Heavy Metal Spatial Variation Mechanism and Ecological Health Risk Assessment in Volcanic Island Soils: A Case Study of Weizhou Island, China

Abstract

Heavy metals in volcanic island soils are key for assessing pollution risks and guiding environmental management strategies. However, research on heavy metals in volcanic island soils remains limited. In this study, the concentrations of heavy metals (Cu, Zn, Pb, Cr) in surface soil samples from Weizhou Island, China, were determined using ICP-OES, with average concentrations of 59.18 mg/kg, 119.06 mg/kg, 35.63 mg/kg, and 159.78 mg/kg, respectively. The basalt profiles generally exhibit higher heavy metal content and pH values compared to volcaniclastic rock profiles, as basalt accumulates higher concentrations of heavy metals. However, surface soils over volcaniclastic rocks show significantly higher heavy metal concentrations than those over basalt, indicating spatial variability in metal accumulation. Heavy metal concentrations in Weizhou Island soils are notably elevated at both the western wharf and the island’s tail, both characterized by volcaniclastic rock lithology, with human activities further increasing concentrations at the western wharf compared to the island’s tail. Land use types influence heavy metal content, with higher concentrations in abandoned land and lower concentrations in forest land with dense vegetation and organic matter. Principal component analysis reveals that heavy metals are primarily derived from natural parent material, with the first two principal components comprising 59.77% of the variance. Ecological risk assessment indicates that Weizhou Island soil is generally considered relatively clean, but Pb presents an ecological hazard, with 86.54% of the sites at risk. Overall, heavy metals in volcanic island soil mainly come from natural sources but lead contamination and human-impacted areas require attention.

1. Introduction

Islands play a significant role in resources, ecology, and economics [1,2,3]. Distinct geographical landscapes and climatic conditions [4,5] lead to complex soil formation processes. Material cycles on islands differ significantly from those in inland areas, which results in distinct soil properties and formation environments [6,7]. Island ecosystems represent a fusion of terrestrial and marine influences, often shaped by developmental activities [8,9].

Soil quality assessment studies in China mostly focus on farmland or coastal wetlands, with limited research on island ecosystems [10,11,12]. In recent times, increased tourism, port construction, aquaculture, shipping, and land reclamation activities have damaged the environmental quality of islands. As a result, numerous islands are confronted with significant ecological pressures [13,14,15]. The persistence of heavy metals in island soils causes several adverse effects, including crop degradation, bioaccumulation, abnormal growth, human health risks, and ecosystem disturbances [16,17]. Both domestic and international research endeavors have focused intensively on elucidating the origins and mechanisms of heavy metals within the soils of volcanic islands [18,19,20]. Chinese scholars have undertaken a systematic examination of heavy metal origins and distributions within the volcanic island soil of Hainan Island. Their inquiry encompasses both ecological considerations and implications for human health [21,22]. Significantly, geological conditions play a pivotal role in determining the concentrations of Cu, Zn, Cr [23,24,25], providing novel insights into the geochemical modulation of heavy metals attributed to parent rocks. Korean researchers conducted a comprehensive analysis of the geochemical and mineralogical characteristics, and the distribution patterns of heavy metals in the soil of Jeju Island, they also examined the factors influencing the adsorption of heavy metals. They traced the origin of heavy metal constituents back to natural volcanic activities within the soil [26,27]. Further research is required to understand the mechanisms driving the migration and transformation of heavy metals in volcanic island soils. Identifying key environmental factors that influence the distribution and evolution of heavy metals will provide a clearer understanding of island ecosystems. This knowledge is essential for developing strategies for island management, environmental protection, and ecological restoration [28,29,30].

Weizhou Island is the youngest volcanic island in China [31,32]. Due to its substantial distance from the mainland and limited industrial impact, Weizhou Island has consistently preserved an advantageous ecological environment [33]. While most research on heavy metal pollution on Weizhou Island has focused on water quality and sediment, there has been limited investigation into soil pollution. Copper (Cu) and zinc (Zn) are essential trace elements, but their excessive concentrations can pose significant environmental risks [34,35]. Lead (Pb) is a typical toxic heavy metal, often associated with industrial activities and traffic-related pollution [36], while chromium (Cr) is highly toxic and environmentally persistent [37]. Due to their stable geochemical characteristics in volcanic and volcaniclastic rocks, these elements are effective indicators of heavy metal distribution, sources, and potential contamination risks in soils of volcanic regions. This study investigates the geochemical distribution of heavy metals (Cu, Zn, Pb, Cr) in surface soils, basalt, and volcaniclastic rock profiles from Weizhou Island. It focuses on identifying the sources of these metals, assessing the influence of parent rock composition on their spatial distribution, and evaluating the environmental status of the island concerning heavy metal contamination. The findings aim to enhance understanding of the geochemical characteristics and distribution patterns of these metals, thereby contributing to the improvement of the soil’s environmental quality and the safeguarding of food safety.

2. Material and Methods

2.1. Geological and Geographical Settings

Weizhou Island, located in Guangxi, China, spans 6 km from east to west and 6.5 km from north to south, covering an area of approximately 25 km². Weizhou Island experiences a subtropical monsoon climate (Köppen classification: Cwa), characterized by hot, humid summers and dry, mild winters. The island has an average annual temperature of 23 °C, with a total accumulated temperature of 8265 °C. This island receives an annual precipitation of 1863 mm, with rainfall predominantly occurring during the summer months due to the monsoon [38,39]. This climate is also characterized by abundant sunshine and frequent rainfall, which significantly influences the island’s vegetation and ecosystem. This climatic condition is typical of southern China and has substantial implications for local agriculture and biodiversity.

Formed by volcanic eruptions and subsequent deposits, Weizhou Island is covered with weathered purple-red basalt. Its topography is relatively simple, with the southern region predominantly marked by sea-eroded landforms, while the northern half features marine sediment landforms [40]. During the early to middle Pleistocene, the Henglu Mountain volcano in the northwest underwent prolonged eruptions, producing lava flows that formed the basalt. In the late Pleistocene, volcanic eruptions and an injection of gas into the southern Nanwan region led to significant volcaniclastic deposition. These are the predominant lithological types on the island [41,42]. Additionally, the northeastern coastal area is rich in beach sediments composed of bioclastic materials, including layered beach rocks, quartz sandstone, and coral gravel. These rocks are primarily composed of quartz sand and bioclastic material [43,44]. The geological map of the study area is shown in Figure 1.

2.2. Sampling and Analytical Methods

Sampling was conducted in July on Weizhou Island during the rainy season. While soil weathering is a long-term process, the increased precipitation and humidity during this period may have a transient effect on the geochemical dynamics of heavy metals in volcanic rock soils. This study primarily focuses on long-term weathering processes and their role in the release and distribution of heavy metals from volcanic breccia and basalt substrates. By examining these processes in the context of varying rainfall patterns and biological activity, this research aims to provide insights into the complex interactions that govern the behavior of heavy metals in soils.

The sampling sites are shown in Figure 2. Surface soil sampling was conducted across the entire Weizhou Island. A total of 103 surface soil samples (0–20 cm depth) were collected using a grid method (500 m × 500 m), along with 16 fresh rock samples taken from the same locations as the soil samples. These comprised 10 volcaniclastic rocks, 3 basalt rocks, and 3 beach sediments containing bioclastic materials. Profiles W01 and W02 were specifically selected for additional sampling to examine vertical weathering patterns and provide a more comprehensive understanding of the soil-formation processes at different depths. The weathering crust in profile W01, located in Wucaitan, is characterized by volcaniclastic rock lithology, which is representative of a key geological feature of the island. In contrast, profile W02 represents a basalt weathering crust, offering a distinct comparison with W01. These profiles were chosen because they represent the two main lithological types found on the island, and sampling at these locations allows for a deeper understanding of weathering processes and the vertical distribution of heavy metals.

The sampling procedure adopted a systematic approach, beginning with fresh parent rock samples, progressing through semi-weathered, highly weathered layers to cover the entire weathered profile, as illustrated in Figure 3. A total of 12 individual samples were collected from profile W01 (W01-1 to W01-12) and 12 from profile W02 (W02-1 to W02-12), with each sample representing a distinct depth interval within the weathered profile. For each location, individual samples were collected at 5–10 cm intervals, depending on the weathering features observed. The soils from the W01 volcaniclastic rock profile were classified as Red Soils in the Chinese Soil Classification System and Ferralsols in the WRB (FAO) system. The soils from the W02 basalt profile, developed on basalt rock and exhibiting a purple-red color, were classified as Purple Soils in the Chinese Soil Classification System and Andosols in the WRB (FAO) system.

To clarify, no composite samples were made; each sample collected was treated as an individual sample for analysis. After collection, all soil samples from profiles W01 and W02, as well as from the surface soil, underwent initial treatment, including air-drying at room temperature to remove impurities such as plant roots and large gravel. The samples were then cleaned with Milli-Q water under controlled conditions, followed by drying at room temperature and sieving through a 200-mesh nylon screen. Each sample weighed approximately 500 g. The processed samples were sealed in bags and stored in a drying vessel for further analysis.

Quantification of heavy metals (Cu, Zn, Pb, Cr) was performed using an inductively coupled plasma spectrometer. Exactly 1000 g of each soil sample was weighed and moistened with Milli-Q water. The samples were then subjected to successive digestion and heating using a mixture of HCl, HNO₃, HF. After cooling, ultrasonic treatment was repeated until all acid was exhausted. The resulting mixture was diluted with Milli-Q water and transferred to a volumetric flask. Finally, the flask was filled to the mark with a 10% HNO₃ solution and labeled.

In this study, we employed the ICP-OES Avio 200 instrument (Stellarnet Inc., Tampa, FL, USA) to analyze the concentrations of heavy metals (Cu, Zn, Pb, Cr) in samples. Aluminum (Al) was chosen as an independent quality control sample due to its widespread abundance, excellent stability and suitability for ICP-OES testing [45]. The concentration of Al in the quality control samples (QCs) was measured and compared with its known value. The measured values closely matched the expected values, confirming the accuracy of the method.

Table 1. Comparison of soil element concentrations and chemical index of alteration (CIA) in volcaniclastic rock and basalt profiles.

Profile	Depth/ cm	Cu/ ppm	Zn/ ppm	Pb/ ppm	Cr/ ppm	Al/ ppm	Fe2O3/ mass%	P2O5/ mass%	pH	OM/ mass%	CIA
Profile W01	0–20	32.78	81.81	107.79	99.61	10.726	10.032	0.17	7.15	3.018	69.0
	20–40	32.72	58.95	130.12	126.04	9.402	10.751	0.119	6.76	2.765	73.8
	40–60	72.94	110.20	218.76	182.18	10.589	11.187	0.118	6.82	2.772	73.8
	60–80	50.84	102.30	162.58	140.94	10.939	9.065	0.105	6.50	2.811	78.2
	80–100	67.90	86.57	199.28	356.64	10.468	10.411	0.138	6.14	2.445	84.8
	100–120	58.99	76.33	181.28	177.01	11.178	10.831	0.129	6.00	2.402	81.4
	120–140	58.67	82.43	223.33	298.31	11.232	11.520	0.165	6.04	2.091	87.5
	140–160	50.83	86.52	178.82	250.73	9.305	9.373	0.145	5.68	1.324	88.0
	160–180	52.24	109.19	170.75	246.68	7.901	9.365	0.154	5.68	1.449	86.6
	180–200	52.17	124.46	149.58	255.75	7.827	7.651	0.125	—	—	86.3
Profile W02	0–15	129.72	174.86	172.68	406.67	18.375	17.155	0.802	5.80	3.067	66.4
	15–30	123.62	168.80	106.95	328.71	18.168	16.692	0.807	6.10	2.810	63.2
	30–45	123.77	158.01	137.23	332.80	18.4951	17.024	0.686	5.60	3.031	70.2
	45–60	162.36	187.45	165.31	449.12	18.901	17.210	0.721	5.70	2.737	69.5
	60–75	120.67	136.47	72.86	320.93	18.6999	17.077	0.716	5.30	2.908	69.2
	75–90	123.69	168.50	131.45	326.70	18.024	16.290	0.718	5.80	2.562	63.5
	90–105	84.91	160.07	46.68	187.00	14.436	13.354	0.875	6.50	1.653	41.7
	105–120	99.25	154.71	29.68	197.49	14.032	13.559	0.905	6.80	1.640	40.2
	120–135	115.55	190.24	124.69	315.18	16.545	15.702	1.053	7.10	1.223	53.6
	135–150	116.13	168.77	243.23	379.79	15.891	15.261	1.025	7.20	1.881	50.2
	150–165	107.66	143.21	170.80	300.66	15.683	14.312	0.922	7.40	1.519	46.8
	165–180	118.52	195.19	18.54	280.58	12.8634	12.304	0.839	—	—	34.3

presents the concentrations of Cu, Zn, Pb, Cr, QCs (Al) obtained using the Avio 200 instrument under optimized operating conditions.

The accuracy of our measurements was further assessed by evaluating the reproducibility through repeated analyses of quality control samples (QCs). Each sample was subjected to three parallel tests, accompanied by simultaneous blank experiments [46]. The observed low relative standard deviation (RSD) values were within the acceptable range specified by the Avio 200 instrument, indicating high accuracy.

Major element (Fe₂O₃, P₂O₅, Al₂O₃, CaO, Na₂O, K₂O) determination was carried out using the XRF ZSX Primus II instrument (Rigaku, Tokyo, Japan). This instrument analyzes the intensity and energy of X-ray fluorescence in samples using principles of spectral excitation and analysis, enabling precise qualitative and quantitative determination of elemental content and composition. The technical specifications of the XRF ZSX Primus II instrument ensure the reliability of the experimental results.

pH was measured using a glass electrode with distilled water (H₂O) at a soil:solution ratio of 1:1, and soil organic matter content was determined using the potassium dichromate sulfuric acid oxidation method. The analyses were conducted at the Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, Guilin University of Technology, which follows stringent quality control protocols and has extensive experience in similar studies [47,48].

2.3. Data Analysis

2.3.1. Chemical Index of Alteration

The chemical index of alteration is used to characterize the degree of soil weathering and the weathering of rocks at various stages. Generally, a lower content of SiO₂, coupled with a higher content of Fe₂O₃ and Al₂O₃, indicates a higher degree of soil weathering [49].

$$CIA=\left[{Al}_2{O}_3/\left({Al}_2{O}_3+{CaO}^{*}+{Na}_{2}O+K_{2}O\right)\right]\times 100\%$$

(1)

In Formula (1), the number of moles of each oxide represents their contribution in the chemical reaction, with CaO* referring to the CaO combined with silicates. The calculation method follows the standard recommended by McLennan [50]. The CIA value serves as an indicator of weathering intensity, with its magnitude reflecting the extent of chemical weathering in the soil and the prevailing climatic conditions.

2.3.2. Geostatistical Interpolation

Geostatistical interpolation is a key method for analyzing the spatial variability of surface materials, such as soil. To ensure data continuity and accuracy, this study employed the general Kriging method [51] to construct spatial variation maps. The spatial dependence index [52,53] was utilized to assess spatial autocorrelation.

$$SDI=\left(C_O/C_o+C\right)\times 100$$

(2)

This index helps determine the range of spatial correlations between samples and calculates the correlation degree using the relationship between the nugget effect (Co) and the sill (Co + C). The standard Kriging method is widely used in spatial interpolation of heavy metal concentrations in soil, which helps identify areas with elevated heavy metal content. As a local, linear, optimal unbiased estimation method, Kriging employs the semi-variogram of a single variable for spatial modeling and prediction (Figure 4). All spatial statistical analyses in this study were conducted using ArcGIS software (version 10.2).

2.3.3. Leaching Ratio

The characteristics of heavy metal leaching and migration in vertical soil profiles are essential for assessing soil environmental quality. Through leaching ratio analysis, the migration rates and behaviors of heavy metals across different soil layers can be determined, providing insights into the potential contamination of groundwater by heavy metals and revealing the associated pollution risks [54]. The leaching ratio for each soil layer element is calculated using the following formula:

$${\mathrm{A}}_{\mathrm{ij}}={\displaystyle \frac{{\mathrm{M}}_{\left(\mathrm{i}-1\right)\mathrm{j}}}{{\mathrm{M}}_{\mathrm{ij}}}}$$

(3)

In Formula (3), A_ij represents the leaching rate of element j in layer i; M_(i−1)j is the content of element j in layer (i − 1); and M_ij is the content of element j in layer i. The leaching ratio does not have a fixed value range or standardized rating, and its value is typically determined by the specific objectives and focus of the study. In the context of heavy metals in soil, the leaching ratio is generally associated with the rate and distance of heavy metal migration in the studied medium. A low leaching ratio typically indicates a slower migration rate of heavy metals in the soil, while a high leaching ratio suggests a faster migration. When interpreting the leaching ratio, it is crucial to consider the specific context and characteristics of the study subject for accurate evaluation.

2.3.4. Geo-Accumulation Index

The geo-accumulation index (I_geo) is a key parameter for assessing the impact of human activities on the environment [55]. By calculating the I_geo, the distribution of heavy metals in the soil can be assessed, and the extent of human influence can be preliminarily evaluated. This provides a critical basis for environmental protection and soil pollution management. The calculation formula is as follows:

$${\mathrm{I}}_{\mathrm{geo}}={\log }_2\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}}{{\mathrm{kB}}_{\mathrm{i}}}}\right)$$

(4)

In Formula (4), C_i represents the concentration of a specific heavy metal in the sample; B_i is the background concentration of this element (based on the soil background values in Guangxi Province); and k is a conversion factor, with a correction index of 1.5, commonly used to account for factors such as sedimentary characteristics and rock geology. When I_geo = 0, it indicates that the concentration of heavy metal elements is at equilibrium with the crustal concentration, suggesting that the presence of these elements is primarily controlled by natural processes. When I_geo > 0, it indicates that the concentration of heavy metal elements exceeds the crustal equilibrium concentration, implying the influence of anthropogenic activities on the environment. As the I_geo value increases, the accumulation of heavy metals also increases (

Table 2. Grading scheme for the geo-accumulation index.

Igeo	Level	Contamination Level
${\mathrm{I}}_{\mathrm{geo}}$ ≤ 0	0	Non-pollution
0 < ${\mathrm{I}}_{\mathrm{geo}}$ ≤ 1	1	Slight-Moderate pollution
1 < ${\mathrm{I}}_{\mathrm{geo}}$ ≤ 2	2	Moderate pollution
2 < ${\mathrm{I}}_{\mathrm{geo}}$ ≤ 3	3	Medium-Strong pollution
3 < ${\mathrm{I}}_{\mathrm{geo}}$ ≤ 4	4	Strong pollution
4 < ${\mathrm{I}}_{\mathrm{geo}}$ ≤ 5	5	Strong-Extremely strong pollution
5 < ${\mathrm{I}}_{\mathrm{geo}}$ ≤ 10	6	Extremely strong pollution

).

2.3.5. Potential Ecological Risk Index

The potential ecological risk index method provides a comprehensive assessment by considering the toxicity of heavy metals and the environmental sensitivity to heavy metal pollution. It incorporates various factors, including element interactions, toxicity levels, pollution concentrations, and environmental sensitivity [56]. The calculation formula is as follows:

$$\mathrm{RI}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{T}}_{\mathrm{r}}^{\mathrm{i}}{\mathrm{C}}_{\mathrm{r}}^{\mathrm{i}}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{T}}_{\mathrm{r}}^{\mathrm{i}}{\mathrm{C}}_{\mathrm{s}}^{\mathrm{i}}/{\mathrm{C}}_{\mathrm{n}}^{\mathrm{i}}$$

(5)

In Formula (5), ${\mathrm{C}}_{\mathrm{r}}^{\mathrm{i}}$ represents the pollution coefficient of a single heavy metal; ${\mathrm{C}}_{\mathrm{s}}^{\mathrm{i}}$ denotes the concentration of heavy metal i (in mg/kg); ${\mathrm{C}}_{\mathrm{n}}^{\mathrm{i}}$ refers to the background concentration of heavy metal i (based on the soil background values in Guangxi Province). ${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$ represents the single-factor potential ecological risk coefficient of heavy metal i, and ${\mathrm{T}}_{\mathrm{r}}^{\mathrm{i}}$ is the toxicity response coefficient of heavy metal i. The standardized toxicity response coefficients for heavy metals are used as the evaluation basis (Zn = 1, Cu = 5, Pb = 5, Cr = 2). RI is the comprehensive potential ecological risk index for soil heavy metals. The classification standard for the potential ecological risk index is modified based on the classification limits proposed by Hakanson.

3. Results

3.1. Spatial Distribution of Heavy Metals in the Soil of Weizhou Island

Table 3. Concentrations of heavy metals in surface soil of Weizhou Island.

	Cu/ppm	Zn/ppm	Pb/ppm	Cr/ppm
Mean	59.18	119.06	35.63	159.78
Median	53.16	109.03	30.86	146.18
Std.	43.52	91.84	26.97	104.73
Min	0.45	0.17	0.62	1.45
5%	6.01	2.78	1.60	5.52
10%	18.94	19.50	4.03	32.88
20%	28.21	54.91	18.64	86.62
50%	53.16	109.03	30.86	146.18
80%	87.35	167.41	51.34	228.26
90%	105.42	213.99	63.90	273.08
95%	119.32	242.95	89.18	345.61
Max	343.20	757.80	178.30	743.17
Cumulative of variance%	73.54	77.14	75.69	65.55

presents the statistical analysis of heavy metal parameters in the surface soil of Weizhou Island. The average concentrations of Cu, Zn, Pb, Cr are 59.18 mg/kg, 119.06 mg/kg, 35.63 mg/kg, 159.78 mg/kg, respectively. The overall sequence of concentrations is Pb < Cu < Zn < Cr. The coefficient of variation (CV) for heavy metal concentrations in the soil ranged from 65.55% to 77.14%, with the order being Cr < Cu < Pb < Zn. Overall, the heavy metals exhibited high variation, with Zn displaying a relatively uniform distribution across the island. Notably, the Zn concentration varied significantly among sample points, indicating its susceptibility to environmental factors.

As depicted in Figure 5, the map illustrates the distribution of heavy metal concentrations in the surface soil and land use types on Weizhou Island. High concentrations of Cu, Zn, Cr are observed around the wharfs and the southern “tail” of Weizhou Island, where the predominant lithology consists of volcaniclastic rocks. Due to frequent human activities, the heavy metal content near the western wharfs exceeds that of the southern tail. The geological complexity and formation processes of the study area contribute to varying degrees of Cu enrichment across different lithologies and minerals, resulting in a patchy distribution of high Cu-value points [57,58,59].

In the western wharf area of Weizhou island, human activities, particularly agriculture and industrialization, contribute to the introduction of additional heavy metals into the environment through mechanisms such as fertilization, industrial discharges, and other anthropogenic sources [15,16,17]. These metals can infiltrate the soil system via pathways such as precipitation and irrigation, thereby influencing the concentrations of heavy metals within the soil. Furthermore, the application of organic and chemical fertilizers in agricultural practices can enhance the mobility of heavy metals, especially in soils with lower pH values, where these metals are more soluble and exhibit greater mobility.

The distribution of heavy metal concentrations in the soil system is influenced by inputs from the underlying soil parent material and human activities, including the redistribution of heavy metals in response to changes in land use [60,61]. Different land use types exert varying effects on soil heavy metal concentrations. According to the land use type map, Cu concentrations in the study area follow the following sequence: abandoned land > banana field > forest land > corn land. This distribution is attributed to higher Cu concentrations in desert areas and lower Cu concentrations in forests, influenced by human activities. Additionally, the elemental composition of the soil parent material influences soil heavy metal concentrations. The higher Cr concentrations in forest land are linked to elevated concentrations in the soil parent material in these regions.

3.2. Heavy Metal Speciation in Soil Parent Materials of Weizhou Island

The type of soil parent material plays a pivotal role in the distribution of heavy metals, determining their initial content in the soil [62,63,64]. According to the data presented in Table 1, the concentrations of Cu, Zn, Cr, Fe, and P oxides are higher in the W02 basalt profile compared to the W01 volcaniclastic profile. The pH in W01 is higher than that in W02. In the W01 volcaniclastic rock profile, heavy metal concentrations decrease with increasing depth, with minimal fluctuations in the concentrations of Cu and Zn. The concentrations of heavy metals Cu, Zn, Pb, and Cr in the surface soil of volcaniclastic rocks are higher than those in basalt, although the total heavy metal content in the basalt profile is greater than in the volcaniclastic rocks.

The presence of sulfide and chromite in volcaniclastic rocks accounts for the increased Cr content, facilitating the leaching of heavy metal elements during weathering and leading to their accumulation in the surface soil [65,66]. This process further promotes the migration of heavy metals within the soil.

Basalt deep soils are rich in heavy metals, iron, aluminum oxides. Basalt, as a primary mafic silicate mineral, weathers easily. During extreme weathering, it is primarily influenced by secondary phosphates and decomposes readily in early stages [67,68]. In natural weathering processes, soluble components such as Ca, Na are quickly leached, leading to the enrichment of Si, Al, and Fe [69,70,71]. The significant enrichment of P₂O₅ and Fe₂O₃ in the basalt profile samples, as shown in Table 1, indicates the dissolution of primary apatite and the formation of secondary phosphate. Cr enrichment is attributed to the adsorption of iron oxides, hydroxides, and organic compounds onto secondary phosphates [72,73,74].

3.3. Influence of Parent Rock on Heavy Metal Leaching in Soil

The chemical characteristics of volcaniclastic rocks and basalts play different roles in the leaching of Cu, Zn, and Pb from soils [75,76]. Analysis of

Table 4. Leaching ratios of Cu, Zn, Pb, and Cr in basalt and volcaniclastic soil profiles in the study area.

Profile	Depth/cm	WWC/Cu	WWC/Zn	WWC/Pb	WWC/Cr
W01	20–40	1.00	1.39	0.83	0.79
	40–60	0.45	0.53	0.59	0.69
	60–80	1.43	1.08	1.35	1.29
	80–100	0.75	1.18	0.82	0.40
	100–120	1.15	1.13	1.10	2.01
	120–140	1.01	0.93	0.81	0.59
	140–160	1.15	0.95	1.25	1.19
	160–180	0.97	0.79	1.05	1.02
	180–200	1.00	0.88	1.14	0.96
W02	15–30	1.05	1.04	1.61	1.24
	30–45	1.00	1.07	0.78	0.99
	45–60	0.76	0.84	0.83	0.74
	60–75	1.35	1.37	2.27	1.40
	75–90	0.98	0.81	0.55	0.98
	90–105	1.46	1.05	2.82	1.75
	105–120	0.86	1.03	1.57	0.95
	120–135	0.86	0.81	0.24	0.63
	135–150	0.99	1.13	0.51	0.83
	150–165	1.08	1.18	1.42	1.26
	165–180	0.91	0.73	9.21	1.07

reveals that in the W01 profile, the leaching ratios of Cu, Zn, and Pb in the 60–80 cm soil layer are relatively high. This is attributed to the presence of iron minerals that facilitate the migration of these elements [77,78,79]. Volcaniclastic rocks exhibit high total calcium (Ca) content and carbonate solubility [80,81] and are readily soluble in water, which leads to the release of ionic metal elements such as iron (Fe), and manganese (Mn) in the soil. Moreover, they are rich in soluble aluminum (Al) and potassium (K) compounds, further promoting the leaching of Cu, Zn, Pb, and other metal elements [82,83,84]. In contrast, basalts generally have low total calcium (Ca) content and carbonate solubility, making them less prone to dissolution in aqueous environments. With an abundance of siliceous minerals possessing high adsorption capacity, basalts effectively adsorb heavy metals in the soil, reducing their leaching rate [85,86].

The chemical index of alteration (CIA) serves as a reliable indicator for assessing the degree of rock weathering [87]. Higher CIA values reflect a greater intensity of chemical weathering. In general, profile W01 exhibits higher CIA values compared to profile W02, indicating that volcaniclastic rocks are more susceptible to chemical weathering than basalts (Table 1). Based on the trend of the CIA, it can be inferred that profile W01 is relatively wetter and may have experienced chemical weathering for a longer period. Volcaniclastic rocks are rich in clay minerals, such as smectite and illite [88,89,90], which have high specific surface areas and complex porous structures. These properties enable clay minerals to strongly interact with dissolved substances through adsorption and ion exchange. Adsorption helps fix heavy metal ions, preventing their deeper penetration into the soil, while ion exchange allows clay minerals to bind with heavy metals in soil solutions, further limiting their mobility [91,92]. Additionally, the particle size and porous structure of clay minerals act as physical barriers, impeding the diffusion of heavy metals. The crystallinity of clay minerals and their ability to regulate the pH of soil solutions also influence their capacity to fix heavy metals [93,94].

Thus, the reduction in heavy metal leaching, particularly Cu, Zn, Pb, and Cr, in volcaniclastic rocks suggests that clay minerals play a key role in preventing or slowing the downstream movement of these metals. This process contributes to the accumulation of heavy metals in the surface soil of volcaniclastic rock profiles, rather than allowing them to migrate to deeper soil layers.

The leaching ratios of Cu, Zn, Pb, and Cr in different soil layers are influenced by various factors, including soil pH, parent rock type, surface water and rainfall, and soil organic matter content [95,96]. Soil pH is a key factor affecting the leaching of metal elements. As indicated in Table 1 and Table 4, metal element leaching is more likely in acidic soil environments [97,98]. Surface water and rainfall promote the leaching of metal elements from the soil through dissolution and leaching [99]. Additionally, soil organic matter content can impact the availability and leaching degree of metal elements in the soil [100,101,102]. The complex interplay of these factors results in different leaching ratios of various metal elements in distinct soil layers.

In the W01 profile, Cu, Zn, and Pb display a certain correlation, owing to their common origin and similar geochemical behavior [103,104,105]. Their frequent coexistence and interaction in the natural environment, as well as their migration and dissolution processes, are influenced by similar geological environmental factors [106]. The similarity of Cu, Zn, and Pb in terms of their physical and chemical properties affects their behavior in the environment. These elements share similar ionic radii and electronic configurations, tending to form analogous bonds with other substances, thereby exhibiting similar adsorption and transport behavior in the soil [107,108].

The mineral composition differences between basalts and volcaniclastic rocks significantly influence their affinity for heavy metals. Basalts typically contain minerals such as pyroxene and olivine, which are more capable of enriching heavy metals, while volcaniclastic rocks are composed of minerals that tend to facilitate the release of heavy metals. The chemical index of alteration (CIA) of the volcaniclastic rock profile W01 is notably higher than that of the basalt profile W02 (Table 1), suggesting that volcaniclastic rocks have undergone more extensive weathering during geological processes. This enhanced weathering promotes the release of heavy metals in a more soluble form, resulting in lower concentrations of heavy metals in volcaniclastic rocks compared to basalts. Additionally, during the solidification of magma, basalt accumulates a higher concentration of heavy metals due to factors such as the composition of the source rock and the melting temperature. As a result, basaltic profiles typically exhibit higher concentrations of heavy metals than volcaniclastic rocks.

4. Discussion

4.1. Sources and Distribution of Heavy Metals in the Soil of Weizhou Island

Soil properties, including soil type, are key factors influencing the content and distribution forms of heavy metals in soil. Soil pH, organic matter, Fe₂O₃, P₂O₅ content, and CIA influence the solubility and chemical forms of heavy metals, which are critical to their bioavailability [109,110,111]. Pearson correlation analysis between heavy metal content and soil physical-chemical properties in the surface soil of Weizhou Island (Figure 6A) revealed a significant negative correlation with soil pH and positive correlations with organic matter, CIA, Fe₂O₃, and P₂O₅ content. The activation of heavy metals is more pronounced in acidic environments. The degree of weathering (as reflected by the CIA value) affects the release and accumulation of heavy metals in sediments [112]. Furthermore, organic matter content facilitates the mobilization of heavy metals, and evidence suggests that increased soil organic matter enhances the uptake of Pb by plant roots [113].

Principal component analysis of nine factors (Cu, Zn, Pb, Cr, pH, OM, CIA, Fe₂O₃, P₂O₅) in the surface soil of Weizhou Island (Figure 6B) identified three clusters (A, B, C) and five outlier data points classified as cluster CK, which facilitated an accurate assessment of soil heavy metal pollution. The results of the KMO and Bartlett’s sphericity tests showed a KMO value of 0.764 and a chi-square statistic of 800.008 (36 degrees of freedom), confirming the suitability of the data for factor analysis [114]. In principal component analysis, the first two principal components were chosen as explanatory variables based on the cumulative contribution rate and Kaiser criterion [115]. The cumulative contribution rate of these two principal components reached 59.77%. Among them, the variance contribution of the first principal component factor (PC 1) was 52.87%. In the load matrix, Cu, Zn, Cr, and Pb exhibited high loadings of 0.977, 0.973, 0.953, and 0.899, respectively, with significant spatial variability, indicating a natural parent material source of heavy metals in the surface soil of Weizhou Island, consistent with the limited mobility of these metals.

The variance explained by the second principal component (PC 2) was 6.9%. In the loading matrix, the loadings of pH, organic matter, Cu, Zn, Pb, and Cr were negative, while the loadings for CIA, Fe₂O₃, and P₂O₅ were 0.814, 0.877, and 0.205. This suggests that elements such as Cu, Zn, Pb and Cr are largely immobilized in the soil due to adsorption and complex chemical reactions with soil minerals, particularly clay particles, which limits their mobility. The loadings below 0.4 are considered insignificant [116], indicating that anthropogenic factors have a minimal influence on the sources of these heavy metals in the soil and that the heavy metals are primarily controlled by the parent material.

4.2. Health Ecological Risk Assessment and Implications of Weizhou Island

The classification and results of the ground accumulation index (Figure 7A and Table 2) showed that I_Cu, I_Zn, I_Pb, I_Cr ranged from −0.35 to 0.81, −0.94 to −0.04, 1.58 to 2.63, and −0.31 to 1.53, respectively. The points for Cu and Cr were both classified as mildly to moderately polluted, accounting for 77.8% and 55.6% respectively, while the proportions of no pollution were 22.2% and 11.1%, and the points for Cr moderate pollution were 33.3%. Zn sites were found to be pollution-free. Among the four heavy metals, the proportion of Pb sites with moderate pollution and moderate to strong pollution was 22.2% and 77.8%, respectively. According to the mean value of the ground accumulation index, the ranking from high to low is Pb (2.24) > Cr (0.65) > Cu (0.3) > Zn (−0.38). This indicates that Zn is at a pollution-free level, Pb is at a moderate pollution level, and Cu and Cr are mostly at a light pollution level, with some local areas being pollution-free. The results of the soil accumulation index showed that the soil of Weizhou Island was primarily affected by Pb, followed by Cr, Cu, and Zn to a lesser extent.

The evaluation results of potential ecological risks posed by heavy metals in the soil of the study area are presented in Figure 7B. The results indicate that the ecological risks follow the order Pb > Cu > Cr > Zn. Among these, Pb poses the highest risk, with slight ecological risk accounting for 8.65%, moderate ecological risk accounting for 1.92%, high ecological risk accounting for 0.96%, very high ecological risk accounting for 1.92%, and extremely high ecological risk accounting for 86.54%. Cu and Cr exhibit similar ecological risks, with slight ecological risk accounting for 9.62%, high ecological risk accounting for 20.19%, and very high ecological risk accounting for 60.58%. The ecological risk associated with Zn is relatively low, with slight ecological risk accounting for 18.27%, moderate ecological risk accounting for 11.54%, very high ecological risk accounting for 22.12%, and extremely high ecological risk accounting for 7.69%.

It is evident that the potential ecological risks posed by soil heavy metals, especially Pb, in the study area require attention and control. Pb, Cu, and Cr are common heavy metal pollutants that mainly originate from industrial activities, automobile exhausts, and waste batteries [117]. Controlling pollution sources and reducing heavy metal emissions can protect both the environment and public health. Additionally, selecting appropriate plant species and implementing effective phytoremediation techniques can provide a scientific basis for the restoration of soil contaminated with heavy metals and facilitate further research on the mechanisms of heavy metal pollution. These measures aim to minimize the damage caused by heavy metals to soils and ecosystems.

Considering the ecological risks posed by all four heavy metals comprehensively, the ecological risk index in the study area ranges from 0.17 to 126.99. Slight, moderate, and high ecological risks account for 46.15%, 44.23%, 9.61% of the study area, respectively. As shown in Figure 8, most areas in the study area exhibit slight to moderate ecological risks. However, areas with frequent human activities, such as the wharf in the northwest, present relatively high ecological risks. Overall, the study area can be considered relatively clean, with some areas showing higher ecological risks.

The results of the geo-accumulation index indicate that the lead (Pb) concentration in the soils of Weizhou Island is predominantly classified as moderately polluted. Specifically, 77.8% of the sampling points exhibit moderate to intense Pb contamination (Figure 7A, Table 2), highlighting the significant presence of Pb pollution in the soil. With an average I_geo value of 2.24, Pb is categorized as moderately polluted, which is notably higher than the values for Cu (0.30) and Cr (0.65). This suggests that Pb plays a more critical role in soil pollution on Weizhou Island compared to other metals.

In the ecological risk assessment, Pb emerges as the most ecologically hazardous heavy metal among those evaluated (Figure 7B). The ecological risk index for Pb reveals that 86.54% of the sampling points face very high ecological risk, while 0.96% and 1.92% of points are subject to high and very high ecological risk, respectively.

According to Table 3, the average Pb concentration is 35.63 mg/kg, which is lower than the concentrations of Cu (59.18 mg/kg) and Zn (119.06 mg/kg). However, Pb remains a significant pollutant due to its high toxicity, long-term persistence, and potential adverse effects on the ecosystem. Moreover, the coefficient of variation (CV) for Pb ranges from 65.55% to 77.14%, suggesting substantial spatial variability in its distribution across the island. This variability points to differing sources and migration pathways of Pb in various regions, indicating the need for targeted pollution management strategies. In comparison with other metals, Pb poses a more persistent and serious threat to the ecological environment of Weizhou Island, necessitating focused attention and mitigation efforts.

The leaching ratio data presented in Table 4 further illustrate the mobility of Pb at different soil depths. Notably, in the 60–80 cm soil layer of profile W01, the leaching ratio of Pb is 1.35. A higher leaching ratio indicates that Pb is more susceptible to environmental factors, such as precipitation and changes in soil pH, which may facilitate its migration to deeper soil layers or into water bodies. This exacerbates the ecological risk associated with Pb contamination. Therefore, in assessing Pb pollution, it is crucial not only to consider its soil concentration but also its mobility and potential migration within the environment.

5. Conclusions

This study systematically investigated the distribution and controlling factors of heavy metals in the soil of Weizhou Island, a volcanic island in China, using geochemical and geostatistical analyses. The findings indicate that the distribution of heavy metals (Cu, Zn, Pb, and Cr) in the soils of the volcanic island Weizhou Island is influenced by multiple factors, including geological parent material, land use practices, and human activities. Volcaniclastic rocks, with their high calcium content and solubility, enhance the leaching of metals, particularly heavy metals, through the presence of aluminum and potassium compounds. These rocks also promote the enrichment of heavy metals in the surface soil due to their rich clay mineral content, which limits the migration of metals via adsorption and ion exchange. In contrast, basalt, with its lower calcium content and poor solubility, reduces the leaching of heavy metals. Additionally, basalt’s silica-rich minerals effectively adsorb heavy metals, thereby decreasing their mobility. The contrasting parent materials of volcaniclastic rocks and basalts further influence the enrichment of specific heavy metals, with volcaniclastic rocks contributing to elevated chromium concentrations due to sulfide and chromite, while basalts enhance the accumulation of iron and aluminum oxides and phosphates. As a result, volcaniclastic rocks are more prone to biochemical weathering, leading to higher concentrations of heavy metals in surface soils, whereas basalts exhibit higher pH values and higher heavy metal content.

Human activities, particularly agricultural and industrial activities in the western wharf and southern “tail” areas of the island, exacerbate the concentrations of Cu, Zn, and Cr, with the western wharf area showing higher levels due to frequent anthropogenic influence. Land use types also significantly affect soil heavy metal concentrations, with abandoned lands and agricultural areas (such as banana and corn fields) exhibiting higher Cu levels, while forest lands show comparatively lower concentrations of heavy metals. The natural parent material, especially Pb pollution from volcanic rocks, remains the primary source of heavy metal contamination in the soils of Weizhou Island. Soil characteristics, such as acidity, high organic matter content, and advanced weathering, significantly promote the dissolution and migration of heavy metals, particularly Pb. Principal component analysis further indicates that the distribution and forms of heavy metals are primarily controlled by the natural geological substrate, while the migration of these metals in the soil is largely constrained by mineral adsorption and chemical reactions, particularly interactions with clay minerals.

Ecological risk assessments reveal that the soil of Weizhou Island is generally clean, but Pb presents some ecological risks, with the majority of monitoring sites showing very high ecological risk. Pb contamination in the soils of Weizhou Island is of particular concern due to its high toxicity, persistence, and widespread distribution, which is significantly influenced by human activities, including industrial and traffic emissions. The spatial variability in Pb contamination highlights the urgent need for targeted management and remediation strategies.

In conclusion, the distribution of heavy metals in the soils of Weizhou Island is governed by both natural geological factors and significant anthropogenic influences. Although the island maintains a generally good level of cleanliness, the elevated Pb concentrations pose a major risk to the local ecosystem, necessitating immediate action. To mitigate the ecological impact of heavy metal contamination, the following management strategies are recommended: (1) strengthening environmental monitoring and regulation, particularly in areas with high industrial and agricultural activity; (2) optimizing land use patterns to reduce heavy metal accumulation, such as limiting agricultural activities in sensitive areas; (3) implementing soil remediation techniques, especially in areas with acidic soils, to reduce heavy metal mobility and enhance adsorption by mineral particles; and (4) focusing on Pb contamination control through targeted pollution reduction measures and public awareness campaigns. These strategies are essential to prevent further environmental degradation and protect the long-term ecological health of Weizhou Island.