# Evolution of the Residual Pollution in Soils after Bioremediation Treatments
## 1 Introduction  
Soil contamination with potentially harmful elements (PHEs) is a growing concern and ubiquitous around the globe. According to [PERSON] et al. [1], more than 10 million contaminated sites exist worldwide related to anthropogenic sources and activities; for instance, 137,000 km\({}^{2}\) of agricultural land in Europe needs local assessment and eventual remediation action in relation to the heavy metal concentrations [2]. The restoration of polluted soils affected by heavy metals and related elements is a great challenge that, in many cases, concludes with incomplete results [3]. The full removal of these polluted soils is very difficult due to the reactions between the contaminants and soil properties and constituents producing the precipitation or immobilization of these PHEs into the soil matrix [4]. Remediation actions to remove these PHEs from the soil are usually expensive, time-consuming or need the application of ex-situ technologies that are not able to develop in large areas. Moreover, in many cases the remediation is focused in the application of amendments to reduce the bioavailability and to promote the immobilization of pollutant into the soil matrix, followed by a stabilization of the soil surface by vegetation (phrostabilization) [5], implying that these areas increase the background concentrationsof PHEs in relation to previous values. In these cases, monitoring over time is necessary to assess the evolution of the soil properties and potential changes in the mobility and availability of pollutants. Few studies involve this type of assessment over time under real situation, but in some cases, potential environmental problems were detected; for example, [PERSON] et al. (2016) reported an increase in the availability of As, Pb, Cu, Zn, and Cd over time 20 years after the remediation actions in the affected area were completed; [PERSON] and [PERSON] (2017) also reported an increase in Sb availability after the rise in soil organic matter related to phytostabilization; and [PERSON] et al. (2018) pointed out that the effect of 22 year fertilization and cropping increased Pb and Cd availability in soils. Therefore, restored areas where the remediation actions involved the application of amendments and the immobilization of pollutants into the soils may lead to hidden residual pollution that needs to be assessed to avoid a potential risk of pollution to the environment of living organisms. In this work, we present the study of an area polluted twenty years ago (April 1998), by one of the largest mining spills in the world (2019) the Aznalcollar's mine spill. This mine is located in the largest polymetal-sulphide reserve in Europe (2019), where the break of the dump containing wastes with high concentrations of PHEs (mainly Pb, As, Zn, Cu) spilled about \(3.6\times 10^{6}\) m\({}^{3}\) of acidic water and \(0.9\times 10^{6}\) m\({}^{3}\) of toxic tailings, affecting an area of 45 km\({}^{2}\) of soils, mainly with agricultural use (2019). After the accident, one of the main soil restoration programs carried out in Europe was realized by the implication of the Administration, the Universities and other Research Institutions (CSIC) and the investment of 280 MUR for the recovery of the area (2019). The remediation actions were focused on the removal of the tailings and the highly polluted topsoil, the application of organic and inorganic amendments and the phytostabilization with autochthonous vegetation (2020). The remediation actions finalized with the implementation in the whole area of the Guadiamar Green Corridor (GGC) in 2003, designated by the administration as a \"protected landscape\" were the extractive activities are forbidden (agriculture, grazing, hunting, and fishing), and the main use was focused in recreational or environmental outreach activities. Nowadays, the whole area is considered as restored, but a very heterogeneous evolution of the vegetation recovery is detected.