Multi-Index Assessment of Heavy Metal Contamination and Ecological Risks in Paddy Soils Along National Highways in Southern Henan Province, China

Abstract

(1) Background: Road traffic emissions significantly influence heavy metal accumulation in roadside agricultural soils, posing risks to food safety. (2) Methods: This study investigated the concentrations of heavy metals (As, Cd, Cu, Cr, Hg, Ni, Pb, and Zn) in paddy soils at 96 soil samples along National Highways G107 and G312 in southern Henan, China, to evaluate the contamination situation and ecological risks using a multimetric approach. (3) Results: Cd, Hg, Cu, and Zn exceeded provincial background levels. Cd dominated contamination, showing heavy pollution (single factor index, P_i > 5) within 40 m of G107 and moderate/heavy levels (P_i = 2–5) along G312. The Nemerow index (P_N) classified both highways as slightly polluted (P_N = 0.70–0.81), with higher contamination along G107. Geoaccumulation indices identified Cd as mildly/moderately polluted within 40 m of G107 and G312 and Zn as slightly contaminated within 20–40 m of G107. Despite low total ecological risk, Cd contributed >75% to cumulative risk due to its high toxicity (Tr = 30). (4) Conclusions: Road traffic constitutes one of the contributors to heavy metal accumulation in paddy soils along national highways in southern Henan Province, while agricultural cultivation adjacent to transportation corridors poses potential food safety risks.

1. Introduction

Soil ecosystems constitute the fundamental basis for global agricultural productivity and environmental sustainability. However, anthropogenic activities have induced severe heavy metal contamination in terrestrial environments, posing significant threats to ecological integrity and human health [1,2]. These persistent pollutants exhibit characteristics that include environmental persistence, acute toxicity, and recalcitrance to remediation [3,4], with demonstrated capacity for bioaccumulation through trophic transfer mechanisms [5]. In agricultural systems, heavy metal accumulation degrades soil tilth quality and impairs crop productivity, while contaminated food crops present direct exposure pathways to human populations [6,7].

Recent nationwide surveys reveal alarming contamination patterns in China, with 16.1% of soil sampling sites exceeding regulatory thresholds for heavy metals [8]. Cadmium emerges as the most prevalent contaminant (7.0% exceedance rate), followed by nickel (4.8%), arsenic (2.7%), copper (2.1%), mercury (1.6%), and lead (1.5%). Regional disparities are particularly pronounced in Southwest China, where background levels of As, Cd, Hg, and Pb significantly surpass national averages [9,10]. Similar contamination patterns have been documented in industrialized nations, including the United States and European countries [11], underscoring the global nature of this environmental challenge. Therefore, elucidating the status of soil heavy metal pollution is crucial for maintaining global ecological equilibrium and ensuring food security.

The sources of heavy metals in soil are diverse [12], with vehicular traffic emissions being a major global concern [13,14] as vehicle exhaust emissions, vehicle tires, brake pad wear, and lubricant combustion are important sources of heavy metal contamination of soils around roads [15,16,17]. Studies have demonstrated a positive correlation between traffic volume and heavy metal concentrations in roadside soils [18]. Elevated heavy metal levels are particularly prevalent at intersections, curved road sections, slope areas, traffic congestion zones, and other roadway segments subject to recurrent braking maneuvers or heightened tire/pavement friction interactions [19]. Heavy metal concentrations in soil typically exhibit an exponential decline with increasing distance from roadways [20], with Zn reaching background levels at 30 m [16,21] and Pb, Cd, and Cu levels peaking in the range of 25 to 50 m from the road [22]. Therefore, it is of great significance to explore the pollution characteristics of heavy metals in soil on both sides of the highway traffic for the prevention and control of heavy metals in farmland soil.

Contemporary pollution assessment employs multidimensional analytical frameworks combining the Nemerow pollution index (P_N), single factor index (P_i), geoaccumulation index (I_geo), and potential ecological risk index (RI) [23,24,25]. While P_N and P_i quantify composite and individual metal contamination, respectively [26,27], the I_geo provides sediment-specific historical accumulation analysis [28,29]. The RI index offers critical insights into ecological risk thresholds through synergistic consideration of metal toxicity and bioavailability [30,31]. Integrated application of these complementary metrics enables comprehensive pollution characterization and informed remediation strategies.

Xinyang serves as a principal rice cultivation zone within Henan Province, China, with a dedicated planting area of 431,000 ha, constituting the second most extensive cereal crop production system in the province [32], and the current research on heavy metal pollution in roadside farmlands within this region remains limited, with existing studies predominantly characterized by temporal gaps and dated methodologies. This study analyzed heavy metal contamination in paddy soils along national highways using a multi-index framework and further identified the impact of road traffic on soil heavy metal pollution in proximity to these highways.

2. Materials and Methods

2.1. Study Area

The study area is located in southern Henan, China (Figure 1). National Highway G107 runs from the Hong Kong Special Administrative Region in the south to Beijing in the north, spanning a total length of 2698 km. It traverses the North China, Central China, and South China regions, serving as a crucial north/south arterial route. The average daily traffic volume on this road is 20,000 vehicles, with large vehicles (trucks, large and medium-sized buses) accounting for about 53%. National Highway G312 stretches from Huangpu District, Shanghai, in the east to Horgos city in the Ili Kazakh Autonomous Prefecture, Xinjiang, in the west, spanning a total length of 4967 km. It passes through nine provincial-level regions: Shanghai, Jiangsu, Anhui, Henan, Hubei, Shaanxi, Ningxia, Gansu, and Xinjiang. The average daily traffic volume on this road is 17,000 vehicles, with the proportion of heavy vehicles being as high as 41%. National Highways G107 and G312 are major transportation arteries in Xinyang City, Henan Province, China. This region is classified as a transitional zone between subtropical and warm temperate climates, characterized by a distinct monsoon climate. It enjoys ample sunshine with an annual duration of 1900–2100 h. The mean annual temperature ranges from 15.3 to 15.8 °C, accompanied by a prolonged frost-free period averaging 220–230 days. The area experiences abundant rainfall, with annual precipitation of 993–1294 mm, high humidity, and a mean annual relative humidity of 74–78%.

2.2. Method of Soil Sample Collection and Determination

Despite agricultural activities not being allowed within 20 m of national highways, field surveys along National Highways G107 and G312 revealed persistent crop cultivation within these restricted zones, driven primarily by farmers’ economic incentives. In order to quantitatively assess the effect of road traffic on heavy metal accumulation in field paddy soils, soil samples were collected from paddy fields within 40 m of adjacent national highways along these specific road sections. Based on this, eight sampling sections were established along the rice cultivation zones bordering National Highways G312 and G107 in 2023. At each section, monitoring transects perpendicular to the highway were configured at 10, 20, 30, and 40 m intervals from the road edge. Within each distance-specific transect, three replicate sampling points were systematically positioned. Composite soil samples (0–20 cm depth) were collected using a multi-point homogenization protocol, yielding 96 initial specimens. These were subsequently reduced to representative 1 kg aliquots through rigorous quartering procedures. Following air-drying under laboratory conditions, samples underwent standardized preparation, including removal of plant residues and gravel, followed by mechanical grinding and sieving through a 100-mesh nylon screen prior to heavy metal quantification.

The total amount of soil heavy metals was determined by four-acid (hydrochloric acid, nitric acid, high hypochlorous acid, and hydrofluoric acid) digestion method [33], and the content of heavy metals Cr, Cd, Ni, Cu, Zn, and Pb was determined by ICAP-6000 inductively coupled plasma emission spectrometer, and As and Hg were determined by atomic fluorescence spectrophotometry (Beijing Jitian AFS-930). Quality control was implemented using National Standard Reference Materials (GBW07401), with 20% of samples randomly selected as analytical duplicates to verify methodological precision

2.3. Single Factor Index

The single factor index method is employed to assess the contamination level of individual heavy metals [23], with the calculation formula expressed as:

$$P_i={\displaystyle \frac{C_i}{S_i}}$$

(1)

where P_i is the single factor index of ith heavy mental; C_i is the concentration of ith heavy mental (mg·kg⁻¹); S_i is the background concentration of ith heavy metal (mg·kg⁻¹). In this study, the heavy metal element content in the surface soil of Henan Province was selected as the background value (the background values of heavy metals As, Cd, Cu, Cr, Hg, Ni, Pb, and Zn in Henan Province were 10.0, 0.147, 22.0, 67.6, 45.0, 27.6, 23.6, and 61.5, respectively), and the degree of pollution in the study area was evaluated, and the specific grading criteria are shown in

Table 1. Classification index of single factor index and Nemerow index in soil.

Single Factor Index (Pi)	Pollution Level	Nemerow Index (PN)	Pollution Level
Pi ≤ 1	No pollution	PN ≤ 0.7	Clean
1 < Pi ≤ 2	Mild to moderate pollution	0.7 < PN ≤ 1	Slight Pollution
2 < Pi ≤ 3	Moderate pollution	1 < PN ≤ 2	Light/Moderate Pollution
3 < Pi ≤ 5	Heavy pollution	2 < PN ≤ 3	Moderate/Heavy Pollution
5 ≤ Pi	Extreme pollution	3 ≤ PN	Severe Pollution

.

2.4. Nemerow Index

The Nemerow pollution index method can comprehensively evaluate the soil heavy metal pollution status [27]; the grading criteria are shown in Table 1.

$$P_N={\displaystyle \frac{\sqrt{{P_{avg}}^2+{P_{max}}^2}}{2}}$$

(2)

where P_N is the Nemerow pollution index; P_avg is average of single factor index; P_max is maximum value.

2.5. Geoaccumulation Index

The geoaccumulation index (I_geo) is a quantitative method for assessing heavy metal contamination, and this index has been widely applied in environmental studies [34,35] and is calculated as follows:

$$I_{geo}={log}_2{C}_n/K\times B_n$$

(3)

where C_n represents the concentration of the ith heavy metal in the soil (mg·kg⁻¹), K denotes its corresponding geochemical background value, and B_n (mg·kg⁻¹) denotes its corresponding geochemical background value The classification criteria for contamination levels are presented in

Table 2. Geological accumulation index classification standard.

Geoaccumulation Index (Igeo)	Grade	Pollution Level
≤0	0	No pollution
0 < Igeo ≤ 1	1	Mild to moderate pollution
1 < Igeo ≤ 2	2	moderate pollution
2 < Igeo ≤ 3	3	Moderate to heavy pollution
3 < Igeo ≤ 4	4	Heavy pollution
4 < Igeo ≤ 5	5	Heavy to extreme pollution
5 ≤ Igeo	6	Extreme pollution

.

2.6. Ecological Risk Index

The potential ecological risk index (RI) is to assess sedimentary contamination and has gained increasing application in evaluating polymetallic pollution across environmental compartments [36,37,38,39]. The RI is calculated using the following expressions:

$$C_f^i=C_D^i/C_R^i$$

(4)

$$E_r^i=T_r^i/C_f^i$$

(5)

$$RI=\sum _1^m{E}_r^i$$

(6)

where RI is the composite potential ecological risk index for the area, $E_r^i$ is the ecological risk index, $T_r^i$ is the toxicity coefficient of a given heavy metal, $C_f^i$ is the contamination factor of the heavy metal, $C_D^i$ is the measured concentration of a certain trace element in the soil, and $C_R^i$ is a reference value. The standardized toxicity coefficients ($T_r^i$) for common heavy metals are as follows: Cd = 30, As = 10, Pb = 5, Cu = 5, Hg = 40, Ni = 5, Cr = 2, and Zn = 1 [30,31,40]. The classification criteria for potential ecological risk levels are summarized in

Table 3. Ecological hazard index classification standard.

Ecological Risk Index (${\mathit{E}}_{\mathit{r}}^{\mathit{i}}$)	Grade	Pollution Level	Potential Ecological Risk Index (RI)	Grade	Pollution Level
$E_r^i$< 40	1	Low potential ecological risk	RI < 150	1	Low potential ecological risk
40 ≤ $E_r^i$< 80	2	Moderate potential risk	150 ≤ RI < 300	2	Moderate potential risk
80 ≤ $E_r^i$ < 160	3	Considerable potential risk	300 ≤ RI < 600	3	Considerable potential risk
160 ≤ $E_r^i$ < 320	4	High potential risk	600 ≤ RI < 1200	4	High potential risk
320 ≤ $E_r^i$	5	Very high potential risk	1200 ≤ RI	5	Very high potential risk

.

2.7. Data Analysis

Microsoft Excel 2010 and Origin 2021 were utilized for statistical analysis and graphing of data. To identify the relationship among the trace elements, Pearson correlation and ANOVA were performed using SPSS 20.0.

3. Results

3.1. Statistical Characterization of Soil Heavy Metals

The statistical characteristics of soil heavy metals in roadside soil are shown in

Table 4. Statistical characteristics of heavy metals in soils at different roadside distances.

Roadside Distance/m	Items	As	Cd	Cu	Cr	Hg	Ni	Pb	Zn
10	average value	4.78 ± 1.88	0.49 ± 0.09	28.73 ± 6.56	55.92 ± 6.21	0.33 ± 0.31	8.48 ± 1.38	17.34 ± 2.67	85.71 ± 11.47
	maximum value	8.76	0.63	50.54	70.06	1.52	12.69	23.66	180.91
	minimum value	1.04	0.31	21.24	45.11	0.05	6.64	13.43	52.89
	standard deviation	1.88	0.09	6.56	6.21	0.31	1.38	2.67	26.42
	coefficient of variation	39.43	18.36	22.82	11.11	95.19	16.28	15.39	30.83
20	average value	4.72 ± 1.71	0.45 ± 0.07	28.67 ± 5.61	54.77 ± 8.73	0.29 ± 0.18	8.29 ± 1.12	16.47 ± 2.80	85.70 ± 11.47
	maximum value	8.25	0.61	48.58	67.82	0.73	10.38	26.32	104.02
	minimum value	1.99	0.35	17.74	40.32	0.04	5.84	13.19	58.29
	standard deviation	1.71	0.07	5.61	8.73	0.18	1.12	2.80	11.47
	coefficient of variation	36.16	15.23	19.56	15.95	63.01	13.53	17.00	13.38
30	average value	3.89 ± 1.66	0.45 ± 0.09	27.93 ± 5.84	54.41 ± 8.73	0.15 ± 0.13	7.97 ± 1.54	15.81 ± 1.87	85.48 ± 18.82
	maximum value	6.97	0.62	15.70	67.75	0.60	10.16	20.71	147.91
	minimum value	0.59	0.27	38.55	36.54	0.02	4.81	13.14	52.01
	standard deviation	1.66	0.09	5.84	9.19	0.13	1.54	1.87	18.82
	coefficient of variation	42.61	21.11	20.91	16.89	86.00	19.36	11.82	22.01
40	average value	4.41 ± 1.55	0.45 ± 0.17	27.57 ± 6.30	55.47 ± 6.39	0.25 ± 0.18	7.99 ± 1.66	16.12 ± 1.38	83.61 ± 10.21
	maximum values	7.18	0.92	49.79	66.46	0.02	10.39	18.59	105.19
	minimum value	1.72	0.24	15.43	13.92	0.70	4.65	13.11	71.39
	standard deviation	1.55	0.14	6.30	6.39	0.18	1.66	1.38	10.21
	coefficient of variation	42.61	31.44	22.84	11.53	69.20	20.79	8.57	12.21
reference value	background values	10	0.147	22.0	67.6	0.045	27.6	23.6	82.79

Note: The coefficient of variation is in % and the rest of the values are in mg kg⁻¹.

. The concentration hierarchy was Zn > Cr > Cu > Pb > Ni > As > Cd > Hg, with coefficients of variation (CV) ranking as Hg > As > Cd > Cu > Zn > Ni > Cr > Pb. Among them, the coefficient of variation for Hg ranged from 63.01% to 95.19%, which is highly variable. The coefficients of variation of soil As roadside distances of 10–30 m are classified as medium-height variability, and roadside distances of 40m are classified as medium variability. The coefficients of variation of soil Cd, Cu, Cr, Ni, Pb, and Zn are classified as medium variability. Comparative analysis with Henan Province soil background values revealed exceedances for Cd, Cu, Hg, and Zn, while As, Cr, Ni, and Pb remained within baseline levels. The mean value of As, Cr, Hg, Ni, and Pb at roadside distances are 10 m > 20 m > 40 m > 30 m. The mean values of Cu and Zn at roadside distances are 10 m > 20 m > 30m > 40 m; the mean value of Cd at roadside distances is 10 m > 20 m = 30 m = 40 m (Figure 2).

As presented in

Table 5. Content of heavy metals in roadside soil at different sections (mg·kg−¹).

Sampling Section	Roadside Distance/m	As	Cd	Cu	Cr	Hg	Ni	Pb	Zn
	10	4.03 ± 1.12 a	0.45 ± 0.08 a	29.36 ± 6.94 a	57.00 ± 7.17 a	0.34 ± 0.38 a	8.60 ± 1.69 a	15.97 ± 1.23 a	83.70 ± 9.52 a
	20	4.09 ± 1.82 a	0.45 ± 0.09 a	26.69 ± 7.23 a	53.83 ± 6.51 a	0.33 ± 0.22 a	7.90 ± 1.09 a	16.34 ± 2.22 a	79.96 ± 10.34 ab
312	30	3.31 ± 1.40 a	0.43 ± 0.11 a	28.06 ± 7.50 a	52.10 ± 9.42 a	0.31 ± 0.19 a	7.51 ± 1.72 a	16.03 ± 1.70 a	79.88 ± 13.93 ab
	40	3.86 ± 1.38 a	0.39 ± 0.07 a	25.90 ± 5.12 a	50.24 ± 8.99 a	0.17 ± 0.16 a	7.27 ± 1.67 a	15.65 ± 1.67 a	73.89 ± 10.07 b
	average value	3.82	0.43	27.50	53.29	0.29	7.82	16.00	79.36
	10	5.86 ± 1.95 a	0.46 ± 0.04 b	27.53 ± 1.95 a	54.13 ± 3.90 b	0.31 ± 0.18 a	8.28 ± 0.64 b	15.55 ± 2.69 b	83.46 ± 11.88 a
	20	5.93 ± 1.42 a	0.55 ± 0.04 ab	29.05 ± 4.33 a	58.20 ± 5.48 ab	0.23 ± 0.09 ab	8.95 ± 0.89 ab	19.01 ± 2.61 a	95.25 ± 5.18 a
107	30	4.85 ± 1.67 a	0.47 ± 0.07 ab	29.85 ± 4.78 a	59.22 ± 5.31 a	0.16 ± 0.11 b	8.73 ± 0.78 ab	16.28 ± 0.61 b	94.83 ± 23.15 a
	40	5.33 ± 1.45 a	0.56 ± 0.17 a	31.32 ± 5.62 a	61.36 ± 3.77 a	0.12 ± 0.05 b	9.21 ± 0.88 a	17.84 ± 3.78 ab	105.41 ± 33.62 a
	average value	5.49	0.51	29.44	58.23	0.20	8.79	17.17	94.74
Background values		10	0.147	22.0	67.6	0.045	27.6	23.6	82.79

Note: Different lowercase letters within the same column indicate statistically significant differences (p < 0.05) at different distances from the same national highway.

, comparative analysis with Henan Province’s geochemical background values revealed distinct contamination patterns between the G312 and G107 highways: the contents of heavy metals of soil Cd, Cu, and Hg for G312 exceeded the background values for soils in Henan Province by 193, 25, and 544%, and the contents of heavy metals Cd, Cu, Hg, and Zn for G107 exceeded 247, 34, 344, and 14% of the soil background values in Henan Province, while the contents of other heavy metals did not exceed the soil background values in Henan Province.

The ANOVA results indicated that for National Highway G312, no significant differences were observed in Zn concentrations between 10 and 30 m, nor among 20 m, 30 m, and 40 m distances. All other elements showed no significant variations within 0–40 m of the roadside. For National Highway G107, As, Cu, and Zn exhibited no significant differences across 0–40 m roadside distances, while certain variations were detected in other elements at different roadside distances.

3.2. Pearson Correlation Analysis of Heavy Metals

Figure 3 delineates the inter-element relationships through Pearson correlation analysis. There was a highly significant positive correlation (p < 0.01) between As and Cd, As and Cu, As and Cr, As and Ni, As and Zn, Cd and Cu, Cd and Cr, Cd and Ni, Cd and Pb, Cd and Zn, Cu and Pb, Cu and Zn, Cr and Ni, Cr and Zn, Ni and Zn, and Pb and Zn, where the correlation coefficients of As-Ni, Cd-Cr, Cd-Pb, Cd-Zn, Cr-Ni, and Pb-Zn are greater than 0.5, suggesting a strong correlation between them.

3.3. Evaluation of Soil Heavy Metal Pollution

Figure 4 presents the single factor evaluation results of soil heavy metals. For Cd along the National Highway G312, the pollution index values at 10 and 20 m roadside distances ranged between 3 and 5 (indicating heavy pollution), while those at 40 and 50 m distances showed P_i values of 2–3 (classified as a moderate level of pollution). The P_i of soil Cd at 10 m, 20 m, 30 m, and 40m on roadside from National Highway G107 indicated heavy pollution levels, whereas soil Cu and Zn were considered at the mild to moderate pollution level at 10–40 m from the National Highways G107 and G312. A no-pollution level was determined for the other heavy metals (As, Cr, Hg, Ni, and Pb) at 10–40 m from the National Highways G107 and G312.

The Nemerow pollution index results are presented in Figure 4. The box-overlaid violin plot (Figure 5a) reveals that roadside distances followed the P_N magnitude sequence: 20 m (0.78) > 40 m (0.77) > 30 m (0.74) > 10 m (0.73), though the overall differences were not substantial. The mean P_N values of soil heavy metals across different road sections exhibited the following order: National Highway G107 (0.81) > National Highway G312 (0.70). Figure 5b demonstrates that within the roadside distance range of 10–40 m along the National Highways G312 and G107, the mean P_N values of soil heavy metals ranged from 0.7 to 1.0, collectively corresponding to an overall moderately clean level.

3.4. Ecological Risk Assessment of Soil Heavy Metals

The results of the geoaccumulation index show that (Figure 6) along the National Highway G312, soil heavy metals exhibited the order Cd (0.281) > Zn (−0.070) > Cu (−0.090) > Cr (−0.285) > Pb (−0.347) > As (−0.633) > Ni (−0.733) > Hg (−2.521), while along the National Highway G107, the sequence was Cd (0.360) > Zn (0.003) > Cu (−0.054) > Cr (0.243) > Pb (−0.320) > As (−0.456) > Ni (−0.675) > Hg (−2.622). Comparative analysis demonstrated that the geoaccumulation indices were generally higher for the National Highway G107 than those for the National Highway G312. The geoaccumulation index of soil Cd at 10–40 m from the National Highway G312 and National Highway G107 roadsides was between 0 and 0.5, which is at the mild to moderate pollution level, and the accumulation index of soil Zn for National Highway G107 within 20–40m of roadsides was between 0.002 and 0.042, which is slightly polluted. The values of the accumulation indexes for soil Cu, Cr, Pb, As, Ni, and Hg at 10–40 m from National Highway G312 and G107 were all less than 0, which was classified as unpolluted.

Figure 7 demonstrates the results of the potential ecological risk assessment. The individual potential ecological risk indices for the eight heavy metals followed the sequence Cd > Cr > Cu > As > Pb > Ni > Zn > Hg. The potential ecological risks of all heavy metals within the 10–40 m roadside distances of both the National Highways G107 and G312 fell within low potential ecological risk. Notably, the total risk levels at 10, 20, 30, and 40 m distances along the National Highway G107 consistently exceeded those of the corresponding distances for the National Highway G312. As shown in Figure 7b, the contribution of heavy metals to the RI on National Highway G312 was Cd (75.45%) > Cr (10.42%) > Cu (5.38%) > As (3.29%) > Pb (2.92%) > Ni (1.22%) > Zn (1.11%) > Hg (0.22%), and the results in Figure 7c show that the contribution of heavy metals to the RI on National Highway G107 was Cd (76.41%) > Cr (9.65%) > Cu (4.88%) > As (4.00%) > Pb (2.65%) > Ni (1.16%) > Zn (1.12%) > Hg (0.13%), with soil Cd contributing more to the RI.

4. Discussion

The results of the present study demonstrated that the heavy metals Cd, Cu, Hg, and Zn were found to exceed the soil background values in the soils on both sides of the national highways in southern Henan, while As, Cr, Ni, and Pb did not exceed the back-ground values. Correlation results indicate that As, Cd, Cu, Cr, Ni, and Zn have extremely significant correlations at the p < 0.01 level, suggesting similar sources of origin among these six elements. This study found that Hg and As have larger coefficients of variation, which may be related to their multiple sources. Studies have shown that As is also associated with soil parent material [41,42], while Hg is related to atmospheric deposition [43,44]. Heavy metals Pb, Cd, Cu, and Zn are brought in by motor vehicle fuels and machine parts such as tires, which are the main traffic source of heavy metal elements [15,16,17]. The study area is mainly beside National Highways G107 and G312 and there are sources of heavy metal pollution caused by motorized activities. Therefore, traffic can increase the levels of heavy metals in agricultural soils on both sides of the road [45].

This study found that the average concentrations of As, Cr, Hg, Ni, and Pb followed the order 10 m > 20 m > 40 m > 30 m. This is inconsistent with the pattern observed by Shaikh et al. [20], who found that roadside heavy metal concentrations decrease as the distance from the road increases. This discrepancy may be due to the interference of various external factors (such as climate conditions, agriculture, and industry) on soil heavy metal concentrations, as well as the migration pathways associated with different factors and the particle sizes of atmospheric particulates present. Previous studies have documented that heavy metal concentrations either gradually decrease or remain stable with increasing distance from roadways [46,47,48]. And this study reveals that the concentrations of Cu, Zn, and Cd were high within 10 m of the roadside, which may be attributed to the higher number of particles generated from vehicle brake pads, fuel, and road wear that settle in the soil closer to the road surface [49]. The hilly terrain of the Xinyang region induces terrain-driven wind attenuation, significantly reducing atmospheric dispersion of traffic-emitted particulate matter [50], and roadside afforestation practices amplify the filtration capacity of vegetative barriers [51]. And the high clay content of rice soils in the Xinyang area resulted in significant specialized adsorption of Cd and Cu [52].

The results of the single factor pollution assessment for soil heavy metals indicate that the pollution index (P_i) for heavy metal Cd along the G312 National Highway at distances of 10 m and 20 m from the roadside ranges between 3 and 5, indicating heavy pollution. At distances of 40 m and 50 m, the P_i is between 2 and 3, indicating a moderate level of pollution. Along the G107 National Highway, heavy metal Cd is classified as heavy polluted at all distances from the roadside. For Cu and Zn, the P_i values at all distances from the roadside along both the National Highways G312 and G107 range between 1 and 2, indicating mild to moderate pollution levels. Other heavy metals are at no-pollution levels. The Nemerow index indicates that the average P_N value for soil heavy metals along the National Highway G107(0.81) is higher than that along the G312 National Highway G312 (0.70). The comprehensive pollution results show that the overall pollution level along both the National Highways G312 and G107 at all distances from the roadside is still relatively clean.

The results of the ecological risk assessment of soil heavy metals indicate that the order of the individual potential ecological risk indices for the eight heavy metals is Cd > Cr > Cu > As > Pb > Ni > Zn > Hg. The total potential ecological risk of all heavy metals within the 40 m roadside distance of both National Highway G107 and National Highway G312 is at a level of low potential ecological risk. The total potential risk level of heavy metals within the 40 m roadside distance of National Highway G107 is higher than that of National Highway G312, with heavy metal Cd contributing significantly to the risk index (RI); this is due to the fact that Cd is more toxic and the RI is mainly affected by the more toxic heavy metal elements in the soil [53,54,55]. Previous studies have indicated that heavy metal pollution is associated with traffic volume [47,56,57], and this study revealed that the geoaccumulation index overall shows that the values for National Highway G107 are higher than those for National Highway G312. This discrepancy may be attributed to the fact that National Highway G107 serves as a major north/south transportation artery passing through the urban area of Xinyang City, handling substantially higher freight and passenger traffic volumes, which concentrate and intensify vehicular activity, thereby resulting in more pronounced pollutant accumulation. Both national highways have Cd at a level of mild to moderate pollution. Specifically, within the 20–40 m roadside distance of National Highway G107, the geoaccumulation index for Zn ranges from 0.002 to 0.042, indicating slight pollution, while the geo-accumulation indices for Cu, Cr, Pb, As, Ni, and Hg are all less than 0, indicating no pollution. Therefore, this agricultural practice of proximal cultivation by farmers is contraindicated, as the excessive Cd contamination in soils can migrate and bioaccumulate in rice grains, ultimately posing significant health risks to consumers through dietary intake [58,59].

Although we have determined that the heavy metal content of farmland soils on both sides of the national highways in southern Henan is related to road traffic and that some heavy metals contaminate the area, the sampling design was restricted to distance of the soil from the road in an area 40 m wide, specifically targeting agricultural zones with established roadside cultivation practices where farmers routinely plant crops adjacent to transportation corridors. However, such proximity-based cultivation practices near major national highway corridors lack global universality, so the results of the study still have some limitations.

5. Conclusions

In summary, the concentrations of Cd, Cu, Hg, and Zn exceed the soil background values in Henan. And Cd reaches heavy pollution levels within a 40 m roadside distance along National Highway G107 and is at moderate to moderate/heavy pollution levels within a 40 m roadside distance along National Highway G312. The overall pollution level along National Highway G107 is higher than that along National Highway G312 but is still relatively clean, with low potential ecological risk. In summary, the heavy metal concentrations in the soil on both sides of the national highways in southern Henan are related to road transportation, and some heavy metals pose certain levels of pollution and ecological risk. It is recommended that farmers are not advised to grow crops close to roads because of the potential pathway by which heavy metal contamination in the soil can be transferred to agricultural products, thus posing a food safety risk.