1. Geographical location
  2. Main soil threat
  3. Natural environment
  4. Drivers and pressures
  5. Status of soil threat
  6. WOCAT Maps
  7. Effects of soil threat on soil function
  8. Administrative and socio-economic setting
  9. Management options
  10. Stakeholder involvement
  11. References

Details about the RECARE experiment in Guadiamar can be found here 


Geographical location

The study area is located in southern Spain (see below). It was declared the "Protected Landscape Guadiamar Green Corridor" on April 2003, occupying about 2,700 ha. It is 60 km long, following the Guadiamar River, and 0.5–1.1 km wide connecting the Sierra Morena Mountains and the coastal Doñana Park, and crossing extensive agricultural and rural lands (Fig. 1). The climate is typically Mediterranean, with mild rainy winters (about 500 mm mean annual rainfall) and hot, dry summers. The mean annual daily temperature is about 17ºC, with a maximum temperature of 33.5ºC in July and a minimum temperature of 5.2ºC in January. The predominant soils in the area belong to the great groups Xerofluvent, Xerochrept, Haploxeralf and Rhodoxeralf.


Location and Digital Elevation Model (DEM) of the Guadiamar Case Study (Source: SRTM)


Main soil threat

GuadiamarLandscapeThe main threat in the Guadiamar valley is soil contamination after a mine spill occurred on April 1998. About four hm3 of acid waters and two hm3 of mud, rich in heavy metals, were released into the Agrio and Guadiamar rivers affecting more than 4,600 ha of agricultural and pasture land (see Grimalt & Macpherson, 1999). Main trace- elements contaminating soil and water were As, Cd, Cu, Pb, Tl and Zn (Cabrera et al. in Grimalt & Macpherson, 1999). The area was subjected to a large-scale phyto-management project, including the removal of sludge and topsoil, the addition of amendments, and plantation of native shrubs and trees, and consequently protected as the "Guadiamar Green Corridor". The total cost of the remediation program, including the purchase of the land, rose up to €165 million paid by public funds (Arenas et al. 2003). While concentrations of available As and Pb in soil and plants have decreased over time, the levels of Cd and Zn in poplar leaves (used as bio-indicators) are still relatively high (Madejón et al. 2013). Long-term monitoring of the potential toxicity of residual contamination, in particular of Cd, is needed. Main constraints and challenges include among others the need for harmonization of soil contamination threshold levels and indicators, the need for new experiments on remediation measures and evaluation of their stability and longevity, development of new materials for remediation, to develop adaptive management programs, and to transfer technologies for diagnosis, monitoring and restoring contaminated soils.


Other soil threats

Other soil threats in Guadiamar include: 1) Floods that are relatively frequent after strong rains, eroding river banks, moving sediments and submersing vegetation in the floodplain; 2) Soil erosion by water where river banks have poor vegetation cover; 3) Desertification potential: the typical Mediterranean summer drought is a source of biological stress that combined with heavy metal stress could result in desertification at local scale; 4) Loss of organic matte due to accelerated mineralization of organic matter during the summer (frequently over 40oC); 5) Loss of soil biodiversity due to soil contamination by toxic elements and very low pH.


Natural Environment

Geology and soils
The Aznalcóllar mining district is located on the south-eastern edge of the Iberian Pyrite Belt (IPB). The IPB constitutes the largest and most important volcanogenic massive sulfide province in W Europe. It extends 200 km from SW Portugal to the W of Sevilla in Spain. Geologically, the mine area is situated on the northern edge of the Guadalquivir Tertiary basin, where transgressive Miocene sediments cover Paleozoic materials. The lower course of the Guadiamar River is underlain by Miocene blue marls and yellowish calcareous sandy silt, although most of the flood-plain is carved on Pleistocene alluvial terraces and Holocene deposits. The mining tailings reservoir was located on the Miocene blue marls of this lower course. On this geological context, the alluvial sediments of the Guadiamar and southern marsh deposits within the coastal wetlands of the Guadalquivir were directly affected by the pyritic sludge. The alluvial deposits of the Guadiamar fluvial system consist of silt, sand and gravel. Gravels are dominant, though quartz sands are locally abundant. Three terrace levels can be recognized along the affected valley segment. The high terrace is preserved only in the northern area, near the confluence of the Agrio and the Guadiamar rivers, whereas it is totally eroded to the south (Gallart et al., 1999 and López-Pamo et al., 1999). The soils in the study area correspond to the Mediterranean edaphic zone, which is very heterogeneous due to a highly variable lithology and mesoclimate. A survey of the affected area up to the Marismas at scale 1:20,000 showed that the alluvial soils are calcareous and non-calcareous typic and aquic Xerofluvents (FAO, Fluvisols), with sandy and sandy-loam textures. Soils of the lower terraces are typic and aquic Haploxerlfs (FAO, Luvisols) and Aquic Xerofluvents (FAO, Fluvisols), while in the higher terraces appears typic Rhodoxeralf soils (FAO, Luvisols) associated to sandy soils and pseudogley. In the contact of terrace and alluvial there are soils of erosion classified as Calcixerollic Xerochreps (FAO, Calcisols) (Clemente et al., 2000; Nagel et al., 2003).


Left: Soil map of (Source: JRC) and land use map (Source: CORINE) of the Case Study.

Land Use

Previous to the mine accident, the Guadiamar Valley was mainly occupied by croplands (sunflower, fruit orchards) and pasture lands. After the spill-contamination and the expropriation, the total area of 2,706.8 ha was declared as the “Protected Landscape of Guadiamar Green Corridor” (April 2003) and was relieved of agriculture and livestock use. Soils have been remediated and the land afforested with native shrub and tree species (Domínguez et al., 2010). Currently, the main land use is for the conservation of biodiversity and the establishment of an ecological corridor connecting the Doñana National park (to the south) and the Sierra Morena Mountains (to the north). Another important land use is currently recreation, mainly cycling, trekking and horse-riding along two main tracks running parallel to the river. Environmental education and touristic activities are also organized in the Information Center of Aznalcázar (www.guadiamareduca.com).

Horse grazing is an activity being considered for the current land use at the Green Corridor. The presence of horses in this protected area was initially triggered by the pressure exerted by the surrounding municipalities for using pastures, despite the fact that grazing was initially forbidden after the accident. Due to this pressure, the option of horse grazing (livestock not intended for human consumption) was considered by the Regional Government as a benign and sustainable management tool for control of the herbaceous cover. At present, control of the horses is carried out by “Equine Guadiamar Society”. One of the disadvantages of this practice is that vigorous and healthy herbaceous cover competes with planted woody species for water and nutrients, and its desiccated remains present a fire hazard during summer droughts. Also, mechanical control of herbaceous species is expensive, may affect biodiversity and generates greenhouse gas emissions. At present, all horses grazing in the corridor must be obligatory identified (marked by plaques) but their presence is still considered illegal. Nevertheless, pasture evaluation has eliminated the possibility of acute toxicity for horse grazing (Madejón et al., 2009b; 2012).


Horses grazing in the Guadiamar Green Corridor. In the background the mounds of the Aznalcóllar mine. Photo by J.M. Murillo.



The climate is typically Mediterranean, with mild rainy winters (about 500 mm mean annual rainfall) and hot and dry summers. The mean annual daily temperature is about 17oC, with a maximum temperature of 33.5oC in July and a minimum temperature of 5.2 oC in January-see below.


Left: Average annual; right: mean monthly precipitation and temperature at Guadiamar


The main tributary to the Guadiamar River is the Agrio River, where the mine spill occurred. The catchment area supplying the Agrio dam is 228 km2 and the mean annual runoff volume is 44.7 hm3. The total catchment area of the Guadiamar is 1,879 km2, the annual flow volume is 209 hm3, and the mean flow rate is 6.6 m3s-1 (Aznalcázar gauging station), nevertheless with a high interannual irregularity. The higher flow period is from January to March (mean rate of 13 m3s-1) and the lower from June to October (3 m3s-1) (Gallart et al., 1999).According to their geomorphic characteristics and human impacts, three main sectors can be distinguished: (a) Along the first 15 km downstream of the tailings reservoir, is a suspended load and sand and gravel bed load river; (b) Between 15 and 30 km downstream of the tailings reservoir, with a much lower valley gradient, is a river dominated by pebble and sand textures with smaller peak discharges than those of the former reach. The floodplain (300-700 m) shows multiple low sinuosity flood channels forming a branching pattern of channels, which are active during flood stages once the floodwaters overtop the natural levees. (c) The lowest reach (from “Vado del Quema” to Doñana marshes) is a suspended load river reach with fine sand and silt textures, and a stream gradient similar to or lower than that of the previous reach. The natural fluvial system in this area was drastically changed during the late 1950s to allow farming. This final reach presents a gentler gradient downstream from the “Vado del Quema” area where marsh sediment is dominant, partially covered by aeolian deposits (Gallart et al., 1999).


Drivers and pressures

The increasing demand for metals for industrial production is one of the main drivers promoting mine activities and the consequent environment impact. The Guadiamar River originates in the south-eastern edge of the Iberian Pyrite Belt, a volcanic-sedimentary complex which has been exploited for gold, silver and copper ore from pre-Roman times (more than 2000 years ago) (López-Pamo et al., 1999). The mine activity in Aznalcóllar produced a diffuse contamination of heavy metals in the Guadiamar River, already detected before the 1998 accident (Cabrera et al., 1987). In a survey of European contaminated sites (about 342,000 sites) the second source of contamination was the industrial/commercial sector, including mining activities, and the heavy metals represented the first type of contaminant (in 35% of sites) (Panagos et al., 2013). After the mining accident and consequent contamination of land, the area was declared Protected Landscape of the Guadiamar Green Corridor and devoted to conservation and recreation. Currently main pressures and threats are: contamination by industrial and urban wastes (there are 7 sewage treatment plants connected to the river); water extraction for agricultural and industrial use; obstruction of the ecological connectivity within the corridor by urbanization, industrial tree plantations and croplands; fire and overgrazing (sometimes despite being forbidden); and the uncertainty of reopening the Aznalcóllar mine and its future impact (JA, 2014)


Status of soil threat

Few days after the mine accident, Cabrera et al. (1999) analysed total heavy metal concentrations in soil samples of seven selected areas along the Guadiamar River valley affected by the toxic flood after removal of the deposited sludge. They reported that the total concentrations of the elements As, Au, Bi, Cd, Cu, Pb, Sb, Tl and Zn were higher in sludge-covered soils than in unaffected soils, with increases into the range of 6 times for Au and 370 times for Sb. After remediation measures, Hg and the other trace-element contents in soils were still higher than background values, and occasionally higher than values before restoration (Cabrera et al., 2008) with an increase of 10 times for Hg. The increase of other trace elements was into the range of 5 times for Cu and 8.4 times for As and Pb. This was attributed to remains of sludge left on the soil surface and buried during restoration operations (sludge removal, liming, and manuring). Total trace-element concentrations were highly variable, indicating a patchy and irregular distribution of the trace elements on the surface soils along the Guadiamar river basin (Cabrera et al., 2008). Nevertheless, further monitoring in 2005 (Domínguez et al., 2008) reported that despite the high concentrations of several trace elements in the affected soils there was a limited transfer of these elements to the aboveground parts of woody plants. This seemed to indicate a reasonable trace element stabilization in soils although the authors highlighted that soil pH requires close monitoring, since acidification will result in increased trace element mobility and it may increase the rates of leaching into receiving waters. Domínguez et al. (2009) and Ciadamidaro et al. (2014) remarked that soil pH is the most important factor affecting the soil contamination and quality.



 Upper left: Dam of the Aznalcóllar tailings pond that was broken on 25 April 1998,
releasing the contaminating content (Photo: Junta de Andalucía);
upper right: remains of mine sludge in soils of the river banks, 16 years after the spill.
Bottom: a detail of the soil auger showing the layer of sludge over the soil. Photos by J.M. Murillo.


A recent study (Burgos et al., 2013) has demonstrated the effectiveness of the remediation tasks carried out in all the area for this trace elements stabilization which avoided the leaching of the most mobile elements and minimized the risk of contamination of groundwater sources, many of them close to the Doñana National Park. In a recent survey of the river banks and the margins of the floodplain in the Guadiamar area, a high contamination of soil with trace elements has been detected, mainly in the northern part of the area; that is from the mine Aznalcóllar to Sanlúcar la Mayor (Alegre 2014). In this area the acidity (low pH) of the soils may aggravate the problems of toxicity for plants and animals. The results also show that in the survey of contamination within a river basin, most of the sampling is focused on the floodplain (normally with high-valued agricultural use). However, the problem of contamination may persist on river banks and levees, less accessible to clean up and remediation practices, and also more sensitive due to their proximity to the water flow. In the banks of the north part of the studied area, the concentration of S (one of the most representative constituent of the sludge) can reach values as high as 9 % (mean of 2.4 %), far greater than those values in the floodplain (mean of 0.3 %).


Maps on the current state of land use, soil degradation and soil conservation in the case study area have been produced using the WOCAT (World Overview of Conservation Approaches and Technologies) methodology

The steps of this process are as follows:

1) The area to be mapped is divided into distinctive land use systems (LUS).
2) The team gathers the necessary data on soil degradation and conservation for each LUS using a standardised questionnaire, in close consultation with local land users, and supported where possible by remote sensing or field data.
3) For each LUS, the soil degradation type, extent, degree, impact on ecosystem services, direct and indirect causes of degradation, as well as all soil conservation practices, are determined.
4) Once collected, the data is entered in the on-line WOCAT-QM Mapping Database from which various maps can be generated.

Following the principles of all WOCAT questionnaires, the collected data are largely qualitative, based on expert opinion and consultation of land users. This allows a rapid and broad spatial assessment of soil degradation and conservation/SLM, including information on the causes and impacts of degradation and soil conservation on ecosystem services.

More details about the methodology used to produce these maps and their interpretation can be found here.

Land Use (click on maps to expand)

Spain Guadiamar land use types Spain Guadiamar area trend land use systemS Spain Guadiamar trend in land use intensityS

Soil Degradation

The degree of degradation reflects the intensity of the degradation process, whilst the rate of degradation indicates the trend of degradation over a recent period of time (approximately 10 years).

Spain Guadiamar dominant types of soil degradationS Spain Guadiamar degree of degradationS Spain Guadiamar rate of degradationS

Conservation Measures

The "effectiveness" of conservation is defined in terms of how much it reduces the degree of degradation, or how well it is preventing degradation. The Effectiveness trend indicates whether over time a technology has increased in effectiveness.

Spain Guadiamar dominant conservation measures Spain Guadiamar effectiveness of conservation measures Spain Guadiamar conservation effectiveness trends

Effects of soil threat on soil functions

The soil contamination affects several functions and therefore the ecosystem services provided (see summary following table).

Function of soil Explanation Effect
Biomass production Trace element toxicity reduces plant growth and biomass production. M
Environmental interactions

The toxicity of elements such as Cd, As, Pb and Tl for plant, animals and microorganisms is the worst effect for the environment.

Potential higher bioavailability of metals through decreasing pH.

Potential diffusion of metals through fluvial erosion.


Gene reservoir/

Biodiversity pool

Soil contamination and reduced pH may reduce biodiversity eliminating low-tolerant species. The effects on soil biota are not well known. L
Physical medium Base for built development is not affected N
Source of raw materials Although raw material collection is not affected directly, logging and gravel extraction have been forbidden in the area. L
Carbon pool Indirectly, the afforestation of former agricultural land has increased the C stocks in soils. L
Cultural heritage Indirectly, the contamination event and posterior recuperation has increased the cultural value of the site. M

Summary of the effects of soil contamination on the soil functions for the Guadiamar site. The ranking (N: None; L: Low; M: Medium; H: High) is expressed in the right column.


Administrative and socio-economic setting

The mine of Azanalcóllar is located in an economically deprived area and it was the main employer in the zone. At the time of the accident, it employed about 400 persons and permitted the employment of 1,800 persons indirectly. As a consequence of the accident, in 1999 the Regional Government and the Central Administration have launched two important restoration programmes: The Guadiamar Green Corridor and the Doñana 2005 Plans. The Guadimar Green Corridor promoted and funded by the Andalusian Regional Government, aims at the restoration of the Guadiamar basin and the reestablishment of an ecological corridor between the mountains area of Sierra Morena and the litoral systems of Doñana. At the same time, the programme seeks the improvement of the quality of life of the Guadiamar basin inhabitants, by developing a socio-economical system that is environmentally sustainable and integrated in the natural context. The programme has received the support of the American Agency for Environmental Protection, the European Council, the International Union for Conservation of Nature, the Environmental European Agency and Conservation of Nature, the Environmental European Agency and conservation NGOs, due to its integrated and scientifically sound approach and to the importance given to the public participation for the achievement of the programme objectives (Bartolomé and Vega, 2002).

Main institutional players in the study site are the Regional Government (Junta de Andalucía), owner and manager of the land and local-scale regulator of environmental legislation, and the European Commission regulator of environmental issues at European scale. The European Directive 2008/1/EC regulates integrated pollution prevention and control; at Spanish level the Law 16/2002 regulates industrial and mining activities to reduce pollution and protect soil and groundwater. In the Thematic Strategy for Soil Protection (COM 2006-231) the European Commission proposes a framework to prevent soil degradation, to preserve soil functions and to remediate degraded soil. At a Global scale the European Commission has proposed a new Environment Action Programme entitled “Living well, within the limits of our planet” that will guide environment policy up to 2020.

On April 2003, the Case Study was declared “Protected Landscape” and included in the Network of Andalusian Protected Areas (RENPA). At European level it is part of the Natura 2000 network regulated by the Habitats Directive (92/43/EEC).


Left: Population; right: GDP per capita trends for Spain and the Euro Area


Management options

As a consequence of the mine accident (in April 1998) the Regional Government of Andalusia and the Spanish Authorities launched two restoration programmes for short- and long-term management. Firstly, the soil cleaning-up was followed by the addition of organic matter and calcium rich amendments. Then, within the Guadiamar Green Corridor programme, the afforestation of ca. 4,500 ha with autochthonous species were performed (CMA, 2003). However, a few fenced plots did not undergo any remediation operations for research purposes, remaining with the sludge layer over the soil surface (Burgos et al., 2013). Additionally, relevant policies and regulations at local and European levels influenced the decontamination and remediation of the Guadiamar site.

Short term management

At local level, the Action Plan of 1998 (just after the mine accident) regulated the investment of 165 M € to evaluate the risk and health control, to purchase the land, to remove sludge and remediate soils, to plant shrubs and trees, as well as to promote research and environmental education. The plan of Emergency Measures involved the Spanish Authorities, the Regional Government of Andalusia, and the mining company Boliden Apirsa (Arenas et al., 2003).

The removal of the tailings was carried out in two campaigns, one in 1998 and the other in 1999. As a result, 8 hm3 of sludge together with a variable layer of top soil (10–30 cm) was removed. Following that, a treatment of soil was carried out through different chemical procedures (amendments) to immobilize the heavy metals remaining in the soil. On the other hand, measures were taken to control the environmental quality in the surface waters, the subterranean waters, the estuary, the air and the living beings. In addition, a sanitary control program monitored the health of human population of the area affected by the spill.

This whole process was established in a series of laws, decrees and orders that were published from 1998 to 1999. The Regional Government approved the actions necessary for the execution of a project of regeneration and adaptation for public use called the Guadiamar Green Corridor. The properties affected by the spill were declared of urgent occupation, to the effect of compulsory purchase (Hernández et al., 2004).

The accident did not cause personal injury, but the socio-economic effects were important. More than nine townships in Seville Province were affected because they lost agricultural crops and the mining activity was stopped. Moreover, indirect effects, such as potential health risks, the impact on the local image and the devaluation of regional agricultural products on the international market, played an important role.

Long term management

The Guadiamar Green Corridor Strategy began in 1999 and the main objectives were (Hernández et al., 2004):

  • Decontamination of the soil, water and organisms of the fluvial riverbed and of the flooded plains and the marsh damaged by the tailings and acid water
  • Restoring the functionality of the aquatic and terrestrial ecosystems damaged or destroyed by the spill.
  • Promoting a model for the management of the multiple uses of the territory in order to promote considerable ecological heterogeneity, reinstating the flood of species and natural processes between the mountain range and the coast.
  • Improving the quality of life of the inhabitants of the area through strategies of development compatible with the conservation of the functions of their natural systems.
  • Contributing to the transformation of the Network of Protected Natural Areas of Andalusia, as a network of areas connected through ecological corridors, among which the fluvial ones stand out.
  • To serve as a model of integrated planning of a Mediterranean basin that can be extrapolated to other areas and regions.

Current situation

According to the existing data, the decontamination of the soil has been effectively carried out and the levels of metals are less than that established by the Regional Department for the Environment according to the legislation.

Since 2002, there has been a continuing follow up of the soil quality, cantered fundamentally in the northern branch (between the Doblas Bridgeand and the Aznalcóllar mine). This area was the most affected due to its proximity to the spill source, causing high levels of residual contamination of arsenic and other trace elements. Despite cleaning up operations, eliminating important focal points of contamination, there is still residual contamination in some patches of land (see above).


Stakeholder involvement

Relevant end-users and local stakeholder groups

The Ministry for the Environment generally intervenes through the Guadalquivir River Basin Authority, the National Parks Administration and the Department of Environment that co-manage the Doñana National Park. The Ministry is responsible for the clean-up operations of the public hydraulic domain and the “Doñana 2005” marshland restoration project. The GeoMining Institute (IGME) is a scientific-technical body specialized in geological, geochemical and mining issues and belonging to the Central Administration. In relation to the Aznalcóllar mine, it issued reports regarding the dam stability before the accident and it advised the public Administration about the use of Aznalcóllar depleted pit as waste disposal. The Guadalquivir River Basin Authority, is a public body belonging to the Ministry of the Environment, and is in charge of the management of water resources in the Guadalquivir river basin. Its territorial responsibility is on the public hydraulic domain and it regulates surface water and groundwater protection, being one of the main regulators of the mining activity. It is responsible for monitoring the water quality of all the rivers within the Guadalquivir basin, including the authorization of the spill of the mining activity. In relation with the Aznalcóllar accident, the Regional Government of Andalusia acts mainly through three different Departments. The Department of Employment and Technology (formerly Industry and Employment) is the supervising authority for all mining activities and the main permits. The Department of Environment has jurisdiction on the Doñana Natural Park and the Environmental Impact Studies concerning the mining activity. It also launched the Guadiamar “Green Corridor” restoration project to ensure environmental rehabilitation. Finally, the Department of Agriculture has actively participated in the clean-up of agricultural land. The Guadiamar Visitor Center conveys educational programs for local schools to learn about the contamination-related environmental problems and the main remediation measures. Currently the Green Corridor is a restored area attracting visitors from nearby cities and villages to enjoy the new landscape, and practice outdoors sports. Sports associations and conservationist NGOs are end-users of the results and information provided by this remediation case study. Finally, companies developing new materials that could contribute to stabilize trace-elements and to improve soil conditions will be interested in testing and demonstrating remediating measures in this Case Study.

Involvement in the Case Study

Researchers of environmental and soil sciences in Universities of South Spain (USE, UPO, UHU and UCO among others) have been collaborating in the remediation and research programs. The Spanish Research Council (CSIC) is a research body belonging to the Central Administration and covering a wide range of scientific fields. CSIC coordinated the scientific advisory group created ad-hoc for the follow-up of the mine disaster, producing several reports about the spill and its consequences on the environment. The Doñana Biological Station (EBD), part of the CSIC, co-ordinates the research in Doñana National Park. During the last 16 years, scientists of the IRNAS, CSIC have been closely working with Government managers, investigating remediation strategies and monitoring contamination in soil and plants of the study area (e. g., Cabrera et al. 1999, 2008; Domínguez et al. 2008, 2009, 2010; Madejón et al. 2002, 2004, 2006a,b, 2007, 2009a, 2010). Main constraints and challenges to overcome include among others: the need for harmonization of soil contamination threshold levels and indicators, the need for new experiments on remediation measures and evaluation of their stability and longevity, the need for modelling soil vulnerability under different scenarios, development of new materials for remediation, to develop adaptive management programs, and to transfer technologies for diagnosis, monitoring and restoring contaminated soils. Stakeholders are actively involved in the project following the RECARE framework for stakeholder involvement, in the Case Studies, at local to the (sub-) national level. Based on a detailed stakeholder analysis, the main activities include promotion of stakeholder learning processes, stakeholder valuation of ecosystem services using local and scientific knowledge and support to other work packages.



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