Assessment of bioaccessibility and health risk of mercury within soil of artisanal gold mine sites, Niger, North-central part of Nigeria

Abstract

The occurrence of mercury (Hg) in the environment globally has been linked largely to its use for gold processing. In this research, ore samples, agricultural soil and mine wastes were taken within the vicinity of an artisanal gold mine and processing sites in Niger state, a north-central part of Nigeria to determine Hg contamination in the environment and estimate the potential hazard to health. The values of Hg measured in ore, agricultural soil and mine wastes ranged between 0.03 and 5.9, 0.002 and 5.57 and 0.19 and 20.99 mg/kg, respectively, with the majority of samples observed above the crustal average values of 0.003 mg/kg. All of the samples were 100 times greater than the USEPA residential soil screening level of 0.0023 mg/kg, but were lower than comparable mine sites within the same region. Contamination indices were used to demonstrate the potential exposure to Hg contamination in the study area which ranged from a medium to high level of contamination. Average daily dose and hazard quotient (HQ) were calculated for adults and children in the study area and decreased in the following order: ADDvapour > ADDingestion > ADDdermal > ADDinhalation. The non-carcinogenic health risk index (HI) of Hg calculated for children and adults in the study area was children: 7.42, 2.19, 1.49 and adults: 4.45, 1.26, 1.19, for mine wastes, agricultural soil and ore, respectively. All of these values were higher than a considered safe level (=\u20091) and therefore showed that Hg posed a serious non-carcinogenic HI for both adults and children exposed to the soil in the study area. The bioaccessible fraction as a measure of ingestion for Hg was generally\u2009<\u200913% across all sample matrices, suggesting a low bioaccessibility. An HQ incorporating bioaccessible data (BHQ) ranged between 0.000005 and 4.06 with a mean value of 0.62. Values for the BHQ were still\u2009>\u20091, threshold limit in some samples and showed that Hg could present a risk to health via ingestion, although further research is required to assess dermal and inhalation bioaccessibility to assess fully the risk to residents. However, the values were lower than the non-carcinogenic health risk index, which is assumed to be overestimated.