A Large-Scale Assessment of Soil Heavy Metal Pollution Using Field-Collected Earthworms as Bio-Indicators in Shaoguan, South China

Abstract

Soil heavy metal pollution is one of the biggest issues that we are facing today, which poses serious threats to ecosystems and communities. However, a lack of efficient means and incomplete monitoring data are the main obstacles impeding soil pollution management and prevention in China. Moreover, it is rare to examine the possibility and reliability of using field-collected earthworms to assess the pollution degree of soil heavy metals in real-world situations at a large field scale. In this study, we investigate the potential use of field-collected earthworms as bio-indicators to assess the pollution characteristics and risks of heavy metals (Zn, Pb, Cu, As, Cd, and Cr) across three typical regions (Xiaokeng Reservoir, Dabaoshan Mine, and Smelter/Steel Plants) in Shaoguan, one of six trial zones identified for the Soil Pollution Prevention and Control Action Plan in China. Our results unveil the existence of significant differences in the spatial distribution and bioavailability of heavy metals in soils and earthworms across these regions. The average contents of Zn, Pb, Cu, As, Cr, and Cd in soils were 329.7 ± 476.5, 180.9 ± 262.4, 82.9 ± 197.8, 66.6 ± 61.6, 53.3 ± 53.6, and 7.2 ± 8.2 mg/kg, while those in earthworms were 113.6 ± 112.1, 99.4 ± 106.8, 90.0 ± 126.9, 24.7 ± 30.8, 8.8 ± 9.4, and 5.1 ± 3.4 mg/kg, respectively. The internal exposure dose of heavy metals in earthworms is found to more accurately reflect their bioavailability under true environmental conditions than the external exposure dose in soils. The bioaccumulation factor (BAF) exhibited the highest values for Cd (0.01–24.40), followed by those for Pb (0.06–9.94), Cu (0.12–5.23), Cr (0.02–1.90), As (0.02–1.18), and Zn (0.06–1.17). Further principal component analysis (PCA) and random forest (RF) classification ascertain that earthworms are more capable of differentiating heavy metal pollution in different regions than soil alone. Compared to the routine analysis of the total heavy metal content in soils, our novel strategy demonstrates the superiority of employing field-collected earthworms as bio-indicators for monitoring heavy metal pollution and discriminating potential sources of soil pollution in a real scenario.