity under the former apartheid regime now wanting to be connected. Geothermal energy, energy generated by geological processes, is not usually considered as a viable option in South Africa, as the nation lies far from any active volcanic field, the most obvious source of geothermal energy, but the country could still potentially generate some of its power from low-energy geothermal sources, which rely on temperature gradients within deep rocks, rather than active heat eruption at the surface.

In a paper published in the South African Journal of Science on 29 December 2017, Taufeeq Dhansay of the South African Council for Geoscience and the Africa Earth Observatory Network at Nelson Mandela University, Chiedza Musekiwa, Thakane Ntholi, Luc Chevallier, and Doug Cole, also of the South African Council for Geoscience, and Maarten de Wit, also of the Africa Earth Observatory Network at Nelson Mandela University, present the results of a study into the viability of low enthalpy geothermal energy production in South Africa.

Low enthalpy geothermal energy production relies on temperature differences within geological formations rather than direct heating by magma or hydrothermal sources, and can produce useful amounts of electricity at temperature gradients as low as 40°C per km. This does not necessarily require the presence of plutonic intrusions from the Earth's interior, in many cases the presence of radiogenic elements such as uranium within the rock strata produces sufficient heating for low enthalpy geothermal plants to operate..

Schematic illustration of a binary fluid enhanced geothermal system related to surrounding fracture-controlled geological features. Dhansay et al. (2017).

Much of South Africa is underlain by the Kaapvaal Craton, an ancient subcontinental mass of lithospheric material that averages 40-50 km in thickness, but is 250 km deep at its thickest. This ancient mass at first seems to have rather poor potential for geothermal energy production of any kind, but the craton is in fact made up of several large blocks of more ancient material that were fused together during the Archean Eon (4.0-2.5 billion years ago). Thus, while the majority of the Kaapvaal Craton is highly thermally stable, something which has discouraged investigation of the potential for geothermal energy in the area, it is cross-cut by a number of ancient geological sutures, with rather different properties.

These sutures are the remains of ancient orogenic belts, bands of volcanic activity similar to that seem around the Pacific Rim today. The geological process associated with such structures tend to concentrate heavier elements within certain strata. This has led to the high concentrations of gold and other precious metals that drive the South African mineral industry, and also higher concentrations of radiogenic metals such as uranium, which at high enough densities can raise the temperatures of the deposits that host them.

Within the Kapvaal Craton these include the Cape Granite Suite (part of the Cape Fold Belt, which formed during the assemblage of the supercontinent of Gondwana) which contains uranium concentrations of up to 34 parts per million, the Namaqua-Natal Belt that formed during the assemblage of the supercontinent of Rodinia, and has uranium concentrations of between 10 and 54 parts per million, the Thabazimbi-Murchison Lineament, which marks the boundary between the ancient Kaapvaal Greenstone Belt and the slightly younger Limpopo Mobile Belt, with uranium concentrations of up to 30 parts per million, and traces of older Archean granite-gneisses around Mombela and Johannesburg, with uranium concentrations of 20-28 parts per million.

Overview of the major tectonic structures and zones across South Africa with the locations of significant earthquake focal mechanisms and inferred structures related to these events. Locations of the various data sources used within this study (e.g. hot springs and temperature measurement points) and high heat producing plutonic rocks are also highlighted. Note that the Namaqua-Natal Belt probably continues beneath the Cape Fold Belt as far as the offshore Agulhas Fracture Zone. Dhansay et al. (2017).

Dhansay et al. obtained measurements of temperature in South African rocks at depths of between 2 and 5 km from previous studies, as well as inferring deeper temperatures from hot springs at the surface, and the presence of deep faults from Earthquake data, and used this data to build up a map of temperature gradients across the country.

Graphical overview of the calculated geothermal gradients across South Africa. Map includes major tectonic contacts and structures, seismic activity and earthquake focal mechanisms and hot spring locations. Dhansay et al. (2017).

Using this data Dhansay et al. were able to identify a number of regions with geothermal temperature gradients high enough to support potential low-enthalpy geothermal energy generation, most notable along the the Colesberg, Thabazimbi-Murchison and Makonjwa Lineaments, ancient tectonic zones associated with early stages of the assemblage of the Kaapvaal Craton, which are also associated with a number of recent seismic events, and a number of hot springs, which may be due to reactivation of ancient faults with a northeast to southwest orientation.

(a) Potentially viable low-enthalpy geothermal investigation regions (1–5); based on (b) high heat producing plutonic rocks and overlying volcanosedimentary rocks; and (c) approximate groundwater yield. (d) Regional seismicity. Dhansay et al. (2017).

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