Ecotoxicity assessment of primary producers in metal mining areas: biological indicators for ecosystem restoration

Abandoned mines represent a considerable risk to ecosystems surrounding former exploitation sites. In metal mining, the exposure of waste significantly increases the mobility and bioavailability of potentially toxic elements (PTEs), affecting adjacent soils and organisms. Assessing the toxicity of mining waste involves challenges related to selecting appropriate bioassays and those recommended by current environmental regulations. Considering the expected increase in metal and metalloid extraction to supply critical raw materials, improving our understanding of the advantages and limitations of specific bioassays is essential for accurate risk assessment. Three representative abandoned metal mining sites in the Iberian Peninsula were selected to apply and compare different bioassays for a robust ecotoxicological assessment of mining waste. Total and soluble PTEs concentrations at all sites significantly exceeded geochemical threshold values (GTVs) and water quality standards. Bioassays using Lepidium satuvium, Spirodela polyrhiza and Raphidocelis subcapitata revealed that acidic conditions combined with elevated PTEs concentration (e.g., Cd, Pb), are highly toxic to primary producers. Conversely, root growth measurements suggested that low soluble concentrations of metalloids (As, Sb) may stimulate root development. Overall, the results indicated that Sb was not a major contributor to observed toxicity in the bioassays under the studied conditions. The Zucconi test showed low sensitivity and reliability, limiting its suitability for risk assessment. Moreover, stimulation effects observed in the algal bioassay question its effectiveness for delineating contaminated areas, as they may lead to false negatives. Therefore, combined bioassay approaches are recommended to avoid underestimation of toxicity. Tailings and dumps from the three mines were classified at least as moderately toxic, particularly in areas affected by acid mine drainage (AMD), identified as the main factor increasing toxicity.