Use of native plants for the remediation of abandoned mine sites in semiarid environments

The cessation of mining activities without proper rehabilitation measures has left significant negative legacy on the surrounding environments. Among the main sources of degradation are abandoned waste dumps and flotation tailings ponds, which are subjected to water erosion and wind dispersion, representing a source of contamination for nearby communities. In semiarid environments the mobilization of the contaminants and the degradation of soils are enhanced owing to the long periods of drought and heat, the seasonal heavy rainfalls and the scarcity of vegetation cover. The traditional disruptive technologies for the remediation of mine tailings are often costly due to the extension of the affected areas. Recently, phytoremediation, based on the use of plants to reduce the concentrations or toxic effects of contaminants in the environment, has been recognized by the scientific community as an environmental friendly technology which is cost-effective for in situ abandoned mine site reclamation. The establishment of a plant cover could indeed promote the development of a diverse microbial community and a better soil structure, which in turns promote plant growth and help to develop a self-sustaining system. In this experimental study, different phytoremediation tests were performed in laboratory under controlled conditions (humidity, temperature, photoperiod). Soils collected at two different Sardinian mine sites were used: Campo Pisano (flotation tailing dam) and Sa Masa (marshy area), both containing zinc (up to 20,000 ppm), lead (up to 5,000 ppm) and cadmium (up to 150 ppm) as the most abundant toxic metals. Tests were performed by applying two selected native plants: Pistacia lentiscus L. and Phragmites australis. Consistently with their respective presence in the selected mine sites, P. lentiscus was studied on both Campo Pisano and Sa Masa soils, whilst P. australis was tested on Sa Masa soil only. The objective of this study was to evaluate: i) the capability of the two species to tolerate high metals concentrations and to extract or immobilize them within the roots; ii) the possibility to mitigate stress on the plants by adding an amendment such as compost. Experiments were conducted by using 1 L reactors, filled with the selected soil and seeded with P. lentiscus or transplanted with P. australis. For each test, ten replicates were used. The results showed that P. australis and P. lentiscus had different adaptation responses and behaved differently in relation to the tested soil and to the addition of compost. When compost was used, the survival and growth of P. australis in Sa Masa soil improved, and a lower uptake of metals in plant tissues was measured. Compost was effective in reducing the metal uptake of P. lentiscus, whilst almost no effect was assessed in relation to the plant survival and growth.