engleski

Distribution of heavy metals and assessment of their mobility by fractionation in agricultural soils of an urban area

Soils in urban and industrial areas are exposed to a large number of potential sources of contamination by heavy metals, which include deposition of various waste materials, release of harmful gases into the atmosphere and input of agrochemicals into soil. These same elements can be found in the parent material from which the soils developed. Whether the said inputs will become toxic and to what degree mobile depends on a number of factors: soil type, land use, geomorphological characteristics within the soil type and/or exposure to emission sources. As they are potentially toxic to plants and animals, their real status has to be assessed. Previous investigations pointed to a significant effect of urban and industrialized environment upon accumulation of heavy metals in agricultural topsoil. Determination of the total content of heavy metals in soil is the main indicator for assessment of the extent of its contamination ; however, long-term risk estimates and direct contamination effects are based on their bioavailability and/or mobility. Intensive urbanisation of the Croatian capital of Zagreb has led to a situation where very good agricultural soils are entrapped within urban and suburban areas. For this reason, investigations were undertaken on 100 ha of agricultural soils, developed on alluvial Holocene sediments, on which vegetables and field crops (predominantly maize) are produced. The test plot is located in the immediate vicinity of ecologically risky facilities: the city waste dump, a very busy ring road, the district-heating plant and the main airport. Drinking water is intensively pumped from the Quaternary fluvial deposits in a wider area, so part of the surrounding area is treated as a water protection zone. The research goal was to determine the accumulation of heavy metals in the topsoil and their distribution within the geochemical phases vertically along the soil profile. Topsoil samples were taken according to a regular grid of 250 x 250 m. Two pedological profiles were opened as well, while samples for chemical analyses were taken from genetic horizons to a 2 m depth. Soil texture, pH, total organic matter and total contents of Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn were determined in topsoil samples using flame AAS after extraction in aqua regia, while in soil samples taken from genetic horizons also CaCO3 content, effective cation exchange capacity and exchangeable cations were analyzed, and selective sequential analysis was done. Comparison with the background concentrations of the wider area pointed to raised concentrations of lead, zinc and nickel in the surface layer of agricultural soils, but their average values are below the norm values according to Croatian government regulations.. In surface horizons of the studied profiles, only nickel concentrations exceed the maximum allowable value. Previous investigations have shown that nickel concentrations in the soils of the wider Zagreb area are primarily of geogenous and pedogenic origin, with occasional anthropogenic influence. The highest nickel concentrations in Gso horizon seem to be related to the accumulation of fine particles rich in iron oxides, and the mechanism of their distribution within the profile. None of the investigated elements shows substantial accumulation in topsoil down to 30 cm, while lead, zinc and manganese concentrations gradually decrease with depth. Five fractions in the soil profile genetic horizons were separated by sequential analysis: exchangeable, carbonate, reductive, organic and residual. Participation of copper in the exchangeable fraction amounts to <7% of its total content in surface horizons. Percentage of the exchangeable fraction of copper diminishes with depth to <2%. The largest part of copper in Molic Fluvisol profiles is bound in the residual fraction (65&#8211 ; 77%), though in shallower horizons its considerable amount is bound in the organic form (10-19%). Nickel is predominantly bound in the residual fraction (72% - 88%), then in the fraction of Fe and Mn oxides (7-17%). Participation of exchangeable lead in Molic Fluvisol profiles amounts to <2% of total lead. Percentage of lead bound to carbonates increases in deeper horizons with increased pH and CaCO3. In shallower horizons, a considerable part of lead is bound in the organic fraction (17-22%), while its largest part along the overall profile depth is bound in the residual fraction (56-80%). Zinc is predominantly bound in the residual fraction (80-90%), its participation in the exchangeable fraction is <1%, while its remaining part is almost uniformly distributed between the reductive and organic, and somewhat less the carbonate fractions. For heavy metals to be translocated by water through the soil profile, they have either to be in the soluble phase or bound to mobile particles. Metals can form complexes with particles of organic matter in topsoil and as such can be translocated vertically along the profile depth. Such complexes actually represent the exchangeable organic form. Complexation of metals with humic acids greatly depends on pH. It is generally taken that the water-soluble and exchangeable fractions, and sometimes also the organic fraction, are bioavailable forms. As potentially mobile forms, they may endanger groundwater quality. Results of selective sequential analyses in two Molic Fluvisol profiles on the studied location provided no evidence of any translocation of metals through the soil profile, and thereby no hazard of their leaching into groundwater.

fractionation; selective sequential analysis; soil profile

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