CHAPTER SEVEN

Discussion and conclusions

Within the mining are there is clear evidence of the negative impacts of alluvial gold mining.

Physical degradation of the surface has occurred. Abandoned pits, trenches, holes and tailings are common along the river banks. Disturbance of the sediments at the current working sites has increased the amount of clay and silts particles suspended in the water which has become cloudy.

Exposure of the overburden and the auriferous gravel and the gold washing process itself has facilitated the dissolution of metals in the water. The distribution graphs drawn for each determinand show that the peak of concentrations of most metals are found in the lower part of the mining area (between sites 7 and 9) where the mining activity is very intense. Metal concentrations in this area are, with the exception of Cd, Mo and Ni, higher than in the unpolluted upstream segment of the river. Thus, the alluvial gold mining is directly responsible for elevated metal concentrations in the water and constitutes the major point source of pollution in the Revue river. Non-point sources are limited to local domestic activities and have a negligible effect on the overall pollution load.

Several metals, Ba, Mn and Pb, have concentrations which exceed the WHO recommended standards for drinking water. Ideally this water should not be used as source of potable water.

Local people dislike the cloudiness of the water and dig small holes in the riverbanks to obtain clear water for drinking. This water will probably have the same chemical composition as the river water but is free of suspended sediments.

The suspended sediments in the water, not only impair water clarity but act as a major transport pathway for various trace elements due to they high sorptive capacity.

Downstream of the mining area the impounded water in the Chicamba Dam reduces the water flow in 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. This results in general auto-purification and improving the water quality with only Ba and Pb exceeding WHO recommended values for drinking water. Geochemical speciation modelling, using MINTEQA2 suggests that the behaviour of most of the metals in the water is controlled by redox, precipitation and adsorption reactions. Modelling indicates that the concentrations of metals Cr, AI, Mn and Fe are controlled by precipitation processes while concentrations of As, Cd, Zn, Cu, Ni, Pb, Ba and Ca are controlled by adsorption on the sediment surfaces. The oxidation of any ferrous iron and the formation of oxyhydroxides of Fe and Mn and their co-precipitation with the settling clay and silt particles is likely to have aided

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Discussion and conclusions

the adsorption processes by coating the clay and silt particles and thus providing additional adsorption capacity.

As described in Chapter 1, page 8, a major limitation of this work is the lack of data for mercury.

Inspection of literature (Holdgate, 1979; Hem, 1985; Lacerda & Salomons, 1991 ; Alloway &

Ayres, 1993; Ward, 1995) indicates that mercury is likely to be found as an adsorbed phase or as organo-mercury complexes. Unfortunately data on the fate and behaviour of mercury in the Revue river is not available.

The settling sediments act as a sink and scavenger of dissolved metals thus inducing an auto- purification process in the impounded water of Chicamba Dam. Modelling predicts that this process should be capable of removing Ba, Pb, Cd, Zn, Cu and Ni totally from the water. In reality Ba and Pb concentrations remain above the WHO recommended standards for drinking water .

Concentrations of elements such as Al, Ba, K, Pb and Sr show an increase in the Chicamba Dam, This is likely to be due to the input to the dam of water from Zonue, Messica and Nhamanguena rivers which cross the Granite-gneiss complex.

If, as the geochemical modelling suggest, adsorption and precipitation are the major processes by which metals are removed from the water, then this removal may only be temporary. Changes in environmental conditions, such as pH change, could result in metals being released back into the water. Modelling at various pH values indicated that adsorption and precipitation were pH dependent, with percentage adsorption and precipitation decreasing with decreasing pH.

Modelling of the effect of dissolved organic matter on the adsorption process indicated that adsorption would decrease with increasing dissolved organic matter. This could be of concern as changes in the concentrations of dissolved organic matter are likely to be related to season. The extent of any seasonal changes are not known as the data in this study represents only one dry season sampling event.

Ideally the water in Chicamba Dam should undergo water treatment prior to distribution as potable water in Chimoio City to reduce Ba and Pb concentrations to acceptable levels.

Currently only disinfection is practiced. Because of the poor economy of the country and lack of trained human resources, any water treatment scheme must need to be relatively inexpensive and be constructed of reliable and proven technology .

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Discussion and conclusions

Traditional lime treatment followed by active carbon filtration would be suitable but the cost of activated carbon might be prohibitive . Solubility products suggest that a scheme based on precipitation of Ba and Pb as either a carbonate or sulphate might have potential.

The increased sedimentation load due to mining activity in the Revue river will negatively impact on the Chicamba Dam by reducing the life span of the dam. The sedimentation will also impact on aquatic ecosystems. Rapid sedimentation can cause burial of small organisms, clogging of aquatic flora and eventually a reduction in biodiversity.

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