Environmental Engineering Reference
In-Depth Information
Figure 9.2. Scheme of pollutant
transport mechanisms from a
continuous source.
soil, and the concentration of pollutant in groundwater. The soil adsorption capacity
could also be estimated by using the Langmuir adsorption isotherms, where the ratio of
the equilibrium concentration of a given pollutant to the amount absorbed by the soil is
given as dependent on specific coefficients characterizing the soil adsorption capacity.
This equation assumes heterogeneous soils conditions, constant energy of adsorption and
a fixed number of binding sites.
3 IMPACTS ON SOILS
3.1
Hydraulic and pollution loads
3.1.1 Methodology
The study was performed during the period December 2002 to March 2003. The study
area - Crowborough Farm - is located to the north of Crowborough Sewage Treatment
Works (CSTW) as shown on Figure 9.3, and has been in use since 1972. A detailed
description is presented in Chapter 8. During the period of study the actual area under
irrigation at any given time was 3 paddocks, or 51.3 ha, which were supposed to be
rotated. However, a visual inspection and interviews with the personnel in the field show
that this principle has not been followed strictly. The most frequently irrigated part of the
farm lies along the road to CSTW. A preliminary investigation of the present hydraulic
loads directed to the farm was carried out, based on interviews and data obtained from the
managing authority - the Municipality of the City of Harare.
Sludge and effluent mixture (SEM) samples were collected as time integrated samples
at 30 minutes intervals for 2 hours on 2 sampling occasions at a discharge chamber on the
farm (Fig. 9.4). Parameters tested included TDS, TSS, T, TP, ortho-P, Nitrate, pH, Cd,
Cr, Pb, Zn, Ni, Cu, and Fe. All tests were performed according to Standard methods
(1989). The analytical procedures are the same as described in Chapter 4, except for
nitrates. A “Spectronic 21D” spectrophotometer was used for nitrates determination by
the ultraviolet spectrophotometric screening method at wavelengths of 220 nm and 275
nm. Duplicate samples were included in all analyses for quality control.
