1. Introduction
Soil contamination by potentially harmful elements (PHEs) is a growing problem in the world. Heavy metals and associated elements released into the environment are persistent and their toxicity represents a serious threat to organisms exposed to high concentrations of such pollutants (Bernard and Oluranti, 2017). For the treatment of contaminated soils, one of the major current remediation techniques is the assisted natural remediation, which consists of improving the soil properties to favor the reestablishment of its main functions and promote the growth of vegetation (Adriano et al., 2004). In this sense, the application of organic and inorganic amendments to the contaminated soil favors its natural remediation, since it is based on the reduction of the mobility and bioavailability of the polluting elements (Xiong et al., 2015). In addition, the application of amendments favors the biological activity of the soil, promoting the implantation of bioremediation techniques such as phytostabilization, in which the mechanisms in the soil-microorganism-plant system are enhanced to speed up the recovery of contaminated areas (Bernard and Oluranti, 2017). Revegetation of areas affected by PHEs is a widely used alternative to reduce related environmental risks, but requires regular monitoring to control potential passive transmission of pollutants into the food chain (Pardo et al., 2018). Therefore, assisted phytoremediation and the use of amendments are efficient, cost-effective and environmentally friendly methods for the recovery of contaminated soils (Wiszniewska et al., 2016). Medium-term studies based on the application of organic and inorganic amendments to an acidic soil contaminated with different PHEs, demonstrated the effectiveness of carbonated materials in increasing pH and reducing Cd, Cu and Zn concentrations in the soil solution (Xiong et al., 2015). Likewise, Pardo et al. (2018) evaluated the revegetation of an acidic soil contaminated with metals, after six years of incorporating lime amendments with and without compost, concluding that lime plus compost amendment was the best treatment to increase vegetation cover.
In this work, we evaluate the remediation of an area contaminated by the Aznalcóllar mine accident (Seville, Spain), twenty years after the spillage of tailings and acidic waters with high concentrations of heavy metals and other PHEs (Simón et al., 1999). Recovery actions initially focused on the removal of potentially toxic tailing and the highly contaminated topsoil, followed by assisted natural remediation based on the application of organic and inorganic amendments and on phytostabilization with native vegetation (Madejón et al., 2018). The recovery of the affected area was realised through the largest soil restoration program carried out to date in Spain, with the participation of the Administration, Universities and research institutions (CSIC), with an investment of 280 M \euro (OECD, 2004). The initial removal of the soils affected by the spill was performed depending on the particular characteristics of the terrain. In the northern part of the study area, the one closest to the mine, more than a meter of the original soil was removed, while in the rest of the areas only the top layer of soil, less than 30 cm, was affected (Martín Peinado, 2001).
After the removal of the contaminated soils, different organic and inorganic amendments were used for the recovery of the soils. The application of the amendments depends on the soil properties and the residual concentration of contaminants. Initially, materials rich in CaCO3, such as waste of sugar factory and cellulose pulp ash, were applied throughout the affected area, the doses applied in acidic soils (first 15 km from the mine) ranged between 60 and 90 t ha-1 and were fixed in 20 t ha-1 in basic soils (rest of the affected area). Subsequently, organic matter amendments were also applied throughout the whole area. Finally, in the most affected areas, clay materials rich in iron oxides were applied to favor the immobilization of arsenic, since it was considered one of the most persistent pollutant (Aguilar et al., 2004a, Madejón et al., 2018). The intensive remediation treatments ended three years after the accident, when the area modified its agricultural use and the Guadiamar Green Corridor (GGC) was implemented in 2003 as a Protected Landscape area, where agriculture, grazing, fishing or hunting were banned (CMA, 2003).
Over time, systematic samplings have been performed in the entire affected area to assess the degree of recovery in the zone, which has been generally considered as positive (Simón et al., 1999, Simón et al., 2001, Aguilar et al., 2004a, Simón et al., 2008). However, in recent years the presence of residual contamination has been detected in certain sectors of the GGC, which were initially identified through field observation, due to the presence of patches where no vegetation grows and a potential risk of dispersion of pollutants was detected (Martín et al., 2015, Romero-Freire et al., 2016a). In this sense, the objective of this work is to evaluate the evolution of the treatments and doses applied to the soils in the different sectors and how these treatments have developed twenty years after the accident. Changes in the physicochemical properties of soils, in total, soluble and bioavailable concentrations of the main pollutants (Pb, As, Zn and Cu) and the potential toxicity in the affected area are also evaluated.