Agriculture Reference
In-Depth Information
directly into irrigation canals that supplement farms. More than 97 per cent of
water samples taken from irrigation canals were found to contain five million
FC, which is above the maximum permitted limits. The quality of irrigation
water worsened downstream as the river passed through more densely
populated settlements in the Rı´ mac Valley.
Heavy metal contents in soils
From the analysis of the soils it was observed that more than 40 per cent of
the samples contained high levels of As and Cd, as well as Pb that exceeded
the safe limits according to Chen (1992), Chen et al (1996) and Wang et al
(1994). The total concentration (Figure 7.7) indicates the location of potential
contamination pathways, such as soil ingestion by children, inhalation of
dust, soil adhesion on edible leaves and other sources associated with handling
the soil (Nabulo, 2002). Extractable concentrations of HM in soils are
considered to be indicators of availability to plant roots (Kimberly and
William, 1999). Observations in this study (Figure 7.8) showed that uptake by
plants was only a fraction of the total HM concentrations (Sauve et al, 2000).
Because there are no guidelines for safe levels of available As in soils, it was
impossible to compare amounts.
Some of the available Cd measured in soil solution may have come from
the weathering of parent rock, as it was also observed that 68 per cent of the
variability of available Cd came from total Cd in soils (P
<
0
:
001). The levels
of trace elements present in soils result from the interactions between the
geology of the parent rock and the soil-forming factors and human activities
(Kabata-Pendias and Adriano, 1995). However, available As found in the soil
may have originated from pesticides, fertilizers or sewage sludge (Alloway and
Ayres, 1993).
The results also suggest different amounts of HM in soils across
evaluation sites. Localities near the main gate of Nuevo Horizonte
accumulated more Cd (P
<
0
:
001) and Pb (P
¼
0
:
019) than other sites that
were far from the gate.
Quality of vegetables
In spite of the mechanisms involved in elemental uptake by roots (non-
metabolic or metabolic), plants are known to respond to the amounts of
available inorganic nutrients in soil solution (Li and Shumas, 1996; Baban,
1999; Madrid et al, 2002). Analysis of some of the vegetables in the irrigated
fields showed that both Cd and As were accumulated more in foliage than in
roots. But only the indigenous aromatic plant huacatay exceeded the
maximum permitted levels (guideline: 0.50mg As/kg and 0.20mg Cd/kg)
(Figure 7.9). The plant is used in small quantities for seasoning or as a
condiment in various Peruvian foods and so may not pose a health risk to
consumers.
Seventeen per cent of lettuce and 31 per cent of radish samples had very
high levels of FC contamination, presumably due to exposure to the highly
contaminated canal water (WHO, 1989). Contamination loads in the two
