Water Quality Downstream ofthe Mining Area

4.2 ASSESSMENT OF WATER QUALITY 4.3.1 Water Quality Within the Mining Area

4.3.2 Water Quality Downstream ofthe Mining Area

Site 14, in the Chicamba Dam was selected as being typical of water quality downstream of the mining area (Figure 1.2, p. 6). Presently this water is used as potable water in Chimoio city.

This dam is located 42 km downstream from the mining area and receives large amount of clay and silt sediments released to the water during the gold mining activities. Water from other rivers, such as the Messica, Z6nue and Nhamanguena (Figure 1.2, p. 6) which have no any mining activity along their courses and which run over the Granite-gneiss Complex also enter the dam. The chemical composition of the water in these rivers will differ from the rivers underlain by rocks of the Manica greenstone belt and will influence the water chemistry in the dam .

The impounded water in the Chicamba Dam influences the water flow in the Revue river, beginning at its confluence with the Messica river. The water flux becomes slow. From this

52

Water quality in the Revue basin

confluence to the Chicamba Dam the quiescent of water allows the sedimentation of suspended clay and silt particles which is reflected directly in the reduction of turbidity. The sedimentation of suspended material causes accelerated silting and depth reduction of the Chicamba Dam.

This has implications for the life expectance of the reservoir (Cook & Doornkamp, 1990). The sediments bury organisms of small dimensions in the river banks and in the Chicamba Dam, eliminating some species, and causing reduction in the biodiversity.

The turbidity of water in the Chicamba Dam, at site 14 is 1.42 NTU, the concentration of TDS is 106.2 mg/I and the electric conductivity is 218 .20 I-lS/cm (Table 4.1, p. 21).

The impoundment of water will also have an effect on metals concentrations. Dilution can occur in the dam and settling of metal-contaminated solids has a much higher likelihood of occurring in the dams than along the river channel itself, both during high flow and low flow periods, as turbulence effects are smaller (Novotny, 1995). Inspection of Figs. 4.4 to 4.21 and Table 4.4 show that the concentration of Ca, As, Co, Cr, Fe, Mg, Mn , Mo and Na drop significantly from site 9 to site 14 in the Chicamba Dam. The percentage decrease for As is misleading as the highest concentrations of As in the area do occur downstream of the mining area (Fig. 4.11). Cd, Ni and Cu show a percentage increase at site 14 compared to site 9. This is also misleading as the highest values of Ni and Cd occur upstream of the mining area (Figs.

4.13 and 4.19). The spatial distribution of copper along the river is erratic (Fig. 4.16).

This reduction in the concentrations of Co, Cr, Cu, Fe, Mn, Mo, Na and Ni may be a result of precipitation and/or adsorption and coprecipitation with hydrous oxides of iron and manganese into sediments. The relative importance of precipitation and adsorption is discussed in Chapter 5. Adsorption occurs when a dissolved ion or molecule becomes attached to the surface of a pre-existing solid substrate generally by complexation with the surface sites. Coprecipitation occurs when a dissolved specie is incorporated as a minor component in a solid phase which itself is being precipitated (Drever, 1997). The extent to which metals are adsorbed depends on the properties of the metal concerned (valency, radius, degree of hydratation and coordination with oxygen), the physico-chemical environment (pH and redox status), the nature of the adsorbent (permanent and pH dependent charge, complex forming ligands), other metals present and their concentrations and the presence of soluble ligands in the surrounding fluids (Alloway

& Ayres, 1993).

Alloway & Ayres (1993) report that V, Mn, Ni, Ca, Zn and Mo are believed to co-precipitate with hydrous Fe oxides while Fe, Ca, Ni, Zn and Pb are believed to co-precipitate with Mn oxides. The reductions in metal concentrations in the Chicamba Dam are also consistent with

53

Water quality in the Revue basin

the work of Rose

et of.

(1979) where it is shown that at pH values of between 6.5 and 8, values found in the Revue river, large quantities of Ba, Ca, Co, Cu, Mg, Mn, Ni, Sr and Zn can be adsorbed (Fig. 4.22). The reduction of Ca and Mg may be due to chemical precipitation of Ca and Mg carbonates in the water column to form particles and colloids (Filella

et af.,

1995) while the reduction of Cd may be due to the adsorption by calcite surfaces (Drever, 1997). In this way the elements are removed by sedimentation after incorporation on to or into particles. Particles deposited in this way into water, either react with the constituents of the water or settle to the bottom where they react with sediments (Alloway & Ayres, 1993). The Chicamba Dam therefore acts as a sink for metals, as the particles surfaces deposited in the dam are important scavengers for heavy metal ions in natural water systems because of their ability to compete with soluble complexing agents for metal ions (Bourg, 1988).

8~---~

CA)

7

•

6

--

Sr 4

2 .~

~

~

'"

~

~ ^{8 (8)}

7 Cu

6 ... -

... Ni

5

.. 0 .,

. .

^~

...

4

3

2

2 ~ ^{3 4 6 7 8}

pH

Fig. 4.22 - Adsorption of divalent cations on MnO as a function of pH. (A) alkaline earth; (B) transition metals. Cation concentration = 10.³ (From Rose et al.; 1979).

54

Water quality in the Revue basin

The concentrations of AI, Ba, K, Pb and Sr at site 14 (Chicamba Dam) show an increase compared to site 9 within the mining area (Table 4.4). The highest As concentrations of are also found downstream of the mining area but not at site 14. The Chicamba Dam receive water from Messica, Z6nue and Nhamanguena rivers that cross the Granite-gneiss Complex (igneous acid rocks). The increase in concentration of these metals may therefore be a result of the input of water that has crossed these acid rocks. Acid rocks are relatively enriched in these elements compared to basic and ultrabasic rocks which predominantly compose the Manica greenstone belt (Table 4.2, p. 23).

The concentration of AI, As, Ca, Cd, eu, Cr, Fe, K, Mg, Mo, Mn, Ni, and Zn are lower than the WHO standards for drinking water but Ba and Pb have concentrations higher than these standards (Table 4. I, p. 2 I). Thus, the water in the Chicamba Dam is not recommended for use as potable water without prior treatment.

55