Environmental Engineering Reference
In-Depth Information
Application Rates (kg/ha.year)
16
154
2
117
11
3
40
127
629
Application Limit ZW
-
-
4
-
-
-
-
-
-
Application Limit EU
15
-
0.15 -
30
12
3
-
-
Application Limit SA
0.41
0.13
2.83
Application Limit USA
15
-
1.9
150
140
75
21
-
-
With respect to the application rates, the TP annual loading rate considerably exceeds the
phosphorous uptake rate for grasses of 24 to 85 kg/ha.year (Reeds et al. 1988), and the
same result was obtained previously (Chapter 8, Table 8.2). During this study, the TP
load was 11% higher than the load in 1999, but the increase should be associated with the
increased hydraulic load, as SEM concentrations did not vary considerably. The
application rate with respect to nitrate is comparable to the one due to TKN loading in
1999 and this indicates that the total nitrogen load to the farm is very high. However, it
should be mentioned that the pollution loads during this study were obtained based on
two measurements only with a high standard deviation, which might not be representative
for the annual pollution load. Unfortunately, the plant operators were not able to provide
additional data regarding this media, as the plant-monitoring program during this period
was restricted due to financial constraints. Zimbabwean regulations limit the nitrogen
application rate to 600 kg TN/ha.year (high environmental risk) and in this case it has
been exceeded considerably.
The metals loads, given as application rates (Table 9.1), are of major concern due to
the possible hazardous effect on ground and surface water. The application rates
regarding Pb, Cr and Ni are alarmingly high. Comparing with the previous study during
1999, a considerable increase of the loading rate has been found with respect to Pb and
Zn, while Ni rates are consistently high.
Comparing the different international criteria, the significant difference between EU
criteria and USA criteria is of interest, indicating the need for additional research on this
topic. As the Zimbabwean regulations recommend a limitation with respect to Cd rates
only, the rates obtained during this study are within the safe recommended limit,
however, they are comparable to the USA maximum permissible value and exceed more
than 10 times the EU limits. No signs of phytotoxicity on the farm were observed,
reflecting either high crop tolerance or high capacity of the soil to reduce toxicity.
Madyiwa et al., 2002 had similar observations.
3.2
Assessing soils characteristics
3.2.1 Methodology
The location of the different boreholes was selected to represent different soil types and
different irrigation routine. The six boreholes (BH) were drilled specifically for the
purpose of this study, with BH6 serving as a control point. An illustration of a sampling
borehole is shown in Figure 9.7. The study area has been surrounded by new residential
developments and some of the undeveloped open spaces in close vicinity have been used
for informal agriculture. This considerably restricted the possibility to choose a pristine
