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The role of soil could be decisive for the neutralization of the acid solution and immobilization of the metallic contaminants produced by mining industry. We studied micromorphological indicators of the interaction between the acid solution and soil material in the profile of Fluvisol at the Bacanuchi river (Sonora River Basin, Mexico) terrace affected by the catastrophic mine spill and in analyzing sample from a soil column treated with an acid liquid imitating the spill. Original unaffected soil is sandy with poorly developed pedogenetic features, however, frequent primary and secondary micritic and sparitic carbonates define high pH values. In the soil influenced by acid solutions under natural and laboratory conditions, carbonates were absent whereas neoformed gypsum crystals with radial intergrowth were observed together with accumulation of fine material enriched in ferruginous pigment. Micromorphometric quantification of the iron-rich fine material has shown its increase after interaction with the acid solution. We conclude that the interaction consisted of the neutralization reaction between the more reactive phases of the soil and the acid solution during which carbonates were consumed and gypsum was neoformed. Fine iron oxides precipitated after neutralization of acidity, the pH increased and the color changed. It is highly probable that other metallic contaminants co-precipitated with the ferruginous components.

Agricultural irrigation using raw wastewater is a popular practice in developing countries. However, as endocrine disrupting chemicals have been found in this water, the potential pollution of soil and water sources has become a source of concern. Such pollutants may be removed during the passage of wastewater through the soil by degradation and/or sorption. In this study the sorption and mobility of bis-2-ethyl(hexyl)phthalate (DEHP) and 4-nonylphenol (4-NP) in three different soils (Leptosol, Phaeozem and Vertisol) was compared. The distribution coefficients showed that DEHP is rapidly sorbed onto the three tested soils (K(d) between 1.8 × 10(4) and 4.2 × 0(4) L/kg), while sorption of 4-NP (K(d) between 15 and 80 L/kg) was weaker. In batch experiments the soil sorption capacity observed was as follows: Vertisol > Phaeozem > Leptosol for both compounds. However, in column experiments the retardation factor (R(F)) for 4-NP was higher than for the DEHP in the three soils. This suggests the possible migration of DEHP through the soil via colloids. The column results were found consistent with those observed in the field. It was concluded that the risk of groundwater contamination is higher for Leptosol soil than for Phaeozem and Vertisol soils and that DEHP can reach the aquifer prior to 4-NP.

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