Radionuclides and heavy metals concentrations in particulate matter around uranium and gold mining towns of Erongo region, Namibia

Abstract

The aim of the study was to measure the activity concentrations of naturally occurring radioactive materials (NORM) and these are: 226Ra, 210Pb , 238U, 232Th and 40K and levels of toxic heavy metals in particulate matter (PM) and soil associated with mining activities in the two towns of Karibib and Arandis in Erongo region, Namibia. Furthermore, the radionuclides concentrations and toxic heavy metals were evaluated for their impact on human health and the environment. In both locations, some PM samples were collected with soil samples. Radioactivity measurements were performed with γ-ray spectrometer coupled with a high purity germanium detector (HPGe) while toxic heavy metal concentrations were determined by inductively coupled plasma mass spectrometry (ICP/ Ms). The activity concentrations from this study revealed higher levels of radionuclides associated with 238U series and these were unevenly distributed among the sites. The most dominant concentration was found in 226Ra, which ranged from 19.76 ± 0.47 to 135.29 ± 12.82 Bq.kg-1. The activity concentrations in the samples were within the acceptable figures given by worldwide ranges of 11 to 64 for 238U, 17 to 60 for 232Th and 140 to 850 Bq kg-1 for 40K (UNSCEAR, 2000). It is also interesting to note that the activity concentrations in PM was higher than the corresponding soil samples because the radioactivity concentrations increase as the average grain size decreases. The radiological parameters associated with the measured radionuclides were: absorbed dose (D), annual effective dose equivalent (AEDE), radium equivalent activity concentrations (Raeq), external hazard index (Hex) and internal hazard index (Hin) and excess lifetime cancer risk (ELCR) and the results showed that the mean radiological parameters in most samples were within the permissible limits with Raeq < 370 Bq.kg-1, which corresponds to an upper safe limit of AEDE of 1 mSv.yr-1, the prescribed acceptable limit by International Commission on Radiological Protection (ICRP) and thus, the potential radiological health effects may not be significant. However, some sites have hazard index closer to a unit which may imply that prolonged exposure to NORM in soil and PM in those mining areas may pose a health hazard to members of the public. Furthermore, the ELCR was found to be greater than 0.29 x 10-3, the safe limit recommended by ICRP in all the samples, which may be a health risk to individuals as the chances of developing cancer due to radionuclides could be high. The average indoor radon concentrations from the two mining towns was found to be 88 Bq.m-3, which corresponds to an annual effective dose of 2.22 mSvy-1. The mean values due to exposure to radon and its decay daughter is two times higher than the worldwide average annual effective dose of 1.26 mSv (UNSCEAR, 2000). The toxic heavy metals concentrations measured in this study were Fe, Mn, Cr, Zn, Cu, Ni, Pb, As, and Cd. The results for the average toxic metals concentrations were compared with the FAO/WHO soil guidelines and other countries to determine whether they were within the allowable limits. The results showed that most of the measured values were found to be within the permissible limits. Pollution contamination indicators were applied to quantify the level of contamination and it was found that contamination factors for most of the samples in the towns of Arandis and Karibib show a low level of contamination while the pollution load index (PLI) and the Geoaccumulation Index (Igeo) illustrate that the soils are not polluted. Similarly, the elemental composition was determined by a scanning electron microscope attached to an energy dispersion X-ray (SEM/EDX) and the major particles identified in the samples were: biogenic particles, soot, aggregates, aluminosilicates, mineral particles, quartz particles, clay particles, and non-biogenic C rich particles. The non-carcinogenic risk of heavy metals of the soil samples in the two towns of Karibib and Arandis was assessed and the total HQ in Arandis town was 1.52x10-1 and 7.04x10-1 for adults and children respectively, while in Karibib town, it was 8.83x10-2 for adults and 3.86x10-1 for children. These values were less than 1 and therefore, not significant. The computed total carcinogenic risk due exposure to heavy metals in Arandis town was also found to be 1.40 x 10-3 for adults, and 1.30 x 10-2 for children. In Karibib town, the cancer risk value for adults was found to be 4.25 x 10-4 and in children it was 3.97 x 10-3. Since these values were higher than the worldwide average Excess Lifetime Cancer Risk due to exposure to radionuclides from terrestial origin (0.29 x 10-3) (Taskin et al, 2009) and thus there higher chances of increase in cancer among the inhabitants of these mining towns due to heavy metal exposure.