Comparative assessment of restored mine tailings based on an integrative ecological quality index

Mining wastes keep soil metal contents above toxicity thresholds many years after abandonment. The physical and chemical infertility of mining tailings makes them prone to high rates of erosion, posing an environmental threat for neighbouring areas. New restoration programs of these wastes might be based on comparative analyses of successful restoration projects. Here we designed a Tailing Ecological Quality Index (TEQI) to compare the environmental status of nine tailings restored through different techniques across 30 years. These tailings belong to a semiarid mining district from SE Spain. TEQI integrated eight weighted physical, chemical and biological semi-quantitative indices, thus summarizing in a single value an estimate of the current status of a site. The vegetation growth rate was the main determinant of TEQI, thus contributing to improve physical and ecological conditions. The high bioaccumulation of metals in some of the studied plant species advised against the risk of using them in future restoration projects without an effective waste isolation method. Contrary to what was expected, the results indicated that a higher cost·ha -1 of the project (directly related to higher intervention level), did not guarantee higher success (TEQI) than cheaper and less complex restoration methods. In fact, TEQI was negatively affected by the cost·ha -1 in projects below 0.4 M€·ha -1 . Moreover, the closeness to undisturbed vegetation in the tailing neighbourhood contributed to reach a higher TEQI. This quality index can be adapted as an integrative method to compare other restored ecosystems beyond mining areas through fitting the relative weight of each component. • Ecological quality index developed to compare rehabilitated mine tailings. • Endorheism contributed to phytostabilization of mine tailings. • Some species hold metal contents above toxicity thresholds after rehabilitation. • Undisturbed nearby vegetation contributed to a higher ecological index. • Higher cost of the rehabilitation did not guarantee higher success.