Heavy metals pollution

Bossissi Nkuba’s PhD research at the Faculty of Biology, University of Antwerp (defended in 2021), studied heavy metals pollution in artisanal and small-scale mining. You can download the first chapters here: PhD thesis Bossissi Nkuba


This thesis investigates the use of mercury (Hg) in artisanal small-scale gold mining (ASGM), the pollution of aquatic ecosystem by Hg and other metals deriving from ASGM as well as human intoxication by these metals. It has been conducted in Kamituga, a mining town in eastern Democratic Republic of Congo (DRC) of 150,000 people of whom 16,000 are gold miners. It is built around three parts.

The first part (chapter 2) uses qualitative surveys involving individual and focus groups interviews of key stakeholders as well a quantitative survey within communities to assess the perception of mercury use. It found that despite existing laws banning Hg use in ASGM, lack of enforcement makes these laws ineffective, leading to widespread use of Hg in the mine. However, without law-enforcement on how to use Hg, miners use it only on concentrates, reducing significantly the level of pollution their activity cause. But at the same time, miners use Hg in/near residences and within watersheds of streams, thus exposing both the communities and aquatic ecosystems. It also found that people are poorly informed on effects of mercury on environment and health. It also shows there is hope for change, since people prioritize more environment and health protection than economic profit from ASGM.

The second part (chapter 3) analyzes samples collected on a tributary of the Congo river (Zalya) and its network, this study investigates the concentration of  metals (Hg, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, U, V, Zn and Au) in water and sediment, as well as in indigenous fish (Labeo sp) and fish (Oreochromis nilotica) experimentally exposed to these streams’ waters for 28 days. It also analyses the bioavailability of these metals from sediments, relationship between accumulated metals and fish condition and risk for humans of water and fish consumption from these streams. It found that metal concentrations in water were nearly always below drinking water standards and environmental standards. Metal concentration, however, in sediments often exceeded the TEC (threshold effect concentrations) and sometimes even exceeded the PEC (probable effect concentration) for the case of Hg, Cr and Ni, thus potentially endagering aquatic life, especially since these metals are not sequestred in sediments based on bioavailability analyses. Indigenous fish from mining-affected streams had higher metal concentration in their muscle, but experimentally exposed fish did not show significant differences in terms of metal concentrations after their 28-day exposure. Fish condition was negatively related to Hg, Cd, Pb, Ni and Zn levels in fish muscle, but not to total metal load. Fish consumers may be at risk of Hg, Cd and Cr intoxication if their daily consumption exceeds 77, 145 and 138g of fish respectively, for an average 60kg person.

The third part (chapter 3) assesses whether dietary and occupational exposure of miners and non-miners lead to metal intoxications and how metal intoxication is reflected in the health symptoms people experience. It analyses the diet, health status and symptoms relatable to metal poisoning of miners and non-miners and compared to metals levels in their blood, urine, nails and hair. It shows that more than 80% of people had blood Hg above normal values, and nearly 25% had alert or high values. It found that many people also had Hg levels in their urine, nail and hair as well as concentration of other metals in different tissues above normal range. Miners had higher nail Hg than the rest of the community; while blood, urine and hair analysis showed that the community was as affected by Hg as miners were. Prevalence of potential Hg poisoning was equally spread in the community disregarding occupations, gender, age and education, but much higher for people with fish-rich diet. Indeed, tilapia being farmed in waters exposed to metal pollution, increased prevalence of potential Hg poisoning on in its consumers. Most symptoms were not reflecting higher level of Hg in the people experiencing them. Neither was the total number of symptoms reflecting higher levels of one of the assessed metals. Symptoms were likely due to other factors such as age and undernourishment.

This thesis recommends raising awareness in the community about the ongoing dietary and occupational exposure, risks of mercury and quantities of fish that are safe to consume from polluted streams. It further recommends to increasing formalization of the ASGM sector to reduce Hg use. It suggests increasing the capacity of law-enforcement agencies to enable them to supervise Hg use in mine. It finally recommends monitoring potentially polluted streams and sharing the results with communities and local medical personnel.


During his PhD (2016-2021), Bossissi Nkuba has highlighted how mercury is commonly used by miners in Congo despite being forbidden by national regulations, and how this translates into water, sediment, fish and human’s contamination (by mercury as well as other inorganic pollutants deriving for ASGM). He then helped UNITAR, UNEP and the Congolese Environment Agency (ACE) assess the extent to which mercury is used nationwide, after which he led the development of a National Action Plan for Congo’s reduction of its mercury use in ASGM. Finally, Prof. Nkuba has worked with Impact for the implementation of the National Action Plan of mercury reduction of Côte d’Ivoire and Guinea. He is currently working on other environmental impacts of ASGM and other sectors of the mining industry.

In 2016 Bossissi participated in a study carried out by COWI and commissioned by the World Bank on Mercury in Sub-Sahara Africa. The report and its annexes can be downloaded here

Mercury Sub-Saharan Africa Trade Report 12 December

Mercury in Sub-Saharan Africa country reports 12 December 2016

In 2019 the article “Invisible and ignored? Local perspectives on mercury in Congolese gold mining” was published in the Journal of Cleaner Production. https://doi.org/10.1016/j.jclepro.2019.01.174

Picture: Gilbert Bossissi
Bossissi at work in the lab at the University of Antwerp