Environmental Engineering Reference
In-Depth Information
ASL laboratory. EC and pH were determined in the field. During the preliminary survey
of ground water samples Ni, Pb, Mn and Hg were tested as well, but concentrations were
below the detection limit and these metals were not included in the study.
The effluent and groundwater characteristics were determined following Standard
methods (1989). Total trace metals were determined using the USEPA 3050 method of
acid (HNO
3
/HClO
4
) extraction followed by AAS (model: VARIAN TECHTRON
SPECTRAA 50B [110 SOFTWARE]) employing an air-acetylene fuel. Electrical
conductivity, nitrate and pH were determined in the field, using a filed unit (model:ELE
Paqualab, 1996). Ammonium was determined by the direct Nesslerisation method,
followed by a photometric determination at a wavelength of 425 nm (model: Spectronic
21D, SPECTRONIC Instruments). Ortho - P, FC and TC were determined by the same
procedures, described in Chapter 9. TKN was determined by the macro-Kjeldahl method.
5.2
Groundwater characteristics
The ground water quality results are shown in Table 10.8. The comparison with water
quality guidelines for different types of beneficial uses has been based on WHO (1996)
and the South African guidelines for domestic use, livestock watering and irrigation
(DWAF 1996 a, b, c). Risks to the environment have been evaluated, based on the
Zimbabwe regulations (WWEDR 2000).
With respect to metals characteristics, the only problematic metal, which exceeds the
recommended limits for all listed beneficial uses, is Cr. This element could be classified
as posing a high environmental risk, as the measured concentrations exceed considerably
the maximum permissible values. However, the concentrations at the control site are high
as well, which could be attributed to natural soil conditions. There is a significant
increase in the concentrations at the furrow-irrigated sites, which shows that the irrigation
practice has an additional adverse effect, especially considering the high Cr
concentrations in the effluent. Cd exceeds the stipulated WHO guideline for potable
purposes, but is lower than the SA requirement. In general, all metals show a distinct
spatial variation with a significant increase in the concentrations at the furrow-irrigated
site. This should not be attributed to the difference in the irrigation methods, but with the
transport of pollutants down-stream the main gradient of the aquifer and the fact that this
area, which is the lowest of all irrigated blocks, receives pollution transported from up-
gradient areas in addition to the one released at this specific block. Observed
concentrations of Cd and Zn pose a low to medium environmental risk.
In respect to pH there is no significant difference but in general the ground water is in
the lower range of the neutral zone. EC values show a distinct difference between the
control and irrigated blocks. However, the measured values are lower, when compared to
the results in Chapter 9, where a sludge and effluent mixture is applied. According to this
parameter, the aquifer water could be used for livestock watering only.
The nutrients variations in ground water quality show clear signs of pollution from the
irrigated sites. These concentrations at the control site are very low. Ortho-P shows a
maximum value at GW1, which is the point, receiving the highest hydraulic load, with
decreasing concentrations along the gradient. Ammonia shows a similar distribution, with
well-pronounced higher concentrations at the sprinkler-irrigated site. This could be
explained by the creation of anaerobic conditions at this block due to high hydraulic and
