Mapping the Soil Chemical Properties of the Sarigkiol Basin, Western Macedonia, Greece, in View of the Transition to the Post-Lignite Era ^†

Abstract

The chemical properties of soils in the Sarigkiol Basin, Western Macedonia, Greece, were studied to investigate the potential for new land uses in post-mining areas. An extensive dataset of chemical parameters was analyzed in a total of 34 representative soil samples from the Sarigkiol Basin. The data were processed using statistical methods (Spearman’s correlation coefficient (CC), factor analysis (FA), and hierarchical cluster analysis (HCA)) and thematic maps. Soil properties were assessed using different geoenvironmental indices (e.g., pollution load index (PLI), enrichment factor (EF), and geoaccumulation index (I_geo)). The results show that geogenic factors are crucial for soil chemical properties and that agricultural activities play an important role in soil contamination. The mapping of soil properties in such a mining area is of great interest for the next step of the transition to the post-lignite era and new land uses for sustainable rehabilitation.

1. Introduction

The great challenge in lignite mining areas is the rehabilitation of mining landscapes after the end of lignite mining. The success of rehabilitation of post-mining landscapes is determined by many factors, including aspects of landscape management, changes in land use, fertility of planting substrates, planting, and maintenance. All these factors depend on the soil’s physical, chemical, and biological properties [1]. Land use change, both for productive purposes and for biodiversity conservation, is closely related to soil chemical properties. Generally, soil contamination indices are used as tools to assess soil contamination or enrichment in potentially toxic elements (PTEs), to define soil properties.

The objective of this study was to map and assess soil chemical properties in order to evaluate the current state of the soils in the Sarigkiol Basin, Western Macedonia, Greece, where a change in land use is planned after lignite mining. This is the first time that soil contamination indices have been used in the mining area of the Sarigkiol Basin.

2. Case Study

The Sarigkiol Basin is located in Western Macedonia, Greece and covers an area of approximately 470 km². It is part of the Ptolemais–Kozani Basin, which is of great importance for energy production in Greece today. The main anthropogenic activities in the Sarigkiol Basin are: (a) lignite mining, (b) power plant operation (lignite combustion, fly ash production, transportation, and co-dumping with waste materials in dump sites), (c) agriculture, and (d) livestock farming [2,3].

The dominant geological formations in the Sarigkiol Basin are Triassic and Cretaceous limestones, Jurassic ultramafic rocks (mainly serpentinites and peridotites), Upper Cretaceous flysch, and Neogene–Quaternary sediments. The Quaternary sediments (sands, screes, clays, gravels, talus cones, breccias, and lacustrine deposits) overlie the Neogene formations (lignite series and alternating layers of sandy or clay marls, clays, silts, sands, and conglomerates). The ultramafic rocks are developed in a large area mainly in the Vermio Mountain (Mt.), the Scopos Mt., and the Askio Mt., supplying the lowland part of the basin with eroded ultramafic material (Figure 1) [3,4].

Ultramafic rocks are enriched in Cr, Ni, and Co; lignite, which is formed in such an environment, is also enriched in these elements. Fly ash, which is a by-product of lignite combustion at the Agios Dimitrios Power Plant, also has elevated concentrations of these elements [5]. The mineralogical composition of the soils in the Sarigkiol Basin, especially the high content of serpentine, indicates a high clastic input from the ultramafic rocks [5].

3. Materials and Methods

3.1. Soil Sampling and Chemical Determination

A total of 34 composite topsoil (5–20 cm) samples were collected from the Sarigkiol Basin. After vegetation was removed from each sampling site (<5 cm), a sample was obtained by assembling five separate subsamples from the corners and center of a total 25 m² square, in sealable plastic bags using a plastic spatula. All samples were processed and geochemically analyzed to obtain a large dataset including the most significant PTEs. The samples were dried at 40 °C at a constant temperature in a thermostatically controlled oven, gently crushed in a mortar with a ceramic pestle, homogenized and passed through a 2 mm sieve, and finally, pulverized in an agate ball mill to obtain an analytical grain size of <75 μm, where they were used for chemical analyses. Chemical analyses of the pulp soil sample (major and trace elements) were determined by lithium borate fusion and aqua regia, followed by X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) at the Analytical Laboratories of Bureau Veritas Commodities Canada Ltd., Vancouver, BC, Canada.

3.2. Data Processing

Statistics (i.e., Spearman’s correlation coefficient (CC), factor analysis using the principal component analysis (PCA) method, and hierarchical cluster analysis (HCA)) of the data were carried out using IBM SPSS v.22 software, to assess soil quality and show the origin of the PTEs recorded in the chemical analyses. Spearman’s rank CC is a non-parametric method that mitigates the possible influence of outliers. The“enrichment” of Cr, As, Hg, Sb, Ti, Cd, Co, Cu, Ni, Pb, Zn, V, and Ba in the soil samples was also assessed using different indices, such as the pollution load index (PLI), the enrichment factor (EF), and the geoaccumulation index (I_geo), which are also widely used to estimate the contamination levels of PTEs in the soils of the world (Figure 2) [6,7,8,9]. They are applied to assess environmental risks and calculate the degree of soil degradation. The background values of the average contents of these “enriched” elements in the Earth’s crust were used to calculate the indices PLI, EF, and I_geo [10]. Thematic maps were created using ESRI’s ArcGIS v.10.8 software. Figure 2 illustrates the most important information for these indices.

4. Results and Discussion

According to the chemical analyses, the most representative elements including the PTEs of the Sarigkiol Basin are (a) Cr from 320 to 753 mg/kg, (b) Ni from 216 to 690 mg/kg, (c) As from 4 to 30 mg/kg, (d) Ca from 27,658 to 345,700 mg/kg, (e) Mg from 4221 to 58,373 mg/kg, (f) Si from 12,574 to 241,151 mg/kg, (e) Mn from 77 to 1316 mg/kg, and (g) Fe from 5245 to 54,555 mg/kg.

4.1. Statistical Analyses

Out 34 soil samples were employed in the FA to establish the primary factors affecting the soil geochemistry in the Sarigkiol Basin. We selected 17 of 54 parameters for all samples included in the FA, HCA, and Spearman’s CC (Figure 3).

The soils of the Sarigkiol Basin are mainly enriched in Cr, Ni, and Co due to the ultramafic environment. This hypothesis was strengthened by Spearman’s CCs (i.e., Cr-Ni (0.84), Cr-Co (0.83), Ni-Co (0.88)), FA (Factor 1), and HCA (Cluster 1). Negative Spearman’s CCs were estimated for Cr-Sr (−0.44), Cr-Ca (−0.55), As-Ca (−0.48), As-Sr (−0.37), and As–Ba (−0.14).

4.2. Soil Contamination Indices

The PLI, is the index used to assess the pollution/enrichment load considering all PTEs, in each soil sample. The PLI values for the Sarigkiol Basin ranged from 0.78 to 1.90, with a mean value of 1.43. Four of the samples were classified as unpolluted (8.8%), while thirty were classified as low to moderately polluted (88%) (Figure 4). The samples characterized as unpolluted originated from the western part of the Basin, where agricultural activities are limited and the influence of the weathered material of the ultramafic rocks of the Vermion Mountain is less.

The EF examines the enrichment load emanating from each PTE and is based on the standardization of a tested element against a reference one. The most common reference elements are Sc, Mn, Ti, Al, and Fe [8]. In this study, Mn was used because the content is closer to the average content in the Earth’s crust. The values of EF for Hg, Sb, Ti, Co, Cu, Pb, Zn, V, and Ba were <2 for all samples and thus classified as ‘deficiency to minimal enrichment’. In the Sarigkiol Basin, the PTEs that contribute to the enrichment are Cr, As, Ni, and Cd (Figure 5).

The I_geo assesses the enrichment load and the level of contamination caused by each PTE by comparing the current state with the content of each element before extensive anthropogenic activities took place. The values of I_geo for Hg, Ti, V, and Ba were negative for all samples and are classified as ‘practically uncontaminated’. The values for Co, Pb, and Zn were <1 and are classified as ‘uncontaminated to moderately contaminated’. In the Sarigkiol Basin, the soils are enriched in Cr, As, Ni, and Cd. Figure 6 shows the percentage distribution of Cr, As, Ni, and Cd, which contribute most to the enrichment of the soil.

To summarize, the soil samples of the study area can be characterized as ‘unpolluted to moderately polluted’, with the main sources of enrichment being Cr, Ni, and As.

Combining the results of the statistical analyses (Spearman’s CC, FA, HCA), we conclude that:

• The PTEs Cr, Ni, and Co have high contents in the soils. The correlations between these PTEs reveal geochemical features that reflect the influence of ultramafic rocks on soils. In general, ultramafic rocks are enriched in Cr and other PTEs (i.e., Ni, Co, As); this has also been found in ultramafic soils in other areas of Greece such as the Psachna Basin [11]. Furthermore, all soil contamination indices highlight that Cr and Ni contribute to the “degradation” of soils in the Sarigkiol Basin.
• The elements As, Fe, Mn, and Mg are strongly correlated. Their contents are strongly influenced by the presence of ultramafic rocks and oxyhydroxides present NE of the Sarigkiol Basin. The hypothesis of the geogenic origin of As in the Sarigkiol Basin is consistent with the results of Petrotou et al. (2012) [12].
• The elements Si, Al, and K are associated with the weathering processes of aluminosilicate minerals and pedogenesis. Copper, Zn, Pb, Cd, and P can be attributed to anthropogenic activities such as intensive agriculture, through the use of phosphate (and other) fertilizers, in agricultural crops in the basin. Specifically, for Cd, soil contamination indices indicate low to moderate enrichment that can be attributed to the use of phosphate fertilizers containing Cd as an impurity [13].
• The lignite fly ash of the Agios Dimitrios Power Plant is enriched in Cr, Ni, Co, B, V, Sr, Sb, Zn, and Se [5,14]. However, the multivariate statistics (i.e., HCA, FA) and the very low contents of elements found in fly ash (e.g., Se) in all soil samples (<0.5 mg/kg) reveal that the operation of the Agios Dimitrios Power Plant has no influence on soil quality; previous soil geochemical studies from the study area confirm these results [13]. In addition, recently published research on the water quality of the Sarigkiol Basin supports the argument that intensive fertilization is the anthropogenic activity in the area associated with elevated PTE concentrations [2,3,14].

5. Conclusions

High levels of Cr, Ni, Co, and As and an elevated content of Cd were found in the soils of the Sarigkiol Basin, Western Macedonia, Greece. The general conclusion is that they are the result of a combined effect of the ultramafic environment and the use of fertilizers (Cd).

The geogenic Cr, Ni, Co, and As contents in the Sarigkiol Basin enhance the enrichment of the soils due to the weathering of ultramafic rocks. However, the characterization of the area as contaminated, or a low to moderate contaminated area according to the geo-environmental indices, is not correct in this situation, as the soils are naturally enriched. It is important to note, that these indices for calculating the contamination level take into account background thresholds that are not representative of all regions, as geological environments differ considerably.

Future land use should take into account the existing soil chemistry. It is important to note that there is a significant lack of regulations setting target values for soils depending on the geological environment and land use. This work highlights the need for a new approach, which will contribute to the efficient management of these soils. Finally, this work shows how systematic environmental monitoring can support authorities, individuals, and stakeholders in decision-making for an optimal use of the lands in post-mining areas.