Movement of heavy metals through undisturbed and homogenized soil columns

When sewage sludge is applied to land for disposal or intended beneficial use, heavy metals accumulate in the soil. Because of the concern about the environmental danger that these sludge-borne metals could represent if mobilized, many studies have been undertaken in an attempt to clarify the different factors that contribute to metal solubility, plant uptake, and leachability. The objectives of this study were to determine if two independent factors - soluble organic chelators and preferential flow paths - enhance metal mobility through soil. Dilute solutions containing CdCl2, ZnCl2, CuCl2, and Pb(NO3)(2) were applied to soil columns with a rainmaker, and leachate metals and chloride concentrations were measured, For four columns, the input metal concentrations (mg L(-1)) were 7.25 Cd, 4.55 Zn, 3.51 Cu, and 13.85 Pb. In four other columns, the solution also contained dissolved organic matter so that the metals were organically complexed, and input metal concentrations (mg L(-1)) were 6.30 Cd, 4.11 Zn, 3.19 Cu, and 12.55 Pb. For each treatment, two of the four soil columns were undisturbed and two were constructed from homogenized soils. The rate of continuous solution application was approximately 3 cm/day. The undisturbed soil columns treated with organically complexed metals had peak effluent concentrations, expressed as percent of influent, of 30, 26, 28, and 27% for Cd, Zn, Cu, and Pb, respectively, Peak effluent concentrations for undisturbed columns treated with metals in water were 30 and 23% of influent concentrations for Cd and Zn, but only 15 and 12% for Cu and Pb, respectively. However, the four homogenized soil columns retained all added metals, whether the metals were added in water or as organic complexes. The results indicate that previous laboratory metal leaching studies performed on homogenous soils might have greatly underestimated metal mobility in the field and that preferential flow, both alone and in combination with organic-facilitated transport, can accelerate metal leaching through soils.