A Review Of Mercury in Lake Victoria, East Africa: Implications for Human and Ecosystem Health

Abstract

Lake Victoria, East Africa, has been the site of many recent studies measuring mercury (Hg) concentrations in water, fish, sediment, soil, and humans. Most of these studies were motivated by concerns about Hg contamination from processing of gold ore on the southern shores. Total Hg (THg) concentrations in fish were usually below permissible World Health Organization (WHO) concentrations and international marketing limits and do not threaten the lucrative export industry. Nile perch 3-10 kg and most >10 kg had THg concentrations above the WHO threshold concentrations for at-risk groups (200 ng/g). Elevated THg concentrations in large Nile perch are not of major concern because Nile perch are rarely consumed by the people living on Lake Victoria and very large Nile perch are becoming increasingly rare in catches. Water THg concentrations were below Canadian drinking water guidelines but were elevated relative to those in the northern Great Lakes. Sediment and soil THg concentrations were within inter-national guidelines and are comparable to those in northern latitudes but are lower than those in the Amazon basin. Biomass burning and soil erosion are estimated to be the major sources of THg for the lake and probably constitute a larger source of THg than gold mining in Tanzania.THg concentrations in urine and hair from human volunteers indicate that while gold miners and frequent skin-bleaching cream users are at risk of inorganic mercury poisoning, the rest of the population, including fishermen, is not. Human exposure assessments demonstrated that fish consumption and soil geophagy constitute major sources of THg for humans, but the total estimated daily intake of THg was below the Health Canada tolerable daily intake (TDI) limits. The use of beauty creams containing high inorganic Hg concentrations, however, caused the estimated THg exposure to exceed the TDI. The high THg content in the hair of regular cream users supports this assessment. The nutritional benefits of fish and soil geophagy outweigh the risk of THg poisoning. Still, due to the importance of those natural items as a THg source to humans, as well as the changing nature of Lake Victoria, regular monitoring and risk assessments need to be carried out in the Lake Victoria catchment.

Mercury and Persistent Organic Pollutant Concentrations in African Fish Eagles, Marabou Storks, and Nile Tilapia in Uganda

“…Mercury concentrations found in crosssection samples of Nile tilapia from both sites in this study were generally lower than those recovered from seven other studies that measured mercury concentrations in Nile tilapia from various sites around Lake Victoria (Campbell et al, 2003). However, the mean weight of fish in these studies was generally greater than our sample fish.…”

“…No comparisons can be made between the fish from our study sites because of the small sample size from Lake Mburo and the difference in size of the sampled fish from each lake. The concentrations of total mercury in these studies and our samples are below international marketing limits of 500 ng/g wet weight in the European Union and the USA (Campbell et al, 2003).…”

“…Urban agriculture is still widely practiced in the Kampala district and also contributes to biomass burning (National Environment Management Authority, 1997). Fifty percent of global fires seen by satellite observations are located in sub-Saharan Africa (Dwyer et al, 2000) and this prompted Campbell et al (2003) to state that biomass burning is a highly significant potential source of total mercury to African lakes. However, atmospheric deposition rates of mercury in the Lake Victoria catchment need to be measured to help determine the significance of biomass burning as a mercury source.…”

“…Persistent organic pollutants and total mercury were determined in whole-body cross sections of Nile tilapia (Oreochromis niloticus), a fish eagle prey item. Fish eagles feed on a variety of fish with cichlid species (including the mostly detritivorous Nile tilapia) that occupy the littoral zone comprising 80% of the fish taken (Stewart et al, 1997;Campbell et al, 2003). Because fish are a primary source protein for human populations in this region; sampling of fish also may indicate potential human exposure to these pollutants.…”

Total mercury concentration in common fish species of Lake Victoria, Tanzania

“…Studies that have been conducted between 1996 and 2003 (Ikingura and Akagi 1996, Migiro 1997, DHV consultants 1998, van Straaten 2000a' 2000b, Campbell 2001, Ikingura and Akagi 2002a, Campbell et al 2003 do not indicate a pronounced Hg pollution of Lake Victoria by gold mining activities in the basin. It is also true that pathways, receptors, controls and fate of mercury released into the environment by LVGF mining activities are not well construed.…”