3.1. Heavy Metal Concentrations in Urban Media
The content of heavy metals/ metalloid in the liquid phase (LSS), the solid phase (SPS) of snow, the soil and the mass concentration of trace elements in vegetables are presented in
Table 1. The highest concentrations of trace elements are observed in the solid phase of snow, with the following decreasing series: Fe (26,000) > Mn (592.5) > Cr (371.3) > Zn (338.8) > Pb (161.9) > Cu (142.5) > Ni (30.9) > As (15.1) > Co (12.1) > Cd (2.6). The concentration of trace elements varied significantly between samples in the following range: Cr (220–690 mg/kg), Mn (530–670 mg/kg), Fe (21,000–35,000 mg/kg), Co (8–22 mg/kg), Ni (19–49 mg/kg), Cu (40–430 mg/kg), Zn (120–590 mg/kg), As (6–37 mg/kg), Cd (1–6.1 mg/kg), Pb (20–300 mg/kg).
Pollutant concentrations in the liquid phase of snow (meltwater) were lower than in solid snow sediments and soils but higher than in vegetables, although the closest value was observed. The concentration of trace elements in meltwater was as follows, in descending order: Fe (222.8) > Mn (29.1) > Zn (28.3) > Pb (5.5) > Cu (3.5) > As (3.0) > Cr (1.7), while the concentrations of cobalt, nickel and cadmium were less than 0.1 mg/kg. In general, the decreasing series corresponds to that for solid snow sediment, but the concentration of chromium in meltwater has low values, indicating that chromium is a metal present in an insoluble form. The range of changes in metal concentrations in the liquid phase of snow is as follows: Cr (0.5–3.2 mg/kg), Mn (16–72 mg/kg), Fe (74–1000 mg/kg), Cu (1.1–8.8 mg/kg), Zn (6–47 mg/kg), As (1.4–4 mg/kg), Pb (2.4–8.9 mg/kg).
The decreasing series of the average content of heavy metals / metalloid in the soils on which vegetables are grown in the city of Pavlodar has the following form: Mn (22,125) > Fe (20,375) > Zn (246.9) > Cr (109.5) > Cu (39.3) > Pb (25.6) > Ni (22.4) > As (9) > Co (6.6) > Cd (0.2). Comparing the concentrations of elements in solid snow sediment with soil data, it was found that the metal content of snow is much higher than that of soil. For example, cadmium in the solid phase of snow exceeds soil data by almost 11 times, followed by lead at 6.3 times, chromium and copper exceed 3.3 times and other metals (iron, nickel, zinc and arsenic) vary between 1.3 and 1.8 times. The exception is manganese, whose content in the soil is more than 37 times higher than that of this metal in the snow.
The mass concentration of the heavy metals/ metalloid in question is lowest in vegetables. The highest values were found for iron, which ranged from 0.4 to 1.1 mg/kg. Despite the extremely high concentration of manganese in the soil—22,125 mg/kg—its content in potatoes decreased by a factor of 1865 and amounted to 11.86 mg/kg (range 7–31 mg/kg). Zinc (15–25 mg/kg), copper (2.7–20 mg/kg), nickel (0.5–3.6 mg/kg), chromium (0.4–1.1 mg/kg) and lead (0.07–0.37 mg/kg) followed in decreasing order.
Comparing the data on the mass concentration of heavy metals/ metalloid in tomatoes and potatoes, the following conclusions can be drawn. The series of decreasing concentrations is identical, but there are variations in the data. For example, the levels of iron and copper in potatoes are on average 1.5 times higher than in tomatoes. All the other metals have similar values in the two vegetables, with a slight bias towards potatoes. The variation in the mass concentration of microelements in tomatoes was as follows: Cr (0.5–1.7 mg/kg), Mn (9–18 mg/kg), Fe (25–75 mg/kg), Co (0.02–0.16 mg/kg), Ni (0.4–2.2 mg/kg), Cu (3.6–9.6 mg/kg), Zn (11–32 mg/kg), Pb (0.06–0.12 mg/kg), As and Cd—less than 0.1 mg/kg.
Compared to other studies [
43] of snow cover in Kazakhstan that were done in a small settlement in the East Kazakhstan region, our study found that the amount of cobalt, nickel and copper in the snow cover of gardens in the city of Pavlodar and nearby settlements is almost the same, though there is a higher concentration of most metals. For example, the level of chromium in the Pavlodar snow is 6.2 times higher than in North Kazakhstan’s settlement, but the levels of zinc and cadmium in our study are 1.2 and 1.7 times lower, respectively. In studies of the East Kazakhstan region, snow samples were taken in areas directly influenced by anthropogenic factors such as vehicles, stove heating and boiler houses. Thus, the anthropogenic influence of the urban environment on snow cover is high in dachas and vegetable gardens.
The results of studies of soil cover on agricultural land in the Kostanay region of the Republic of Kazakhstan [
44] show low levels of heavy metal content compared to Pavlodar vegetable gardens. Thus, in Pavlodar, the content of lead in soils exceeds the content in agricultural lands of Kostanay by 3.4 times, followed by arsenic by 3 times, etc., except for cadmium, the concentration of which, on the contrary, is 2 times lower in our studies.
Compared to the urban soils of the Republic of Kazakhstan, which are the most contaminated with heavy metals (Balkhash, Ust-Kamenogorsk, Ridder and Shymkent), the average metal concentrations varied between 251 and 442 mg/kg for Pb, 5–9 mg/kg for Cd, 8–138 mg/kg for Cu, 87–178 mg/kg for Zn and 2–5 mg/kg for Cr (Ramazanova et al., 2021) [
45].The soils of vegetable gardens and dachas in Pavlodar and its surroundings contain large amounts of chromium and zinc, which exceed the average and maximum values of the data presented by Ramazanova et al., 2021 [
45]. For cadmium and lead, the values in our study are significantly lower than in the above studies of urban soils; the remaining metals are within the range of values.
3.2. Assessment of Heavy Metal Contamination
An indicator of heavy metal contamination of snow (solid fraction) and soil is the contamination coefficient relative to the background concentration.
Figure 2 shows the contamination coefficients of different media. The highest values of the snow contamination coefficient for all metals are found in the city’s vegetable gardens. The descending series of average contamination index values in the snow cover of the city gardens is as follows: Pb > Cu > Cd > As > Zn > Co > Ni > Fe > Mn > Cr. The minimum exceedance of the background indicators relates to chromium (3.75), which corresponds to severe pollution according to the pollution classification of this coefficient. Thus, all of the other metals in urban snow are classified as highly polluted. For village gardens, the pollution index varies from 0.3 for chromium to 5.3 for cadmium. The level of contamination of snow is low or medium for most metals and high for cadmium and lead.
There is not such a big difference between urban and rural areas for heavy metal soil pollution, although there is still a tendency for urban areas to exceed these values. The highest pollution coefficients are Cu > Pb > Zn > Cd > Mn > Co > Fe > Ni > Cr > As. In the soils of urban gardens, arsenic has a very low coefficient, indicating an uncontaminated area; the maximum values are recorded for copper (3.3) and lead (3.04), corresponding to severe contamination. The remaining metals range between 1 and 2, indicating low levels of contamination. For most metals, rural soils are uncontaminated compared to background levels, with only the values for iron, magnesium and lead indicating low levels of contamination.
The enrichment factor (EF) allows us to estimate the degree of intensity of anthropogenic activity relative to the clark concentration (geochemical background). In our study, the concentration of iron (Fe) was used as the reference background concentration. As a result of the calculations, data on soil accumulation in different vegetable growing areas were obtained (
Figure 3).
In general, the enrichment of soils with heavy metals/metalloid in the city is represented in the following series: Mn > As > Zn > Pb > Cr > Cu > Ni > Co > Cd. With maximum values characteristic of manganese (50.5), arsenic (10.4), zinc (6.6), lead (3.5) and chromium (3.03). Nickel, cobalt and cadmium have values below 1, which corresponds to a normal distribution in relation to concentrations in the earth’s crust.
Maximum allowable concentrations (MACs) of pollutants are used to regulate the content of heavy metals in soils in the Republic of Kazakhstan. In order to determine the degree of compliance with these soil standards, in our study we determined the degree of exceedance of the maximum allowable concentrations of heavy metals in the soils of urban and rural gardens. We used the MPCs according to Hygienic standards for the safety of the environment, 2021 [
39], for lead and arsenic, the WHO standard [
40] for chromium and the Russian GOST standards (Hygienic Standards HS 2.1.7.2041-06) [
38] for other metals (
Table 2).
Most metals do not exceed established standards for soil content. However, magnesium exceeds the MPC by 28.6–35.7 times, an excess indicating extreme pollution. Zinc also exceeds standards in a wide range, from 1.6 to 10.9 times the MPC in all study areas. The arsenic content in urban gardens reaches 6, while the minimum value at point 6 is 0.05. The arsenic content is higher in villages than in cities, which distinguishes this metal’s behavior from others. The concentrations of cobalt and nickel are higher than the standard values in urban gardens, while in rural gardens, the concentrations of these metals are normal. The maximum exceedance values of the MPCs for chromium, copper and lead also apply to urban gardens, and the exceedance is 10.4, 3.9 and 2.25, respectively. Thus, manganese and zinc are the main pollutants in both urban and rural areas. Only cadmium has minimum values with respect to the maximum permissible concentration, whereas the maximum values of the other metals exceed the city’s maximum concentrations.
The hygienic standards for controlling the concentration of heavy metals in food have been adopted as the acceptable level of contamination (mg/kg) for different products in the Republic of Kazakhstan (Sanitary and epidemiological requirements for food products, 2010) [
41]. This legislation is no longer in force and standards for heavy metals in vegetables have had to be obtained from other sources. Thus, the limiting indicators of heavy metal pollution in vegetables adopted by WHO [
41] for chromium, nickel, copper and lead, literature sources [
19]. for cobalt and sanitary rules and regulations of the USSR for zinc (SanPiN 42-123-4089-86) [
42] were taken into account (
Table 3).
Of the metals considered, only zinc exceeds the MPC for both potatoes and tomatoes. In some areas, the copper content also exceeds the standards, although the average value is within the normal range. In general, chromium and cobalt are characterized by an excess of the standard concentration for tomatoes, and other metals accumulate more in potatoes.
Since the highest excesses were found for zinc and copper,
Figure 4 shows the spatial patterns of the content of these metals in vegetables in rural and urban gardens, as well as in the city as a whole.
It was found that the concentration of zinc and copper in the city largely exceeds the MPC, in contrast to rural gardens, where the range varies from 1.8 to 2.1 for zinc and from 0.54 to 0.66 for copper. The opposite situation was observed for tomatoes, where the high content of the metals studied corresponds to village gardens.
The pollution index relative to the background for potatoes and tomatoes is shown in
Figure 5.
Relative to the background, nickel is the most important contaminant for the vegetables studied, with an excess of 6 (very high contamination) for potatoes and 4.4 (high contamination) for tomatoes. The average value of soil contamination with this metal refers to the average level of contamination. For tomatoes, cobalt is also a strong contaminant; its value is 2.2 times higher than the background value, and the maximum value is 5.3. The remaining metals are at a low level of contamination. In potatoes, lead, zinc, copper and nickel are classified as moderately polluted, and the metals are divided into non-polluted and moderately polluted.
3.4. Health Risk Assessment
For both local adults and children, the human health risk of exposure to trace elements through consumption of contaminated local crops was assessed.
To calculate the carcinogenic and non-carcinogenic risks, the daily intake of vegetables for adults and children was calculated and presented in
Table 4. In general, according to the maximum tolerable intakes (MDIs) presented in different sources [
47,
48], the daily consumption of all heavy metals does not exceed the standards. However, potatoes have the highest values for both adults and children, as their average consumption is higher than that of tomatoes.
Analysis of the non-carcinogenic risk of exposure to heavy metals revealed that no single metal posed a significant threat to children and adults, as a hazard coefficient value less than one indicates no consumer risk. Thus, in our studies, the carcinogenic risk of the heavy metals in question is low for children and adults. Potatoes have slightly higher levels than tomatoes. Lead has the lowest levels, so it has virtually no impact on human health, with slightly higher levels of nickel and chromium in both vegetables for children.
The Hazard Index (HI) and the Carcinogenic Risk (CR) were considered as indicators of the potential risk to human health from exposure to several potentially toxic elements (
Table 5 and
Table 6). If the hazard index value is less than 1, adverse health effects are unlikely. In general, the non-carcinogenic risk for potatoes and tomatoes is low. Spatially, the hazard index for consuming potatoes in cities is 1.3 higher for both adults and children than in rural gardens. Eating potatoes also has a low carcinogenic risk. Values above 10
−4 represent significant health effects and risks, while values below 10
−6 are unlikely to cause health effects. The carcinogenic risk of consuming potatoes in an urban area is 1.6 times higher for adults and children than in the countryside, according to the hazard coefficient.
The risk of adverse effects from urban and rural consumption of tomatoes is lower than for potatoes, but this is also associated with lower tomato consumption. However, the overall non-carcinogenic and carcinogenic risks of tomatoes do not differ much between urban and rural gardens and are almost identical, indicating that tomatoes have a high selectivity.
3.5. Sources of Pollution
The results show the impact of industry and transport on plants and, consequently, on human health when consuming vegetables. Although the average health risk is not high, metals such as chromium, lead, nickel, cadmium and zinc are of concern as the maximum permissible concentrations of these metals exceed the standards.
Another aim of the study is to try to identify the sources of contamination of vegetables in the territory of the city of Pavlodar as well as in the nearest settlements. The dacha plots of site 1 are situated in the southern direction of the Northern Industrial Zone, approximately 1.5 km away from the largest industrial facility, the Pavlodar Oil Refinery. Meanwhile, sites 2 and 4 of the city’s vegetable gardens were directly impacted by motor transport, including personal vehicles, near the vegetable planting areas. The southern part of the city (sites 6, 7) was also affected by the Eastern Industrial Zone with aluminum production, CHPP-1, ash dumps and sludge pits.
In order to identify the sources of pollution by heavy metals and metalloids, the emissions of the main industrial enterprises in the city of Pavlodar were analysed according to the results of the industrial environmental control (
Table S1). The amounts of heavy metals/metalloids emitted in pure form are not high. However, the ash and dust emissions of the industrial enterprises in Pavlodar contain the following compositions of elements: lead—2.86 mg/kg; cadmium—0.23 mg/kg; mercury—6.59 mg/kg; arsenic—0.28 mg/kg; fluorine—10.0 mg/kg; antimony—0.37 mg/kg; beryllium—0.017 mg/kg; selenium—0.53 mg/kg; tellurium—0.1 mg/kg [
49]. The main sources of dust and ash emissions are CHPP-2, CHPP-3, followed by KSP “Steel” (metallurgical production). Other emissions are as follows: aluminum—78.877 t/year; petrol—48.74 t/year, emitted by the Pavlodar Petrochemical Plant; manganese—18 t/year; iron—16.915 t/year; emitted by KSP “Steel”; as well as 1.45 t/year of iron emitted by “Kasting” LLP (metallurgical production). The largest amounts of chromium (6 t/year), zinc (0.3135 t/year) and wood dust (6.741 t/year) are emitted by the pipe mill. JSC “Kaustik” (chemical plant) emits 0.175 tonnes/year of chlorine, 0.313 tonnes/year of zinc [
50]. The analysis showed that the maximum emissions from aluminum production are from aluminum: 133.5 t/year from the aluminum smelter and 433.6 t/year from the electrolysis plant [
51]. It should be noted that, according to the results of industrial environmental control, none of the industrial facilities exceeds the normative indicator of pollutant emissions adopted in the Republic of Kazakhstan.
It is important to identify the sources of pollution that influence the accumulation of pollutants in vegetables. For instance, Weissmannová’s 2019 [
30] studies on the relationship between pollution sources, heavy metals and metaloids reveal a connection between pollutants and industrial and traffic emissions. Studies confirm that most metals (Pb, Cu, Cd) are grouped by NSA analysis and originate only from anthropogenic sources (metallurgy, coal burning, transport). The high iron and manganese content may also be due to weathering of the parent rock and emissions from iron production, rolling mills, blast furnaces and so on. Emissions of the Fe, Cr and Cd groups of metals and the Pb-Cu group are due to coal combustion in industry and local water heating. In the city of Pavlodar, this refers to the operation of CHPP-1, 2 and 3, as well as to the areas of urban and village vegetable gardens. Vegetable gardens are usually adjacent to houses with stove heating, and snow accumulates all emissions during the winter period (from November to April). High concentrations of lead in all environments and in vegetables are caused by coal burning, metallurgy and industrial waste. Zinc is emitted in the production of stainless steel (steel production). Zinc, lead and copper are also widely emitted by car exhaust [
33]. In 2022, emissions of pollutants from mobile sources in the Pavlodar region will amount to 43 thousand tonnes [
20].
Previous studies [
52] also support the assumption that motor vehicle emissions and the deposition of chemical elements in atmospheric dust are the primary sources of heavy metal/metalloid contamination of vegetables, based on the reviewed data. Domestic coal combustion can be considered potentially problematic, with serious environmental and health consequences, as can motor vehicle emissions. We recommend using high-quality coal and petrol for domestic use, as well as protecting plants from car exhaust. The data obtained on heavy metal/metalloid accumulation in vegetables and comparison with concentrations in snow and soil can be used for further research to identify patterns and use only one medium to determine contaminant levels in the snow-soil-plant system. Thus, methods already exist to assess spatial air pollution based on snow cover [
27]. In addition, the results of this study may motivate ecologists, managers and health workers to educate the public about the risks of consuming vegetables grown on contaminated soils, which may help reduce health hazards. In addition, it is recommended for managers to provide publicly available information on the standards of permissible level of heavy metal contamination of food products, including fruits and vegetables. The authors recommend regular monitoring of heavy metal and metalloid levels in snow, soil and vegetables to prevent significant metal accumulation in the food chain and mitigate public health risks.