Assessing Porewater and Sediment Quality in the Sidi Salem Dam: Insights from an Artificial Aquatic Geosystem in Tunisia

Abstract The concentration profiles and diffusion fluxes of PO₄³⁻, NH₄⁺ and NO₃⁻ at the water-sediment interface, along with the chemical speciation of phosphorus and heavy metals (Pb, Zn, Cu, Cd, and Cr), were analyzed in three sediment cores from the Sidi Salem Dam lake, the largest dam in Tunisia. The results show that phosphorus in the sediment is associated with carbonates (P-Ca 40 to 60%) and iron from 0 to 10 cm depth (P-Fe 25 to 30%). This association seems to be related to pH regarding P-Ca and Eh regarding P-Fe. The PO₄³⁻ and NH₄⁺ fluxes are either zero or directed towards the water column (7 to 16 µmol m² d⁻ ¹) for NH₄⁺, excepting NO₃⁻, whose flux at the dam entrance is directed towards the sediment (-8.44 µmol m⁻² d⁻ ¹). Cd contents are negligible in all cores; Zn and Cr contents are highest in the three studied sites; and Pb and Cu are mainly high at the dam entrance (20 to 43.7 µg g⁻ ¹ at 10 cm depth for Pb). Chemical fractionation shows that Pb, Zn, Cu, and Cr are mainly associated with the residual phase (41%, 71%, 53%, and 95%, respectively).