Environmental Engineering Reference
In-Depth Information
and effluent disposal practice on the part of the pasture, which does not have a protective
dike.
Soil samples were collected during borehole drilling at three depth profiles: 0-0.3 m,
0.30-0.60 m and 0.60-1.2 m. The collected soil samples were placed in polyethylene bags
clearly marked with a borehole number, depth profile and date. The location of the
sampling boreholes is shown in Figure 9.3. Samples were also collected at depths where
there was remarkable change in soil texture and color. Borehole logging was done during
drilling at each site by 'finger' assessment as recommended by Foth (1984). The
following parameters were tested: pH, Cation Exchange Capacity (CEC), Organic Carbon
(OC), TP, Available Phosphorous (Pav) and TN at the Analytical Laboratory Services by
using standard procedures (Page et al. 1982). Nitrate was extracted using potassium
chloride, followed by steam distillation. The determination was done using the Salicylate
method employing an uv/vis spectrophotometer. Ortho-P was determined by the Bray 1
method (for acidic soils) and Olsen method (for basic soils), followed by a UV/ VIS
spectrophotometer determination. The extraction of TP followed the dry-ashing and
dissolution method. The determination of total phosphates was as for Orthophosphates.
Exchangeable bases (Ca, Mg, K and Na) were extracted using acidified ammonium
acetate as an index. Ca and Mg were then determined by an atomic absorption
spectrophotometer (AAS) employing nitrous/acetylene as fuel. Na and K were
determined by a flame photometer employing a low-pressure butane gas as fuel. The
determination of pH was done in a 1:5 soil: CaCl
2
suspension, using a pH meter.
Metals were analyzed at the laboratories of the Geology Department and the Institute
of Mining Research, University of Zimbabwe by an atomic absorption spectrophotometer
(model VARIAN TECHTRON SPECTRAA 50B-110 SOFTWARE) employing an air-
acetylene fuel. Duplicates were prepared and included in the analysis for quality control
as per recommendations of WRC (1992). Soil samples for metal analysis were dried at
room temperature, grounded, and then passed through a 180 µm sieve until the entire
sample had passed. About 0.5 g (actual weights were noted) of each sample was
transferred into a 30 ml test tube and digested using nitric acid following
recommendations of Page et al. (1982). Then, 10 ml of the supernatant was transferred
into 100 ml flasks and mixed with distilled water for the AAS analysis.
3.2.2 General parameters and nutrients
Chemicals and pollutant constituents exist in soils in different phases: solid, liquid or
gaseous. Constituents in solid phase are associated with soil particles - adsorbed on their
surfaces or precipitated in different forms and chemical complexes. Liquid constituents
are dissolved in water or soil moisture. Gaseous fractions may result as the product of
chemical or biological processes. In general, only the constituents in the solid phase are
immobilized in the soils and are non-available for biological degradation by
microorganisms or plant uptake. The parameters, which characterize the soil capacity to
immobilize pollutants, also known as “capacity controlling parameters” are OC and the
CEC. OC represents the organic material in soils, which has a strong binding capacity,
while CEC is dependent on the surface area and the nature of soil particles. The smallest
particles, typical for clay soils, have the highest adsorbing capacity. CEC is also
