Environmental Engineering Reference
In-Depth Information
Henning et al. (2000), in a study of the behavior of heavy metals in sludge-amended
soils on a sandy soil in South Africa, observed that Cd was very mobile especially in acid
soils. This indicates a high level of plant uptake and leaching at the irrigated sites, leading
to a reduction in the Cd concentration at these sites, compared to the control. Although
Zimbabwe does not have legislated guidelines for a maximum allowable concentration of
Cd in the soil, all concentrations exceeded the FAO guideline limit of 3 mg/kg. The Cd
concentration in the control was significantly higher than that of the sprinkler-irrigated
site reflecting the effect of over-irrigation on the mobility of Cd.
Unlike Cd, Zn is an essential plant nutrient. The EDTA-extractable Zn in the major
soils of Zimbabwe ranged from 1.8 to 3.8 mg/kg. Since the double acid (HNO
3
/HCIO
4
) is
a stronger extractant than EDTA, higher values than those of Nyamangara & Mzezewa
(1999) were expected. The results fall within the ranges of 10-300 mg/kg for most soils,
but were about 10 times lower than those of Reemtsma et al. (2000), who reported a Zn
concentration of 400 mg/kg due to wastewater application on a sandy soil. Similar to Cd
variations, the highest concentrations were obtained for the control site. Rechgil (1995)
noted that zinc was most readily absorbed by plants than any other trace metal and has a
high potential to move beyond the depth of incorporation. Oloya & Tagwira (1996)
reported similar low zinc concentrations in effluent-irrigated clay soils at Good hope and
Aiselby farms, which did not correspond to the loading rates of Zn. They attributed this
to the leaching of Zn by large volumes of irrigation water. During this study, the highest
Zn concentrations were about 3-fold lower than the FAO cumulative maximum allowable
guideline limit of 150 mg/kg. The leaching of Zn to the ground water could be expected
and is supported by the clearly defined trend of increased concentrations with an
increasing depth at the sprinkler-irrigated site.
For all the soil depths, the highest concentrations of Cr were obtained in the furrow-
irrigated site with an average of around 18 mg/kg with no major variation with depth. The
values obtained are in line with the native concentration of Cr in a natural soil (Landon
1991). There was a significant difference between the control and the furrow-irrigated
site at all depths. Unlike other trace metals discussed before, Cr is relatively immobile. In
the furrow-irrigated site, there was evidence of a slight accumulation at the 0-60 cm
depth. Higher concentrations in the irrigated sites than the control were attributed to
wastewater irrigation and to the high concentrations of Cr in the ponds effluent.
However, since the application rates at the two sites could not be identified, the higher Cr
level in the furrow-irrigated site compared to the sprinkler-irrigated cannot be fully
accounted for. According to WRC (1997), the recommended maximum permissible value
is 80 mg/kg, which is much higher than the observed values in this study.
Table 10.7. Soil characteristics-metals.
Depth
[cm]
Site
K
[mg/kg]
Ca
[mg/kg]
Cd
[mg/kg]
Cr
[mg/kg]
Cu
[mg/kg]
Zn
[mg/kg]
0-30
C1
191.1
40
7
12
6
50
0-30
C2
78
94
6
12
0.01
55
0-30
C3
78
18
6
14
0.01
40
0-30
C4
128.7
72
5
10
0.01
57
