Authors:
Prof. José Luis Rodríguez Gallego | University of Oviedo | Spain
Eduardo Rodríguez-Valdés | University of Oviedo | Spain
Noemi Esquinas | University of Oviedo | Spain
Alicia Fernández-Braña | University of Oviedo | Spain
Nora Matanzas | University of Oviedo | Spain
Carlos Boente | University of Oviedo | Spain
Elías Afif | University of Oviedo | Spain
Brownfields threaten soil and (ground)water resources, and cause environmental & health risks as well as economic and social charges. In this context, “megasites”, large and complex contaminated brownfield sites, are a challenge for the development of effective strategies of investigation, clean-up and decision making. Regarding characterization studies, analytical chemistry generally focus on compounds with regulatory limits in order to measure how much contaminant is present, and not to provide information regarding the source of the contamination. On the contrary, environmental forensics investigations are devoted not only to describe pollution importance and distribution, but to identify former industrial practices and ulterior weathering processes linked to the current pollution in the site. In this sense, a possible strategy is the search for compounds, elements, isotopic signatures and/or molecular-markers that are diagnostic to be useful as tracers.
The LIFE+ I+DARTS project (Innovative and Demonstrative Arsenic Remediation Technologies for Soils, 2012-2016, www.lifeidarts.eu) is aimed at the recovery of soils contaminated with arsenic and heavy metals in former mining and industrial sites included in the Spanish inventory of polluted soils. One of the brownfields studied is known as Nitrastur (located in Langreo, Asturias, northern Spain). It was one of the main fertilizers plant in Spain for more than fifty years until its abandonment in 1997. The total surface of the affected site is 20 ha, and at least another 20 ha should be considered outside the main parcel, given that the surroundings have been affected by the historical activity of other heavy industries (ferrous metallurgy, coal thermal power station, chemical industry, etc.). In this site, we have carried out a detailed environmental forensics study to obtain a conceptual model for risk assessment and for the selection of clean-up strategies. The main tools used and a summary of the foremost results obtained, are the following:
• Five different types of pure waste (three inorganic: pyrite ashes and two types of slags, and two organic: spilled fuel and coal waste) were identified as the main pollution sources. Thus, a comprehensive chemical and mineralogical characterization of these products was carried out. As a result, and given their wide distribution and chemical contents, pyrite ashes, comprising mainly oxides and hydroxides of iron and other metal(loid)s, were considered the most problematic one. This waste was produced as a by-product of toasting sulphur ores, that were used in the former industrial activities to produce sulphuric acid subsequently used to manufacture ammonium sulphate fertilizer.
• As a consequence of pyrite ashes dumping, soil pollution is mainly composed of As and Pb, while Zn and Cu are considered secondary contaminants, all of them showing with levels higher than 1.000 ppm in many samples, and above 5.000 ppm in hot spots. Although inadequate waste disposal was also detected concerning other materials (slag, coal waste, etc.), only pyrite ashes had significant As and heavy metal contents capable to affect dramatically soil quality. In this context, the origin of the contamination has been verified by determination of Pb isotopic signature, and by the geochemical association of As with Sb and other trace elements determined by means of multivariate statistics methods.
• Distinctive forensics tools such as fingerprinting of weathered hydrocarbons spilled, and PAHs diagnostic ratios have revealed the co-occurrence of organic contaminants in some areas within the site.
• The soil affection is high in about 25% of the total surface of the site; this means that many areas could be considered initially free of pollution. Conversely, the subsoil of several buildings (classified as industrial heritage) was filled in the past with pyrite ashes.
• Groundwater is also affected especially wherever a thick layer of pyrite ashes (sometimes more than 2 m) is found. The amount of water in the alluvial aquifer, together with the low permeability of the buried waste, promotes only punctual affections as it was revealed by means of a high-resolution groundwater study. All things together, the potential mobility of the contaminants was determined to be quite low (chemical speciation and sequential extraction analyses were also made).
On the whole, the forensic study demonstrated the presence in high concentrations of a variety of contaminants affecting soil and groundwater quality. However, the low mobility of the main contaminants, the limited extension of polluted groundwater, and the high background levels in the surroundings suggest that sustainable clean-up technologies such as phytoremediation and bioremediation could be applied, at least partially; therefore supporting remediation costs optimisation strategies for the megasite.