Sixteen groundwater and surface-water samples (wells, boreholes, and sachets) were collected from six different locations within Ojo Local Government area in Nigeria. Sites of collection are presented in Fig. 1 and listed in Table 1. In addition, samples number with sites (sample ID) of water collected from Ojo Local Government area in Nigeria is presented in Table 1.
Radon Measurement
Radon detector (RAD-7) manufactured by DURRIDGE COMPANY Inc, in USA, is a portable radon detector with fast measurement, widely used to measure the radon concentration in water samples. It’s lower limit detection (LLD) is less than 0.37 Bq L− 1.
Using RAD-7, H2O gives results after 30 min analysis with a sensitivity that matches or exceeds that of liquid scintillation methods (RAD-7, RAD H2O). The setup of RAD7 used to measure Rn-222 activity in water samples is shown in Fig. 2. The accuracy of radon in water measurement utilizing RAD-7 are affected by some factors such as the technique of sampling, sample size, counting time, temperature, relative humidity, and background effects. The water sample is a representative of the water being tested and such that it has never been in contact with air. In the method, a faucet was attached to a bowl so that the water overflowing into the bowl is prevented. Furthermore, a 250 ml vial was filled with water and taken for radon concentration measurement. Radon concentration of less than 100 BqL− 1 was observed and for the radon concentrations higher than 100 BqL− 1, a 40 ml vial is used.
Since in this investigation, the radon concentration in water was unknown, the samples in both sizes were taken. When sampling a tap water, water was let run for 10 min before taking the sample to let out the water from the possibly stagnant pipe section. The sampling vial (volume 250 ml) was placed in the bottom of the bowl, and the tube end was put into the vial. The water flow for a while, keeping the vial full and flushing with fresh water. The vial was cap while still under the water.
Due to the impact of relative humidity on measurements, the RAD-7 counter should be dried out thoroughly before making the measurement. High humidity reduces the efficiency of collection of the Po-218 atoms, first decay of radon, inside the chamber. However, the Po-218 half-life 3.05 min means that almost all the decays that are counted come from at mode posited.
Annual effective dose due to the ingestion of radon from the underground E(ing) was calculated using Eq. (1), [33].
\({E}_{ing}=\) CRn (Bq\({L}^{-1})\) x \({D}_{ing}\) x \({D}_{w}\) x F x T (1)
where, CRn is the mean radon (222Rn) activity concentration in water, Dw is the daily water ingestion (2 L day− 1), Ding is the ingesting dose conversion factor of radon (10− 8 Sv Bq− 1), and T is equal to 365 day y− 1 [34]. The annual effective dose of inhalation E(inh) of radon from water is obtained from Eq. (2), (UNSCEAR, 1993)
\({E}_{inh}={Rn}^{222}\) conc. (Bq\({L}^{-1})\) x R x D x F x T (2)
where, CRn is the mean radon (222Rn) activity concentration in water, R is the ratio of radon in air to radon in water (10− 4), D is the dose conversion factor of radon (9 nSvh− 1(Bq/m3))−1, F is the indoor equilibrium factor between radon and its progeny (0.4) and T is indoor time (7000 hy− 1), (ICRP, 1993; UNSCEAR, 2000).