Impact of alluvial gold mining on surface water quality in the Revue basin-Manica District, Mozambique.
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Date
2000
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Abstract
The upper part of the Revue basin in the Manica District, Mozambique is located in a mountainous
area underlain by rocks of the Manica greenstone belt. This greenstone belt has alluvial gold deposits
in the Revue river and its tributaries Chua and Zambuzi. Alluvial gold in the Manica District has
been mined by local people using artisanal mining methods (panning) and by small scale companies.
The recovery process of gold involves washing of the auriferous gravel with large quantities of water
and the surface water quality has been impaired in this process. The aim of this dissertation is to
assess the impact of alluvial gold mining on surface water quality in the Revue basin. Physical and
chemical characteristics of the surface water were determined upstream of, within and downstream of
the mining area and in the main tributaries immediately before flowing into the Revue river.
Upstream of the mining area the water is clear and the rock types of the Manica greenstone belt are
likely to be the only source of metals dissolved in the water. Metal concentrations are generally low
except Cd, Mo and Ni but the water in this area meets all World Health Organization (WHO)
recommendations for drinking water. In contrast within the mining area there are signs of pollution.
The water is cloudy and the highest concentrations of most metals are found in the lower part of this
area where mining activity is very intense. Thus, the alluvial gold mining is responsible for elevated
metal concentrations and constitutes the major point source of pollution in the Revue basin. Water
quality within the mining area has been affected and metals Ba, Pb and Mn have concentrations
exceeding the WHO recommended values for drinking water. Downstream of the mining area the
impounded water in the Chicamba Dam, which is the source of potable water for Chimoio City,
reduces the water flow in the Revue river and sedimentation of suspended sediments occurs, together
with associated adsorption and precipitation processes. This result in general improvement of water
quality with only Ba and Pb concentrations remaining above the WHO recommended values for
drinking water. Increase in concentration of metals AI, Ba, K, Pb and Sr occurring in the Chicamba
Dam is likely to be due to input to the dam of water from rivers which cross the Granite-gneiss
Complex. Geochemical speciation modelling using MINTEQA2 program suggests that the behaviour
of metals Cr, AI, Mn and Fe is controlled by redox and precipitation reactions while the behaviour of
As, Cd, Zn, Cu, Ni, Pb, Ba and Ca is controlled by adsorption on the sediment surfaces. Changes in
environmental conditions, such as pH and dissolved organic matter (DOM) could result in metals
being released back into the water. Modelling the effect of a change in pH and variation in DOM
indicate that adsorption and precipitation would decrease with decreasing pH values and with
increasing DOM. The chemical form of dissolved metals, the type of interactive processes
(absorption and precipitation) and concentration of particulate matter gives the distribution of
pollutants while the transport process affect the fate of pollutants in the Revue river water.
Description
Thesis (M.Sc.)-University of Natal, Durban, 2000.
Keywords
Water quality--Mozambique., Gold mines and mining--Environmental aspects--Mozambique., Water--Pollution--Environmental aspects--Mozambique., Hydrogeology--Mozambique., Theses--Geology.