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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British tourist killed by rockfall on El Capitan, Yosemite National Park.

A British tourist has been killed and his female companion, also British, has been seriously injured, following a rockfall on El Capitan, a mountain in Yosemite National Park, California, on Wednesday 27 September 2017. The couple were initially thought to have been climbing the peak when the incident occurred, but are now known to have been walking beneath a cliff popular with climbers when a chunk of rock about 40 m high, 20 m wide and 3 m thick, and estimated to weigh about 1170 tonnes, fell from the rockface. The couple will not be named until relatives in the UK can be contacted. About 30 climbers were on the cliff-face when the incident occurred, though none of them are reported to have suffered any injuries.

Dust from a rockfall on El Capitan, Yosemite National Park, on 27 September 2017. Tom Evans/US National Park Service.

El Capitan is a granite outcrop reaching 2308 m above sea-level, and about 900 m above the valley bellow, in the Sierra Nevada Mountains. Mountains appear to be permanent structures to humans, but are actually dynamic active systems, being constantly uplifted (in the case of the Sierra Nevada by the subduction of the last chunks of the ancient Farallon Plate along the Pacific Coast of North America), and constantly worn down by erosion. The granite outcrops of the Sierra Nevada ate the remains of an ancient batholith, formed in the Cretaceous Period by a mantle plume that rose up beneath the area, but did not reach the surface to any great extent. This allowed the liquid magma of the plume to cool slowly, allowing the large mineral crystals that make up granite to form. This granite has been exposed at the surface by uplift and erosion over the last four million years. Because granite is made up of large crystals of different minerals, with different physical properties, it is particularly prone to erosion, as heating and cooling of the rock on a seasonal, or even daily, cycle, causes the minerals to expand at different rates, leading to the formation of cracks. These cracks can be further widened by water entering them and freezing, pushing them further open, and allowing more water to enter on the next cycle.

Granite from the Sierra Nevada, showing the large crystals that give it both its distinctive appearance and make it vulnerable to erosion. Roc Doc Travel.

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Explosion kills three workers at Rajasthan granite mine.

Three workers have been killed and several more are being treated following an explosion at a granite mine near Dhavala in the Jalore District of Rajasthan on Monday 17 April 2017. The incident reportedly happened after one of the men attempted to cut a fuse wire with a rock rather than a pair of metal scissors. The dead men have been identified as Ramesh Kumar Koli, 40, and Jaisaram Koli, 32, both hailing from Sirohi district, and Mahendra Meena, 20, of Udaipur.

The approximate location of the Dhavala granite mine. Google Maps.

Historically Rajashan has been a major producer of limestone and marble for the construction industry, with most granite produced in India coming from the south of the country. However in recent decades an extensive granite mining industry has developed in Rajasthan, centred on Jalore District, providing granite to the fast growing cities of northeast India.

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Bubble-like caves in the Laoshan Granite of Shandong Province.

Large bodies of ice are known to have covered wide ares of Europe and North America during the Pleistocene galciations, however evidence for similar glaciation in China has been harder to find, which is puzzling as it is difficult to explain how China wouldhave excaped such a glaciation. One piece of evidence that has been used to support the idea of glaciation in northern China is the presence of a number of small cave systems in the Laoshan Granite, which outcrops around Qingdao in Shandong Province in northeast China. Granite is a volcaninc rock formed by the slow cooling of magmatic material deep beneath the ground, which enables large, visible, mineral grains to form. These larger grains are more prone to weathering by freeze-thaw conditions than the smaller mineral grains that make up extruded lavas and basalts, thus enhanced erosion of granites can be evidence of freezing conditions in the past.

In a paper published in the journal Acta Geologica Sinica in February 2016, Song Zaojun of the Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals at the Shandong University of Science and Technology, Lui Xiqing of the Qindao Institute of Marine Geology, Tang Wenjia, Gao Luo and Li Jianping, also of the Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals at the Shandong University of Science and Technology, and Luan Shaogang, of the Qingdao Laoshan Scenic Area Adminstrative Bureau, re-examine the caves of the Laoshan Granite, and propose an alternative hypothesis for their formation.

Song et al. note that the Laoshan Granite contains a number of small caves, with bubble-like shapes, averaging 1-2 m across, with the largest measuring 3.1 m in diameter. However they also note that the Laoshan Granite is mariolitic in nature; that is to say it contains, as well as the larger caves, smaller bubble-structures (vesicles) believed to have formed by pockets of gas evolved by the cooling magmatic rock which were unable to excape the burried granite. Such vesicles are common in granites, often cotaining larger crystals formed later from minerals disolved in water that percolated through the rocks (geodes). 

Bubble cave north of Laoshan Reservoir. Song et al. (2016).

Mairiolitic caves as large as the Laoshan Granite structures have never previously been recorded. Nevertheless Song et al. conclude that, based upon the morphology of the caves, a mariolitic origin is more likely that a glacial erosion based one.

Large cave near Tiaqinggong. Song et al. (2016).

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At least 49 dead following Hiroshima landslides.

49 people are known to have died and a further 41 are still missing following a series of landslides that hit the Japanese city of Hiroshima on Wednesday 20 August 2014. The landslides occurred following a period of exceptional rainfall, with 215.7 mm of rain falling in just three hours, between 1.30 and 4.30 am local time; more than falls in the city in most months. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall. 

One of a series of landslides that hit Hiroshima on Wednesday 20 August 2014. Kyodo News/AP.

The worst affected areas were in the north of the city, in the wards of Asakita-ku and Asaminami-ku. These areas lie on a bedrock of decomposed granite, which can be particularly hazardous when it begins to erode. While granite is generally thought of as being extremely hard, it is made up of large grains of several different minerals, typically plagioclase feldspar, orthoclase feldspar, quartz and mica, and under some circumstances one or more of these minerals can become chemically weathered, breaking up the structure of the rock and allowing the other minerals to crumble as free grains. The most common form of chemical weathering involved is the breakdown of the feldspar minerals in granite to kaolinite clay (the white clay used to make porcelain ceramics), something that happens readily in warm wet climates, such as that of southern Japan.

Rescue workers searching for survivors in Asaminami-ku Ward, Hiroshima, following the 20 AUgust 2014 landslides. Reuters.

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