3.1. Heavy Metal Concentration, pH and SOC in Paddy Soil
In
Table 2, the levels of five heavy metals in soil in the study area are presented as the mean ± standard error and are expressed in mg kg
−1. The arithmetic mean (AM) contents of As, Cd, Cu, Pb, and Zn were 19.7 ± 17.1, 0.577 ± 0.690, 40.5 ± 32.8, 35.5 ± 32.0, and 135 ± 128 mg kg
−1, respectively. Compared with the soil background level in Guizhou [
30], the total concentration(s) of Zn (48.8~907 mg kg
−1) was significantly elevated, with it being 38.46% higher than the background value, and the upper limit was 9.12 times higher than the background value. Overall, 92.3% of the samples exhibited the total concentration(s) of Cd of less than 1.0 mg kg
−1, and the median value was 0.416 mg kg
−1. Kabata-Pendias [
44] and Alloway [
45] reported that the background Cd concentration(s) (of most of the surface soil on the earth) did not exceed 1.0–1.1 mg kg
−1, indicating that the concentration(s) of total Cd in Guizhou Province was in the range of geochemical background values. The total concentration(s) of As, Pb, and Cu were comparable to their local background levels throughout the study area. The soil samples with high Zn content also showed a high content of Cu, Cd, and Pb, and there was a very significant correlation between them (r = 0.35~0.504, all
p < 0.01) (
Table 3), indicating that Zn, as well as Cu, Cd, and Pb, had a common source and received substantial replenishment from it (
Table 3). The AM of pH and organic matter (OM) was 6.5 + 0.3 and 5.2 + 2.0, respectively. The pH value was lower than the local background value, while the SOC was 1.2 times higher than the background value.
3.2. The Heterogeneity of Soil Heavy Metal Content
It can be seen from the variation coefficient of paddy soil in the study area (
Table 2) that heavy metals in different regions showed different degrees of variation, in which Cd and Zn exhibited strong variation and the other elements showed moderate variation, with the coefficients of variation with values more than 10. This indicates that heavy metals originate from different parent material (rock). The parent material (rock) was the main influencing factor for the accumulation of heavy metals in large-scale soils [
46,
47,
48,
49,
50,
51,
52,
53,
54,
55,
56,
57,
58]. Most of the parent materials in Guizhou Province were limestone and sand shale. The background values of Zn, Pb, As, and Cd in the soil developed by limestone parent material were high, and the background value of Cu in the soil developed by sand shale parent material was high, as well [
21,
49]. This could be evidence that high content of heavy metals and metalloids in soils is largely a result of HGB, since As and heavy metals are less mobile than other elements. In addition, other anthropogenic activities, such as water irrigation and fertilizer application in agricultural lands, may enhance the accumulation of heavy metals in soils, but it is not significant [
50,
51], and thus not considered in this study.
3.5. Health Risk Assessment
As the staple food of local residents, rice consumption contributes to an important part of the daily intake of heavy metals. The HQs of individual elements and HI for rice consumption for adults in the HGB area were calculated and are shown in
Figure 2. The detailed HQs and HI (with AM, minimum, and maximum) are presented in
Table 5.
In the study area, the HQs were ranked in the order of Cu > Zn > Cd > As > Pb. HQ-Cu, HQ-Zn, HQ-Cd, HQ-As, and HQ-Pb accounted for 74.27%, 18.20%, 6.34%, 0.86%, and 0.32% of the HI, respectively. Adults in the HGB area have HQ values greater than 1 for Cu (HQ = 1.10), indicating that Cu absorbed by ingesting rice poses a potential health risk to local residents. The remaining heavy metals have HQ values less than 1. The results show that the health risks posed by As, Cd, Pb, and Zn are negligible.
3.6. Risk Consideration
It is essential to evaluate the human health risks caused by rice consumption in the different parent material HGB areas, where heavy metal levels exceed the CSEQS. The results show that in the HGB area of the parent material soil, there is almost no health risk from heavy metals posed by eating locally grown rice. This suggests that in previous studies, we may have overestimated the health risks posed by the HGB formed by the soil’s parent materials.
Rice is the main food in the study area. Although the daily intake of metals or toxic elements through rice is an important pathway for the dietary exposure of local people to heavy metals through food, many studies have reported that human beings are also significantly exposed to metals through other foods such as wheat, vegetables, fruit, fish, meat, eggs, and milk, as well as water [
52,
53,
54]. However, exposure via these foods is rarely considered, so this article has not mentioned it. This paper only provides a reference for the potential health risks caused by the intake of heavy metals or toxic elements in rice grown in the HGB area created by the parent material. In addition, our calculations do not consider special groups such as the elderly, pregnant women, children, and medical patients.
3.7. Heavy Metal Bioaccumulation and Influencing Factors
To understand the migration and enrichment of heavy metals in the paddy soil–rice system and the influencing factors, this study introduced the geology and background data of the high background types of the metallogeny belt and alluvial plains to help explore whether these two types also have the same phenomenon as in the HGB created by the soil parent materials. Daye City and Changshu City, both in China, comprise a typical HGB metallogenic belt and alluvial plain [
55,
56], respectively. The detailed heavy metal concentration(s) (with AM) are presented in
Table 6. There were no data for the Zn element in Daye City. The concentration(s) of Cd, Cu, Pb, and As in rice grains grown in the metallogenic belt were 0.04–3.29, 3.10–38.3, 0.08–2.02, and 0.14–1.33 mg kg
−1, respectively. The Cd, Pb, and As concentrations in rice grains were 2.95, 1.85, and 2.07 times higher than the Chinese suggested MPL, respectively. These results indicated that there was no similar phenomenon of heavy metal absorption by rice grains grown in the metallogenic belt-type HGB areas compared to soil parent material-type HGB areas. The average concentrations of As, Cd, Cu, Pb, and Zn in the HGB region of alluvial plains were lower than those of the Chinese suggested MPL. However, there remained 46 rice samples (29.7% of the total sample number) containing Pb levels in excess of its maximum accepted level in foods. For Cd, the number of samples with similar excessive levels was 1 (0.7% of the total sample number). In all rice grain samples, the highest concentration(s) of Pb and Cd were 0.957 and 0.201 mg kg
−1, respectively [
35]. This demonstrates that the alluvial plain HGB areas did not have the same phenomenon as the soil parent material-type HGB areas.
We also calculated the bioaccumulation factor, BAF (BAF = rice heavy metal content/topsoil heavy metal content); BAF is a dimensionless value used to quantitatively analyze the effect of soil heavy metals on crops. The BAF statistical analysis results for each heavy metal from different types of HGB are shown in
Figure 3 and
Figure 4. There are significant differences in the bioaccumulation of heavy metals in the soils of the three types of HGB regions, which are closely related to the bioavailable heavy metals in their respective soils [
57,
58,
59]. The BAF values of all the heavy metals except for Cd were the highest in the alluvial plain-type HGB zone, as seen in
Figure 4. This indicated that the bioavailability of heavy metals in the alluvial plain was the highest. The most important reason for this was that the sediments that carry heavy metals from the higher areas are deposited in the downstream areas; therefore, the heavy metals have been accumulated due to the long-term sediment deposition on the plains.
In HGB areas which originated from different parent materials, all the BAFs of heavy metals were the lowest. Studies have shown that the eutectic substitution of cadmium and calcium often occurs in limestone areas, resulting in strong cadmium stability and low effective cadmium content which is not easily absorbed by plants [
60]. Different lithologic regions had complex isomorphic substitutions, which mainly occur in karstic lava areas of Guizhou Province [
61]. It is more acceptable that the large-scale karst areas in Guizhou Province are mainly formed by rocks such as limestone, dolomite, marl, and gypsum. They mainly act upon the soil via chemical dissolution and erosion, as well as leaching and collapse; therefore, the free and adsorbed ions of heavy metals in the surface soil are transferred to the underground part, eventually resulting in the decreased bioavailability of heavy metals in the topsoil [
62]. Compared to the HGB types of the metallogenic belt and alluvial plain, the heavy metal elements on the surface have no or less related geological effect in the HGB of parent material soil, which explains the reason for the differential heavy metal bioaccumulation in rice grown in the HGB area with the parent material in Guizhou Province compared to in other types of HGB regions.
In addition, soil pH is also an important factor that causes heavy metals to exceed the CSEQS levels. The bioavailability of heavy metals decreases with the increase of soil pH. The lower the soil pH is, the worse the adsorption capacity of soil iron oxides for heavy metals will be [
62,
63]. In addition, mineral elements can also affect the accumulation and absorption of heavy metals by rice. The higher content of mineral nutrients in soil media, such as Ca, can significantly reduce the rate of Cd absorption in crops, and when these mineral nutrients are lacking in the medium, Cd will be actively transported into the cell through the carrier protein of Ca, due to the similar ionic radius of Cd and Ca [
64]. Although the soil medium affects the heavy metal BAF and ability to migrate into the crop under different pH levels, the ion concentration in the soil also changes the enrichment of heavy metals in the crop. Under high-strength fertilization conditions, the ionic strength in the soil is high, and the effectiveness of cadmium is significantly enhanced [
60].
In August 2018, China re-released the Chinese Soil Environmental Quality Standard (GB 15618-2018), which classifies heavy metal values into four grades according to the pH of the soil. However, the pH of the soil in China has decreased by 0.13 to 0.80 pH units in recent years, which means that the acidity of the soil has increased by 1.35 to 6.31 times; this extent of acidification takes tens of thousands of years to occur in normal soil processes [
65,
66]. Our results indicate that the total amount of heavy metals does not necessarily correlate with food security. Studies have shown that the bioavailability of heavy metals in southern paddy soils was more than 60%, and the bioavailability in northern farmland was only about 30% [
67]. The proportion of available heavy metals in soils with different geological backgrounds is quite different. Therefore, finding out the soil properties in different regions is a prerequisite for assessing the risk of heavy metals. The current Soil Environmental Quality Standards may not be suitable for the safety risk management of heavy metals in different HGB regions. The phenomenon whereby the soil concentrations are higher than the standard and the rice concentrations are lower than the standard in the HGB area of the parent material type highlights the limitations of the current soil environmental quality standards in China. Incorporating the bioavailability of soil heavy metals into the soil environment quality standard system may be more appropriate and realistic. The relevant soil bioavailability index of soil environmental quality standards in Japan has a certain reference value [
68].