Mobility of potentially toxic elements in family garden soils of the Riotinto mining area

Agricultural soils in mining areas usually accumulate potentially toxic elements (PTEs) that can become a health risk to humans by entering the trophic chain. In this study, five small agricultural plots close to Riotinto mines (SW Spain) were studied, with the aims of comparing the concentration of PTEs with respect to the regional (South Portuguese Zone) baseline and conducting availability studies in order to determine the contamination of soils. Chemical composition, total and clay mineralogy, and edaphic parameters were determined in topsoil and subsoil samples to characterize the soils, and single extractions were conducted to assess the mobility. The mineralogy of the soils was composed of quartz and phyllosilicates, with small amounts of feldspars and occasionally containing hematite and calcite. The texture ranged from sandy to silty loam, the pH was slightly acidic, and high contents of organic matter were found. Total concentrations of trace elements correlated with the texture, the content in iron oxy-hydroxides and the pH. The values of As, Pb, Cu, and Zn exceeded the regional baseline even in sites unaffected by mining. The results suggest that a widespread sampling is necessary to determine the local background. The most water-soluble element was As, due to the competition of organic matter for sorption sites. The content of Cu, Cr and Zn extracted with different methods were higher in sandy soils with low iron oxy-hydroxides content. Monoammonium phosphate and EDTA extractions seemed to remove elements from organic matter and iron oxy-hydroxides. The extracted fractions of As and metals reached up to 10-30 wt%.  Despite the high total concentrations of the element in soils, they generally showed low available proportions, especially with water and ammonium acetate extractants. The results suggest that the soils are not necessarily a risk to humans and higher investigation efforts are necessary to assess the availability of PTEs and their transfer to plants.