Arsenic compounds have been known, at least in impure form, for several millenia. It occurs in the earth's crust at an average of 2-5 ppm (Manahan, 1990). Historical events make it understandable that it is perceived as a life threatening poison (Irgolic, 1992). Arsenic may form metal arsenides in which its oxidation state is negative. Arsenic may also form sulfides and can be present as an accessory element in sulfide ore deposits. Arsenite and arsenate are the most common arsenic compounds in the environment. Arsenite is formed as the weathering product of arsenic-containing sulfide ores and is considered the most toxic among arsenic compounds.
Arsenic also occurs with phosphate minerals. In solution in aerobic waters arsenate (As
5+ ) or Arsenite (As
3+ ) oxyanions are the thermodynamically stable forms of inorganic arsenic (Hem, 1980).
The dissolution of the above mentioned sources and the combustion of foss il fuels, particularly coal, introduces large quantities of arsenic into the environment, much of it reaching natural waters (Manahan, 1990). The arsenic concentration values measured in the Revue river water range from 0 to 0.042 mg/I with the highest value (0.042 mg/I) recorded at site 13 (Fig. 4.11).
Examination of Table 4.2 indicates that apart from shales and clays , typical arsenic concentrations in major rock types varies from 1 to 1.5 ppm. Concentrations of arsenic in natural waters can be expected to be lower than the average concentration found in rocks but will be independent of lithology. Within the mining area variations in concentrations may be
36
Water quality in the Revue basin
the result of adsorption of arsenic by hydrous iron oxide, or co-precipitation, or combination with sulfide in the bottom sediments (Hem, 1980).
Because small amounts of arsenic can be toxic to humans, it is considered a highly undesirable impurity in water supplies. Although, arsenic compounds can have beneficial influences on human and animal life. The recommended arsenic concentration for drinking water set by the WHO in 1996 is less than 0.05 mg/1. This concentration is above the values measured in the Revue river water.
1996 WHO recormx:nded value for drinking water < 0.05 I11YI 0.045 ~
~ 0.04
5
0.035§ 0.03
.~
0.0258
c 0.02 o U 0.015.~
5
0.01«
~ 0.005rT-r----~-r---~r---~~~~~----~~---RV
o
~~____
~-L~~ _ _ ~ _ _ ~~~~~~~~L~~~~~~~~~~2 3 4 5 6 7 8 9 10 11 12 13 14
Sarrq:Jling sites down the Revue river
Fig. 4.11 - Spatial distribution of Arsenic concentration down the Revue river. [ - Upstream of the mining area; [[ - Within the mining area; IIl-Downstream of the mining area; RV - Reference Value (Site I).
ii) Barium
Barium is somewhat more abundant in acid igneous rocks than in basic and ultrabasic. 1t occurs principally as the mineral barite (BaS04), which is fairly common mineral. The concentration of barium in natural waters is likely controlled by the solubility of this mineral. This reduces the range between the upper and lower extreme values of barium to be expected in natural waters. Another factor that seems likely to influence the concentration of barium in natural water is adsorption by metal oxides or hydroxides (Hem, 1980).
Barium oxidation state is 2+ and a median concentration of 0.045 mg/I is observed in rivers (Hem, 1980). In Manica barium concentrations of 0.08 to 0.19 mg/I were recorded. The lowest barium concentrations are observed upstream of the mining area (Fig. 4.12). Within the mining area sites 3 to 7 have fairly constant barium concentrations. These sites have similar lithologies of peridotites, serpentinites and amphibolites. The Zambuzi river (also with mining activity),
37
Water quality in the Revue Basin
which flow into the Revue river between sites 7 and 8 brings high barium content (0.27 mgll) that increases the barium concentration at site 8. At site 8, the country rocks are carbonatic and metasedimentary (marble, quartzites , metagreywakes). These rock types are likely to contain higher barium concentrations than mafic rocks and also contribute to the higher concentrations at site 8 and 9. The rel eas e of barium can be accelerated by the intense mining activity in these sites.
Downstream of the mining area the barium concentration remains fairly constant. The Chicamba Dam (site 14) receives water from other rivers flowing over the granitic gneiss complex and has the highest concentration of barium (0.19 mgll).
From site 8 downstream to the Chicamba Dam, barium concentrations are higher than the recommended WHO standard for drinking water (0.1 mg/I). Because of the toxicity of barium, it is considered an undesirable impurity in drinking water. Water from Chicamba Dam is pumped to Chimoio City and used as potable water. Apart from desinfection with chlorine, no water treatment is undertaken.
0.2 -,-- - - 0.18
---.
~ 0.16
S
c: 0.14 .2 012e c
0.1<l.l
g o 0.08 U 0.06 .§
§
004 co 0.02o
2
1996 WHO recommended value for drinking water < 0.1 mg/l
RV
3 4 5 6 7 8 9 10 11 12 13 14
Sampling sites down the Revue river
Fig. 4.12 - Spatial distribution of Barium concentration down the Revue river. I - Upstream of the mining area; 11- Within the mining area; TII - Downstream of the mining area; RV - Reference Value (Site I).
iii) Nickel
Nickel is widely distributed in the environment and is the twenty-fourth most abundant element in the earth's crust. It may substitute for iron in ferromagnesian igneous-rock minerals. The source of nickel are ferrous sulfides in which nickel is substituted for part of the iron, and nickel-bearing laterites developed on ultramafic bedrock terranes (Hem, 1980).
38
WaleI' qualily in the Revue river
Nickel oxidation states are + 1, +2 and +3 and the aqueous chemistry is primarily concerned with the Ni
2+oxidation state. Important compounds are nickel oxide, nickel hydroxide, nickel sulphate, nickel chloride and nickel subsulfide (Stoeppler & Ostapczuk, 1992).
The median concentration of nickel in river water and probably in most other natural freshwater is somewhat less than I to 60 Jlg/l. In the Revue river water values between 0 and 0.0069 mg/I (6,9 Jlg/I) were measured (Fig. 4.13). The highest concentrations were observed in sites underlain by ultramafic and mafic rocks, which have the highest typical concentrations of nickel (Table 4.2, p. 23). The concentrations of nickel in the Revue river may be a reflection of its natural abundance. Nickel tend to be coprecipitated with iron oxides and especially with manganese oxides (Hem, 1985).
The maximum recommended nickel concentration for drinking water is 0.02 mg/\. Therefore, the concentrations in Revue water are below the recommended value. Divalent nickel compounds are non-toxic for animals, plants and man at prevalent concentrations in natural waters. In humans beings adverse effects (dermatitis) from inorganic, water-soluble nickel compounds frequently occur following skin contact.
0.008 ____ 0.007 r
~
..s
0.0061
.~ 0.005 c:
f
t: c:
8
c:u o 'ii ~
u
Z
0.004 ~ 0.003 I 0.002
l
0.001
o
2
1996 WHO recommended value for drinking water
<0.02 mg/I
11 III
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 RV
3 4 5 6 7 8 9 10 II 12 13 14
sampling sites down the Revue river
Fig.4.13 - Spatial distribution of Nickel concentration down the Revue river.
I -Upstream of the mining area; II - Within the mining area; Ill- Downstream of the mining area; RV - Reference Value (Site I)
iii) Cobalt
Cobalt is comparatively rare than nickel and is the thirty-second most abundant element in the earth's crust. It can substitute for part of the iron in ferromagnesian rock minerals (Hem, 1985).
39
Water quality in the Revue river
Cobalt oxidation states are +2 and +3. Important cobalt compounds are cobalt oxide, cobalt tetraoxide, cobalt chloride, cobalt sulfide and cobalt sulfate (Stoeppler & Ostapczuk, \992).
With the possible exception of certain complex ions, aqueous species of C0
3+are not thermodynamically stable under Eh and pH conditions that commonly occur in natural water.
(Hem, 1985).
The median concentration of cobalt in river water is between 0.4 to 4 ).Lg/\. In the Revue river water values between 0 and 0.024 mg/I (24 ).Lg/I) were measured (Fig. 4.l4). The first change in the Cobalt concentration is observed at site 3 with the change in lithology from schists to peridotites and serpentinites. The highest concentration is observed in the lower part of the mining area at site 9 which is underlain by metasedimentary and carbonate rocks. The high concentration here may be due to remobilization of the sediments during mining activity. A similar value at site 10, underlain by peridotites and serpentinites, may be a consequence of rock type (Table 4.2, p. 23). Downstream of the mining area the Cobalt concentration drops and returns to values close to those observed upstream of the mining area.
The maximum acceptable concentration of cobalt in drinking water has not been established.
Cobal t is a constituent of vitamin B 12 and in this form is essential for mammals (i.e. also for human beings) especially for ruminant animals. Extremely low contents in food may lead to deficiency syndromes, which has been especially observed in ruminants. Although, because of its industrial use, cobalt also poses a potential danger in occupational exposure primarily for metal workers (Stoeppler & Ostapczuk, 1992). Exposure to cobalt-containing dust and fumes can cause adverse effects to lungs, heart and skin.
1996 WHO recommneded value for drinking water - NS 003
r
11 III
0.025 1
~ a
';; 0.02
I
·15 I
'"
t: 0.015
4
8
§ I~ 0.01
I
~ I
U 0.005 1
1~~----HM'-~'-~*---B
__ -w*-____
~M-~M--ftM--ft~-+M-IRV2 3 4 5 6 7 8 9 10 11 12 13 14
Sampling sites down the Revue river
Fig. 4.14 - Spatial distribution of Cobalt concentration down the Revue river. \- Upstream of the mining area; 11- Within the mining area;
111-
Downstream of the mining area; RV - Reference Value (Site 1).40
Water quality in the Revue river
iv) Manganese
Manganese is the tenth most abundant element in the earth's crust. Manganese occurs as a minor constituent in igneous and metamorphic minerals. It is not an essential constituent of the more common silicate rock minerals, but it can substitute for iron, magnesium or calcium in silicate structures (Hem, 1985). Manganese is a significant constituent of basalt and many olivines and of pyroxene and amphibole. It is also present in small amounts in dolomites and limestones substituting for calcium.
The chemistry of manganese is somewhat like that of iron, but manganese has three oxidation states, +2, +3 and +4, and can form a wide variety of mixed-valence oxides. The +3 species are unstable. When divalent manganese is released to the aqueous environment during weathering, it is more stable toward oxidation than is ferrous iron (Hem, 1985). Manganese co-precipitates with iron, and other metal ions such as cobalt, lead, zinc, copper, nickel and barium (Alloway &
Ayres, 1993). Under conditions in natural waters systems, any dissolved manganese will be in the +2 oxidation state. The ion Mn2+ will predominate in most situations. It forms hydroxides, complex ions and ion pairs.
The presence of manganese in a stream water depends on the pH. Acidic streams can have more than I mg/I of manganese. In the Revue river concentrations of 0 to 0.2 mg/I were measured with the highest concentration recorded in the lower part of the mining area (Fig.
4.15). The concentration of manganese increases throughout the mining area, reaching a maximum at site 9 which is underlain by metasedimentary rocks. As high concentrations of manganese are normally associated with mafic rocks, the peak concentration can be attributed to alluvial gold mining activity. Furthermore, between sites 8 and 9 the Revue river receives water from the Inhamazonga river, a small affluent, which brings water with high manganese concentration (0.33 mg/I). This water also helps to raise the manganese concentration in the Revue river. Downstream of the mining area manganese concentration drops until zero in the Chicamba Dam as the river runs over acid rocks such as granites, gneissic granites and cratonic granitoids which give a very small input of Mn to the water due to their low Mn concentration.
It is believed that the impoundment of water in the Chicamba Dam has also effect on the manganese concentration because dilution can occur as well as settling of metal-contaminated solids. Hem (1985) suggests that manganese takes long time to disappear from solution.
A recommended upper limit for manganese in drinking water is 0.02 mg/1. Fig. 4.15 shows that from site 3 to site 11 Mn exceeds this value. Manganese is an undesirable impurity in water supplies, mainly owing to a tendency to deposit black oxide stains.
41
0.25
i
~ 0.2c 9
e
015~ u
C o U 0.1
<l)
~ §
0.05:2
2
Water quality in the Revue river
1996 WHO recommended value for drinking water < 0.02 mg/l
II
3 4 5 6 7 8 9 10
Sampling sites down the Revue river 11
JI[
- - - R V I
12 13 14
Fig. 4.15 - Spatial distribution of Manganese concentration down the Revue river. I -Upstream of the mining area; [[ - Within the mining area; JII -Downstream of the mining area; RV - Reference Value
(Site 1).
Dalam dokumen
Impact of alluvial gold mining on surface water quality in the Revue basin-Manica District, Mozambique.
(Halaman 46-52)