Determination of the geochemical background in a metal mining site: example of the mining district of Linares (South Spain)

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Abstract

The study region is situated in the mining district of Linares (province of Jaén, South Spain) and it is characterised by the presence of important seams of galena. The mining, minerallurgic and metallurgic activities in the area, as well as the local geological (metallogenetic) conditions, are important sources of heavy metals in the environment, resulting in considerable soil contamination.

The study area occupies a surface area of 126 km2, divided into grid squares of 1 km2 for soil sampling, in order to characterise the geochemical background of this abandoned mining region. The soil samples were analysed for total concentrations of 32 elements; the discussion focuses on eight in particular (Cu, Pb, Zn, Ag, As, Cd, Sb and Mn), these being the main elements associated with human activity. The mean contents of all these elements exceed the Clarke comparison values for trace elements in acid igneous rocks. The total concentrations of Pb, Ag, As, Cd and Sb are especially high. In this very complex zone, on the basis of cluster analysis and of field observations, the local background contents of each geological substrate are defined, distinguishing the different types of activity that took place in the zone and different degrees of intensity.

Introduction

Geochemical maps are of great interest, not just from the scientific or mining standpoint, but also, they constitute an effective tool for environmental planning, and for establishing healthcare policies and promoting sustainable development (Ferguson and Kasamas, 1999). This explains why, in recent years, many governments have promoted research to define geochemical background levels to serve as a basis for legislation to diagnose, prevent and reduce soil contamination (Mielke and Ragan, 1998, Berglund et al., 2000, Rieuwerts et al., 2000). In the USA, background concentrations are often used as soil clean-up criteria following industrial activities. Determining reliable background concentrations of chemical elements in soil is an important issue in environmental sciences, and requires the availability of reliable procedures to distinguish between “natural” and “man-made” element concentrations in soils (Reimann and Garrett, 2005).

The term “geochemical background” has been defined as “the normal abundance of an element in barren earth material” (Hawkes and Webb, 1962). It was introduced to differentiate between normal element concentrations and anomalies. By definition, a geochemical anomaly is a departure from the geochemical patterns that are normal for a given area (Hawkes and Webb, 1962). In the same way, the background values for a lithology indicate a typical composition of that rock type.

In regions where hydrothermal processes have given rise to mineralization, the common compositions of local bedrocks are modified, raising metal concentrations (high local backgrounds due to geogenic sources). In addition, ore extraction and mineral processing facilitate metal mobilisation in surrounding areas and increase the average levels of metals in the soil (anomalous values from anthropogenic sources).

The local background levels must be determined for each geologic unit, because its geochemical pattern will be unique and differentiating. The soils that have developed there reflect the geochemistry of the unit over which they have formed (Salminen and Gregorauskiene, 2000, Reimann and Garrett, 2005). A geochemical investigation that is based exclusively on systematic sampling will detect areas enriched in certain trace elements, but will run into difficulties in discerning whether the anomaly is geogenic or anthropogenic (Crock et al., 1992, Darnley, 1997).

The Linares mining district (Spain) (Fig. 1) was selected to analyse the importance of the different geogenic and anthropogenic factors in the present-day composition of the soil. This area has been characterised historically, though the activity has now been abandoned, by the exploitation of seams of galena (PbS). Paragenesis has also given rise to other minerals, including zinc blende (ZnS), calcopyrite (CuFeS2) and barite (BaSO4) (Azcárate and Argüelles, 1971, Azcárate, 1977, Fontboté, 1982). The abundance of these ores enabled the development of a large-scale extractive mining industry, minerallurgic industry (gravimetric and flotation) and smelting industry over hundreds of years. This mining district is located above a substrate in which various geological units can be distinguished, these being mainly constituted of Palaeozoic granites, Triassic lutites and Miocene marls (Fig. 1).

Despite all the human activity and geogenic variability described, to date no detailed studies have been carried out in the region to determine the concentrations of heavy elements in the soil. Therefore, there is a real need to evaluate the levels of concentration of trace elements in the soil of the Linares area and to make this information available, both generically and cartographically, as a matter of public interest and as a reference document for administrations, as a basis for risk studies, among other applications. Taking these factors into account, the present study is focused on the following objectives:

  • 1.

    Geochemical characterization of this area, to identify the trace elements present in the soils and the incidence of mining activity.

  • 2.

    To define the local background values of selected trace elements for different geological substrata.

  • 3.

    To identify the average concentration of the anomalous elements in each geological substratum and discriminate natural and anthropogenic contributions.

Section snippets

Site description and experimental procedure

The mining district of Linares was divided into 126 grid squares, each measuring 1 km2 (Fig. 1). Firstly, after a review of the geologic mapping and having identified the main human activities carried out, three grid squares were selected as representative of the geochemical pattern of the area (Martínez, 2002, Martínez et al., 2006).

On the basis of this pilot study (Martínez et al., 2006), the definitive sampling campaign was designed (i.e. the number of samples and the volume of material to

Statistical treatment

The soils of the mining district of Linares contain very high total concentrations of copper, lead, zinc, silver, manganese, arsenic, cadmium and antimony, compared with the Clarke values. Consequently, high Enrichment Factors are also obtained for this area (Table 1).

The EFs for these eight elements exceed the Clarke values; the most notable is Pb at 1852 times the Clarke values (extremely high enrichment), followed by Sb (26), As (16), Ag (12) and Cd (6.9), with significant to very high

Conclusions

The results obtained in this study demonstrate that statistical techniques combined with element concentration analysis can be a useful tool to identify different geogenic and anthropogenic sources affecting soil composition, provided that local geochemical backgrounds are correctly established.

Soil samples from the Linares mining district show high Cu, Pb, Zn, Ag, Mn, As, Cd and Sb concentrations. The high degree of dispersion of the total concentration is associated with the different

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