Phytostabilization of polluted soils by mining activities in Cartagena-La Unión Mountain, SE Spain

Resumen

The intense mining activity carried out in Cartagena-La Unión Mountain, in Murciaregion, Southeast Spain, has provoked extremely high accumulation of heavy metalsin the environment. These mine soils have contents of toxic metals high enough to prevent or restrict plant growth. They are also very deficient in macronutrients, mainlynitrogen and phosphorous and physical properties. Due to the absence of vegetationcover, these soils are very liable to erosion, either eolic or hydric, and they representthe major source of heavy metals pollution on the local environment due to dust blowand leaching of the results of mineral weathering into nearby watercourses. This pollution may also have serious detrimental effects upon crops and public health. Thereforethere is a great need to stabilize these lands because of the existing risks. A range ofreclamation techniques are available for mine soils. Covering them with imported soilor other minerals may be a successful remediation option, but it can be prohibitivelyexpensive for large areas owing to high transportation costs and limited soil availability. Thus, long-term rehabilitation can only be achieved through the use of vegetationto stabilise these lands. It can be a permanent and visually attractive solution and, atthe same time, relatively inexpensive. Stabilisation, of these lands by the establishmentof vegetation slows the processes of erosion and metal leaching. Brassica fructiculosaand Zygophyllum fabago were the two selected species for revegetation processes because they are tolerant adapted to stress factors of mine sites, such as nutrient limitation, drought and low or moderate concentrations of heavy metals. Although revegetation is desirable, these mine soils are very unfavourable sustrate for plants becausethe presence of many growth-limiting factors such as, particularly phytotoxic levelsof residual heavy metals, macronutrient deficiencies, acidity, reduced water retentionand poor physical structure. As a result it is necessary to overcome limitations tovegetation establishment. Thus for soils with low pH values and acid producers, onlyneutralization could prevent acid drainage and remobilization of metallic ions. For thatreason, in order to revegetate this area, the addition of carbonate, as marble waste, andorganic matter were required to raise pH and consequently decrease metal mobilityand risks associated to their toxicity. Organic wastes, such as sewage sludge and pigmanure, were also needed for overcoming nutrient deficiencies.