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

Abstract

The mercury content and the contamination characteristics of water, sediments, edible muscles of a non-piscivorous fish (Oreochromis nilotica Linnaeus 1758 [Cichlidae]) and yams (Dioscorea alata) from Namukombe stream in Busia gold district of Uganda were evaluated. Human health risk assessment from consumption of contaminated fish and yams as well as contact with contaminated sediments from the stream were performed. Forty-eight (48) samples of water (n = 12), sediments (n = 12), fish (n = 12) and yams (n = 12) were taken at intervals of 10 m from three gold recovery sites located at up, middle and down sluices of the stream and analyzed for total mercury (THg) using US EPA method 1631. Results (presented as means  ±  standard deviations) showed that water in the stream is polluted with mercury in the range of < detection limit to 1.21  ±  0.040 mg/L while sediments contain mean THg from < detection limit to 0.14  ±  0.040 ugg−1. Mean THg content of the edible muscles of O. nilotica ranged from < detection limit to 0.11 ± 0.014 ugg−1while D. alata contained from < detection limit to 0.30  ±  0.173 ugg−1mean THg. The estimated daily intake ranged from 0.0049 ugg−1day−1 to 0.0183 ugg−1day−1 and 0.0200 ugg−1day−1 to 0.0730 ugg−1day−1 for fish consumed by adults and children respectively. The corresponding health risk indices ranged from 0.0123 to 0.0458 and 0.0500 to 0.1830. Estimated daily intake was from 0.0042 ugg−1day−1 to 0.1279 ugg−1day−1 and 0.0130 ugg−1day−1 to 0.3940 ugg−1day−1 for D. alata consumed by adults and children respectively. The health risk indices recorded were from 0.011 to 0.320 and 0.033 to 0.985 for adults and children respectively. The mean THg content of the sediments, edible muscles of O. nilotica and D. alata were within acceptable WHO/US EPA limits. About 91.7% of the water samples had mean THg above US EPA maximum permissible limit for mercury in drinking water. Consumption of D. alata grown within 5 m radius up sluice of Namukombe stream may pose deleterious health risks as reflected by the health risk index of 0.985 being very close to one. From the pollution and risk assessments, mercury use should be delimited in Syanyonja artisanal gold mining areas. A solution to abolish mercury-based gold mining in the area needs to be sought as soon as possible to avert the accentuating health, economic and ecological disaster arising from the continuous discharge of mercury into the surrounding areas. Other mercury-free gold recovering methods such as use of borax, sluice boxes and direct panning should be encouraged. Waste management system for contaminated wastewater, used mercury bottles and tailings should be centralized.