ASSISTED NATURAL REMEDIATION OF TRACE ELEMENT POLLUTED SOILS P. Burgos, A. Pérez De Mora, E. Madejón and F. Cabrera Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS-CSIC), Spain. Apartado 1052. 41080. pburgos@irnase.csic.es ABSTRACT Organic and inorganic amendments are increasingly used to enhance natural attenuation in soils pol- luted with trace elements. Four organic materials: a litter from a deciduous forest (LIT), a municipal waste compost (MWC), a biosolid compost (BC), and a low rank coal, rich in humic acids, leonardite (LEO) and an inorganic material, sugar beet lime, (SL) were tested as amendments to assist natural remediation of a trace element polluted soil affected by the Aznalcollar mine accident. The efficiency of these amendments to immobilize trace elements in polluted soils was determined by CaCl 2 (labile) and EDTA (plant-avail- able) extractions after two years of successive applications. Amendments reduced the concentration of labile trace elements in solution of polluted soils. The reduction of the trace elements mobility seemed to be caused to the increase of soil pH due to amendment application. Keywords: amendments, remediation, soil pollution, trace elements. INTRODUCTION Reclamation of soils polluted with trace elements has led to a variety of techniques based either on the extraction or the stabilization of the contaminants. The first ones are generally carried out “ex situ”, and imply soil structure deterioration and high costs, which limits their use on vast contaminated areas. However, soils can naturally reduced mobility and bioavailability of trace elements as they are retained in soil by sorption, precipitation and complexation reac- tions. This natural attenuation process, (natural remediation), can be accelerated by the addition of amendments (Adriano et al., 2002) converting the soluble form to more geochemically stable solid phases and reducing biological availability and plant toxicity of trace elements (Vangronsveld and Cunninghan, 1998). On the other hand, plants can mechanically stabilize polluted soils to prevent bulk erosion and airborne transport (i.e. Phytostabilization) or/and pre- vent movement and transport of dissolved contaminants in soils (i.e. Phytoimmobilization) (Wenzel et al., 1999). The use of plants together with the application of amendments is a more natural approach to remediation when compared to some current remediation practices. This combined “in situ” inactivation technique can be classified as a “soft” or “low impact” rehabi- litation technique (Vangronsveld and Cunninghan, 1998) and is within the so-called assisted natural remediation. Amendments commonly include phosphates, liming agents, metal (Fe/Mn) oxyhydroxides, and organic matter (Iskandar and Adriano, 1997; Vangronsveld et al., 1998), especially urban compost and biosolids are increasingly used for soil remediation (USEPA, 1997). This study deals with the effect of five amendments and a vegetal cover on trace element solubility and availability in soils contaminated by the Aznalcollar mine accident occurred in 1998 (Cabrera et al., 1999). The functional groups of the organic materials used as adsorbents were also studied to understand the mechanismstheir affinity for metal binding. The knowledge of the mechanisms of binding onto organic matter and their long-term beha- 65 Waste Contaminants: Lifecycle and Entry into Food Chain