This thesis examines historical and contemporary artisanal and small-scale gold mining (ASGM) in Kadoma-Chakari, Zimbabwe in order to identify effective strategies to reduce mercury loss and exposure and to increase miners’ incomes by improving gold recoveries. Cyanidation of mercury-rich tailings and the use of nitric acid to leach mercury from cathode sludge and amalgams are identified as significant pathways for losses of mercury into the environment in Zimbabwe. Indirect evidence suggests that as much as 90% of the mercury contained in amalgamation tailings at mills in Kadoma-Chakari is dissolved during passive vat cyanidation. Mercury traps placed after copper amalgamation plates and centrifuges could reduce the amount of mercury subjected to cyanidation, but mercury can be kept out of cyanidation circuits altogether by replacing whole ore amalgamation with vinyl loop carpets. The optimal cyanide concentration for passive vat leaching is between 0.1 to 0.15%. Better management of nitric acid waste solutions can also significantly reduce mercury losses. The current political and socio-economic crisis significantly limits the effectiveness of ASGM programs in Zimbabwe. Nevertheless, strategies for more effective management of ASGM interventions are suggested by a review of the history of didactic theatre (Theatre for Development) in Africa. Theatre used as an awareness building tool is exemplified by “Nakai”, a drama produced in Kadoma-Chakari to increase knowledge of the hazards of mercury use. Theatre can also be a means to ensure horizontal communication between donors and project beneficiaries if it is used to stimulate discussions that give communities a real voice in development programs. It is proposed that community participation in project design, implementation and evaluation increases the likelihood of project success and sustainability because community-identified problems and solutions are more realistic than those defined by donors, and because community “buy in” and ownership increases pressure on project administrators to deliver the services communities need.

This thesis examines historical and contemporary artisanal and small-scale gold mining (ASGM) in Kadoma-Chakari, Zimbabwe in order to identify effective strategies to reduce mercury loss and exposure and to increase miners’ incomes by improving gold recoveries. Cyanidation of mercury-rich tailings and the use of nitric acid to leach mercury from cathode sludge and amalgams are identified as significant pathways for losses of mercury into the environment in Zimbabwe. Indirect evidence suggests that as much as 90% of the mercury contained in amalgamation tailings at mills in Kadoma-Chakari is dissolved during passive vat cyanidation. Mercury traps placed after copper amalgamation plates and centrifuges could reduce the amount of mercury subjected to cyanidation, but mercury can be kept out of cyanidation circuits altogether by replacing whole ore amalgamation with vinyl loop carpets. The optimal cyanide concentration for passive vat leaching is between 0.1 to 0.15%. Better management of nitric acid waste solutions can also significantly reduce mercury losses. The current political and socio-economic crisis significantly limits the effectiveness of ASGM programs in Zimbabwe. Nevertheless, strategies for more effective management of ASGM interventions are suggested by a review of the history of didactic theatre (Theatre for Development) in Africa. Theatre used as an awareness building tool is exemplified by “Nakai”, a drama produced in Kadoma-Chakari to increase knowledge of the hazards of mercury use. Theatre can also be a means to ensure horizontal communication between donors and project beneficiaries if it is used to stimulate discussions that give communities a real voice in development programs. It is proposed that community participation in project design, implementation and evaluation increases the likelihood of project success and sustainability because community-identified problems and solutions are more realistic than those defined by donors, and because community “buy in” and ownership increases pressure on project administrators to deliver the services communities need.

This thesis examines historical and contemporary artisanal and small-scale gold mining (ASGM) in Kadoma-Chakari, Zimbabwe in order to identify effective strategies to reduce mercury loss and exposure and to increase miners’ incomes by improving gold recoveries. Cyanidation of mercury-rich tailings and the use of nitric acid to leach mercury from cathode sludge and amalgams are identified as significant pathways for losses of mercury into the environment in Zimbabwe. Indirect evidence suggests that as much as 90% of the mercury contained in amalgamation tailings at mills in Kadoma-Chakari is dissolved during passive vat cyanidation. Mercury traps placed after copper amalgamation plates and centrifuges could reduce the amount of mercury subjected to cyanidation, but mercury can be kept out of cyanidation circuits altogether by replacing whole ore amalgamation with vinyl loop carpets. The optimal cyanide concentration for passive vat leaching is between 0.1 to 0.15%. Better management of nitric acid waste solutions can also significantly reduce mercury losses. The current political and socio-economic crisis significantly limits the effectiveness of ASGM programs in Zimbabwe. Nevertheless, strategies for more effective management of ASGM interventions are suggested by a review of the history of didactic theatre (Theatre for Development) in Africa. Theatre used as an awareness building tool is exemplified by “Nakai”, a drama produced in Kadoma-Chakari to increase knowledge of the hazards of mercury use. Theatre can also be a means to ensure horizontal communication between donors and project beneficiaries if it is used to stimulate discussions that give communities a real voice in development programs. It is proposed that community participation in project design, implementation and evaluation increases the likelihood of project success and sustainability because community-identified problems and solutions are more realistic than those defined by donors, and because community “buy in” and ownership increases pressure on project administrators to deliver the services communities need. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

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Les biofilms périphytiques, ou périphyton, sont des assemblages de microorganismes colonisant les surfaces d’une variété de substrats de la zone photique des habitats aquatiques. Retrouvés à la base des réseaux trophiques, ils présentent une grande importance écologique. Les microorganismes du périphyton sont maintenus les uns aux autres par une matrice extracellulaire composée de substances exopolymériques endogènes. Cette matrice permet la formation de microenvironnements aux propriétés physicochimiques distinctes du milieu extrapériphytique. En raison de la diversité microbienne qui les compose, de leur capacité à moduler l’environnement immédiat et leur propension à séquestrer puis à transformer les métaux, les biofilms périphytiques représentent des modèles de recherche des plus pertinents d’un point de vue biogéochimique. L’objectif général de la présente thèse est d’étudier le rôle des biofilms périphytiques, de leur matrice extracellulaire et de la matière organique qu’elle contient sur le cycle des métaux, plus particulièrement le mercure. Dans un premier temps, nous avons mené une étude sur la composition organique de la matrice extracellulaire du périphyton de lacs d’environnements non contaminés et sur la mobilité des métaux dans les différentes fractions de cette matrice. Nos résultats ont démontré que la matrice extracellulaire du périphyton était riche en matière organique fluorescente et nous y avons identifié deux composantes majeures d’origine périphytique. Nous avons observé une cooccurrence de certains métaux avec ces composantes dans les fractions mobiles et attachées de la matrice extracellulaire du périphyton. Nous avons constaté que l’une des composantes périphytiques entraînait une diminution des concentrations de métaux dans les cellules du périphyton, alors que l’autre était associée à des concentrations plus élevées de métaux essentiels. Notre étude a montré que la matière organique de la matrice extracellulaire joue un double rôle sur la mobilité des métaux et apporte un regard nouveau sur les mécanismes naturels de gestion des métaux du périphyton. Dans un deuxième temps, nous avons réalisé une étude de terrain dans une rivière altérée par des centrales hydroélectriques au fil de l’eau et par la création de milieux humides artificiels. Cette étude avait pour objectif de déterminer si le périphyton de ces habitats modifiés pouvait produire du méthylmercure. En combinant des mesures des taux de transformation in situ à des approches génomiques ciblant les gènes hgcAB, responsables de la méthylation du mercure, nous avons démontré que le périphyton des milieux artificiels avait la capacité de produire du méthylmercure. Nous avons mesuré des taux de méthylation positifs et nous avons détecté la présence du gène hgcA dans les communautés périphytiques des milieux humides. Nous avons majoritairement associé le gène hgcA aux bactéries réductrices de fer de la famille des Geobacteraceae. Notre travail a apporté de nouvelles informations sur la méthylation du mercure au sein du périphyton colonisant des rivières ayant subi des transformations d’habitats. Nos résultats ont également permis d'associer cette méthylation aux bactéries réductrices de fer, jusqu’alors rarement considérées comme potentielles méthylatrices dans le périphyton. Finalement, nous avons étudié les impacts de la mise en eau d’une rivière par des centrales hydroélectriques au fil de l’eau sur les dynamiques du mercure en nous attardant au périphyton et aux premiers niveaux des réseaux trophiques. La création de nouveaux habitats aquatiques favorise l’accumulation et la transformation du mercure en plus de fournir des conditions propices à l’établissement et à la croissance du périphyton. Nos résultats ont démontré que le périphyton de ces habitats pouvait accumuler d’importantes concentrations de méthylmercure et qu’il était une voie d’entrée efficace de ce contaminant pour les macroinvertébrés benthiques. Cette étude souligne les rôles clés que joue le périphyton dans le cycle du mercure des écosystèmes aquatiques. Les résultats de la thèse appuient la pertinence de considérer le périphyton comme un compartiment biologique de haute incidence sur les cycles biogéochimiques des métaux lors de l’étude ou de la gestion des écosystèmes aquatiques. / Periphytic biofilms, or periphyton, are collections of microorganisms that colonize the surfaces of a variety of substrates in the photic zone of aquatic habitats. Found at the base of trophic webs, they are of major ecological importance. Periphytic microorganisms are linked to each other by an extracellular matrix composed of endogenous exopolymeric substances. This matrix allows the formation of microenvironments with physicochemical properties distinct from the extraperiphytic environment. Because of their microbial diversity, their ability to modulate the immediate environment and their propensity to sequester and then transform metals, periphytic biofilms represent relevant research models from a biogeochemical perspective. The general objective of this thesis is to study the role of periphytic biofilms, their extracellular matrix and the organic matter they contain on the cycling of metals, more specifically mercury. First, we conducted a study on the organic composition of the periphyton extracellular matrix of lakes from uncontaminated environments and on the mobility of metals in the different fractions of this matrix. Our results showed that extracellular matrix of periphyton was rich in fluorescent dissolved organic matter and we identified two major organic components of periphyton origin. We observed co-occurrence of some metals with these components in the loosely- and tightly-bound fractions of the extracellular matrix. We found that one of the periphytic components resulted in decreased metal concentrations in periphyton cells, whereas the other was associated with higher concentrations of essential metals. Our study showed that the dissolved organic matter of the extracellular matrix plays a dual role on metal mobility and provides new insight into the natural mechanisms of metal management of periphyton in its immediate environment. Second, we conducted a field study in a river impacted by run-of-river hydroelectric power plants and the creation of artificial wetlands. The objective of this study was to determine if the periphyton of these modified habitats could produce methylmercury. By combining measurements of in situ transformation rates with genomic approaches targeting the hgcAB genes, associated with microbial mercury methylation, we demonstrated that the periphyton of the artificial wetlands had the capacity to produce methylmercury. We measured positive vi methylation rates and detected the presence of the hgcA gene in the periphytic communities of the wetland site. We mostly associated the hgcA gene with iron-reducing bacteria of the Geobacteraceae family. Our work provided new information on mercury methylation within periphyton colonizing rivers with altered habitats. Our results also allowed us to associate the methylation of mercury with iron-reducing bacteria, which are rarely considered as potential methylators in periphyton. Finally, we studied the impacts of river impoundment by run-of-river hydroelectric power plants on mercury dynamics by focusing on periphyton and lowertrophic web layers. The creation of new aquatic habitats promotes the accumulation and transformation of mercury and provides conditions for periphyton growth. Our results showed that periphyton in these habitats can accumulate important concentrations of methylmercury and is an efficient gateway to this contaminant for benthic macroinvertebrates. This study highlights the key roles that periphyton plays in the cycling of mercury in aquatic ecosystems. The results of this thesis support the relevance of considering periphyton as a biological compartment of high impact on biogeochemical metal cycles when studying or managing aquatic ecosystems.