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.