Mercuric pollution of surface water, superficial sediments, Nile tilapia (Oreochromis nilotica Linnaeus 1758 [Cichlidae]) and yams (Dioscorea alata) in auriferous areas of Namukombe stream, Syanyonja, Busia- Uganda

The mercuric content, pollution and contamination characteristics of water, sediments, edible muscles of a non-piscivorous fish (Oreochromis nilotica Linnaeus 1758 [Cichlidae]) and yams (Dioscorea alata) in mercury-based artisanal and small-scale gold mining (ASGM) impacted Namukombe stream and its propinquity, Busia gold district, Uganda were evaluated. Human health risk assessment from consumption of the fishes and yams as well as dermal contact with sediments from the stream were performed. Forty-eight (48) samples of water (12), sediments (12), fish (12), and yams (12) were taken at intervals of 0, 10, 20 and 30m from up, middle and down sluices of the stream and analyzed for total mercury (THg) using US EPA method 1631. Results showed that water in the stream is polluted with mercury (Hg) in the range of 0.00 to 1.21±0.070mg/L while sediments contain Hg up to 0.14±0.04ugg^-1. THg content of the edible muscles of Oreochromis nilotica ranges from 0.00 to 0.11±0.010ugg^-1 while yams contain 0.00 to 0.30±0.001ugg^-1 of Hg. The estimated daily intakes (EDIs) ranged from 0.0049 to 0.0183ugg^-1day^-1 and 0.020 to 0.073ugg^-1day^-1 for fish consumed by adults and children respectively. The corresponding health risk indices (HRIs) ranged from 0.0123 to 0.04576 and 0.05 to 0.183. EDIs were from 0.0042 to 0.1279ugg^-1day^-1 and 0.013 to 0.394ugg^-1day^-1 for yams consumed by adults and children respectively. The HRIs recorded were from 0.011 to 0.320 and to 0.033 to 0.985. All the mean THg contents of the investigated matrices were within acceptable WHO/US EPA limits except for water samples. Consumption of yams grown at 0m up sluice of Namukombe stream may pose deleterious health risks as reflected by the HRI of 0.985 being very close to 1.0. From pollution and risk assessments, Hg usage should be delimited in Syanyonja ASGM areas; solutions to abolish mercury based ASGM in the area ought to be sought at its soonest to avert the accentuating health, economic and ecological disaster arising from the continual discharge of Hg into the surrounding areas. Other safe gold recovery methods such as use of borax should be encouraged. Waste management system for contaminated wastewater, used Hg bottles and tailings should be centralized to enable Hg waste management in ASGM areas in Syanyonja.