Evaluation of Restoration Effectiveness and Phytostabilization Potential in the Las Matildes Mining Tailings (Cartagena–La Unión, SE Spain)

Mine tailings in the Cartagena–La Unión district (SE Spain) remain a metal(loid) source decades after mining ceased. This case study evaluated Las Matildes tailings fifteen to seventeen years after a partial restoration carried out in 2003–2005, comparing the restored sector with two untreated sectors, boreholes, native species and bordering farmland. The restored sector reached pH 8.19 and 16.2% CaCO3, against pH 3.11 and no carbonate in the unrestored tailings; water-soluble Zn and Cd were over three orders of magnitude lower in the restored sector, which retained a high pseudo-total burden, with Pb, Zn, Cd and As exceeding the local background 376-, 37-, 14- and 4-fold. At the single sampled restored borehole, acidic, sulfate-rich material occurred at 2.2–2.6 m. Among the five species, Piptatherum miliaceum showed the most favourable retention profile, with mean translocation factors below unity for all nine metal(loid)s. Every agricultural sample exceeded the local background for Pb, Zn, Cd, As and Cu, and one geochemical gradient explained 78.3% of the variance. At Las Matildes, restoration was associated with lower operationally defined mobility, not contaminant removal. Continued subsurface monitoring, crop-transfer assessment and replicated trials with Piptatherum miliaceum are warranted.