Environmental Assessment of Heavy Metals in the Typical Vegetable Field and Irrigation Sediment in Xijiang River Basin, China: Spatial Distribution and Ecological Risk

Heavy metal (HM) contamination of agroecosystems has raised great attention across the world due to the rapid development of industrialization and urbanization. Importantly, HMs have been consistently entering the food webs, imposing great risks on public health. However, previous studies have focused more on large-scale areas. Exemplarily, some typical areas (e.g., riverside farmlands) receive insufficient attention, but they may have a higher ecological risk. In this study, we used a series of indices to evaluate risks, including single factor index, Nemerow index, geological accumulation index, potential ecological risk index, risk assessment code, etc. Based on the field investigation, we systematically analyzed the pollution characteristics and ecological risks of five HMs (i.e., Pb, Cu, Zn, Cr, and Ni) in the typical vegetable soil and its irrigation sediment in Xijiang River Basin, Guangdong, China. The results indicate that the pH values of the soil and sediment are mainly neutral or slightly acidic. Moreover, large-scale natural sources may have the effect on the soil and sediment, among which storm runoff and scouring of the river may be main natural factors of HM activation. And importantly, there was a migration cycle (with the in-out trait) for the main non-residual forms of HMs between riverside soil and sediment. According to the principal component analysis (PCA), HMs may be mainly affected by anthropogenic sources. Specifically, PC1 may be related to transportation and agricultural machineries, and PC2 may be related to pesticides and fertilizers. Also, according to risk assessment code and the ration of secondary phase to primary phase, these meant high proportion of the bio-available forms of HMs. Conversely, Pearson correlation coefficient between bioavailable metal index and the sum of toxic unit was not high, which was related to low contents of the bio-available HMs. And though, these indices may show great differences due to different method focuses, an important suggestion is that integrating all the methods can receive more comprehensive results of ecological risks. According to the bioavailable metal index, the bio-availability of HMs decreases in the following order: river sediment (3.69 +/- 1.24) > riverside farmland soil (2.28 +/- 0.61) > inside-dyke farmland soil (2.26 +/- 1.24) > reservoir sediment (1.99 +/- 0.18). Besides, comprehensive evaluation indicates that Cu and Zn pose the greater environmental risks as they have the highest effective content, bioavailability, and mobility. Thus, developing high-tech environmental monitoring, properly purifing irrigation water, and improving the drainage systems are a necessity for these HMs.

Automated summary

Heavy metal contamination in agroecosystems is a global concern due to industrialization and urbanization, with HMs entering the food chain and posing risks to public health. This study focused on a specific area, the Xijiang River Basin in Guangdong, China, and analyzed the pollution characteristics and ecological risks of five HMs. The results showed that natural sources and anthropogenic activities both contribute to the activation and migration of HMs, and Cu and Zn were identified as posing the greatest environmental risks. Integration of different assessment methods is recommended for more comprehensive risk evaluation.