Natural radioactivity concentrations and occurrence of heavy Metals in Shore Sediments along the Coastline of the Erongo Region in Western Namibia
Onjefu, Sylvanus Ameh
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Some human activities like uranium mining, sea port activities and improper wastes handling has been identified as potential sources of exposure to naturally occurring radioactivity materials (NORM) and heavy metals. However, some of these activities are not being effectively regulated for natural radioactivity and heavy metals in Namibia. Whilst some developed nations have identified these environmental challenges and have put measures in place to address it, very little attention is given in developing countries. However, most of the human activities that releases NORM and heavy metals are located in the developing nations that have paid little or no attention to address it. The Erongo region of Namibia is currently home to several active uranium mining companies, port harbor and other fish processing companies that are only a few kilometers from the shoreline. Namibia is an arid country with an average day temperature of 30 $^o$C. This prevailing climate has necessitated the migration of both locals and international tourists to the beaches in the coastline. Most worrisome is the use of the shore sediment as building materials. These studies have been carried out to measure the natural radioactivity concentrations and occurrence of heavy metals in shore sediments along the coastline of the Erongo region. A high purity germanium (HPGe) detector was used in radioactivity assessment. In evaluating the occupancy factor appropriate for the coastline, a questionnaire was administered to 2400 population that visit the beaches for different activities. Analysis of the shore sediment for heavy metals was done using inductively coupled-optical emission spectrometry (ICP-OES) Perkin Elmer Optima 7000 DV. The mean activity concentration along the coastline of the Erongo region ranges from 142.79 to 199.76 Bq.kg $^–$614 for $^2$$^3$$^8$U, 29.69 to 42.47 Bq.kg $^–$$^1$ for $^2$$^3$$^2$Th and 354.38 to 611.19 Bq.kg $^–$$^1$ for $^4$$^0$K. The results from the study were compared with world safe values of 50, 50, and 500 Bq.kg$^–$$^1$, respectively (UNSCEAR, 2000). The modelled results showed outdoor occupancy factor of 0.48 and indoor occupancy factor of 0.52, which are 2.4 times higher than the UNSCEAR outdoor factor and 0.65 times less than the UNSCEAR indoor factor. The absorbed dose rate (ADR) in air at 1 meter above ground was estimated to be in the range 93.27 to 144.01 nGy.h$^–$$^1$. The outdoor annual effective dose rate ranged from 121.01 to 176.61 μSv.y$^–$$^1$ (UNSCEAR factor), 292.60 to 413.63 μSv.y$^–$$^1$ (present modelled factor), with average values of 142.5 μSv.y$^–$$^1$ and 339.36 μSv.y$^–$$^1$ which are both above the world effective dose value of 70 μSv.y$^–$$^1$ The mean values of Ra$_e$$_q$, H$_e$$_x$ H$_i$$_n$ and ELCR obtained for all sediment samples showed that Ra$_e$$_q$ and H$_e$$_x$ are lower than the accepted world safe limit value of 370 Bq.kg$^-$$^1$ and below the limit of unity. However, use of H$_i$$_n$ and ELCR values showed that the use of the shore sediment samples for building material poses internal radiation risk since the values are > 1 for H$_i$$_n$ and above 0.29 x 10$^-$$^3$ for ELCR. Assessment on the contamination status of heavy metals in the shore sediment showed high enrichment for Cd in both seasons of summer and winter, moderate degree of contamination of As in summer in the beach of Walvis Bay. The geo-accumulation index revealed moderate pollution of Swakopmund beach with Cd in summer, unpolluted beach of Swakopmund and polluted to moderately polluted beach of Henties Bay with Cd in winter season. However, the evaluation of the results of pollution load index (PLI) from this present study indicated PLI < 1 which showed that all the beaches investigated in this present study are not polluted with metals.