Distribution and transfer of heavy metals in agricultural systems near highways

Distribuição e transferência de metais pesados em sistemas agrícolas próximos a rodovias

Abstract

The purpose of the proposed research work is to determine the number of heavy metals (Pb, Zn, Cd) in soils and agricultural crops located near highways in South Kazakhstan (Turkestan region, Saryagash district) and to evaluate their transfer into plants. Soil samples along the highway and vegetables (tomatoes, cucumbers) and cucurbits (watermelon, melon) grown in this area were selected as the objects of the research. The concentrations of heavy metals were determined by atomic absorption spectroscopy. The transfer factor (TF) was calculated to characterize the rate of element transfer into plants. This study provides new data on heavy metal distribution and transfer in agricultural systems of South Kazakhstan, a region where roadside contamination of crops remains insufficiently studied. According to the research results, the concentrations of Pb, Zn and Cd in the soil along the road were found to be 1.5–3 times higher than the permissible limit values. The spread of heavy metals in the soil was determined in a decreasing order: Pb > Zn > Cd. The transfer factor into plants were within the following limits: Pb — 0.16–0.32; Zn — 1.99–3.0; Cd — 2.9–4.4. The highest migration ability was characteristic of the Cd element, which indicated its high bioavailability. As a result of the correlation analysis, positive relationship (r > 0.90) was revealed between all indicators, especially between Zn and Cd (r = 0.99). The conducted research showed that the migration of heavy metals to agricultural crops poses a significant threat to human health. Therefore, it is recommended to conduct regular monitoring, maintain sanitary distances and apply bioremediation measures aimed at reducing soil contamination to prevent the accumulation of heavy metals in products grown along roads.

Keywords:
soil; heavy metals; migration; vegetables; pollution; highway

Resumo

O objetivo do trabalho de pesquisa proposto é determinar o número de metais pesados (Pb, Zn, Cd) em solos e em culturas agrícolas localizados perto de rodovias no sul do Cazaquistão (região do Turquestão, Distrito de Saryagash), bem como avaliar sua transferência para as plantas. Foram selecionadas amostras de solo ao longo da rodovia, além de hortaliças (tomates, pepinos) e cucurbitáceas (melancia, melão) cultivadas nessa área como objetos de estudo. As concentrações de metais pesados foram determinadas por espectroscopia de absorção atômica. O fator de transferência (TF) foi calculado para caracterizar a taxa de transferência de elementos para as plantsa. Este estudo fornece novos dados sobre a distribuição e transferência de metais pesados em sistemas agrícolas do sul do Cazaquistão, uma região onde a contaminação das culturas nas rodovias ainda é insuficientemente estudada. De acordo com os resultados, as concentrações de Pb, Zn e Cd no solo ao longo da rodovia foram de 1,5 a 3 vezes superiores aos valores-limite admissíveis. A propagação dos metais pesados no solo foi determinada por ordem decrescente: Pb > Zn > Cd. O fator de transferência para as plantas situaram-se dentro dos seguintes limites: Pb–0,16-0,32; Zn–1,99-3,0; Cd–2,9-4,4. A maior capacidade de migração foi característica do elemento Cd, o que indicou a sua elevada biodisponibilidade. Como resultado da análise de correlação, revelou-se uma relação positiva (r > 0,90) entre todos os indicadores, especialmente entre Zn e Cd (r = 0,99). A investigação conduzida mostrou que a migração de metais pesados para culturas agrícolas representa uma ameaça significativa para a saúde humana. Portanto, recomenda-se a realização de uma monitoramento regular, a manutenção de distâncias sanitárias e a aplicação de medidas de biorremediação destinadas a reduzir a contaminação do solo, a fim de evitar a acumulação de metais pesados em produtos cultivados ao longo das rodovias.

Palavras‑chave:
solo; metais pesados; migração; hortaliças; poluição; rodovia

1. Introduction

Research on soil pollution by heavy metals shows the need to continue research in this area (Yu et al., 2025). The soil of Kazakhstan also tends to accumulate heavy metals, which in turn destroys soil fertility. Turkestan region in Kazakhstan plays an important role in the development of agriculture in this region. The area of land used for agriculture in the republic reached 19.1 million hectares according to official data (Akhmetova et al., 2025).

Turkestan region is located at the junction of international transport corridors, the main highways to neighboring countries pass through this region. Part of these roads are located adjacent to agricultural fields, which increases the possibility of the impact of technogenic factors on the environment.

The identification of pollution sources of heavy metal in soil, evaluation of their biological effectiveness, and analyses and explanation of morphological features are of particular importance for ecosystem studies (Zhang et al., 2010). The migration of heavy metals depends mainly on their mobile forms, which directly affect the natural cycle and ecological stability (Alemu, 2015).

Highways are one of the main sources of soil pollution along highways. Road traffic leads to the accumulation of heavy metals (Pb, Cd, Zn, Cu, Ni) in the soil. Heavy metals enter the environment not only through fuel combustion, but also through the wear products of brake pads, tires, and road surfaces (Ferreira-Baptista and Miguel, 2005).

Numerous studies have reported that roadside soils and dust are significantly enriched with heavy metals due to traffic-related activities, making them an important source of environmental contamination (Wei and Yang, 2010; Alloway, 2012).

The levels of heavy metals in soils along highways are increases with increasing distance from the road, and the highest level is observed in the area of 0–10 m. This indicates that the technogenic factor affects the ecological condition of agricultural lands and the pollutant load on plants (Kachenko and Singh, 2006; Viard et al., 2004).

Heavy metals such as lead, cadmium, zinc and others have negative effects on human health. Toxic heavy metals can be accumulated in the human body for years. Lead negatively affects the development of children, can cause excessive levels of lead in blood, which in turn leads to hypertension, nephropathy and cardiovascular diseases (Adriano, 2001; Flora et al., 2012; Järup, 2003). Chronic exposure of cadmium is highly toxic to the liver and lungs, causing nephrotoxicity and osteotoxicity, and impairing immune system function (Bernard, 2008; Godt et al., 2006).

Heavy metals such as lead and cadmium are known for their high toxicity and persistence in the environment, posing serious risks to ecosystems and human health even at low concentrations (Sharma and Dubey, 2005).

Vegetables are mainly important edible crops and form part of human diet. They include nutrients essensial for the human body (Khan et al., 2008). Vegetables absorb heavy metals from soil via their roots, and they are accumulated in large quantities in edible parts of plants, even if the soil contains small amounts of heavy metals (Radwan and Salama, 2006; Salim and Blanchard, 2017).

At present, the quality and environmental safety of agricultural products are one of the main directions of sustainable development of the agro-industrial complex (Sezgin et al., 2003). In recent years, the volume of vegetable and cucurbits (tomatoes, cucumbers, melons, watermelons) in the agriculture has significantly increased, which in turn has increased the need to monitor the ecological state of the soil (Sharma et al., 2007). Heavy metals and other pollutants eventually affect all environmental compartments. Traffic-related pollution and industrial runoffs can migrate through water systems and sediments, eventually penetrating agricultural soils and contributing to the contamination of crops (Tileuberdi et al., 2023; Rahim et al., 2024). This interconnected migration highlights the need for site-specific agricultural monitoring.

The climatic conditions are favorable for the accumulation of heavy metals in the soil and their entry into the biological cycle by plants. The increase in emissions of heavy metals (lead, cadmium, zinc, copper, nickel, etc.) along highways is one of the main factors of technogenic pressure on regional ecosystems (Sharafi et al., 2024; Nabulo et al., 2006; Szwalec, 2020; Kabata-Pendias, 2011; Satarug et al., 2010).

According to studies of other authors, the amount of lead in soil and vegetation along the road is higher the closer it is to the pollution source, and the further away it gets, the lower it is (Rattan et al., 2005). This is largely explained by the low mobility of lead in the soil and its tendency to be adsorbed on organic-mineral complexes, which limits its migration into the soil, but nevertheless contributes to its accumulation in the aboveground parts of plants.

Zinc and cadmium exhibits greater mobility in soil than lead and can migrate to deeper levels, but their uptake by plants is restricted by biological barriers (Tileuberdi et al., 2024). Several studies have shown that the concentration of zinc and cadmium in vegetables grown along roads is lower than that of lead, although long-term anthropogenic exposure of cadmium can lead to its accumulation in food products and have negative effects on human health (Liu et al., 2013; Chary et al., 2008; Zhou et al., 2016; Al Jassir et al., 2005; Naz et al., 2024).

Despite numerous studies on roadside contamination, there is a lack of data for agricultural systems in South Kazakhstan, particularly for vegetable and cucurbit crops. The aim of the research is determination of the level of accumulation of heavy metals (Pb, Cd, Zn) in vegetable and melon plants grown along the highway and estimation of their transfer coefficient (TF) from soil to plants.

2. Materials and Methods of Research

The research object was soil samples taken along the highway passing through the Saryagash district of the Tukistan region of South Kazakhstan. In addition, vegetables (cucumbers, tomatoes) and fruits (melon, watermelon) grown in the fields located along the highway were selected as research objects. Only edible parts of the plants and soil samples were taken from the root zone (0-20 cm). Each sample was randomly selected from 5-10 plants per area of ~1–5 m². The samples were placed in clear polyethylene bags, labeled, and transported to the laboratory in a cooled state.

Laboratory pre-treatment.

1. “Wet” samples were weighed.

2. Visible inert impurities were removed mechanically without damaging the plant.

3. The samples were washed with distilled water, dried, and analyzed to remove adsorbed particles from the surface layer of the plant.

Drying and grinding.

1. The washed and dried samples were pre-dried in air at room temperature for 30-60 min to remove moisture from the surface layer.

2. To remove moisture from the surface layer of the washed and dried samples, they were pre-dried in air at room temperature for 30-60 min.

3. We dried them in a drying oven at a temperature of 60-70°C to a constant mass.

4. We ground the dry samples in a laboratory mill. After grinding, the samples were thoroughly mixed and separated for analytical research.

Methodology for preparing plant samples.

Collected plant materials (cucumber, tomato, melon, watermelon) and soil samples were cleaned of dust and mechanical impurities after delivery to the laboratory. Each sample was washed with running water and finally rinsed with distilled water.

The samples were dried in a drying cabinet at 70°C to constant weight, then ground in a porcelain mortar and sieved through a 1 mm sieve.

Mineralization was carried out by the open method: 1 g of dry plant powder was mixed with 10 ml of concentrated nitric acid (HNO₃) and 2 ml of hydrogen peroxide (H₂O₂) and decomposed at 120°C for 4 hours.

After cooling, the solution was filtered and made up to 25 ml with distilled water.

The content of Pb, Cd and Zn elements was determined by atomic absorption spectrometry (AAS) (AA-7000 Shimadzu, Japan).

The instrument was calibrated using standard solutions. Each measurement was repeated three times, and the mean and standard deviation were calculated (n = 3).

After acid digestion of heavy elements, the content of Pb, Cd, Zn was determined by atomic absorption spectroscopy.

The results are expressed in mg/kg of dry weight. The migration assessment was calculated using the plant incorporation factor according to Equation 1:

$BAF=\frac{C_{plant}}{{\text{С}}_{soil}}$ (1)

Here, BAF – bioaccumulation factor, $C_{plant}$– refers to the amount of the metal element in the edible part of the plant (mg/kg), and $C_{soil}$ – refers to the amount of the element in the soil (mg/kg).

Statistical analysis was performed to assess the reliability of obtained data during the research. One-way analysis of variance (ANOVA) was used to determine the differences in the levels of heavy metals transferring in plants, and Tukey's test was used to determine which groups had differences (p < 0.05).

Pearson's correlation coefficient (r) was calculated to assess the relationship between the concentrations of heavy metals in soil and plant samples. This method allowed us to determine the migration capacity of metals and the probability of their accumulation in plants.

All results are given as the mean value (Mean) ± standard deviation (SD), the experiment was performed with three to five repetitions (n = 3–5).

3. Results of the Studies and Their Discussion

As mentioned above, main anthropogenic source of heavy metals entering the natural environment is transport, which, due to the high level of atmospheric emissions, forms a powerful flux of heavy metals. Due to the imperfection of ecological processes, it leads to pollution of the atmosphere as a whole, and also negatively affects the surface and subsurface layers of the soil and also plants.

Transport is a source of pollutants such as zinc, lead and cadmium in roadside soils. Specifically, vehicle exhaust is historically a major contributor to Pb accumulation, while Zn and Cd are strongly associated with the continuous wear of automobile tires, brake pads, and the resuspension of road dust deposition (Serrani et al., 2022). The main highway connecting Kazakhstan and Uzbekistan passes through Saryagash. In this regard, the flow of transit and light vehicles on this road is very high. When studying samples taken from the surface layer of the soil and the 25-30 cm deep layer, the formation of ecological anomalies and their specific dependence on the sources of pollutants were revealed (Table 1). These particulates settle on adjacent agricultural soils, creating the localized pollution gradient observed in our study.

Table 1
The content of heavy metals in roadside soils, mg/kg soil.

Road side, km	Distance from highway, m
	2	5	10	150	200	300
	Lead
5	25.8	22.32	18.6	12.7	7.86	5.61
10	26.2	23.45	19.1	13.45	8.14	6.34
15	26.9	24.1	18.9	12.97	8.03	5.98
Average value	26.3±0.56a	23.29±0.90b	18.87±0.25c	13.04±0.38d	8.01±0.14e	5.98±0.37f
Zinc
5	21.56	18.37	13.9	12.65	9.83	4.59
10	20.98	18.96	13.19	12.71	8.97	3.91
15	21.34	17.98	14.12	13.01	8.78	3.98
Average value	21.29±0.29a	18.44±0.49b	13.74±0.47c	12.79±0.19c	9.19±0.56d	4.16±0.36e
Cadmium
5	0.48	0.31	0.14	0.09	0.05	0.01
10	0.47	0.32	0.16	0.1	0.05	0.01
15	0.47	0.3	0.16	0.08	0.05	0.01
Average value	0.47±0.01a	0.31±0.01b	0.15±0.01c	0.09±0.01d	0.05±0.0e	0.01±0.0f

• Note: Different superscript letters within the same row indicate statistically significant differences at p<0.05 according to Tukey’s test.

As can be seen from the table, the highest concentration of heavy metals is in the soil along the road from 2 m to 5 m, and then the concentration of heavy metals is decreased. It is clear that the soil contains a high amount of lead, while the zinc content is lower, and the cadmium content is minimal. Consequently, the established decreasing order of absolute concentration is Pb > Zn > Cd.

The next task of the research was to study the concentration of heavy metals in vegetable and cucurbits to determine the migration of defined heavy metals in the soil to these plants. The results of the study are presented in Table 2 and Figure 1 2-3.

Table 2
The number of heavy metals in vegetable and cucurbits, (mg/kg).

Type of the plant	Pb, mg/kg	Zn, mg/kg	Cd, mg/kg
Tomato	1.81±0.15a	12.5±0.9a	0.044±0.007a
Cucumber	1.33±0.11b	10.1±0.6b	0.037±0.005a,b
Water melon	0.95±0.07c	8.3±0.5c	0.029±0.004c
Melon	1.11±0.09b,c	9.0±0.6b,c	0.032±0.004b,c

• Note: Different superscript letters within the same column indicate statistically significant differences between plant species at p<0.05 according to Tukey’s test.

Figure 1
Graph of comparison of Cd content in plants with the maximum permissible concentration (mg/kg).

Figure 2
Graph of comparison of Zn content in plants with the maximum permissible concentration (mg/kg).

Figure 3
Graph of comparison of Pb content in plants with the maximum permissible concentration (mg/kg).

As can be seen from the results, the Pb content in all studied vegetables and cucurbits is significantly higher than the MPC, the Zn concentration in tomatoes and root vegetables is higher than the MPC, and in cucurbits is in accordance with the norm, the Cd concentration in the mentioned vegetables and melons is lower than the norm.

The penetration of heavy metals into plants was determined by the transfer factor, the results of which are given in Table 3. The values of heavy metals in the soil at a distance of 300 m from the road were taken for the calculation of the formula.

Table 3
The transfer factor of heavy metals from the soil to vegetables and cucurbits.

Cucurbits	Pb (TF)	Zn (TF)	Cd (TF)
Tomato	0.30	3.0	4.4
Cucumber	0.22	2.43	3.7
Watermelon	0.16	1.99	2.9
Melon	0.19	2.16	3.2

Cadmium has the highest coefficient (2.9-4.4), which indicates a high transfer of cadmium into plants, zinc has an average level (1.99-3.0), which indicates its good bioavailability, and lead (0.16-0.30), which indicates its low migration into plants. Thus, according to the transfer factor, the following order was formed: Cd > Zn > Pb. A one-way analysis of variance (ANOVA) was performed to assess the reliability of statistical differences in the accumulation of heavy metals in vegetables and cucurbits. The studies have shown that the statistical difference between cultures is significant for lead (p<0.001), and for zinc and cadmium (p<0.05). This, in turn, confirms the fact that the accumulation of heavy metals depends on the variety of plants. Compared with international studies, the TF value obtained for cadmium in melons is lower than that of India, where the TF value of cadmium in vegetable crops grown in China does not exceed 2.0, but is lower than that of India, where the TF value of cadmium in soils irrigated with wastewater reaches 5.0-6.0 for vegetable crops. This confirms that intensive technogenic influence determines the bioavailability of cadmium.

Regarding the results obtained for zinc, the TF (3.0) is consistent with the data, which shows that the accumulation level of zinc heavy metal (2.5–3.2) in vegetable crops grown near production areas in China is similar. This confirms that zinc heavy metal enters the environment through the wear of vehicle tires and brake pads.

The TF values obtained for lead (Pb) were relatively low (0.16–0.32). These values are consistent with the results presented in many international studies. These works show that lead has low mobility and it is accumulated more in leafy vegetables than in fruit parts.

Thus, the results of the study indicate that Cd has the highest mobility and is dangerous to human health in vegetable crops, while Pb is mainly accumulated in soil. Such patterns are observed in other regions of the world, which confirms the universal nature of the identified trends.

Correlation analysis was performed to assess the relationship between the transfer factors of various heavy metals (Pb, Zn, Cd) (Table 4, Figure 4).

Table 4
Correlation matrix of heavy metal of transfer factor in vegetable and fruit crops.

Indicators	Pb (TF)	Zn (TF)	Cd (TF)
Pb (TF)	1.00	0.92	0.95
Zn (TF)	0.92	1.00	0.99
Cd (TF)	0.95	0.99	1.00

Figure 4
Correlation matrix of transfer factor (TF).

A positive correlation (r > 0.90) was observed between all coefficients. A particularly close relationship was found between the heavy metals zinc and cadmium (r = 0.99), which indicates that the mechanisms of migration of these elements into plants are similar. However, it is important to note that a high positive correlation does not strictly indicate an identical pollution source. For instance, within vehicular emissions, Pb is historically associated with fuel combustion exhaust, whereas Zn is primarily released from tire wear, and Cd is linked to brake pad friction and lubricating oils. Therefore, the strong correlation (r>0.90) observed among these metals likely reflects a shared transport and deposition mechanism—specifically, the simultaneous atmospheric dispersion and settling of traffic-related dust across the agricultural area—rather than a single emission point.

The migration of heavy metals from the soil to plants depends on their mobility and bioavailability. Lead, which has a very low mobility, it is accumulated in the roots and leaves, which in turn limits its transfer into the fruits. Typically, a lower (more acidic) soil pH increases metal solubility and bioavailability, facilitating higher root uptake, particularly for mobile elements like Cd. Conversely, high organic matter can form stable organo-mineral complexes that strongly bind heavy metals like Pb, thereby restricting their transfer to the aerial and edible parts of the plant (Hawrami and Baset, 2024). However, in crops grown near roads, it can be accumulated on the surface layer of the product due to atmospheric dust deposition.

Overall, the observed trends are highly consistent with broader global studies on urban and peri-urban agriculture. In various international contexts, Cd consistently exhibits higher soil-to-plant transfer rates compared to Pb and Zn, posing severe health risks via the food chain even at relatively low soil concentrations (Khan et al., 2008; Hawrami and Baset, 2024). Furthermore, our TF values for cucurbits align with the contamination patterns observed in European and Asian agricultural soils, confirming that intensive traffic pressure dictates heavy metal bioavailability regardless of the specific climatic zone (Serrani et al., 2022).

One of the heavy metals, zinc is characterized by its high mobility and ability to transfer into the fruits of vegetables and cucurbits. In particular, as many studies have shown, tomatoes and cucumbers actively accumulate zinc in the fruits, and their high concentration in the soil can lead to exceeding safety standards.

Although the concentration cadmium is low in the soil, it can be easily transferred through the above-ground parts of plants. Especially in the case of horticultural fruits, cadmium can be accumulated in the edible part of the fruit.

Thus, according to our research results, motor vehicles remain one of the sources of heavy metals in agro-vegetable natural areas.

Therefore, growing tomatoes, cucumbers, watermelons and melons along highways poses a risk of accumulation of heavy metals such as cadmium and zinc in their fruits.

4. Conclusion

According to the results of the study, the determination of heavy metals (exactly Pb, Zn, Cd) in soils near highways in South Kazakhstan was found to be 1.5–3 times higher than the permissible limit concentrations. It was observed that while the concentration was high in areas near the road, their content was decreased with increasing distance. The sequence of decreasing soil accumulation was determined as follows: Pb > Zn > Cd.

As a result of evaluation, the transfer of heavy metals to agricultural crops, the transfer factor (TF) varied in the following range: Pb — 0.16–0.32; Zn — 1.99–3.0; Cd — 2.9–4.4. The highest values belong to Cd element, which proves its high mobility and poses a significant threat to food security.

The conducted correlation analysis showed that there was a clear correlation between metal concentrations in soil and plants. This allows using the TF indicator as a reliable indicator of environmental risks.

In conclusion, growing vegetables and horticultural crops near busy roads increases the risk of heavy metal accumulation in them. These products may exceed permissible levels, which may pose a threat to public health.

Acknowledgements

This study was funded by the Ministry of Science and Higher Education of the Republic of Kazakhstan, under the grant NºAP22684097 « Study of heavy metals in the soil-plant system when growing crops in the south of the country».

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

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Edited by

• Editor:
  Takako Matsumura Tundisi

Publication Dates

• Publication in this collection
  24 July 2026
• Date of issue
  2026

History

• Received
  11 Feb 2026
• Accepted
  08 Mar 2026