Total Contents and Sequential Extraction of Heavy Metals in Soils Irrigated with Wastewater, Akaki, Ethiopia

Abstract

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.

Microbiological quality of lettuce (Lactuca sativa) irrigated with wastewater in Addis Ababa, Ethiopia and effect of green salads washing methods

“…Aside excreta borne pathogens, also heavy metals can be a potential risk factor where industrial effluent enters the common water sources as discussed by Itanna (1998Itanna ( , 2002, Fitamo et al (2007) and Weldegebriel et al (2012). The data verify concentrations of selected metals above irrigation water thresholds but limited plant uptake due to high soil pH, cation exchange capacity and clay content (Weldegebriel et al 2012).…”

Heavy metal sources and associated risk in response to agricultural intensification in the estuarine sediments of Chaohu Lake Valley, East China

“…Sediment contaminants may move into the food chain, particularly if the contaminants are sequestered in bioavailable forms. Cd, Pb and Zn can accumulate in relatively large amounts in plants without any apparent effects, resulting in an inexorable rise in the concentrations of these metals in crops, which could then cause human health problems [26,36,37]. Therefore, it is important that heavy metals pollution in the estuarine sediments of the agricultural region of Chaohu Lake Valley should be considered.…”

Zinc fractions in soil and uptake by wheat as affected by different preceding crops

“…Instead, certain chemical forms of Zn, for example exchangeable fractions, are reported to closely correlate with metal uptake by plants (Kabala and Singh 2001;Chahal et al 2005;Li et al 2007). In previous studies, the chemical pools of Zn and other metals as affected by land application of sewage sludge (Sposito et al 1982;Melo et al 2007;Nogueira et al 2010), waste waters (Fitamo et al 2007;Kabala et al 2011), compost (Zheljazkov and Warman 2004;Achiba et al 2009) and chemical fertilizers (Liu et al 2007;Jalali and Moharami 2010) have been studied. But there is a lack of information on the effect of preceding crops on the distribution of Zn among different chemical forms in the soil solid phase, particularly under field conditions.…”