Radon in groundwater baseline study prior to unconventional shale gas development and hydraulic fracturing in the Karoo Basin (South Africa)

Highlights: • Most extensive study up to date studying radon-in-groundwater prior to hydraulic fracturing in the Karoo Basin. • Modes which could result in radon levels increase within drinking water due to hydraulic fracturing are discussed. • Measurement of radon-in-water levels presents a method to indicate the aquifer type within the Karoo Basin region. • Higher radon-in-water levels are associated with the warmer shallow aquifers and lower levels with cooler deep aquifers. • Inter-annual and inter-seasonal radon fluctuations are observed from the shallow and mixed aquifers. - Abstract: The prospect of unconventional shale gas development in the semi-arid Karoo Basin (South Africa) has created the prerequisite to temporally characterise the natural radioactivity in associated groundwater which is solely depended on for drinking and agriculture purposes. Radon (²²²Rn) was the primary natural radionuclide of interest in this study; however, supplementary radium (²²⁶Ra and ²²⁸Ra) in-water measurements were also conducted. A total of 53 aquifers spanning three provinces were studied during three separate measurement campaigns from 2014 to 2016. The Karoo Basin's natural radon-in-water levels can be characterised by a minimum of 1 ± 1 Bq/L (consistent with zero or below LLD), a maximum of 183 ± 18 Bq/L and mean of 41 ± 5 Bq/L. The mean radon-in-water levels for shallow aquifers were systematically higher (55 ± 10 Bq/L) compared to deep (14 ± 3 Bq/L) or mixed aquifers (20 ± 6 Bq/L). Radon-in-water activity concentration fluctuations were predominantly observed from shallow aquifers compared to the generally steady levels of deep aquifers. A collective seasonal mean radon-in-water levels increase from the winter of 2014 (44 ± 8 Bq/L) to winter of 2016 (61 ± 16 Bq/L) was noticed which could be related to the extreme national drought experienced in 2015. Radium-in-water (²²⁸Ra and ²²⁶Ra) levels ranged from below detection level to a maximum of 0.008 Bq/L (²²⁶Ra) and 0.015 Bq/L (²²⁸Ra). The ²²⁸Ra/²²⁶Ra ratio was characterised by a minimum of 0.93, a maximum of 6.5 and a mean value of 3.3 ± 1.3. Developing and improving baseline naturally occurring radionuclide groundwater databases is vital to study potential radiological environmental impacts attributed to industrial processes such as hydraulic fracturing or mining.