Toxic effects of representative petroleum contaminants on earthworms (Eisenia fetida) under rainfall conditions

During petroleum extraction and processing, petroleum contaminants readily pollute site soils. Under rainfall conditions, both contaminant migration and the activity of soil fauna are significantly affected. However, the implications of these interactions for soil fauna toxicity remain unclear. In this study, dodecane and naphthalene were selected as representative petroleum contaminants to investigate the migration pattern in site soils and their toxic effects on earthworms (Eisenia fetida), simulating interactions between soil surface contamination and different rainfall intensities (0.6 mm/d, 17.3 mm/d, and 60.2 mm/d). Both pollutants were primarily retained within the top 5 cm of soil; neither their vertical transport nor bioavailability significantly correlated with rainfall. Increasing rainfall intensity enhanced earthworm activity, thereby increasing their exposure to the pollutants, with the highest levels of 28.98 and 36.60 μg/g for dodecane and naphthalene, respectively. Accumulation of high concentrations of either contaminant damaged intestinal tissues and induced severe oxidative stress in earthworms. Dodecane and naphthalene exposure processes altered the composition of the earthworm gut flora and led to niche differentiation, regardless of rainfall intensity. The combined stress of contaminants and rainfall significantly disrupted earthworm metabolic pathways (energy, amino acids, carbohydrates, etc.). For hydrophobic organics like these, rainfall likely modulates toxicity risks to soil fauna not primarily through enhanced contaminant migration. Instead, it alters earthworm behavioral patterns, leading to differential contaminant exposure and subsequent toxicological responses.