The district of Abidjan is located in the south of Côte d’Ivoire. It is located between latitudes 5˚10 and 5˚38 North and longitudes 3˚45 and 4˚21 west. With an area of 2119 km², the district of Abidjan has a growth rate of 6% per year and a population density of 2221 inhabitants/km². It includes a population estimated in 2021 at about 6,321,017 inhabitants [18] .

In geological and hydrogeological plans, the district of Abidjan belongs to the coastal sedimentary basin which is 400 km long and 40 km wide from Fresco to the Ghanaian boundary. The sedimentary formations of this basin consist of sandy clays, sands and sandstones, conglomerates, glaucous sands and marl [19] . This basin is a Cretaceous to Quaternary basin with enormous potential in groundwater ‎[20] .

These groundwater resources are contained in three aquifer levels of unequal importance. The three main aquifers are: Quaternary aquifer, Maastrichtian aquifer, and Continental Terminal aquifer. The Continental Terminal is one used for the supply of water in the district of Abidjan. The sampling points are indicated on the (Figure 1) below.

A total of 43 tap water samples were collected from 13 townships in the District. They were collected in 1.5 liter polyethylene bottles previously well washed, rinsed with nitric acid and labeled. The collected samples were immediately acidified with a few drops of nitric acid HNO₃ (1 M) to prevent adherence of the radionuclides to the walls of the containers.

For the measurement of gross alpha and beta activities in samples, 300 mL of water from each sample were transferred into 400 mL beakers, previously thoroughly washed and rinsed with ultrapure water to avoid contamination of the samples. The beaker and their contents were slowly heated to a low temperature (below 65˚C) on an electric hot plate to evaporate the water sample contained at a small volume. The temperature was uniform throughout the plate to avoid areas of “overheating” that could cause losses by projections.

After heating, the small amounts remaining in the beakers were dissolved with few drops of nitric acid (HNO₃) (1 M) to prevent the radionuclides from sticking to the walls of the cups. Then the dissolved quantities were transferred to cylindrical stainless steel cups with a diameter of 47 mm and labeled to differentiate the samples.

The steel cups and the contents were heated on the electric hot plate to be dried to obtain radioactive residues (Figure 2). The steel cups were placed in a dryer to cool them at laboratory temperature but also to prevent them from absorbing moisture. The dried samples were then placed in a gross alpha and beta activity counter for analysis.

The prepared samples were counted to determine alpha and beta activity concentration using the low background Gas-less Automatic Alpha/Beta counting system (Canberra iMatic^TM) calibrated with alpha (²⁴¹Am) and beta (⁹⁰Sr) standards. The system uses solid state silicon (Passivated implanted Planar Silicon, PIPS) detector for alpha and beta detection. The samples were counted for 200 min.

The alpha and beta efficiencies were determined to be 36.39% ± 2.1% and 36.61% ± 2.2% respectively. The background readings of the detector for alpha and beta activity concentrations were 0.04 ± 0.01 and 0.22 ± 0.03 cpm. All the measurements were carried out at the Ghana Atomic Energy Commission, Accra.

The gross α and β activities of the water samples were estimated according to Saleh et al., as shown by Equation (1) [21] .

A α , β = N 60 × E f f α , β 100 × V s (1)

N is the separately net gross alpha or beta count rate (cpm), E f f α , β is separately gross alpha or beta counting efficiency (in percent), V s is the volume of sample aliquot (in L) and 60 is the conversion factor.

The effective dose equivalent (DRw), the total effective equivalent dose (TEED) and lifetime risk index using concentrations of gross alpha and beta activity were calculated using ICRP (1991), UNSCEAR (2000) and Karahan et al. [22] [23] [24] .

D R w ( α , β ) = A w ( α , β ) × D C F ( α , β ) × 730 (2)

where D R w ( α , β ) is the dose equivalent effective (Sv/year), A w ( α , β ) is activity (Bq/L), a daily water intake of 2 L/Day according to WHO [1] results in annual consumption rate of 730 L/year, D C F ( α , β ) is the dose conversion factor for ingestion of the individual natural radionuclides for adult, α = 2.80 × 10 − 4 mSv∙Bq⁻¹ and β = 6.90 × 10 − 4 mSv∙Bq⁻¹ [8] .

The measured activity concentrations of gross alpha in tap water samples of different locations in the district of Abidjan are gathered in Table 1. The observed gross alpha activities found in the collected samples varied from 0.001 ± 0.002 to 0.063 ± 0.050 Bq/l with an average of 0.013 ± 0.012 Bq/l. The highest gross alpha activity in the water was found in sample R18 and the lowest alpha activity was found in sample R10. The gross alpha activity in water sample is primarily comprised uranium decay products such as ²²⁶Ra and ⁴⁰K. WHO recommended the parameter of gross alpha activity concentration to be 0.1 Bq/l. If the gross alpha activity does not exceed 0.1 Bq/l, it can be assumed that the annual total indicative dose is less than 0.1 mSv per year. In the present study, the obtained results show that the measured activity concentrations of gross alpha in all tap water samples were less than 0.1 Bq/l which is the limit recommended by WHO.

For gross beta activity concentration in the tap water samples, Table 1 showed various values ranging from 0.067 ± 0.080 to 0.320 ± 0.120 Bq/l with an average of 0.174 ± 0.076 Bq/l.

The highest concentration was found in sample R22 and the lowest in sample R21. This high activity concentration could be due to the geological setting of the underground water source. However, the results obtained also showed that the measured activity concentrations of gross beta in all tap water samples are less than 1.0 Bq/l which is the limit recommended by WHO.

Compared with other values of gross alpha and beta activities in tap water in other countries, it is observed, as shown in (Table 2), that the results of the present study are lower than values of gross alpha concentration found in tap drinking water samples in countries like Italy, Nigeria, Jordan and Malaysia. It is also shown than gross beta activity obtained in the present study was lower than the one found in tap water in Jordan, Malaysia but higher than the beta activity measured in Turkey, Italy, Nigeria and Bangladesh Dhaka.

The annual alpha and beta effective dose due to intake of water was determined by averaging the individual annual committed effective doses contributed by the major alpha and beta emitters in the U-238 and Th-232 series of the naturally occurring radionuclides [17] . The calculated effective doses are shown in (Table 3).

The equivalent gross alpha effective dose due to water intake in the district varied from 0.0002 ± 0.0004 mSv/year found in sample R10 to 0.0129 ± 0.0102 mSv/year found in R18, with an average of 0.0027 ± 0.0025 mSv/year.

As for the annual beta effective dose due to intake of water, the lowest value of 0.0335 ± 0.0400 mSv/year was found in sample R21 while the highest one of 0.1598 ± 0.0600 mSv/year was found in R22, with an average value of 0.0863 ± 0.0369 mSv/year. Summing the two equivalent gross effective doses, the total annual effective dose due to intake of tap water samples found varied from 0.0341 ± 0.0761 mSv/year to 0.1624 ± 0.0410 mSv/year, with an average value of 0.0890 ± 0.0394 mSv/year. The lowest annual effective dose was found in sample R21 while the highest one was obtained in R22. It thus follows that greater risk is associated with tap water from R22, than at the other areas of the district.

The recommended reference dose level (RDL) of the committed effective dose, equal to 0.1 mSv from 1 year’s consumption of drinking water [31] was however exceeded in thirteen out of the forty three samples (Table 2), meaning 30% of the total samples (Figure 2). This result shows that the water is not safe for consumption in some locations of the district of Abidjan.