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Article

Distribution of Selenium in the Soil–Plant–Groundwater System: Factors Controlling Its Bio-Accumulation

by
George D. Eliopoulos
1,
Ioannis-Porfyrios D. Eliopoulos
1,
Myrto Tsioubri
2 and
Maria Economou-Eliopoulos
3,*
1
Department of Chemistry, University of Crete, GR-70013 Heraklion, Crete, Greece
2
Department of Natural Resources Management and Engineering, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece
3
Department of Geology and Geoenvironment, University of Athens, 15784 Athens, Greece
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(9), 795; https://doi.org/10.3390/min10090795
Submission received: 29 July 2020 / Revised: 3 September 2020 / Accepted: 4 September 2020 / Published: 8 September 2020
(This article belongs to the Section Environmental Mineralogy and Biogeochemistry)
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Abstract

:
Selenium (Se) is an essential micronutrient for humans and animals, but both Se excess and deficiency can cause various health risks. Since Greece is among the European countries where people have very low Se-serum, the present study is focused on the Se distribution in cultivated and non-cultivated plants and relative soil coming from the Neogene basins of Greece (Assopos-Thiva and Attica), aiming to define potential Se-source/es and factors controlling Se bio-accumulation and enrichment in food. The dry weight Se values are relatively low (0.1–0.9 mg/kg) with the highest Se contents in garlic, beet and lettuce from the Assopos basin, where the translocation percentage [(mplant/msoil) × 100] for Se, P and S is much higher compared to non-cultivated Attica basin. There is a diversity between the Se source in soil and coastal groundwater which is used for irrigation in the cultivated Assopos–Thiva basin. The soil pH and oxidizing conditions (Eh) are considered the main driving force to make Se available for plant uptake. Potential sources for Se in Greece are Fe-Cu-Zn-sulphide ores and peat deposits in northern Greece, with a Se content ranging from decades to hundreds of mg/kg. Application of the leaching testing protocol is necessary to select the most appropriate proportion of additives to improve the Se deficiencies in agricultural soil.

Graphical Abstract

1. Introduction

Selenium (Se) is one of the elements playing a crucial role in human and animal health and is essential to all other organisms including bacteria and algae [1,2,3]. It is an essential micronutrient for humans and animals, but both Se excess and deficiency, due to consumption of Se-rich or Se-deficient food, can cause various health risks [4]. Selenium is highly mobile in the Earth’s, crust ranging from 0.01 to 0.5 mg/kg. It is slightly higher in mafic–ultramafic rocks (0.01–0.12 mg/kg) compared to acid rocks (0.01–0.05 mg/kg). Elevated Se contents are found in coals and shales, due to the great affinity of Se to different organic compounds [5,6], in sulphide, ores associated with ophiolites [7,8] salts in sedimentary rocks of marine origin and soils [9,10,11,12,13,14] and in soils surrounding sulphide deposits [15]. Besides total element content in soil, the oxidation state is of particular significance, due to the diverse nature of biological interactions like toxicity, mobility, bio-availability and bio-accumulation. Selenium in nature can be found to +4 oxidation state, existing mainly as inorganic selenite (SeO32−) anions or +6 oxidation state existing as selenate (SeO42−) anions, while organic compounds exist as Se-methionine, Se-cysteine, methyl-selenides [1,3,6,9]. Both forms of Se differ in terms of their mobility and absorption or transfer within the plant where they are metabolized to form Se-compounds [1,9,10,11,12,13,14]. Selenate is the dominant form of bioavailable Se in agricultural soils and more soluble than selenite in water [9,10,11,12,13,14,15,16].
Previous studies have focused on the accumulation and identification of the sources for heavy metals, with emphasis on hexavalent chromium, (Cr(VI) and other heavy metals in the Assopos-Thiva, Evia, Attica, Argolis (Peloponnese) and soils elsewhere (Figure 1) [17,18,19,20,21,22,23,24,25,26,27,28,29,30]. The assessment of the environmental impact of As and heavy metals in lacustrine travertine limestone and soil in Attica, and mapping of potentially contaminated sites have been investigated as well [31,32,33]. According to the above authors, those agricultural areas are representative of the Mediterranean region, and they highlighted a synergetic impact on the agricultural soil–groundwater–plants system by natural processes, anthropogenic agents, due to long time of uncontrolled application rates of fertilizers and pesticides–fungicides, and industrial activities. The Se level in groundwater affected by Se-bearing rocks/ores [34,35] or the intrusion of seawater is well known [17,18,19,20,30]. The low Se in foods produced and consumed in Europe was emphasized [36]. Among the lowest reported serum Se status of many studies in European are Albanian adults living in Greece, with 37.4 μg/L [36,37]. However, the analytical data concerning the Se distribution and evaluation, as well as potential Se-sources in Greece, are still lacking. Since the rate of uptake and the bio-accumulation factor of metals and metalloids in crops depends on their availability in soils, organic matter and their oxidation state [9,10,11,12,13,14,36,37,38,39], the present study is focused on the Se distribution in cultivated and no-cultivated plants from two different types of Neogene basins of Greece. Specifically, new analytical data on the Se contents in plants and relative soil, coming from the Neogene Basins of Assopos-Thiva, and the yellow–brown soil associated with the travertine limestone, covering many places throughout Attica, are presented. They are combined with previously published data on heavy metals and metalloids, in an attempt to define the potential source/s and factors controlling the Se-accumulation and enrichment of foods.

2. Materials and Methods

Study Area and Sampling Procedure

The presented analytical data are coming from the Neogene Basins of (a) Assopos-Thiva, composed of Quaternary and Neogene sediments, with an extension of approximately 700 km2 in the Assopos area and 150 km2 in the Thiva valley, located north of the Assopos. Both are composed of alternations of alluvial, marls, marl limestones, and continental sediments consisting of conglomerates derived from carbonates, mafic/ultramafic rocks and Fe-Ni laterites [40,41]; (b) the yellow–brown soil associated with the travertine limestone that is extended in many places throughout Attica, giving rise to a significant impact on or risk to human health and ecosystems [31,32,33]. More than 50 representative surface (up to 20 cm) soil samples from cultivated and no-cultivated areas have been collected, during the year of 2009 and spring of 2014, covering some sites of the basin from the rhizosphere of plants. They were air-dried, crumbled mechanically and then passed through a sieve with a 2-mm mesh. The elements Se, Cu, Pb, Fe, Mn, Zn, K, Na, Ni, Ca, Mg, P, S were analyzed by Inductively Coupled Plasma Mass Spectroscopy (ICP/MS) after Aqua Regia Digestion at ACME Laboratories Ltd., Vancouver, BC, Canada. (Table 1). The detection limit (Det. limit) of the method is presented in Table 1. Plant samples were analyzed after cleaning and drying at 70 °C. They were powdered in an agate mortar and analyzed by Inductively Coupled Plasma Mass Spectroscopy (ICP/MS), after Aqua Regia Digestion, at the ACME Analytical Laboratories in Canada. The detection limit (Det. Limit) of the method is given at the end of Table 2. The organic matter was determined by igniting the oven-dried samples (from moisture content determination) in a muffle furnace at 440 °C for 3 h at the National University of Athens [42]. Since the availability and mobility of Se depend on the soil Se speciation, the bio-accumulation of Se was determined as the ratio of the Se taken up by plants relative to the total Se content in the relative soil (Tables 3 and 4). Average values for groundwater samples collected from domestic and irrigation wells and municipality of the Assopos-Thiva and Attica basins (during both wet and dry seasons), which have been analysed for many elements (Al, As, B, Ba, Cu, Zn, Fe, K, Li, Mn, Na, Ni, P, S, Se, Si, V), hexavalent chromium, Cr(VI), and physical and chemical parameters of the water samples, including total dissolved solids (TDS), are given (Table 5).

3. Distribution of Selenium in Neogene Basins of Greece

3.1. Soils of the Assopos-Thiva and Attica Basins

The mineralogical composition of soil samples from the rhizosphere of plants was investigated in previous studies by optical microscopy, X-ray diffraction, using a Siemens Model 5005 X-ray diffractometer (Bruker AXS GmbH., Karlsruhe, Germany), Cu Ka radiation at 40 kV, 40 nA, 0.020 step size and 1.0 s. step time and SEM-EDS analyses, using a JEOL JSM 5600 scanning electron microscope (Tokyo, Japan), equipped with ISIS 300 OXFORD automated energy dispersive analysis system (Oxford shire, UK) and Energy Dispersive Spectroscopy (EDS) analysis. The soil samples from the Assopos-Thiva basin are mostly composed of rounded fragments of quartz, calcite, serpentinite [Mg3Si2O5(OH)4], chlorite [(Mg,Al,Fe)12(Si,Al)8O20(OH)16] as well as chromite [(Mg,Fe2+)(Cr,Al, Fe3+)2O4], Fe-chromite, magnetite (Fe2+)(Fe3+)2O4, goethite α-Fe3+O(OH), (Mn-Fe-Ni-Co)-(hydro)oxides, ilmenite (FeTiO3), rutile (TiO2), zircon (ZrSiO4), calcite and epigenetic minerals of rare earth elements. Micro-organisms, with various morphological forms, mostly filament-like and occasionally spherical to lenticular bacteria [21,22,23]. The soils from the Attica basin are mostly composed by calcite (Mg-free or with varying Mg-content), dolomite, siderite quartz, goethite, hematite, Mn-(hydro)oxides, apatite (fluor-hydroxylapatite), rutile and rare earth element (REE) minerals; Cr-spinel, serpentine, chlorite, muscovite are present in lesser amounts [31,32,33].
The Se contents in soil samples from the Assopos-Thiva basin are <0.5 mg/kg (Table 1), while in soils from the Attica basin, contents range from less than 0.5 to 1.0 mg/kg [43,44,45,46]. Those soils are much poorer compared to the Se contents reported in soil samples surrounding sulphide outcrops in distances up to 500 m, from the Argolida (Ermioni) sulphide mine [15] and the Othrys ophiolites hosting the Cyprus-type sulphide mineralization (Table 1, [8,15,43,44]). The highest Se values of 44.0 mg/kg were found in soils from both Argolida and Othrys, while average contents are 8.3 ± 16 and 8.5 ± 14 mg/kl Se, respectively. The heavy metals Cr, Ni, Co, Mn and Fe are present in significant amounts, derived probably by natural processes from chromite and silicates (olivine, pyroxenes or serpentine) hosted in mafic-ultramafic rocks, as exemplified by the presented inter-element relationships (Figure 2). Calcium is the most abundant element in the Attica basin; sulphur content in soil samples ranges from less than 500 to 1800 ppm, while organic matter ranges between 0.8 and 17.0 wt% (Table 1).
Although replacement of lower than the detection limit (D.L.) values with their half should be used with caution, in general this replacement is often used in the environmental sciences [47]. Thus, assuming that Se content is 0.25 mg/kg (half of the detection limit) for soils with low Se content, the plots of Se versus Cu, Fe and Mn showed a positive correlation between Se and Cu and Fe, and a negative correlation with Mn (Figure 3). In addition, a negative correlation between Se and organic matter is better pronounced for the soil samples from the Attica Basin (Figure 3d).

3.2. Selenium Contents in Plants

Special attention was paid to the Se content in plants because of its importance in the food chain. In general, the dry weight Se values in shoots, ranging from 0.2 to 0.9 mg/kg (average 0.41 ± 0.17), are higher compared to those in roots, ranging from 0.2 to 0.5 mg/kg Se (average 0.38) and in bulb plants, ranging from 0.1 to 0.6 mg/kg Se (average 0.27 ± 0.15) in plants from the Assopos-Thiva basin (Table 2). The highest average values of the Se in cultivated plants from the Thiva were recorded in carrots (0.24 mg/kg Se) and the lowest in potatoes (0.16 mg/kg Se) (Table 2). With respect to the plant specious, the Se content is higher in garlic (0.55 mg/kg Se), confirming previous studies [48], while the lowest values were recorded in potatoes (Table 2). Relatively high Se content was recorded in beet as well, in particular in shoots (0.9 mg/kg Se) (Table 2). Non-cultivated plants from the valley of Attica exhibit higher contents in shoots (average 0.4 mg/kg Se) compared to relative roots (average 0.2 mg/kg Se) (Table 2).

3.3. Accumulation Factor

The accumulation or transfer factor is the ratio of the element content in plants relative to the total element content in the relative soil, and it mainly depends on the amount of elements in the soil and their mobility [9,10,11,12,13,14,49,50]. Thus, the calculated percentage of soil Se and other multi-oxidation state metals in plants (Table 3) and related plot diagrams (Figure 4) may provide information on the effect of physical/chemical parameters (pH, Eh), the concentrations of completing ions, organic matter content and other parameters. The translocation percentages for heavy metals (Fe, Mn, Cr, Ni, Co) is much lower, compared to those for Se, S and P, which are much higher in the Assopos basin compared to those for the Attica basin, and in roots compared to the shoots (Table 3). The plots of the percentage for Se versus that for heavy metals (Cu, Fe, Ni, Co, Mn), S and P (3a–d) show a negative correlation between Se and S for the Assopos-Thiva basin (Figure 4e) and a positive correlation between Se and P for both Assopos-Thiva and Attica basins (Figure 4f; Table 4). Since the mobility of Se trace elements and heavy metals in the environment is largely dependent on the pH and redox potential (Eh) a liquid system [9] a plot of the Eh versus pH, showing a negative correlation is given (Figure 5). Most agricultural soils in the Assopos-Thiva and Attica Basins have neutral to alkaline conditions and a redox potential (Eh) ranging from −93 to −61 mV in the Assopos-Thiva basin and from −93 to −73 mV in Attica (Figure 5; [22,31,32,51]).

3.4. Distribution of Se in Groundwater

Detailed studies have shown that the main aquifers in Greece are developed in carbonates and in alluvial formations, such as Quaternary and Neogene unconsolidated deposits, which supply large quantities of water [52,53,54,55,56,57]. The levels of Se in groundwater from the Assopos-Thiva basin range from less than 0.5 µg/L in the Mavrosouvala wells, which is pumping water from a karstic aquifer and it is used for the municipal water supply of the city of Athens, and some of the surrounding towns and villages in the area [17], to 32 µg/L (Table 5). In addition, the Se concentrations in groundwater from the Attica basin (Koropi) exhibit a wide variation, reaching as high as 88 μg/L during April (2009), after a wet period, and 210 μg/L, during November (same year) [30]. Although Se is relatively low compared to major components of coastal groundwater, it seems likely that the salinity, which is expressed by the total dissolved solids (TDS), can potentially affect the coastal groundwater Se levels. A plot of Se values versus TDS for several coastal groundwater samples from the Assopos-Thiva and Attica basins [17,18,19,20,21,22,23,33] exhibit a very good positive correlation (Figure 6), probably due to the contribution by seawater in the aquifers.
A salient feature of Neogene coastal aquifers is Cr concentrations over the maximum acceptable level for Crtotal in drinking water (50 μg L−1) reaching values of 900 μg/L in the Assopos basin and the elevated Se, Na, B, Cl, As, and Li concentrations (Table 5; [17,18,19,20,21,22,23,30,33]). The salinity of coastal groundwater, (TDS) has an upper limit for freshwater being at 1000 mg TDS/L, while in seawater is approximately 35 g/L [56]. There is a very good positive correlation between Se and (TDS) concentrations for coastal groundwater from the Assopos-Thiva basin and karst type from Attica (Figure 6).

4. Discussion

The Se status in a human depends on the daily dietary Se intake, which, in turn, depends on the amount of Se transported from soil to plant or food chain [1]. The presented analytical data show relatively low Se contents in cultivated and non-cultivated plants and the relative soils from their rhizosphere. However, the relatively wide variations recorded between (a) the Assopos-Thiva and Attica basins, (b) shoots and roots of plants and (c) some correlations between Se and other elements (Table 1, Table 2, Table 3, Table 4 and Table 5; Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5), may reflect the effect of pH, Eh, organic matter and metals on the adsorption of Se or its bio-accumulation.

4.1. Bio-Accumulation of Selenium

The accumulation factor is relatively high for Se (100 to 280%, average 190%), S (170–1130, average 440%) and P (400–1600, average 450%) in the Assopos basin (Table 3). Given that the H+ ions take up space on the negative charges along the soil surface, the soil pH, Eh (Figure 4) are considered to effect element availability, depending on the size and charge of the nutrient molecules and whether or not they can be lost by leaching [6,9]. Selenium is taken up by plants in the forms of selenate, selenite and organic compounds [1,6,9,10,11,12,13,14,58,59,60,61,62,63]. High Se mobility can be expected in neutral and alkaline soils (low H+ concentration), where the predominant species selenate (SeO42−) is weakly adsorbed to the surface of soil particles and easily available. In contrast, in acid soils selenite (SeO32−) salts seems to be strongly adsorbed by Fe oxides as ferric selenite, Fe2(OH)4SeO3, and probably clays and it is less available for plant uptake [1,6]. The negative correlation Se-organic matter for plant samples from the Attica basin (Figure 4f), and the negative correlation coefficient (r = −0.31) between accumulation factor Sep/Ses × 100 and organic matter (Table 4) suggest that selenite ion rather than selenate is the predominant species, under less oxidizing conditions (Figure 5) where Se is less available for plant uptake. In addition, the positive correlation between accumulation factor for Se and that for P (Figure 4f, Table 4) is consistent with the suggested transport mechanism of selenites by phosphate [58]. With respect to the negative correlation between accumulation factor for Se and that for S for the samples from the cultivated Assopos-Thiva basin (Figure 4e), it seems likely that there is higher selectivity for sulphate over selenate [56,62,63]. Elevated phosphate ion, [PO4]3− to soil may increase Se uptake by plants (Figure 4f), because this ion displaces selenites from soil particles, and thus increases Se mobility [6,60,64,65]. Nevertheless, it is well known that selenite (SeO32−) is the most thermodynamically stable and predominant species of Se in the soil under reducing conditions, whilst selenate (SeO42−) is the predominant species under oxidizing conditions [65].

4.2. Source(s) of Selenium

Selenium can be extracted as a by-product and its supply is directly affected by the supply of the main products, mainly from Cu-Ni-sulphide deposits [65]. The Se content of different food supplies (veges, cereals, meats, and fish) is related to dominant food sources, affected the Se content in the system plant-soil-groundwater, such as the weathered Se-bearing rocks and ores, volcanic activity, ashes produced during lignite combustion, Se-containing fertilizers, and groundwater [1,6,9]. Elevated Se contents in oxidized sulphide ores and associated soils in Greece have been reported in the Argolis (Ermioni), Othrys, Chalkidiki Peninsula [15,43,66,67]. The soil and plants (cultivated and non-cultivated) from the Assopos-Thiva and Attica basins are not in the vicinity of sulphide deposits and/or occurrences. They exhibit significant contents in heavy metals (Cu, Zn, Mn, Cr, Ni, Co) and much lower Se compared to soils from the areas of Ermioni and Othrys (Table 1). The effect of the transferred metals into soil from the neighboring weathered ophiolitic rocks and ores is obvious (Figure 2).
The composition of groundwater in the Assopos-Thiva basin is often characterized by elevated concentrations of Se, Na, B, Cl, As, and Li, which are characteristic of the seawater composition, in contrast to the much lower concentrations of these elements in drinking water, as exemplified by the water samples from the Mavrosouvala (Table 5). Although the Cr concentrations in seawater are less than 1μg/L, the Cr(VI) concentrations in coastal groundwater affected by the intrusion of seawater is often over the maximum acceptable level for Crtot in drinking water (50 μg/L), reaching values of hundreds μg/L (Table 5). Due to a diversity between the chemical composition of coastal groundwater and that of water leachates from soils and rocks/ores, which are common rock types in the aquifer pathway [22], it is obvious that the Cr in groundwater has been affected by additional sources, and that salinity may facilitate the Cr solubility.
The very good correlation between Se and the salinity of such a coastal groundwater (Figure 5), coupled with the level of these elements in sea water (Table 5) may provide evidence for the origin of Se toward the seawater rather than weathering of Se-bearing rocks and/or the interaction between water and rocks. Thus, it is obvious that, regarding the Se-source, there is a diversity between the Se source in soil and coastal groundwater that is used for irrigation in the cultivated Assopos-Thiva basin. Moreover, the existence of elevated Cr(VI) concentrations in groundwater samples from Evia and Assopos-Thiva (Table 5) implying oxidative and alkaline pH (pH > 7) conditions, may suggest that selenate (SeO42−) ion is the predominant species.
In addition to natural Se-elevated rocks/ores, there are other Se-sources which may contribute to the reduction in the Se deficiency in agricultural regions. More than 50% of the electricity demand in the United States is derived from coal-fired power plants, resulting in the production of approximately 134 million tons of coal combustion by-products, mainly fly ash [68]. The U.S. EPA and several state environmental agencies require the coal combustion by-products to be subjected to the EPA toxicity characteristic leaching procedure to determine if the material can be applied without concern for groundwater and surface water contamination [69]. Although there is not always a linear relationship between leachate metal concentrations and fly ash content, and the application of a leaching protocol based on simple dilution factors, correcting for fly ash content, are often not accurate, an integrated testing protocol is required to select the most appropriate technique [27,68,69].
In Greece there are two main coal-mining districts in production in Ptolemais-Amynteon Lignite Center (Northern Greece) and Megalopolis in the Peloponnese (Southern Greece) (Figure 1; [27,70,71,72,73,74]). From the investigation of the chemical and mineralogical composition of fly and bottom ash from the lignite combustion and leaching experiments, applying these material on agricultural land [71,73], it was concluded that the addition of a small proportion of 5 wt% fly ash in acid soil, can increase the pH to 7.3–7.8, and the Ca, Mg, K and Na content of the soil. Although these elements can be classified as essential nutrients, the co-existing trace elements such as As, Cr, Cu, Ni, Zn, Hg, are considered to have some environmental or public health impacts, including potential groundwater contamination and should be used with caution for agricultural applications [69].

4.3. Implications to the Increase of the Se Uptake by Crops

Soil Se-deficiencies in agricultural regions can be improved by adding Se-compounds to fertilizers [6,71] or natural Se-elevated materials, such as the peat from the Se-elevated Philippi peat deposit, Eastern Macedonia (Greece), having relatively high Se contents, with a small variation throughout the deposit, ranging from 240 to 310 mg/kg (average = 270) [74]. Given that high Se mobility can be expected in neutral and alkaline soils (low H+ concentration), where the predominant species selenate is weakly adsorbed to the surface of soil particles, such additives may facilitate Se translocation from soil to plant. Therefore, applying the leaching testing protocol in order to select the most appropriate proportion of additives, the Se-deficient soils in agricultural regions can be improved, and the toxicity of heavy metals and metalloids can be avoided.

5. Conclusions

The combination of the presented Se contents in the system soil-plants-groundwater with literature data on the system contamination by heavy metals and metalloids led us to the following conclusions:
  • The dry weight Se values are relatively low (0.1–0.9 mg/kg) and they are higher in the cultivated plants from the neutral-alkaline soil of the Assopos basin with respect to non-cultivated plants from the Attica basin, and higher in shoots than in roots of the plants. The highest Se content was recorded in garlic (bulb), beet (shoot) and lettuce (shoot);
  • The translocation percentage [(mplant/msoil) × 100] for Se, P and S is much higher in the cultivated Assopos basin compared to that for the non-cultivated Attica basin;
  • A negative Se-organic matter correlation suggests that under less oxidizing conditions the reduction in Se in soil makes Se less available for plant uptake. A negative trend between Se and Fe for the soil samples from the Assopos-Thiva and Attica basins, in contrast to the positive trend for the Se-bearing soils from Ermioni and Othrys, may be related to the alkaline soil in the former (the predominant species are selenite ions, SeO42−), and the presence of sulphides (low pH, with predominant selenite, SeO32− salts) in the latter;
  • The soil pH and the oxidizing conditions (Eh) are considered to be the main driving force to make Se available for plant uptake;
  • There is a diversity between the Se source in soil and coastal groundwater;
  • Potential sources for Se in Greece are Fe-Cu-Zn-sulphide ores, peat deposits with average = 270 mg/kg Se content, but the application of the leaching testing protocol is necessary to select the most appropriate proportion of additives, in order to improve the Se deficiencies in agricultural soil.

Author Contributions

Conceptualization, M.E.-E.; methodology, M.E.-E., G.D.E., I.-P.D.E. and M.T.; software, G.D.E. and I.-P.D.E.; validation of data, M.E.-E., G.D.E., I.-P.D.E. and M.T.; writing—original draft preparation, M.E.-E. All authors have read and agreed to the published version of the manuscript.

Funding

The financial support by the Municipality of Oropos, Greece (Grand No AK 70/3/9997) and the University of Athens (Grant No ΚΕ 11078) for of the work in the Assopos-Thiva and Attica basins is greatly acknowledged.

Acknowledgments

Many thanks are due to all authors included in previous publications (list of references) for their valuable data on the contamination by heavy metals and as in the studied Neogene basins. Many thanks are expressed to the Acad. editor and two anonymous reviewers for their constructive criticism and suggestions on an earlier draft of the manuscript. Many thanks are due to the editorial staff of Minerals.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 10 00795 g001 550
Figure 1. Sketch map of Greece showing the studied and other Se-bearing the locations.
Figure 1. Sketch map of Greece showing the studied and other Se-bearing the locations.
Minerals 10 00795 g001
Minerals 10 00795 g002 550
Figure 2. Plot of the Ni versus Cr, Co and Mg (a–c) and Al vs Fe contents in soils from the Assopos–Thiva and Attica basins. Data from Table 1. (a) Cr vs. Ni; (b) Co vs. Ni; (c) Mg vs. Ni; (d) Al vs. Fe.
Figure 2. Plot of the Ni versus Cr, Co and Mg (a–c) and Al vs Fe contents in soils from the Assopos–Thiva and Attica basins. Data from Table 1. (a) Cr vs. Ni; (b) Co vs. Ni; (c) Mg vs. Ni; (d) Al vs. Fe.
Minerals 10 00795 g002
Minerals 10 00795 g003 550
Figure 3. Plot of the Se vs. Cu, Mn, Fe and organic matter (M.) contents (a–d) in soils from the Assopos-Thiva and Attica basins, as well as the Ermioni and Othrys areas. Data from Table 1.
Figure 3. Plot of the Se vs. Cu, Mn, Fe and organic matter (M.) contents (a–d) in soils from the Assopos-Thiva and Attica basins, as well as the Ermioni and Othrys areas. Data from Table 1.
Minerals 10 00795 g003
Minerals 10 00795 g004 550
Figure 4. Plot of the percentage for Se versus that for Cu, Fe, Ni, Co, S, P, Mn and organic matter (a–h) in soils from the Assopos–Thiva and Attica basins. Data from Table 3.
Figure 4. Plot of the percentage for Se versus that for Cu, Fe, Ni, Co, S, P, Mn and organic matter (a–h) in soils from the Assopos–Thiva and Attica basins. Data from Table 3.
Minerals 10 00795 g004
Minerals 10 00795 g005 550
Figure 5. Plot of the Eh versus pH for soil samples from the Assopos-Thiva and Attica Neogene basins of Greece. Data from [22,33,51].
Figure 5. Plot of the Eh versus pH for soil samples from the Assopos-Thiva and Attica Neogene basins of Greece. Data from [22,33,51].
Minerals 10 00795 g005
Minerals 10 00795 g006 550
Figure 6. Plot of the Se versus total dissolved solids (TDS) concentrations for coastal groundwater from the Assopos-Thiva basin and karst type from Attica. Data from [17,18,19,20,21,22,23,30,33].
Figure 6. Plot of the Se versus total dissolved solids (TDS) concentrations for coastal groundwater from the Assopos-Thiva basin and karst type from Attica. Data from [17,18,19,20,21,22,23,30,33].
Minerals 10 00795 g006
Table
Table 1. Trace element content in soils from Neogene Basins of Greece, in comparison to Ermioni and Othrys soil affected by sulphide mineralization.(Present study *, [15,21,22,23,31,32,33]).
Table 1. Trace element content in soils from Neogene Basins of Greece, in comparison to Ermioni and Othrys soil affected by sulphide mineralization.(Present study *, [15,21,22,23,31,32,33]).
mg/kgwt%
SeCrNiZnCuMnCoFeAlCaMgSOrganic M.
Assopos Basin
Ass.10.252004706028910392.961.677.7420.060.86
Ass.20.251904806228860361.811.396.851.89<0.050.82
Ass.30.252004506047920392.821.597.681.890.060.97
Ass.40.251503207150920313.311.807.211.740.061.04
Ass.50.251303007036860282.961.577.061.70<0.051.22
Ass.60.251904705726860362.901.627.282.05<0.050.80
Ass.70.251302607637810293.021.847.381.62<0.051.37
Ass.80.25671306628690192.691.255.601.14<0.050.70
Ass.90.2518047011540880372.841.518.711.68<0.051.16
Ass.100.251403405841680302.191.047.191.70<0.051.40
Ass.110.251403304730670292.100.996.841.610.051.90
Ass.120.251302609341800303.051.797.201.64<0.051.20
Ass.130.25891904924660212.391.206.140.83<0.051.15
Ass.140.251302906531830282.921.457.011.61<0.051.20
Or.1 *0.2550508723827101.531.072.020.480.1817.0
Or.2 *0.2554528427846131.971.462.070.480.159.00
Or.3 *0.25675263361120193.232.586.520.360.170.98
Or.4 *0.25705166341060183.342.774.950.320.150.90
Thiva Basin
Th10.2523076070351000504.392.821.992.38<0.053.81
Th20.252407106031990464.252.893.802.74<0.053.71
Th30.252701650623312601105.601.970.933.41<0.055.60
Th40.25340115050361100734.521.733.137.10<0.053.18
Th50.25340140048301200824.771.621.116.17<0.053.64
Th60.25320140048311140804.581.451.506.18<0.053.79
Attica Basin, travertine-bearing
Att.1 *0.2515010076261100192.504.1012.000.800.059.10
Att.2 *0.25190160170301400294.107.101.200.800.0510.50
Att.3 *0.25140988922670182.905.1011.000.800.086.20
Att.4 *0.256404106264620333.003.7011.002.600.073.40
Att.5 *0.6162816439937233.361.8812.981.060.054.10
Att.6 *0.5455511820414342.061.4511.280.390.115.70
Att.7 *0.2531488015302231.570.9417.610.340.054.20
Att.81.0415418019670142.370.9014.660.250.051.30
Att.90.9425324018635132.400.8818.850.170.061.40
Att.101.0355224019595132.300.7219.170.170.051.00
Att.110.9476426523710142.900.9617.490.190.051.40
Ermioni4419136421201301017.4
9.060361503170500603.20
1470441202750450707.40
1058583602660330606.20
7.15867620438014121208.50
1.66342951500540221.10
2.95868113127202632312.60
5.4577075281601180226.70
2.942518556961580263.00
6.966716704000247014015.30
3.7784513019601200493.00
3.8797490124002100333.00
8.8261946010001001715.8
4.45034410860500273.70
3.1749311041701900312.50
4.247711106570520173.80
Othrys0.77876193062033407010.90
4.510029032055019504710.50
0.64463100701800272.10
4411025143018002203221.0
0.9153039012017101900535.90
7.5160120152022505804414.30
5.29725097018306503811.30
4.7901708801650460319.40
Det. Limit0.51.00.11.00.11.00.10.010.010.010.010.05
Table
Table 2. Trace elements contents in cultivated and no-cultivated plants from Neogene Basins of Greece. (Present study *, [21,31,32,33,44,45]).
Table 2. Trace elements contents in cultivated and no-cultivated plants from Neogene Basins of Greece. (Present study *, [21,31,32,33,44,45]).
mg/kgwt%
SAMPLESSeCrNiMnFeZnCuPbCoKNaMgCaSP
Cultivated
Assopos basin
Onion-shoot0.31.82.06012246.00.23.84.10.1000.6302.00.330.680
Onion-bulb0.40.72.9222.0396.00.12.32.30.0400.1600.50.400.530
Garlic-shoot0.51.41.83414283.80.22.03.20.0300.6801.70.390.530
Garlic-bulb0.60.73.1154.0468.10.16.11.70.0200.1200.40.600.520
Cocozelle-shoot0.31.314521534100.13.84.50.0000.8902.50.370.740
Beet-shoot0.90.81.31158.0117.10.42.43.98.2001.4701.10.480.170
Beet-root0.20.60.5233.0127.70.02.72.71.4700.2000.10.190.180
Beet-root0.30.92.0299.021120.15.33.10.9400.2800.20.180.380
Lettuce-shoot0.613.2491548100.41.06.91.0500.7100.80.340.540
Cabbage-shoot0.30.87.7225.0152.50.01.93.00.1200.2000.50.500.270
Leek-shoot0.20.71.6165.0174.70.12.04.00.1600.1800.30.500.310
Leek-root0.58.0112370198.30.31.93.51.6300.8000.50.810.260
Leek-shoot0.20.61.3176.0204.40.12.42.90.0400.1100.10.450.440
Leek-root0.35.48.040120257.40.71.43.31.0400.7700.40.510.350
Beet-shoot0.63.7101382038100.71.66.14.3701.2300.80.390.430
Lettuce-shoot0.21.32.7311717100.32.97.60.1900.3201.00.220.590
Spinach-shoot0.42.65.0455468100.52.25.91.5101.3000.60.380.600
Cocozelle-shoot0.30.913561737100.14.04.50.0040.8922.50.370.742
Coccozele-shoot0.40.6113415237.30.13.76.90.0190.9311.90.270.554
Coccozele-shoot0.30.58.3211818130.10.58.00.0130.5191.10.260.623
Lettuce-shoot *
O.shoot10.67.51.911788042120.40.376.20.9800.4161.650.490.372
O.root10.56.81.812370043110.50.465.10.7950.4451.840.430.307
O.shoor1, org.0.62.20.857160337.90.50.316.20.7550.1800.500.340.404
O.root, org.0.413.11.6135360521931.70.2911.00.1470.3211.010.400.448
O.shoot20.74.11.24745041110.30.497.20.6350.2790.940.500.315
O.roo20.46.01.611257042120.50.394.80.7220.3611.600.430.298
O.shoot2, org0.62.10.94019039120.50.375.40.9900.1700.450.300.479
O.root2,org0.43.70.711015049120.20.1410.50.2140.3210.990.380.482
Thiva basin
Carrot10.32.03.21040197.60.203.23.010.2300.2100.390.130.280
Carrot20.32.03.3840197.10.010.53.300.2300.2100.380.120.320
Carrot30.22.03.0740197.30.010.43.500.3200.2100.380.140.290
Carrot40.32.03.3850187.40.040.83.100.2300.1900.380.140.290
Carrot50.12.45,2740145.10.070.62.190.0500.1200.040.170.280
Potatoes 10.22.16.2790123.00.174.32.350.0100.1500.070.220.360
Potatoes 20.12.15.1850265.70.081.12.330.0100.1500.070.190.320
Potatoes 30.22.45.0840327.61.371.32.310.0050.1200.040.160.280
Potatoes 40.12.36.31230378.60.280.32.030.0040.1200.040.160.260
Potatoes 50.22.28.61450473.90.590.52.100.0250.1200.040.210.260
Onion10.44.61818340327.61.401.32.630.2100.2600.240.630.410
onion20.12.6201260378.60.300.31.330.2800.1900.200.790.480
onion30.22.17.81430473.90.600.51.780.1100.1600.140.490.330
No-cultivated
Attica
Att.1 *0.13.11.320240173.81.0311.00.0430.0580.360.110.027
Att.2 *0.127721705401458.60.2220.91.9570.2841.760.100.040
Att.3 *0.22.11.439190418.40.7120.70.0440.0530.220.210.162
Att.4 *0.13.31.1142709.64.40.77.30.60.1010.1120.950.060.031
Att.5 *0.2272049160038161.2462.90.0610.4182.190.310.259
Att.7 *0.11.42.528100413.70.20.82.00.0400.1101.540.50.330
Shoots
ALF10.58.47.18413047162.45.01.01.1710.2432.390.270.138
ALF20.32.41.94251107141.75.82.81.9600.3191.470.460.184
ALF40.33.63.0349966242.84.72.40.0890.1991.830.180.126
ALF50.62.86.811864597.71.55.81.90.0120.0770.740.360.096
ALF60.34.97.3251190586.65.15.71.40.1060.0881.240.240.087
ALF70.35.47.326625015018105.11.20.3070.1302.130.260.142
ALF80.51.82.47769576.81.84.03.40.5770.1544.571.380.319
Roots
ALF10.28.86.14310839100.650.80.8100.1231.180.140.080
ALF20.22.72.0236253132.0221.10.6140.1270.830.120.111
ALF40.33.02.7357266711.76.11.60.0980.3991.230.210.262
ALF50.311201973771261314471.00.0320.0932.550.130.075
ALF60.2202441187515710532210.70.0480.1044.830.090.068
ALF70.12.72.15280396.81.7310.40.0690.0340.410.060.078
ALF80.12.51.93066408.81.1113.20.5570.1051.010.700.202
Det. Limit0.10.10.11.00.0010.10.010.010.010.010.0010.0010.010.010.001
Ref. Material
FLOUR<10.80.3324253.20.220.020.320.0010.130.030.180.376
STD V14500.81.5213017144.30.790.770.520.0020.080.620.070.092
STD V163837510756535377.32.891.240.240.0020.0560.280.020.052
Table
Table 3. Percentage of soil Se and metals in plants from Neogene Basins of Greece. Data from Table 1 and Table 2).
Table 3. Percentage of soil Se and metals in plants from Neogene Basins of Greece. Data from Table 1 and Table 2).
Location(mp/ms) × 100Soil
AssoposSeCrNiZnCuMnCoFeSPOrganic Matter
Ass.11302.31.596204.97.10.166307501.04
Ass.21301.00.924283.6100.059004001.22
Ass.32309.44.671269.75.82.0028010400.4
Ass.41000.54.448283.86.40.065205100.82
Ass.51005.81.82617106.30.1511304201.35
Ass.61671.21.129238.5100.025202801.2
Ass.71700.60.560222.8112.607805801.22
Ass.81170.60.553214.57.90.026107600.9
O.shoot12808.22.4474865.02.002704100.9
O.root1240153.84852144.05.802804700.9
O.shoot22809.03.16733102.21.802509700.8
O.root2200103.56831112.82.2025310000.8
O.shoot1, org2404.41.6383523.01.0016750617
O.root1, org2404.23.260830163.02.0019057017
O.shoot2, org2004.21.47435100.61.0022716009.0
O.root2, org1606.81.04031172.30.602534309.0
Thiva
Th.1771.20.781161.30.83.1081012703.8
Th.21001.00.59.2540.92.00.092606104.0
Th.3670.60.441140.72.00.113606103.5
Attica
Att.1332.11.323151.81660.80550389.0
Att.2251.61.426103.81181.2043010010.5
Att.3172.41.111202.1411.00200786.2
Att.4290.50.318142.4360.677004103.4
Att.5620.50.633106.23.20.285005703.8
Att.6141.11.81371433.50.318303805.1
Shoots
ALF12505.28.873429223.905001654.1
ALF2385.33.79172108.22.404204305.7
ALF44312448216011206.403603704.2
ALF5676.813334218412.707204801.3
ALF61201214244339447.904804401.4
ALF712015146297454010.95204701.0
ALF82002.83.8223011282.40276016001.4
Roots
ALF140105868362511008.356584.1
ALF2672910050935538012427695.7
ALF41008390100296103130721202084.2
ALF5504.031021416217081074536781.3
ALF66740033027016016037046038781.4
ALF733503026391946103223551.0
ALF8201407970132392759451631.4
Table
Table 4. Correlation matrix of percentage of soil elements in plants from Neogene Basins of Greece. Data from Table 1 and Table 2. Symbols: mp—element content in plant; ms—element content in soil: O.M.—organic matter.
Table 4. Correlation matrix of percentage of soil elements in plants from Neogene Basins of Greece. Data from Table 1 and Table 2. Symbols: mp—element content in plant; ms—element content in soil: O.M.—organic matter.
mp/ms × 100
AssoposSeCrNiZnCuMnCoFeSPO.M.soil
Thiva
Se1.00
Cr0.701.00
Ni0.530.651.00
Zn0.210.170.271.00
Cu0.280.010.230.071.00
Mn0.510.670.480.130.431.00
Co−0.13−0.31−0.11−0.18−0.16−0.191.00
Fe0.500.670.410.340.140.33−0.221.00
S−0.61−0.43−0.36−0.09−0.27−0.330.58−0.221.00
P0.050.080.080.68−0.10−0.01−0.510.19−0.171.00
O.M.soil0.24−0.07−0.09−0.050.610.21−0.45−0.03−0.440.021
cont. mp/ms × 100
AtticaSeCrNiZnCuMnCoFeSPO.M.soil
Se1
Cr−0.081
Ni−0.090.691.00
Zn−0.060.660.871.00
Cu0.040.440.600.561.00
Mn−0.080.520.740.570.521.00
Co−0.220.290.720.460.330.851.00
Fe−0.080.430.940.800.470.700.781.00
S0.49−0.31−0.38−0.30−0.37−0.42−0.42−0.301.00
P0.49−0.28−0.33−0.26−0.22−0.34−0.43−0.270.931.00
O.M.soil−0.31−0.29−0.34−0.27−0.28−0.48−0.27−0.32−0.11−0.301
Table
Table 5. Average composition of groundwater from the Assopos-Thiva and Attica basins [17,18,19,20,21,22,23,33].
Table 5. Average composition of groundwater from the Assopos-Thiva and Attica basins [17,18,19,20,21,22,23,33].
Locationnμg/Lmg/Lg/LpH
CrC(VI)SeLiAsBCaMgSNaKTDS
Drinking Water
Mavrosouvala3<1<1<0.52.52.41890.3174140.60.317.4
Assopos Basin
Avlida1070649.31644403699.837310111.137.4
Oropos36157323312038.555.3151091.60.597.5
OroposAs.K.W.9008508.8324.513040.2140604082.51.427.7
Attica Basin
Attica, Athens199.791.96.43.5431202015442.50.437.3
Attica, Koropi3112101088.413013555372178.80.487.4
Sea WaterEvoikos Gulf<1<1360160804200390120012006400490357.9
Det. Limit 1.01.00.50.10.55.00.050.051.00.050.05

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Eliopoulos, G.D.; Eliopoulos, I.-P.D.; Tsioubri, M.; Economou-Eliopoulos, M. Distribution of Selenium in the Soil–Plant–Groundwater System: Factors Controlling Its Bio-Accumulation. Minerals 2020, 10, 795. https://doi.org/10.3390/min10090795

AMA Style

Eliopoulos GD, Eliopoulos I-PD, Tsioubri M, Economou-Eliopoulos M. Distribution of Selenium in the Soil–Plant–Groundwater System: Factors Controlling Its Bio-Accumulation. Minerals. 2020; 10(9):795. https://doi.org/10.3390/min10090795

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Eliopoulos, George D., Ioannis-Porfyrios D. Eliopoulos, Myrto Tsioubri, and Maria Economou-Eliopoulos. 2020. "Distribution of Selenium in the Soil–Plant–Groundwater System: Factors Controlling Its Bio-Accumulation" Minerals 10, no. 9: 795. https://doi.org/10.3390/min10090795

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