Agriculture Reference
In-Depth Information
at 0.50mg As/kg, 0.20mg Cd/kg, 0.50mg Cr/kg and 0.3mg Pb/kg in fresh
matter.
FC bacteria count was analysed by the five-tube most probable number
method (MPN) (APHA, 1992). Limits for FC were taken from those
recommended by the WHO (1989).
Heavy metals in soils
Soil samples were obtained from the same place as vegetable samples. All soils
were sampled manually with a soil sampler (Model J, Spectrum Technologies).
4
Samples were extracted from topsoil (approximately 20 cm depth) and air-dried
for four days. Tiny roots and other residues were removed before the soil was
ground and sieved in a 2-mm mesh. Fractions less than 2mm were analysed for
total as well as dissolved HM.
Total HM content was determined using an acid mixture. One gram of
soil for each sample, in duplicates, was transferred into a 200-ml digestion
flask. Ten millilitres of a mixture of concentrated HClO
4
and HNO
3
(ratio of
2 :1) was added before covering the digestion flask with a watch glass. The
mixture was heated progressively and boiled under reflux for two hours after
which the digestion flask was cooled (Baker and Amacher, 1982; AOAC,
1984). Tests for available HM in soils were carried out with 1N ammonium
acetate shaking for two hours (Bradi, 1984; Yanai et al, 1998). Heavy metals
in all samples were determined with AAS for Cd and Cr, AAS
þ
graphite
furnace for Pb and AAS
þ
hydride generation for As.
Guidelines for HM concentrations in soils were taken from the Taiwan
standards for assessment of soils contaminated with HM (Wang et al, 1994;
Chen et al, 1996). These included A values (the upper limit of background
concentration), B values (the acceptable level) and C values (the intervention
level, at which pollution control is needed). These guidelines take into account
Table 7.2 The Taiwanese standards for assessment of soils contaminated
with heavy metals
Element
A value
B value
C value
0.1M HCl
extracted
(ppm, dry
soils)
Total
concentration
(ppm, dry
soils)
0.1M HCl
extracted
(ppm, dry
soils)
Total
concentration
(ppm, dry
soils)
0.1M HCl
extracted
(ppm, dry
soils)
Total
concentration
(ppm, dry
soils)
As
-
16.0
-
30.0
-
40.0
Cd
0.4
2.0
11.0
4.0
2.0
5.0
Cr
12.0
100.0
25.0
250.0
40.0
400.0
Pb
18.0
50.0
150.0
300.0
200.0
500.0
A value, reference top value of the background range; B value, further monitoring level; C value,
pollution control level.
Source: Chen, 1992; Chen et al, 1996.
