Pilot groundwater radon mapping and the assessment of health risk from heavy metals in drinking water of southwest, Nigeria

Radon and heavy metals are sources of groundwater pollution and are identified as potential carcinogens. Southwest Nigeria's populace mostly relies on groundwater source for drinking. This study aims to map radon distribution in groundwater of southwest Nigeria and to determine the health risk of radon and heavy metal in drinking water. Radon concentrations of 145 groundwater samples were measured using RAD7 electronic radon detector and heavy metal concentrations of 52 groundwater samples were measured using atomic absorption spectrophotometer. Radon concentration distributions were delineated using geographical information system. Radon concentration of water samples ranges between 1.6 Bq l(-1) and 271 Bq l(-1) with an average value of 35.9 +/- 38.4 Bq l(-1). The average groundwater radon concentration is higher than US-EPA recommended level of 11.1 Bq l(-1) but lower than the WHO recommended limit of 100 Bq l(-1). The estimated average annual effective radiation doses to infants, children, and adults are 29 mu Sv y(-1), 41 mu Sv y(-1) and 92 mu Sv y(-1) respectively. The radon distribution map of the study area reveals regions of high, medium, and low groundwater radon concentrations. The average concentration values of heavy metals in groundwater samples are of the order Mn > Zn > Pb > Cu > Cr > Ni > Cd. 84% of groundwater exhibits good to excellent quality in terms of heavy metal pollution. However, about 16% of the samples which lie in the sedimentary regions of Ogun and Lagos States exhibit poor to very poor quality. Overall, ingestion of groundwater in the study area may not pose a serious health hazards from radon ingestion and heavy metal toxicity.