Ecological risk from wastewater irrigation: a comprehensive assessment of heavy metal contamination in Egyptian soils

Abstract Background Continuous irrigation with poor-quality water poses a major threat to soil health and food security through the progressive buildup of potentially toxic elements (PTEs). This study conducted a detailed ecological risk assessment of nickel (Ni), copper (Cu), and zinc (Zn) in Egyptian agricultural soils subjected to various durations and types of wastewater irrigation. Surface soil samples (0–30 cm) were collected from three representative sites: a heavily polluted area irrigated with mixed industrial and sewage effluents for over 80 years (G1), a site receiving mixed wastewater for 40 years (G2), and a control site irrigated with agricultural drainage water for 35 years (G3). The total concentrations, sequential chemical speciation, and contamination indices, including the contamination factor (Cf), zinc equivalent (ZE), and contamination degree ( CD ), were determined to evaluate the pollution severity and potential mobility. Results Sites G1 and G2 presented severe contamination, with significantly elevated Cf and CD values compared with those of G3, confirming intense historical pollution. Speciation analysis revealed that approximately 20% of the total Cu was bound to the organic fraction, suggesting elevated mobility and bioavailability. Overall PTE mobility followed the order of hardly available ≥ moderately available > readily available fractions, indicating limited immediate leaching but persistent long-term risk. Correlation analyses revealed that the clay content and organic matter content are the primary soil properties governing PTE retention and distribution. Conclusions These findings underscore that decades of wastewater irrigation have created a legacy of soil contamination, facilitating PTE transfer into the food chain and posing ongoing ecological and human health risks. The results highlight the urgent need for targeted soil remediation strategies, improved wastewater treatment, and sustainable irrigation practices to mitigate these environmental threats in vulnerable agricultural regions. Graphical Abstract