Trace metal concentrations in a pristine Ramsar site: the Okavango Delta

Abstract

Pristine aquatic environments are the preferred state for maintaining ecosystem integrity, supporting biodiversity and preserving human health. However, world ecosystems have been altered by anthropogenic activities which release chemical contaminants into the environment and threaten terrestrial and aquatic life. In order to protect water bodies from pollution, monitoring water quality indicators and levels of trace metals is critical. The Okavango Delta is regarded pristine, but concentrations of trace metals in the river have not been adequately studied. This study quantifies concentration of 10 trace metals, determines their spatiotemporal distribution and assesses associated health risks. Water samples were collected monthly between 2014 and 2017 from five sites and concentrations determined by ICP-OES. The results indicate that all metals studied were present at all study sites with a concentration in the order of Fe > Mn > Ni > V>Zn > Pb > Cd ≫ Cu > Cr > Co, a wide occurrence of metals reflecting a possible presence of mafic and ultramafic rocks in the catchment. Concentrations of trace metals were variable, but these were not statistically significant at p ≤ 0.05, indicating a relatively uniform water composition. However, Fe occurred in distinctively higher concentrations compared to other metals. When compared with US EPA guidelines for freshwater ecosystems and WHO drinking water standard, mean concentrations of all metals except Pb were below EPA guidelines for freshwater ecosystems and WHO drinking water standard. The risk assessment revealed that none of the metals posed health risks to the communities that use river water as a direct drinking source. Continuous monitoring is necessary for the protection of human health and sustainability of the delta.