Assessment of metal accumulation in calcareous soil and forage crops subjected to long-term irrigation using treated wastewater: Case of El Hajeb-Sfax, Tunisia

In Sfax (Tunisia), waters deriving from the treatment of a mix of industrial and domestic effluents have been used for the purpose of forage crop irrigation for 17 years. The aim of this study is to evaluate the impact of this wastewater irrigation on the metal contents in soil and to assess the risk of metal uptake by plants over the long term. The bioavailability and phytoabsorption of metals by alfalfa (Medicago sativa) and sorghum (Sorghum bicolor) forage crops have been evaluated through an approach based on spatially distributed topsoil (0–7cm) and plant samples. This approach uses direct measurements of the metal uptake by the roots and shoot tissues of the plants, in addition to the total metal content of the soil and the available fraction for uptake by plants (i.e. the bioavailable fraction), as extracted by ethylenediaminetetra-acetic acid (EDTA) mixed with ammonium acetate (NH₄OAc). Two sites were selected from within the same calcisol field so as to focus on various agricultural practices. It was found that only total Zn content was accumulated in soil irrigated with treated wastewater, compared to the nearby non-irrigated control area, though these contents remained within the critical limits for normal plant growth. However, total metal contents in the soil and in the bioavailable fraction of soil were higher in the topsoil layer of the permanent 3-year alfalfa plot than the contents of the summer sorghum plot. A lack of correlation between metal content in the bioavailable fraction of soil and metal content in plants (either the tissues of aerial parts or roots) suggests that NH₄OAc–EDTA serves as a partial extractant. When only the mineral fraction, for both soil and plants, is considered in the calculation of enrichment and transfer factors, a linear relationship appears in a log scale representation. This approach has refined results obtained by the classical calculations of TF (Transfer Factors) and the BCF (BioConcentration Factor) and moreover has provided complementary information about the specific selectivity of each plant with respect to the five studied metal elements. In addition, this approach has enabled identifying similarities in the metal behaviour for both forage plants. Lastly, this treatment has highlighted specific behaviour associated with the two studied plants as regards metal uptake.