Changes in the content of certain chemical substances in the soil of Abakan under the influence of transport

The article considers correlations between pollutant content in the soil of Abakan and traffic intensity around the sampling sites. Pollution with lead, iron and carbonate ions was analyzed. Significant correlations were found for all three, especially strong one for carbonates.

Keywords:soil pollution, lead pollution, traffic pollution

Soil is the fertile surface layer of the Earth's lithosphere, which is a heterogeneous open four-part system (solid, liquid, gaseous phase and living organisms) formed by the weathering of rocks and activity of organisms. Soil is the uppermost part of the lithosphere and thus in general inherits its chemical composition. About 50–60 % of the volume and up to 90–97 % of the weight of the soil are mineral components [1].

Soil pollution is a form of anthropogenic degradation of soil, in which the content of chemicals in soils exposed to anthropogenic impact exceeds the natural regional background levels of their content in the soils. High levels of contamination of a wide number of metals and metalloids (Bi, Hg, Sb, Pb, Cu, Se, Ag, As, Mo, Sn, Cr, Zn), which need different types of production is high [2]. Technogenic pollution of soil is generally understood as accumulation, associated with industrial or other human activities, of a number of substances and organisms in amounts that reduce the technological, nutritional and sanitary value of crops, degrade the quality associated with other natural objects (surface and ground waters, atmosphere, biocenosis) and leading to the degradation of the soil. Rationing of chemical contamination of soils is set to the maximum permissible concentrations (MPCs). MPCs — this is such a concentration of a chemical (in mg per kg of soil in the plow layer), which should not cause direct or indirect negative impact on contact with the soil environment and human health, as well as the soil's ability to cleanse itself [3].

Table 1

Toxic components (in g) formed from combustion of 1 kg of fuel [4].

Heavy metal pollution is an excess accumulation of chemical elements of ecotoxic group in the soil, typically Pb, Cu, Zn, Cd, Cr, Ni, Co, Sb, Sn, Bi, Hg, Mo, V, Mn, Ti, W. Sometimes other metals and some metalloids are considered as well: Be, Tl, Sr, Ga, Ge, As, Se, B. Thus, the concept of heavy metals is of combined nature and criteria are not very strict. Heavy metal soil contamination has two major downsides. Firstly, it causes interruptions in the food chain from soil to plants and from plants into the body of animals and humans. Heavy metals cause serious illnesses thus impeding growth of population by raising morbidity and reducing life expectancy, as well as reducing the quantity and quality of crop yields and livestock production.

Secondly, accumulation of large quantities of heavy metals in the soil may lead to change many of soil properties. First of all, the changes affect the biological properties of the soil: reduced total number of microorganisms, narrowed down biota variety (species composition), changes in main microbiological processes and falling activity of soil enzymes, etc. Strong contamination with heave metals leads to changes in more conservative features of the soil, such as humus status, structure, pH and others. The result of this is partial, and in some cases, complete loss of soil fertility [5].

We decided to analyze soil samples and find out level of heavy metal contamination in Abakan.

Sampling. The soil was taken at a distance of 1–2 meters from the road with a depth of 20–25 cm. The sample mass of about 0.5 kg.

For this study 7 sampling plots were defined in different directions of the wind rose and characterized by different level of transport intensity (See Figure 1).

Fig. 1. Map of location of plots. Scale 1: 800

Area 1. The sampling area located in the Park of Culture and Recreation in the south-eastern part of the city. Traffic is minimal, whereby it was selected as a reference.

Area 2. The sampling area is located on the outskirts of the city to the south. Chosen as the extreme southern point of observation.

Area 3 and 4 were located near the central market and the central post office, a part of the city characterized by maximum traffic congestion.

Area 5 and 6: Kirov Street and nearby hotels, the northernmost point of observation.

Area 7 in the vicinity of ICS, the south-westernmost point, average traffic.

Thus, we have incorporated the sites allowing to cover the main directions of the winds prevailing in Abakan, as well as places with different traffic intensity that probably should affect the composition of the soil.

Repeatability of wind direction and still air conditions are shown in Figure 2.

Fig. 2. The frequency of wind directions (%) Abakan

The south-western direction prevails during the year. Dangerous direction of the wind in terms of air pollution to the residential areas of the city of Abakan are West (13 %) and Northwest (4 %). Thus, the high frequency of calms and light winds combined with temperature inversions in winter causes the most adverse weather conditions for dispersion of pollutants in the atmosphere of the city. The connection between the wind direction and soil contamination is defined by the fact that the maximum amount of pollution falls on the central part of the city, which lies to the west, which correlates with the most intense annual wind direction.

Determination of lead followed this procedure. Weigh 10 g of soil sample. Transfer the sample into numbered conical flasks (№ 1, 2, 3). Pour 15 ml HNO3 into each flask, agitate for 2–3 minutes. The resulting extracts are put into numbered glasses, each using its own filter. For analysis 5 ml of filtrate are placed in three numbered tubes using a measuring pipette and then 3 drops of potassium chromate are added. Let precipitation to flocculate.

Pb²⁺ + K₂CrO₄= PbCrO₄ (yellow precipitate) + 2K⁺.

The resulting intensity of precipitation is expressed in points on a scale from 1 to 5 and compared with solutions of decreasing concentrations of lead nitrate.

Determination of iron followed this procedure. Weigh 5 g of the soil sample. Extract is prepared similar to procedure for lead. For analysis 5 ml of filtrate are placed in three numbered tubes using a measuring pipette and then 3 drops of potassium hexacyanoferrate are added.

3Fe²⁺ + 2K₃ [Fe(CN)₆] → Fe₃ [Fe(CN)₆]₂ +6K⁺;

Fe³⁺ + K₃ [Fe(CN)₆] → Fe [Fe(CN)₆] +2K⁺ [6].

Determination was conducted on a scale of 1–10 by the color intensity compared with prepared solutions of decreasing concentration of iron sulfate.

Determination of carbonate ions was by acid-base titration. Sample of 2 g of soil was diluted with 5 ml of water and 3 drops of methyl orange. Titration was performed with 0.1M HCl until the red color.

2HCl + CO₃^2- → H₂O + CO₂ + 2Cl^- [6].

Lead ions content. Lead is one of the most dangerous heavy metals and is carcinogenic. Its presence in the soil is determined mostly by use of ethylated gasoline. The lead content in the soil is shown in Table 2.

Table 2

Led ion content in the soil of Abakan, 2013

The lead content is low enough so that the differences in areas quite low. Possibly, a small score is due to the fact that use of lead ethyl as a fuel additive is prohibited in Russia since 2002, but, nevertheless, accumulated lead was found in all the sampled areas.

Iron ions content. Iron is usually not counted as a heavy metal, despite the fact that its mass is greater than 50 a. e. m. However, high iron content in the soil leads to a sharp deterioration of its fertility and enhance the accumulation of other hazardous substances. The iron content is shown in Table 3.

Table 3

Iron in the soil of Abakan, 2013

It is worth noting that the high iron content is observed near water, which may indicate a process of bog formation.

Carbonate ion content. Carbonate ions are an important part of the soil. They neutralize the effect of acid from entering the environment in soil. Due to combustion of gasoline a small amount of sulfur and nitrogen oxides comes in contact with moist soil and they may form a strong acid that will react with the carbonates. Carbonate ion content is shown in Table 4.

Table 4

Carbonate ion content, g/kg dry soil, Abakan, 2013

The biggest amount of carbonates is typical for areas 1 and 2 (4.88 and 4.69 g/kg of soil). Minimum values are typical for the Central Market and Post Office (0.72 and 2.15 g / kg of soil). Probably the sharp drop in weight carbonates may be due to high congestion areas close to these vehicles. Thus, the content of iron and carbonate ions differs significantly in different areas, it is likely to be affected by various factors, including traffic.

Today there are more than 40 thousand cars in Abakan, so traffic is quite intense. It leads to high amount of exhaust gases particularly at busy intersections. Since the activity of transport should influence the composition of the soil pollutants we made counts of cars passing by at the sampling areas (Table 5).

Table 5

5 minute counts of vehicles passing near the sample plots, Abakan, 2013.

In order to determine whether there is a relationship between the number of vehicles and ion content in the soil, we determined the coefficient of correlation using the Microsoft Office Excel 2013 (Table 6).

Table 6

Relationship between the content of the test substances in the soil and transport activity, Abakan, 2013

The contents of lead and iron ions in the soil shows strong correlation with the transport activity (0.63 and 0.61, respectively), probably, most of the lead and iron to the soil is the exhaust gas of automobiles. Exhaust gases affect the content of iron, since they change the acidity of the soil. Very strong link is shown in carbonate ions, increasing the number of machines reduces the carbonate content of the soil which may be explained by the presence of nitrogen and sulfur oxides in the exhaust, which combine with the soil moisture and form acids that destroy carbonates. Of course, the content of carbonate depends on the type of soil as well, but the link shows the dependence of their transport nevertheless.

Concluding, soils contain various substances and its constitution is subject to change by the vehicle exhaust gases. Especially dangerous is the accumulation of lead ions, which are carcinogens, their presence leads to the death of soil animals, their accumulation in plants, as well as and the possibility of getting them into the human body. Exhaust gases influence soils in the presence of carbonate ions, a gradual acidification leads to a loss of fertility and subdued microbial activity due to which the soil eventually lose the ability to repair itself.

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