Environmental Engineering Reference
In-Depth Information
that there is a trend of pollution from the farm with respect to phosphates. Results show
that phosphorous in groundwater is mainly in the form of ortho-P and that despite the
high loading rates, the groundwater has not yet been polluted at a level to pose an
environmental risk. At present the phosphorous is immobilized in soil, which is
confirmed by the relatively high TP concentrations in soil (Table 9.2). This could be
explained with the high organic content of the top layer and the Fe concentrations in the
soil, which contribute to the phosphorous immobilization. At BH5, the values of ortho-P
are higher than the TP values, but this is due to the high standard deviations of the data
set.
FC values indicate that groundwater is biologically contaminated and therefore is not
fit for direct human consumption. In terms of environmental protection the counts are
within the safe limit. Again the most contaminated sites are the ones more frequently
irrigated and the results are consistent in this direction. On average the fraction of FC to
TC is about 60%.
4.2.2 Metals
Results regarding metal concentrations in groundwater are presented in Table 9.5. The
values of metal concentrations in ground water show that only Cu and Zn concentrations
are within the prescribed environmentally safe limit. Comparing with the application
rates regarding these parameters (Table 9.1), show that the rates are far below the
maximum permissible limit. This explains the low concentrations of these metals in
ground water.
Cd and Fe values fall in the yellow classification, indicating a medium hazard to the
environment, while Cr, Pb and Ni are exceeding the red classification limit (high risk to
the environment). The Cr concentrations in particular are two times higher than the red
limit. These findings could be well explained with the high application rates regarding
these metals (Table 9.1), which are close or exceed the maximum permissible limits.
Also, these results confirm the low soil concentrations of these metals (except for Fe,
which forms part of the soil background quality) and the fact that the conditions at the
pasture are conducive for the release of metals in solution and leaching to the
groundwater.
A point of serious concern is related to the spatial variation of the metals. Results
show that no significant variation was found with respect to the most frequently irrigated
areas, the areas, which have not been irrigated and the control point. The only possible
explanation of this fact could be the assumption that the metals pollution plume from the
pasture has been extended beyond the stream, separating the pasture from the control
point and has affected the aquifer quality at BH6, due to a dispersive flow of
contaminants from the pasture aquifer. This assumption could be supported by the fact
that TDS concentrations at BH6 are also high. Little data on groundwater quality in the
City of Harare is available, and none from anywhere near the study area, in order to
confirm this assumption.
The levels of Cr, Ni and Pb at the control point are much higher than the levels of
these metals in the Marimba River and tributaries (Chapters 5 and 8). The interaction and
recharge mechanisms between the Marimba River water and the aquifer are not clear. It
is expected, that during the dry season the aquifer supplies part of the base flow of the
