This greenstone belt has alluvial gold deposits in the Revue River and its Chua and Zambuzi tributaries. The aim of this thesis is to assess the impact of alluvial gold mining on surface water quality in the Revue Basin. The physical and chemical characteristics of the surface water were determined before, within and downstream of the mining area and in the main tributaries immediately before flowing into the Revue River.
Descriptions of each determinant's behavior and spatial distribution graphs down the Revue river are presented.
CHAPTER ONE
INTRODUCTION
- PROBLEM FRAMING
- THE PHYSICAL ENVIRONMENT
- RESEARCH AIMS
- METHODS OF INVESTIGATION
- Sample Collection
- Field Analysis
- Laboratory Analysis
- Limitations
The main tributaries of the Revue River in the study area are the Chua, Zambuzi, Inhamazonga, Messica, Nhamanguena and Z6nue rivers. The Revue River is not a large river, but it is the most important in the region. The land in the upper part of the Revue River basin is used for agriculture, grazing and mining activities.
Thus, the data presented represent a snapshot of water quality in the Revue River during July 1998.
CHAPTER TWO
GEOLOGY OF MANICA ALLUVIAL GOLD DEPOSITS
- GEOLOGIC SETTING OF MANICA
Lower Macequece Formation
GEOLOGY OF THE MANICA ALLUVIUM
- Genesis
- Tectonic Setting
- Size and Mineralogy
AIIuvial gold deposits in the Manica greenstone belt occur in the Revue River and some of its tributaries. These rivers are part of the Revue hydrographic basin, whose headwaters originate in the Manica greenstone belt (Fig. 1.1, p. 2). These epigenetic deposits are due to gold mobilization during the metamorphism of the rocks of Manica greenstone belt (Manuel, 1992).
The geomorphological evolution of the Manica greenstone belt contributes to the understanding of modern alluvial gold deposits.
CHAPTER THREE
ALLUVIAL GOLD MINING METHODS
- HISTORICAL NOTES
- TRADITIONAL GOLD MINING
- SMALL SCALE GOLD MINING
The auriferous gravel is brought to the surface with buckets attached to ropes and then washed directly into the river water to produce gold-rich gravitic concentrate. A small amount of gold-rich material is placed in the pan, and alternately dipped in and out of the river water. This results in density splitting and the light fraction is thrown at the river boundaries.
Any gold nuggets held in the first performance are selected by hand and the remaining material is dumped.
CHAPTER FOUR
WATER QUALITY IN THE REVUE BASIN
- NATURAL BACKGROUND WATER QUALITY
- WATER QUALITY PARAMETERS
- Physical Parameters
- Chemical Parameters
In the Revue Basin catchment, the water traverses a series of rocks of the Manica Greenstone Belt. Trace metals present in the minerals will also dissolve in the Revue River water. Water quality problems are not a cause for concern upstream of the Revue Basin alluvial gold mining area.
Downstream of the mining area, turbidity drops to 1.42 NTU and 1.56 NTU at Chicamba Dam.
Minor metals i) Aluminium
Aluminum is considered a non-essential element for human health and development, as no supporting evidence has been found to indicate that it fulfills any essential biological function. At neutral pH, the aqueous concentration of aluminum is low in underground water, lakes, rivers and seawater, due to the formation of secondary hydroxophases (Hem, 1980). Aluminum is trivalent and the cation A13+ predominates in many solutions in which the pH is less than 4.0.
The presence of granitic rocks at sites 13 and 14 coincides with the highest A13+ concentrations found in the area.
I - Upstream of the mining area; 11 - Inside the mining area; JII -Downstream of the mining area; RV - Reference value (Terrain I).
Trace metals i) Arsenic
Upstream of the mining area; [[ - Within the mining area; IIl-Downstream of the mining area; RV - Reference value (Site I). ii) Barium. The highest concentration is observed in the lower part of the mine area at site 9, which is underlain by metasedimentary and carbonate rocks. In Revyen, concentrations of 0 to 0.2 mg/l were measured, with the highest concentration measured in the lower part of the mine area (Fig. 4.15).
I - Upstream from the mining area; [[ - Within the mining area; JII - Downstream of the mining area; RV - reference value.
ASSESSMENT OF WATER QUALITY 4.3.1 Water Quality Within the Mining Area
- Water Quality Downstream ofthe Mining Area
AI, Ba, Cd, Cu, K, Ni, Pb, Sr and Zn show a percentage increase compared to water in the mining area. Si, Ca, Co, Cr, Fe, Mg, Mn, Mo and Na all show a percentage decrease compared to water in the mining area. Site 14, at Chicamba Dam was chosen as typical of water quality downstream of the mining area (Figure 1.2, p. 6).
The percent reduction for As is misleading, as the highest As concentrations in the area do occur downstream of the mining area (Figure 4.11).
CHAPTER FIVE
DISTRIBUTION AND FATE OF POLLUTANTS
- MINTEQA2
- REVUE WATER MODELLING PROCEDURE
- REVUE WATER MODELLING RESULTS
If all possible physical, chemical and biological processes of trace metals in the aquatic environment were taken into account when developing the model, it would be a very complex model. In order to describe the water quality of the Revue basin, the study area was divided into three parts; upstream of, within and downstream of the mining area. In a subsequent run, MINTEQA2 was used only as a speciation-saturation program with total concentrations of water quality variables as input.
In this initial run, the oxidation state of the Fe, Mn and Cr was also decided in the analysis. Thus, any oxides or oxyhydroxides of these elements that could be present in the water as colloidal or suspended solids were removed. After the samples were filtered, the samples were also acidified, so any dissolved Fe, Mg and Cr reported in the chemical analysis would likely be in a reduced state.
In the initial flow water was specified to be in equilibrium with the gas phase CO2 at a partial pressure of atm and O2 at a fixed partial pressure of 0.21 atm. In the second run, the components of the first run were called as the seed file and the total carbonate concentration obtained in the output file of the first run was entered as the component. In the third run, redox couples for Fe and Mn were imposed on the model.
With fixed redox ratio imposed, MINTEQA2 calculates the concentration of species present in the reduced and in the oxidized state. Since all components at all sites are 100 % dissolved in the water, only the output file from site 14 (Chicamba Dam) is presented as an example.
Run 1 Run 2 Run 3
CHAPTER SIX
SOURCES OF WATER POLLUTION AND WATER TREATMENT
This results in an overall improvement in water quality, with only Ba and Pb exceeding the values recommended by WHO for drinking water. The upper part of the Revue river basin is a remote area without industrial development. There is no production and no emissions of sulfur and nitrogen oxides into the atmosphere.
The soils are very fertile, and there is no reference to the use of nitrogenous fertilizers to increase yields, which could lead to a significant increase in the nitrate content of surface water. According to Alloway & Ayres (1993), all cases of pollution have certain characteristics in common: these involve (i) the pollutant, (ii) the source of pollution, (iii) the transport medium and (iv) the target or receptor. The identified pollutants are heavy metals dissolved in the water; the source of the pollutants is the alluvial gold mining activity; the transport medium for pollutants is the water of the Revue River and the target is humans, plants, animals and probably the turbines of the Chicamba Dam.
The water in the Chicamba Dam is used as a source of drinking water for the town of Chimoio. Manahan (1990) showed that Pb can be removed by lime addition followed by activated carbon filtration. Examination of the solubility products of Ba and Pb carbonates and sulfates (Table 6.1) shows that they are quite insoluble compared to other common carbonates and sulfates.
This suggests that there is a possibility of removing Ba and Pb by precipitation either as carbonate or sulphate. The equilibrium, reaction kinetics and technical aspects of possible water treatment processes are beyond the scope of this study.
CHAPTER SEVEN
DISCUSSION AND CONCLUSIONS
Disturbance of the sediments at the current work locations has increased the amount of clay and silt particles floating in the cloudy water. The exposure of the overburden and gold-bearing gravel and the gold washing process itself facilitated the dissolution of metals in the water. Metal concentrations in this area, with the exception of Cd, Mo and Ni, are higher than in the unpolluted upstream part of the river.
Alluvial gold mining is therefore directly responsible for increased metal concentrations in the water and is the main point source of pollution in the Revue River. The locals hate the cloudiness of the water and dig small holes in the river banks to obtain clear drinking water. Downstream from the mining area, water captured in the Chicamba Dam reduces water flow into the Revue River, creating conditions for clay and silt sedimentation.
The sedimentation process is reflected in the decrease in turbidity and in the concentrations of most total dissolved metals. Geochemical speciation modeling using MINTEQA2 suggests that the behavior of most metals in water is controlled by redox, precipitation and adsorption reactions. Unfortunately, no data are available on the fate and behavior of mercury in the Revue River.
This may be of concern as changes in dissolved organic matter concentrations are likely to be seasonal. The increased sedimentation load resulting from mining in the Revue River will have a negative impact on the Chicamba Dam by reducing the life of the dam.
Determinação das fases de deformação do complexo granítico-ultramáfico de contacto a norte da Vila de Manica. Modificações físico-ambientais causadas pela exploração de ouro aluvial no Distrito da Bacia Hidrográfica do Rio Revue, em Manica.
OUTPUT FILES OF REVUE WATER QUALITY MODELLING WITH THE MINTEQA2 PROGRAM
HAUSMANNITE 1.211E-06
- PERCENT BOUND IN SPECIES # 811 ADS1TYP1
- PERCENT BOUND IN SPECIES # 303301 A1(OH)2 + 48.5 PERCENT BOUND IN SPECIES # 303302 A1(OH)4 -
- PERCENT BOUND IN SPECIES #3301400 HC03 - 4.9 PERCENT BOUND IN SPECIES #3301401 H2C03 AQ
- PERCENT BOUND IN SPECIES #3300020 OH- 1.6 PERCENT BOUND IN SPECIES #8129500 =S02Zn+
Precipitation is allowed for all solids in the thermodynamic database and the solids print option is set to: 1. Input CO/-, PC02, P02, fixed pH, Fe redox pair, Mn redox pair, solids are allowed to precipitate. Do not automatically terminate if load imbalance is greater than 30%. Precipitation is only allowed for solids specified as ALLOWED.
Enter CO/-, PC02, P02, fixed pH, solids are allowed to precipitate Fe redox couple, Mn redox couple.
EFFECT OF pH CHANGES IN METALS
ADSORPTION AND PRECIPITATION IN THE WATER OF THE CHICAMBA DAM (SITE 14)
Output flIes of modelling with the Minteqa2 Program
EFFECT OF DISSOLVED ORGANIC MATTER (DOM) CHANGES IN METALS ADSORPTION
Appendix III
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