Soil samples were collected randomly up to a distance
of 20 km from four directions around the closed Kabwe
mine. Control samples were collected from Tobwe
Estate, Bonanza, Moyo and Makombe Farms near
Kabwe and Golden Valley, Kyindu farms as well as
Chalimbana International Conservation Research for
Agro-Forestry(ICRAF)/Southern African Development
Community (SADC) station near Lusaka. Samples were
collected and analysed over a period of 12 months, from
April 1991 to May 1992. At least three composite soil
samples were collected from each sampling point. Soil
samples were collected into transparent plastic bags
using a steel Auger from a depth of 0–15 cm. Samples
were air-dried in the laboratory by spreading out on
transparent plastic on a bench for several days. They
were then passed through a 2 mm sieve and repacked
in transparent plastic bags until they were analysed.
20 g of sieved air-dried soil samples were placed in well
cleaned 250 cm3 conical flasks and 100 cm3 of 0.5 M
nitric acid was added. The flasks were mechanically shaken
for at least 30 min using a mechanical shaker. The
samples were then filtered into 100 cm3 plastic bottles
through ashless Whatman filter paper 40. Some control
samples were analysed for background target analytes
and then spiked with known amounts of cadmium, copper,
lead and zinc and passed through the whole process
to determine the recovery rates of extraction procedure.
The differences between the background concentrations
and the concentrations of the spiked samples accounted
for the percent recoveries. General purpose reagent cadmium
nitrate of a minimum purity of 99% was used in
the preparation of solutions used to spike the samples
for cadmium. An analytical grade of a nitrate salt of lead
and analytical grade granules of copper and zinc were
used in the preparation of solutions used in the spiking
of samples for lead, copper and zinc. A reagent blank
for each metal was also prepared and passed through
the whole process and used in the determination of samples
for the chosen metals. Calibration curves were prepared
using analytical grade metals and metal salts.
Cadmium, copper, lead and zinc were determined using
a Varian Techtron AA6 atomic absorption spectrophotometer
using corresponding metallic hollow cathode
lamps. Acetylene gas was used as the fuel and air as
the support. An oxidising flame was used in all cases.
Table 1 shows the different parameters at which the
instrument was set for each metal.
Calibration curves were used to calculate the concentrations
of four metals. A reagent blank was used to zero
the instrument. This was followed by aspiration of standard
solutions and finally the soil sample extracts were
aspirated.