Assessment and mapping of the high background radiation anomaly associated with laterite utilization in the south coastal region of Kenya

Abstract

Laterite is commonly used in Kenya and other tropical countries as an aggregate material for road paving. However, the use and relocation of laterite and other geological materials from high background radiation areas (HBRA) may increase exposure to ionizing radiation in areas that were originally at background radiation levels. We present the results of an in-situ radiometric survey conducted at sites associated with laterite utilization around the Mrima Hill HBRA and adjoining environs along a section of the main road in south coastal region of Kenya. The goal of this study was to determine the levels of primordial radionuclides, as well as to assess and map the associated radiation exposure due to the modified radiation field resulting from the use of laterite in the region. It was found that air absorbed gamma dose-rates on parts of the road and side road intersections (median values are 326 ± 52 and 987 ± 138 nGy h−1, respectively) range 4–14 times higher than the adjoining areas. This anomaly was attributed to elevated levels of 238U and 232Th (activity concentrations range 202–1452 Bq kg−1 and 315–1579 Bq kg−1, respectively) measured adjacent to the laterite roadbeds. In addition, results from the energy dispersive X-ray fluorescence analysis of lateritic soils showed that Th correlates positively with monazite constituent elements namely P, La, Ce and Nd suggesting that the laterite used for road paving in parts of the region was sourced from Mrima Hill.

Introduction Laterites are fine-grained soils formed as a result of intensive weathering of the underlying parent rock in areas which experience humid tropical climatic conditions (Tardy, 1997). Studies have indicated that among other properties, lateritic soils posses high shear strength which is preferred for road paving in many tropical countries (Madu, 1977; Charman, 1988; Nogami and Villibor, 1991; Mahalinga-Iyer and Williams, 1994). This is why during the early Post-Colonial Era in Kenya, laterite was commonly used as an aggregate material for road paving; such paving was extended about 1.5–2 m beyond the edge of carriageways (Grace, 1991). However, radioactive minerals such as monazite and pyrochlore have been associated with laterites found in carbonatite intrusions (Coetzee and Edwards, 1959; Lottermoser and England, 1988; Wall and Mariano, 1995; Nasraoui and Bilal, 2000; Giovannini et al., 2017). Therefore the relocation and utilization of laterite from such areas implies migration of elevated radioactivity to intrinsically lower radiologically active environments thereby increasing human and biota exposure to natural ionizing radiation.

Mrima Hill is a well-known carbonatite intrusion located in south coast Kenya and is about 80 km south-west of the port city of Mombasa (Baker, 1953). It rises approximately 300 m above sea level out of the seaward-sloping coastal plain of the Indian Ocean. The absorbed gamma dose-rate values measured within a 2 km radius in and around the hill, as well as the nearby Kiruku Hill (5 km east of Mrima Hill) have been found to be more than 1.5–33.8 times higher compared to the global population weighted average value of 60 nGy h−1 (Kaniu et al., 2018a). Chege et al. (2015) reported high annual effective doses due to thoron inhalation in mud dwellings around Mrima Hill. The estimated annual doses ranged 2.8–27.2 mSv h−1compared to world average value of 0.1 mSv h−1. The presence of huge iron, manganese and REEs rich laterite (locally known as murram) deposits containing radioactive mineral ores, such as monazite and pyrochlore has been reported on the lower slopes of Mrima Hill (Baker, 1953; Coetzee and Edwards, 1959; Loupekine, 1968). Mangala (1987) and Patel (1991) observed several laterite pits and trenches dug on the southern and eastern slopes of the hill from which the dug-laterite could have been relocated and exploited for other uses such as road construction in the region.

This study presents the results of an in-situ (vehicular and ground-based) γ-ray radiometric survey carried out at sites associated with laterite utilization in the south coastal region of Kenya. The sites comprise a section of the Likoni - Lunga Lunga main road (including 2 m extension from both sides of bitumen pavement), side-road intersections ( 10 m away from main road), road sides ( 15 m and 100 m away from main road) and environs adjacent the main road ( 200 m away from main road), as well as selected fields around Mrima Hill. The goal of the study was to determine the activity concentrations of 40 K, 238U and 232Th at the surveyed sites, as well as to assess and map associated radiation exposure due to the modified radiation field resulting from the use of laterite. Additionally, some soils were analyzed using EDXRF spectroscopy in order to indirectly determine the presence of monazite utilizing its elemental associations.