Chemical fractionation of boron and heavy metals in soils irrigated with wastewater in central Mexico

The aim of the present work was to evaluate the accumulation of selected trace elements (boron, cadmium, chromium, mercury, lead and arsenic) in agricultural soils of the Zone 1 of the Irrigation District 03 (DR03) in the State of Hidalgo, Mexico. The soils of Zone 1 have, on average, been irrigated with raw wastewater for about 20 years (range 6-41 years). A clear positive correlation between the organic carbon contents in soils and the irrigation time was found. The total concentrations of the elements analysed were 0.51-1.89 mg Cd kg-1, 11.59-27.42 mg Cr kg-1, 3.99-47.08 mg Pb kg-1 and 9.2-123.8 mg B kg-1. Samples of topsoils (0-30 cm) were extracted using a modified Tessier method according to a six fraction scheme: easily soluble (F0), exchangeable (F1), bound to carbonates (F2), associated with oxides of iron and manganese (F3), bound to organic matter and sulphides (F4), and the residual fraction (F5). In general, the elements were distributed in the fractions F0, F4, and F5. The concentrations of total arsenic and mercury were below the detection limits (0.03 and 0.01 mg kg-1, respectively) except for 0.77 mg Hg kg-1 in one soil. The concentrations of total Cr and Pb did not exceed the maximum permissible levels set by the regulations of the European Union whereas total Cd was on the verge. A linear increase between the total metal concentrations and the irrigation time was found. In this study several crops harvested from soils showed Cd and Pb contents higher than those recommended by the Dutch and German guidelines. It can be concluded that the soils may pose a potential risk to the trophic chain and effective restrictions for the cultivation of crops that are sensitive to or known to accumulate heavy metals (i.e. nopal, onions, garlic, lettuce, potatoes, beets, and turnips) should be considered.