Radon-222 from different sources of water and the assessment of health hazard

There are three naturally occurring radioactive decay series in the Earth crust originating from ²³⁸U, ²³⁵U and ²³²Th. Each of the series has an isotope of radon (symbol: Rn); ²²²Rn (T_½ = 3.8235 days), ²¹⁹Rn (T½ = 3.96 seconds) and ²²⁰Rn (T½ = 0.9267 minutes) from the ²³⁸U-, ²³⁵U- and ²³²Th-decay series, respectively. ²¹⁹Rn and ²²⁰Rn are short-lived nuclides and as such they are not abundant in nature (Dewayne & Gesell 1992). ²²²Rn can reach secular equilibrium in ca. 25 days with its parent element ²²⁶Ra contained in rocks (Andrews & Lee 1979) and thus it can accumulate in appreciable amounts in groundwater (Kendall & McDonnell 1998; Wu et al. 2003). Radon is odourless, colorless and tasteless, thereby making it difficult to detect its presence in water with the human senses. ²²²Rn decays to ²¹⁸Po by emitting alpha particles which are a potential health hazard if radon is inhaled or ingested (National Research Council 1999).

The inhalation of radon progeny is the largest single source of radiation exposure to the population, contributing to about 52% of the total dose due to natural radiation (UNSCEAR 2000). A relationship between lung cancer and inhalation of radon decay products has been demonstrated for underground miners (Lubin et al. 1995) and in domestic environments (Lubin & Boice 1997). Radon is a factor of stomach radiation burden due to consumption of water (Nikolopoulos et al. 2009).

Radon in tap water may lead to exposure from ingestion of drinking water and from the inhalation of radon released to air when water is used for bathing, cooking, washing, etc. (UNSCEAR 2000). Once in the building, water with an elevated level of radon can cause radon to diffuse into the indoor atmosphere and increase the overall radon levels (Appleton 2005). A link between lung cancer and inhalation of radon has been established and ingestion of radon has been weakly linked to stomach cancer (Hopke et al. 2000). A high concentration of radon in drinking water causes stomach cancer (Tabassum & Mujtaba 2012; Thabayneh 2015). As such radon has been identified as a public health concern when present in all types of drinking water. Due to the health burden of radon, this study is aimed at assessing the radon concentration in different sources of water at the University of Ibadan, Nigeria and evaluating the effective dose due to the ingestion and inhalation of radon in water.

Forty-six water samples were collected from different sources. The sources include borehole (drilled deep well), well (dung shallow well), treated water from the university utility water supply and packaged water. The concentration of dissolved radon in water is highly dependent on the extent to which the water is aerated. Therefore, in order to measure the actual public exposure levels, water samples were collected at consumer's fetching points.

Water samples were collected using 400 mL glass bottles. For borehole sources, water from the tap was left running for several minutes, a bucket was filled to overflowing and then raised so that the tap is below the water surface. The collection bottle was then immersed and filled from the bottom of the bucket and capped underwater. This was done to prevent air pockets and bubbles. Water from wells was collected with the aid of a bailer and bottles were carefully submerged into the bailer to collect water samples. Each sample was carefully labeled and the time and date of sampling were noted. Treated water (sourced from a surface dam) from the utility water supply of the university was collected from storage tanks in a residential area where the water had been directly supplied from the treatment plant. Packaged water was purchased from an open market in the university. The source of the packaged water is a borehole in the university; however it had been subjected to some processes before packaging. After collection, samples were quickly taken to the laboratory for measurement. Two aliquots were measured for each sample and the mean value was obtained.

In this study, an AlphaGUARD portable radon monitor manufactured by Saphymo GmbH, Germany, was used for the measurement of radon concentration in water. AlphaGUARD is a potable, battery operated monitor with a high storage capacity. It measures and records radon concentrations simultaneously with the ambient temperature, relative humidity and atmospheric pressure with integrated sensors. AlphaGUARD incorporates a pulse-counting ionization chamber (alpha spectroscopy). It is suitable for the continuous monitoring of radon concentrations between 2 and 2,000,000 Bq/m³ through the optimal geometry of its chamber and intelligent signal evaluation. AlphaGUARD offers high detection efficiency, a wide measurement range, fast response and permanent, maintenance-free operation with long-term stable calibration. The instrument is insensitive to both high humidity and vibrations. With AlphaPUMP, AquaKIT and the Soil Gas Probe, the equipment is suitable for the measurement of radon in water samples and soil gas. The measurement of radon in water was carried out using the AlphaGUARD via a special unit (AquaKIT). The unit consists of a vessel used for forced degassing of radon diluted in water samples, a security vessel used for water drop deposition. Vessels and AlphaGUARD were connected via plastic radon proof tubes. Forced degassing of radon gas is performed by circulating the air in the set up with the use of a pump (AquaPUMP). A 100 mL water sample was placed in an appropriate system of glass vessels connected to the detector through the air pump, following the recommendations of the manufacturer (Saphymo GmbH 2015). The radon detector, AlphaGUARD, is based on the optimized design of a pulse-ionization chamber. In regular operation this detector measures the radioactivity of the air using the diffusion of gas through the large surface of the glass fiber filter installed inside the ionization chamber. This filter allows only the ²²²Rn gas to pass through and prevents the products of the radon decay from entering into the ionizing chamber. It also protects the ionizing chamber from contamination by dust particles (Corrêa et al. 2011).

Table 1 contains the result of radon concentrations in water samples collected from boreholes. The radon concentration in water ranged from 3.56 ± 0.74 Bq.l⁻¹ to 98.57 ± 5.10 Bq.l⁻¹ with a median value of 14.63 ± 1.9 Bq.l⁻¹. This is a major source of water in the students' halls of residence. The water is pumped directly from boreholes everyday without any treatment into reserviour tanks where it is taken for drinking, cooking and bathing. It is possible that if the water had been treated, most of the dissolved radon in water would be vented/released during treatment and this might result to low radon concentrations. The radon concentrations in water samples collected from wells are presented in Table 2. The radon activity concentration levels in the well water ranged from 0.88 ± 0.57 Bq.l⁻¹ to 25.49 ± 1.92 Bq.l⁻¹ with a median value of 6.21 ± 0.68 Bq.l⁻¹. Table 3 presents the results for the packaged water which ranged from 0.73 ± 0.13 Bq.l⁻¹ to 1.35 ± 0.30 Bq.l⁻¹ with a median value of 0.91 ± 0.19 Bq.l⁻¹. For the utility water supply shown in Table 4, the radon concentration ranged from 0.24 ± 0.11 to 1.03 ± 0.20 Bq.l⁻¹ with a median value of 0.46 ± 0.12 Bq.l⁻¹. It is observed that the radon concentrations of the packaged and utility water are much lower than those of the borehole and well even though the packaged water is sourced from a borehole. This could be attributed to the treatment process which could lead to the radon degassing. This study showed a high level of radon in borehole water followed by well water. The utility water had the least amount of radon concentration, which is due to the fact that radon is readily released from surface water. About 67% (borehole) and 27% (well) of the investigated water samples had radon concentrations higher than 11.1 Bq.l⁻¹, recommended by the United States Environmental Protection Agency (EPA 1991; Paschuk et al. 2013).

²²²Rn concentration levels in different sources of the water were measured using a portable pulse-counting ionization chamber (alpha spectroscopy), AlphaGUARD (SAPHYMO GmbH). The AED due to ingestion and inhalation of radon in the water samples were determined. Borehole water had the highest value of radon concentration with about 67% higher than the value recommended by the United States Environmental Protection Agency for drinking water. The mean AED of radon concentration in water from boreholes and wells for adults, children and infants were higher than the recommended reference dose level of 0.1 mSv from one year's consumption of water given by the World Health Organization.

The authors are grateful to the Alexander von Humboldt Foundation, Bonn, Germany for the donation of the AlphaGUARD radon monitor and the accessories used for this study.