Chemical Parameters

CHAPTER FOUR

4. WATER QUALITY IN THE REVUE BASIN

4.2 WATER QUALITY PARAMETERS

4.2.2 Chemical Parameters

Water quality in the Revue Basin

1996 WHO rerommended value tOr drinking water < 5 Nl1J

160 140

S

120

~ IOO C 80

:e

'0 ⁶⁰

"

f- 40

20

o

I

~

1

r::l

0

--,- -.-

2 3 4 5

U I"""

.

'C'

- ^{f ,',·}

I~~

n

^r::1

6 7 8 9

Sampling sites down the Revue river

,

III I

I

r- r-

f:,:

I"

,

-

^{I RV}

10 11 12 13 14

Fig. 4.2 - Spatial distribution Turbidity down the Revue river. 1 - Upstream of the mining area; 11- Within the mining area; 111- Downstream of the mining area; RV - Reference Value (Site I).

Water quality in the Revue basin

and the end point of the titration is the pH at which virtually all solutes contributing to alkalinity have reacted (Hem, 1980). The alkalinity titration is an important analytical procedure in natural water chemistry. The titration curve also provides insights into pH stability and buffering.

Alkalinity is caused by different types of solutes and in almost all natural water the alkalinity is produced by dissolved carbon dioxide species, bicarbonate and carbonate. The important non- carbonate contributors to alkalinity include hydroxide, silicate, borate and organic ligands, especially acetate and propionate (Hem, 1980). The principal source of carbon dioxide species that produce alkalinity in surface or ground water is the CO

2

gas fraction of the atmosphere or the atmospheric gases present in the soil.

In the Revue river the alkalinity concentrations measured are between 24.8 and 61.6 mg/I as CaC0

3.

In a general manner the alkalinity va lues rises gradually from upstream of the mining area with some small oscillations. Fig. 4.3 shows that the highest alkalinity is observed within the mining area at site 9 with 61.6 mg/I as CaC0

₃.

This site is underlain by carbonate rocks.

Downstream of the mining area the alkalinity starts to drop gradually until the Chicamba Dam at site 14. Chemical precipitation of Ca and Mg carbonates which occurs in the water column to form particles and colloids can be the cause of the reduction (Fillela et al. , (995) .

The WHO does not establish the maximum alka linity value in drinking water.

1996 WHO recommended value lor drinking water -NS

70

11

_ _ ...lRV

I

2 3 4 5 6 7 8 9 10 11 12 13 14

Sampling sites down the Revue river

Fig. 4.3 - Spatial distribution of Alkalinity down the Revue river. I - Upstream of the mining area; 11- Within the mining area; III - Downstream of the mining area; RV - Reference Value (Site 1).

27

4.2.2.1 Major metals

i) Calcium

Water quality in the Revue Basin

Calcium is the most abundant of the alkaline-earth metals and is a major component of the solutes in most natural waters. The sources of calcium are the sedimentary rocks where Ca is found as carbonates and as calcareous cement between particles. In igneous rocks plagioclase feldspars are a major source of calcium but calcium is also present in pyroxenes, amphiboles and in other silicate minerals produced by metamorphism.

Calcium has only one oxidation state, Ca

²+,

and generally it is a predominant cation in river water. The behaviour of calcium in natural aqueous systems is generally governed by the availability of the more soluble calcium-containing solids and by solution and gas-phase equilibria that involve carbon dioxide species (Hem, 1980).

In the Revue river the calcium concentrations range between 3.8 and 7 .2 mg/I. The lowest values are found at site 1. Here the possible sources of calcium are the pyroxinites and amphibolites. The highest concentrations of Calcium are observed, at the site 9, within the mining area, where the river crosses carbonate rocks as marble which is found within the Manica greenstone belt.

Downstream of the mining area at sites 12 to 14 the calcium concentration drops gradually until the Chicamba Dam (Fig. 4.4) . When river water is impounded in a storage reservoir, changes may occur in calcium content as a result of calcium carbonate precipitation (Hem 1980). The increased pH near the water surface, caused by algae and plankton, may bring about supersaturation, and precipitation can occur on solid surfaces around the edges of the water.

All sites sampled in the Revue river are well below 200 mg/I, the recommended maximum calcium concentration for drinking water.

ti) Magnesium

Magnesium is an alkaline-earth metal together with calcium ions contributes to the property of water hardness . Magnesium is derived from different sources. In igneous rocks, magnesium is typically a major constituent of ferromagnesian minerals such as olivine, pyroxenes, amphiboles and biotite. In metamorphic rocks magnesian mineral species such as chlorite and serpentine occur. In sedimentary rocks it is found in magnesite and hydromagnesite, the hydroxide brucite, limestone and dolomite (Hem, 1980).

28

8 7 1-

~

6

g

§ 5 -

'.0

~

1l 4 1 _ _ _ -

c o U 3 1

§

~ ^{2 t-}

U 1

r

o

Water quality in the Revue basin

1996 WHO recommended value for drinkingwater < 200 mgll

11 III

RV

4 5 6 7 8 9 10 11 12 13 14

Sampling sites down the Revue river

Fig, 4.4 - Spatial distribution of Calcium concentration down the Revue river. I - Upstream of the mining area; 11- Within the mining area; 111-Downstream of the mining area; RV - Reference Value (Site I),

Magnesium has only one oxidation state of significance in water chemistry, Mg2 + . The magnesium ion, Mg 2 + , will normally be the predominant form of magnesium in solution in natural water. Other magnesium forms in solution are the ion complexes MgOH+ formed at pH

> I 0 and the neutral ion pair MgS04

^(aq)

(Hem, 1980).

The concentrations of magnesium measured in the Revue river range between 3. 1 and 7.7 mg/1.

The distribution of magnesium down the Revue river is similar to that reported for calcium.

The smallest concentrations are found upstream the mining area (Fig. 4.5). The magnesium concentration rises within the mining area reaching a peak at sites 8 and 9. This coincides with the change in lithology to carbonates rocks (marble). Similar concentrations are found at sites 10 and 11. Downstream of the mining area the magnesium concentration drops until the Chicamba Dam. This may be due to the chemical precipitation of calcium and magnesium carbonates in the water column to form particles and colloids (Fillela et al. , 1995) or to dilution by other river waters flowing in the Chicamba Dam. In this area the water is quiescent which would aid settlement of suspended material.

Magnesium is a common element and is essential to plant and animal nutrition. The maximum recommended concentration in drinking water is 150 mg/I which is higher than the observed concentrations in the Revue river.

29

Water quality in the Revue Basin

1996 WHO recommended value for drinking water < 150 mgll

u

1\1

2 3 4 5 6 7 8 9 10 II 12 13 14

Sampling sites down the Revue river

Fig. 4.5 - Spatial distribution of Magnesium concentration down the Revue river. , - Upstream of the mining area; 11 - Within the mining area; "' - Downstream of the mining area; RV -Reference Value

(Site I).