Assessing heavy metal sources in agricultural soils of an European Mediterranean area by multivariate analysis

Abstract

According to the European Thematic Strategy for Soil Protection, the characterization of the content and source of heavy metals in soils are necessary to establish quality standards on a regional level that allow the detection of sampling sites affected by pollution. In relation to this, the surface horizons of 54 agricultural soils under vegetable crops in the Alicante province (Spain), a representative area of the European Mediterranean region, were sampled to determine the content of Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn. Analytical determinations were performed by atomic absorption spectroscopy after microwave sample digestion in acid solution. Results indicated that heavy metal levels were similar to those reported by authors working on agricultural soils from other parts of the Mediterranean region, with the exception of Cu and Pb in some samples. Multivariate analysis (principal component analysis and cluster analysis) was performed to identify a common source for heavy metals. Moreover, soil properties were determined in order to characterize agricultural soils and to analyze relationships between heavy metal contents and soil properties. The content of Co, Cr, Fe, Mn, Ni and Zn were associated with parent rocks and corresponded to the first principal component called the lithogenic component. A significant correlation was found between lithogenic metals and some soil properties such as soil organic matter, clay content, and carbonates, indicating an important interaction among them. On the other hand, elements such as Cd, Cu and Pb were related to anthropic activities and comprised the second (Cu and Pb) and third principal components (Cd), designated the anthropogenic components. Generally, Cd, Cu and Pb showed a lower correlation with soil properties due to the fact that they remain in available forms in these agricultural soils. Taking into account these results and other achieved in other parts of the European Mediterranean region, it can be concluded that soil quality standards are highly needed to declare soils affected by human induced pollution. This is particularly relevant for anthropogenic metals (Cd, Cu and Pb, and in some areas also Zn). Further research in other agricultural areas of the region would improve the basis for proposing such soil quality standards.

Introduction

Soil pollution has become an important environmental issue in developed countries (e.g., The Netherlands, United Kingdom, Spain) due to changes in the land use pattern (urban-industrial expansion, infrastructure development, and tourism increase) over the last few decades. When considering the different kinds of contaminants, heavy metals are especially dangerous because of their persistence and toxicity (Adriano, 2001). Metals can be transferred from soil to the other ecosystem components, such as underground water or crops, and can affect human health through the water supply and food web. In some areas with heterogeneous lithology, heavy metal contents can be highly variable, determined by the parent material and soil properties. For example, organic matter, clay and carbonates play a relevant role in the availability of heavy metals in calcareous soils (Sánchez-Camazano et al., 1998).

The natural input of several heavy metals to soils due to pedogenic processes has been exceeded in some local areas by human input, even on a regional scale. In particular, agricultural soils can be a long-term sink for heavy metals. Intensification of agriculture, encouraged in Europe by the Common Agricultural Policy (CAP) from the fifties, has resulted in the incorporation of several types of pollutants to soil, such as heavy metals, due to excess use of agrochemicals. These soils have also been influenced by other pollutant activities such as the use of manures, sewage sludge disposal or aerial fallout from industrial activities (Van Camp et al., 2004). As a consequence, potentially toxic elements have accumulated in the soil profile. This can result in a loss of soil functions concerning environmental quality protection, maintenance of human health and productivity, which are relevant aspects of soil quality (e.g., Doran and Parkin, 1994). Pollutant activities can have implications for the quality of agricultural soils, including phytotoxicity at high concentrations and the transfer of heavy metals to the human diet from crop uptake or soil ingestion by grazing livestock (Nicholson et al., 2003). However, heavy metal studies in agricultural soils are mainly referred to Northern European countries (e.g., Dudka, 1992, Zhang et al., 1999, Reimann et al., 2001) having a lack of information in Southern Europe, particularly at the Mediterranean region (Stanners and Bourdeau, 1995), where only few studies have been carried out at regional level in some countries such as Greece (e.g., Stalikas et al., 1997) and Italy (e.g., Facchinelli et al., 2001, Abollino et al., 2002). In the Spanish Mediterranean region, most of the studies on heavy metals in agricultural soils have concerned local areas (e.g., Boluda et al., 1988, Andreu and Gimeno-García, 1996, Pomares et al., 1998). This paper contributes to the knowledge of the content and potential source of heavy metals in agricultural soils of the Alicante province, which is a representative area of the Spanish Mediterranean region.

The knowledge of the heavy metal content in soils and the origin of these levels are priority objectives in the European Union (EU). At the start of 2002, the European Commission published a report “Towards a Thematic Strategy for Soil Protection” (EC, 2002) that established the basis and guidelines for maintaining or even improving soil resources quality. Recently, different working groups produced reports addressing the state of soils, impacts and pressures as well as recommendations for soil protection policy making 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 to assess the contamination risks and to identify background values as a basis to establish soil quality standards. Analysis of heavy metal concentrations in soils is, therefore, critical for policy making orientated at reducing heavy metal inputs to soil and guaranteeing the maintenance or even improvement of soil functions.

To establish reliable policies, it is necessary to have a good knowledge of the content and variability of heavy metals in soils, but also of the contribution of both parent rock and human activities. This is a difficult task in ancient populated areas such as the European Mediterranean region, where totally unpolluted soils are almost impossible to find. Multivariate analysis (principal component analysis – PCA and cluster analysis – CA) has been widely used to assist the interpretation of environmental data (e.g., Tuncer et al., 1993, Einax and Soldt, 1999) and to distinguish between natural and anthropogenic inputs (e.g., García et al., 1996, Facchinelli et al., 2001, Lucho-Constantino et al., 2005).

The present study was carried out as part of a survey on heavy metal contents in agricultural soils under vegetable crops on a regional scale supported by the Regional Government (Valencian Ministry of Agriculture, Fisheries and Food). The aims of the study were (1) to determine contents of Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn in soils; (2) to define their natural or anthropic source using multivariate analysis; and (3) to establish relationships between heavy metals and some soil properties.