Baseline values for heavy metals in agricultural soils in an European Mediterranean region

Abstract

Baseline values for heavy metals were proposed in Alicante (Spain), a representative agricultural area of the European Mediterranean region, as a basis to identify and assess soil contamination processes at regional level. Fifty-four agricultural plots were sampled and heavy metals concentrations were analysed for nine elements (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) by atomic absorption spectrometry (AAS) after acid digestion with HNO₃ and HCl in a microwave oven. Heavy metal concentrations obtained in the study area were similar to concentrations analysed by other authors within the European Mediterranean region. However, some agricultural plots with a high heavy metal content were identified by statistical treatment (boxplots) as outliers and, therefore, were not considered for establishing baseline values. Baseline values in Alicante were proposed by taking into account the statistical approach extensively used elsewhere, which refers to the increase in the mean plus twice the standard deviation. The baseline values were: 0.7 mg/kg for Cd, 11 mg/kg for Co, 36 mg/kg for Cr, 28 mg/kg for Cu, 19,822 mg/kg for Fe, 402 mg/kg for Mn, 31 mg/kg for Ni, 28 mg/kg for Pb and 83 mg/kg for Zn. The experience gained in this work further suggests that baseline values for heavy metals should be proposed in other areas. This is necessary to facilitate the identification of soil contamination processes over the whole European Mediterranean region as a basis to undertaking appropriate action to protect soil resource quality.

Introduction

The knowledge of background values of heavy metals in soils is necessary before a soil can be declared to be contaminated. These values can be defined as the natural concentration of heavy metals in soils without human influence (Salminen and Gregorauskiene, 2000, Adriano, 2001). This is a difficult task in ancient populated areas, such as the European Mediterranean region, where totally unpolluted soils are almost impossible to find because of the long distance aerial transport of trace pollutants and human influences. On the other hand, baseline values show an expected range of concentrations for heavy metals specific to one area and time, and considering the diffuse input of these elements to soils (Tack et al., 1997, Adriano, 2001). Therefore, baseline values are not always true background levels. Particularly, baseline values in agricultural soils correspond to total concentrations of heavy metals in soils not influenced by point input due to local human activities (e.g., industries) but include diffuse or non-point input (e.g., atmospheric deposition, fertilization) (Holmgren et al., 1993, ISO/DIS 19258, 2005). Baseline values express the upper limit of the normal range of concentration covering, as a function of a predefined statistic, the range of natural variability associated with the background levels of certain elements or compounds at regional level. In this context, baseline values are useful to identify the current contents of heavy metals and to assess the degree of contamination by human activities.

At a national level, the Spanish Royal Decree 9/2005 establishes that contaminated soils are those with heavy metal concentrations over 100-times baseline values. Soils with heavy metal content between baseline values and 100-times baseline values must be investigated. This means a risk analysis must be carried out in these soils in order to assess risks for human health or the environment. Baseline values for heavy metals have not been established by this regulation at national level, giving regional governments the responsibility to do this. These governments must establish baseline values for different soil uses taking into account the variability of heavy metals in soils at regional scale. For such purpose, Royal Decree 9/2005 proposes both toxicological and statistical approaches to do this. According to the statistical approach, baseline values consist of adding to the mean value (X) twice the standard deviation (s) for each element studied (X + 2s). This method has also been used by different authors (e.g., Tack et al., 1997, Gil et al., 2004) and organisations (e.g., ISO/DIS 19258, 2005).

Assessment of the heavy metal contents in soils and the establishment of baseline values are priority objectives within the European Union (EU). Thus, the European Commission has recently published a report “Towards a Thematic Strategy for Soil Protection” (EC, 2002) that establishes the basis and guidelines for maintaining or even improving the quality of soil resources. In this context, different working groups produced reports addressing the state of soils, impacts and pressures, as well as recommendations for soil protection policy made at the EU level. For example, the report presented by Van Camp et al. (2004) as part of the technical group working on “Contamination and Land Management” established the need to measure heavy metal concentrations in soils in order to identify baseline values to establish soil quality standards (e.g., intervention values).

Most of the works carried out on heavy metals in agricultural soils in the European Mediterranean region have focussed on analysing concentrations and sources of these elements (e.g., Facchinelli et al., 2001, Mitsios et al., 2003) but baseline values were not proposed. In the Spanish Mediterranean region, most of the studies on heavy metals in agricultural soils have focussed on local areas (e.g., Boluda et al., 1988, Andreu and Gimeno-García, 1996). The purpose of this paper was to analyse heavy metal concentrations for some elements (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) currently found in agricultural soils that are not subjected to particular point contamination sources, in order to propose baseline values for heavy metals of a representative area in the European Mediterranean region, as a basis to assess soil contamination processes at a regional level.