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The Akaki River, laden with untreated wastes from domestic, industrial, and commercial sources, serves as a source of water
for irrigating vegetable farms. The purpose of this study is to identify the impact of waste-water irrigation on the level
of heavy metals and to predict their potential mobility and bioavailability. Zn and V had the highest, whereas Hg the lowest,
concentrations observed in the soils. The average contents of As, Co, Cr, Cu, Ni, Zn, V, and Hg of both soils; and Pb and
Se from Fluvisol surpassed the mean + 2 SD of the corresponding levels reported for their uncontaminated counterparts. Apparently,
irrigation with waste water for the last few decades has contributed to the observed higher concentrations of the above elements
in the study soils (Vertisol and Fluvisol) when compared to uncontaminated Vertisol and Fluvisol. On the other hand, Vertisol
accommodated comparatively higher average levels of Cr, Cu, Ni, Zn, etc V, and Cd, whereas high contents of Pb and Se were
observed in Fluvisol. Alternatively, comparable levels of Co and Hg were found in either soil. Except for Ni, Cr, and Cd in
contaminated Vertisol, heavy metals in the soils were not significantly affected by the depth (0–20 and 30–50 cm). When the
same element from the two soils was compared, the levels of Cr, Cu, Ni, Pb, Se, Zn, V, Cd at 0–20 cm; and Cr, Ni, Cu, Cd,
and Zn at 30–50 cm were significantly different. Organic carbon (in both soils), CEC (Fluvisol), and clay (Vertisol) exhibited
significant positive correspondences with the total heavy metal levels. Conversely, Se and Hg contents revealed perceptible
associations with carbonate and pH. The exchangeable fraction was dominated by Hg and Cd, whereas the carbonate fraction was
abounded with Cd, Pb, and Co. conversely, V and Pb displayed strong affinity to reducible fraction, where as Cr, Cu, Zn, and
Ni dominated the oxidizable fraction. Cr, Hg, Se, and Zn (in both soils) showed preference to the residual fraction. Generally,
a considerable proportion of the total levels of many of the heavy metals resided in non residual fractions. The enhanced
lability is generally expected to follow the order: Cd > Co > Pb > Cu > Ni > Se > V and Pb > Cd > Co > Cu > Ni > Zn in Vertisol
and Fluvisol, respectively. For the similar wastewater application, the soil variables influence the status and the distribution
of the associated heavy metals among the different soil fractions in the study soils. Among heavy metals that presented relatively
elevated levels and with potential mobility, Co, Cu, Ni (either soil), V (Vertisol), Pb, and Zn (Fluvisol) could pose health
threat through their introduction into the food chain in the wastewater irrigated soils. 相似文献
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