Spatial Hotspots and Seasonal Fluxes of Heavy Metals in Soils Affected by Artisanal Refining in Bayelsa State, Nigeria

This study assessed the inter-spatial and seasonal variability of heavy metals in the soils of five artisanal refinery sites and a control site from Bayelsa State, Nigeria. Analysis of the samples was carried out for the following: mercury, arsenic, cadmium, chromium, copper, and lead. On all the studied sites, less than 0.001 mg/kg of mercury was recorded at the detection level, and since it was the determination level, mercury was said to be uncontaminated or minimally contaminated. Arsenic was present only in low levels in all the studied soils except for site L2, where 0.07 mg/kg concentration was higher than the highest acceptable level of 0.05 mg/kg by either WHO standard or EGASPIN. Cadmium was present between 0.02 to 0.035 mg/kg; chromium between 0.08 to 0.17 mg/kg; and copper from 2.79 to 4.45 mg/kg, all of which fall within permissible limits. Lead levels tended to increase from 2.52 mg/kg to 5.01 mg/kg, again well below such permissible levels. With changes across the seasons, concentrations of these metals indicate slight increases in arsenic pollution with the maximum value of as seen in March of 0.04 mg/kg, a similar situation also was observed in the case of cadmium and chromium; it was the highest during that dry season. For both copper and lead, the highest concentrations are recorded in the wet season (May). Most levels were well below permissible limits, although there will probably be occasional surges for all different metal levels and exposure arising from arsenic contamination brings localized challenges calling for continuing monitoring and regulation in order to mitigate possible long-term environmental and health risks.