Soil
Contents of risk metals in soils
Lands of localities from which soil and plant samples were taken belong to agricultural lands.
Soil reaction is one of the factors that most affects the behaviour of heavy metals in soil. Low pH values pose a risk of reduced nutrient intake and increase the availability of heavy metals for plants29,30.
The presence of risk elements in the soil was evaluated based on their contents in bioavailable form (mobile forms), determined in soil extracts NH4NO3, and the total contents of risk elements were determined in soil extract by aqua regia (Table 1).
Accessible heavy metals for plants are those which are present in the soil solution as soluble components or those which are easily dissolved by root exudates31. The highest Cu contents determined in soil extract by NH4NO3, were in the cadastre of Gemerská Poloma (max. 0.390 mg/kg) (Table 1). However, even the highest determined concentration of Cu in its bioavailable form did not exceeded the determined critical value for this element18. Nickel is a beneficial element for plants. Elevated Ni concentrations in soils have a potential negative effect on plants32. Content of bioavailable forms of nickel is lower than the determined critical value in all analysed samples. Cadmium and lead present a risk to agricultural activity in this area. Cadmium in soil is highly bioavailable and has higher mobility in plants compared to other heavy metals. It is easily transported by roots to shoots. In contrast, lead is one of the least mobile heavy metals. It is naturally concentrated in the upper layers of the soil33. The contents of the available forms of cadmium and lead exceed the critical values for these elements. In case of lead, the determined contents are from 0.257 Henckovce to 0.676 Gemerská Poloma. Takáč et al.34 determined in 20 soil samples from the Central Spiš region 7.2–257.6 mg Cu/kg soil and 1.0–84.8 mg Pb/kg in their potentially mobilizable form and 0.4–1.4 mg Cu/kg soil and 4.3–7.1 mg Pb/kg in their mobile form. In comparison with our results, Vilček et al.35 determined a lower content of Cd (0.04), Pb (0.17), Ni (0.15) and higher Cu content (0.48) mg/kg in forms accessible to plants in 16 soil samples from locality Nižná Slaná in the years 2006–2008. However, high concentrations of metals in soil do not necessarily mean the availability of metals for plants36. As a result, extractable Mn is often a better indicator of Mn availability. Mn2+ is generally considered to be bioavailable22. The highest concentration of Mn was measured in soil samples from the cadastre of Nižná Slaná. On the contrary, the lowest concentrations were detected in samples from Gemerská Poloma cadastre, which is the furthest cadastre from the source. No critical limit is set up for manganese according to Slovak legislation, it is not possible to classify these soils as contaminated/uncontaminated. For comparison, the EDTA-extractable content of Mn ranged from 22.7 to 127 mg/kg dry soil (China)29; the mobile concentrations between 0.32 and 202.0 mg/kg and the available concentrations from 5.4 to 126.3 mg/kg (Egypt)37.
Based on results of statistical analysis, significant higher content of Cu, Pb and Cd can be stated in samples from Gemerská Poloma cadastre. These soils are classified as gley fluvisols, soils from the other two localities are cambisols (from medium heavy to light) and acid cambisols (Henckovce), cambisols from medium heavy to light and typically acid cambisols (Nižná Slaná). The soil profile of fluvisols is repeatedly disrupted by floods, which often enriches them with a new layer of sludge sediments2.
Another method for determination of metal content in soil is mineralisation using aqua regia, which dissolves most of the soil constituents except those strongly bound in silicate minerals. This content is sometimes referred to as pseudototal (determined in aqua regia). In this way, all elements that are likely to become bioavailable in the long term are determined38.
Pseudototal contents of risk metals (Table 1) determined in soil extract using aqua regia were higher than their limit value in case of Cu (Gemerská Poloma cadastre), Cd (all cadastres) and Hg (cadastre of Henckovce and Gemerská Poloma).
Due to the fact that the hygienic condition of agricultural soils is assessed according to the exceeding of the limit values of at least one risk substance, the monitored plots can be classified as contaminated (Cu > 60.0, Cd > 0.7, Hg > 0.5 mg/kg soil).
Manganese is not classified as risk element in Slovak legislation.
Tóth et al.39 classified European soils into four categories: (1) no detectable content of HM, (2) the concentration of the investigated element is above the threshold value (Hg 0.5, Cd 1, Cu 100, Pb 60 and Ni 50 mg/kg), but below the lower guideline value (Hg 2, Cd 10, Cu 150, Pb 200 and Ni 100 mg/kg), (3) concentration of one or more element exceeds the lower guideline value but is below the higher guideline value (Hg 5, Cd 20, Cu 200, Pb 750 and Ni 150 mg/kg), (4) samples having concentrations above the higher guideline value.
In comparison with the threshold and guideline values, soils in cadastres of Gemerská Poloma (Cu), Henckovce, Nižná Slaná, Gemerská Poloma (Cd, Hg) represent the ecological risk. Threshold and guideline values for Mn were not defined.
The Spiš region and the northern part of the Gemer region belong to the most polluted areas in Slovakia in terms of soil contamination due to mining and metallurgical activities that have been carried out here in the past. Soils near the sludge in Nižná Slaná contain 3.17–53.3 (14.2–301, 0.71–20.6, 3.33–177, 12.9–223 and 675–11,510, respectively) mg Cd (Cu, Hg, Ni, Pb and Mn, respectively)/kg of soil14. In loaded area of Dongchuan, (China), contained Cd (Cu, Hg, Ni and Pb, resp.) 0.20–3.57 (45.38–2026, 0.02–0.23, 24.06–95.9 and 6.83–146.6, resp.) mg/kg40. In contrast, in the agricultural area of Punjab of the India, the soil contamination was caused by an excessive use of agrochemicals and polluted irrigation sources. Increased Cu (Pb and Cd) contents were determined in the soil samples: 9.0–48.5 (5.5–9.67 and 0.516–1.58, resp.) mg/kg41.
However, in most cases, a large portion of the total element content is not available for immediate uptake by plants. Available forms represent a small proportion of this total content which is potentially available to plants. Availability is affected by many factors, including pH, redox state, macronutrient levels, available water content and temperature29,33,36,38.
Indicators of soil contamination
Contamination factors and degree of contamination
The contamination character may be described in a uniform, adequate and standardised way by means of the contamination factor and the degree of contamination. Hakanson24 reported four Contamination degrees of individual metal (({mathrm{C}}_{mathrm{f}}^{mathrm{i}})) – low (({mathrm{C}}_{mathrm{f}}^{mathrm{i}}) < 1), moderate (1 ≤ ({mathrm{C}}_{mathrm{f}}^{mathrm{i}}) < 3), considerable (3 ≤ ({mathrm{C}}_{mathrm{f}}^{mathrm{i}}) < 6) and very high (({mathrm{C}}_{mathrm{f}}^{mathrm{i}}) ≥ 6) and four Contamination degrees of the environment (Cdeg) – low contamination (Cdeg < 8), moderate contamination (8 ≤ Cdeg < 16), Considerable contamination (16 ≤ Cdeg < 32) and Very high contamination indicating serious anthropogenic pollution (Cdeg ≥ 32).
Average Cf values for risk metals (Cu, Co, Ni, Pb, Cd, Hg, and Mn) range from 1.05 (Cu, Nižná Slaná) to 29.1 (Hg, Gemerská Poloma) (Table 2).
Based on values of Cf, soils in the monitored cadastres can be characterised as contaminated. The values of ({mathrm{C}}_{mathrm{f}}^{mathrm{i}}) < 1 were not present in any plot. Cd and Hg are high-risk elements, which showed very high contamination degree. Approximately the same average Cf values of Ni and Pb were recorded on all three plots (Henckovce, Nižná Slaná, Gemerská Poloma). On two plots (Henckovce, Nižná Slaná), similar average Cf values of Cd and Mn on the plots of Nižná Slaná and Gemerská Poloma were recorded. On the contrary, there were significant differences between the average Cf values of Cu (Henckovce ≈ Nižná Slaná < Gemerská Poloma), Hg (Henckovce ≈ Gemerská Poloma > Nižná Slaná) and Mn (Henckovce > Nižná Slaná ≈ Gemerská Poloma).
Guo et al.42 describes Cf as a pollution coefficient for a certain heavy metal, which can reflect the pollution character of the investigated region but can not reveal the ecological effects and hazards. Based on the contamination factor on scale ranging from 1 to 6, Hakanson24 and other authors37,43,44 report four degrees of soil contamination by individual risk metals: low–moderate–considerable–very high contamination. Islam et al.20 classifies the highest degree of contamination as “high”.
Degree of contamination is indicator of degree of soil contamination by all risk elements. The Cdeg values determined for the monitored plots are given in Table 2. The Cdeg values determined for individual sampling sites range from 17.6 to 103 (Gemerská Poloma). According to the classification given by Hakanson24, on the basis of average Cdeg values, lands in the cadastres of Henckovce and Gemerská Poloma show very high contamination degree of the environment, indicating serious anthropogenic pollution and land of Nižná Slaná (mean Cdeg = 28.3) shows a considerable contamination degree of the environment. Some authors7,20,44 use a different scale for soil classification than Hakanson24.
According to the soil classification by Luo et al.44 and Islam et al.20, soils on all three plots show a very high contamination degree.
Among monitored elements, cadmium and mercury are the main contributors to soil contamination. Even if only these two elements were considered for determination of degree of contamination, the soils of Henckovce and Gemerská Poloma plots show a very high degree of contamination (ΣCCd + CHg = 36.2, ΣCCd + CHg = 80.6% Cdeg and ΣCCd + CHg = 39.9, ΣCCd + CHg = 78.1% Cdeg, respectively). Mercury represents 51.7% of Cdeg (Henckovce) and 56.5% of Cdeg (Gemerská Poloma). Cadmium is the highest risk on the plot of Nižná Slaná: CCd = 48.8% Cdeg.
Potential ecological risk factor and potential ecological risk index
The toxicity of heavy metals and risk elements is assessed by the ecological risk index. Highly toxic risk elements, present in the soil, especially in their accessible forms, can enter the food chain through cultivated production and animals.
Potential ecological risk factor (({mathrm{E}}_{mathrm{r}}^{mathrm{i}})) is related not only to the concentration of the risk element present in the soil, but also to the toxicity of each element. The toxicity values determined by Hakanson24 are used in the calculation of ({mathrm{E}}_{mathrm{r}}^{mathrm{i}}): Zn = 1, Cr = 2, Cu = Pb = 5, As = 10, Cd = 30 and Hg = 40.
The values of Potential ecological risk factor, determined based on the total content of risk element and toxicity index, are given in Table 3.
Based on the values reported by Hakanson24 and other authors20,27,43,44,45, the soil of all three plots have a low risk of contamination (({mathrm{E}}_{mathrm{r}}^{mathrm{i}}) < 40) by metals (Cu, Ni, Pb and Mn). The average values of Potential ecological risk factor for these elements range from 1.77 (Mn, Nižná Slaná) to 14.6 (Ni, Gemerská Poloma). In the case of the site Nižná Slaná, soil samples had a high risk of Hg contamination (160 ≤ ({mathrm{E}}_{mathrm{r}}^{mathrm{i}}) < 320). In case of ({mathrm{E}}_{mathrm{r}}^{mathrm{i}}) ≥ 320, soils show a very high risk (very great risk). This value was exceeded for cadmium on all three plots and for mercury on the plots of Henckovce and Gemerská Poloma, where the maximum is ({mathrm{E}}_{mathrm{r}}^{mathrm{i}}) = 2,919.
There are several numerical intervals for the classification of soils based on Potential ecological risk index (RI), e.g., for very high ecological risk is RI ≥ 26044 (40045, 60024,43). However, even when using the assessment with the greatest tolerance (RI ≥ 60024), soils of all three plots pose a very high ecological risk of potential contamination. ({mathrm{E}}_{mathrm{r}}^{mathrm{C}mathrm{d}}) summed to ({mathrm{E}}_{mathrm{r}}^{mathrm{H}mathrm{g}}) present 96% of RI at the land of Nižná Slaná and 98% of RI at Henckovce and Gemerská Poloma lands.
Pollution load index and geo-accumulation index
The PLI gives an assessment of the overall toxicity status of the sample and it is a result of the contribution of all monitored hazardous elements20. In our case, the concentrations and contamination factors of six elements were determined. To calculate PLI was derived the six roots of the six factors multiplied together. Results are shown in Table 4.
The determined PLI values, which inform about the total level of heavy metal pollution, were in range 2.33–7.23. Varol46 evaluates soils according to PLI as uncontaminated, when PLI < 1 (no metal pollution) and contaminated, when PLI > 1 (a pollution exists). According to a more detailed classification7, soil contamination can be expressed by seven levels of pollution: background level (PLI 0), no pollution (0–1), moderate to no pollution (1–2), moderate pollution (2–3), high to moderate pollution (3–4), high pollution (4–5), extreme pollution (PLI > 5).
According to average values of PLI and the scale provided by authors, the soils of individual plots can be classified as follows: polluted, PLI > 1 (all plots)46; high to moderate pollution (Nižná Slaná), high pollution (Henckovce, Gemerská Poloma), while the soils from three sampling points in Gemerská Poloma showed values of PLI above 5, which mean an extreme pollution7.
Another indicator of soil contamination is Geo-accumulation index, according to which is soil quality classified by 7 degrees of contamination: 0—no contamination (Igeo < 0), 1—light (0 < Igeo ≤ 1), 2—slightly moderate (1 < Igeo ≤ 2), 3—moderate (2 < Igeo ≤ 3), 4—hlightly heavy (3 < Igeo ≤ 4), 5—heavy (4 < Igeo ≤ 5), 6—extremely heavy contamination (Igeo > 5)26. The seven-level classification is also given by other authors6,25,27,46.
The values of Igeo of risk metals are given in Table 4. Based on the average values of Igeo, the plot of Henckovce can be classified as uncontaminated with cooper and lead (Igeo < 0), lightly contaminated with Ni, Mn (0 < Igeo ≤ 1) and slightly heavy contaminated with Cd and Hg (3 < Igeo ≤ 4). Nižná Slaná plot is not contaminated with Cu, but is lightly contaminated with Ni, Pb and Mn, slightly moderate contaminated (1 < Igeo ≤ 2) with Hg moderate contaminated with Hg and slightly heavy contaminated with Cd. On the Gemerská Poloma plot, the soil is lightly contaminated (Pb, Mn), slightly moderate contaminated (Cu, Ni) to moderate contaminated (Cd) and slightly heavy contaminated (Hg). Among six risk metals studied, Igeo of cadmium and mercury had the highest values on all three plots.
Source: Ecology - nature.com
