Reactive transport modeling of heavy metal migration from aquaculture water to groundwater and irrigation canals in the northeastern Nile Delta, Egypt

Due to shortages of clean irrigation water, fish farm operators in the northeastern Nile Delta have increasingly relied on low-quality drainage water. This study quantitatively assessed the long-term risk of heavy metal migration from aquaculture systems to shallow groundwater and irrigation canals using reactive transport modeling. Water quality analyses showed that aquaculture water contained heavy metal concentrations exceeding WHO permissible limits, including cadmium (0.86 mg/L), cobalt (0.42 mg/L), lead (0.03 mg/L), chromium (0.07 mg/L), and nickel (0.052 mg/L), whereas groundwater in the study area showed no detectable heavy metal contamination. A coupled groundwater flow and reactive transport model was developed using MODFLOW and MT3DMS. The steady-state flow model was calibrated and validated against 20 groundwater level observations. Contaminant transport was simulated over a 50-year period, accounting for advection, hydrodynamic dispersion, Langmuir equilibrium sorption, and first-order irreversible kinetic reactions. Simulation results indicated persistent southeastward migration of heavy metal plumes controlled by the regional hydraulic gradient. Cadmium exhibited the greatest spatial extent and mass accumulation, reaching approximately 291,200 kg by the end of the simulation period. The irrigation canal network in the fish farm area acted as a major hydraulic sink, intercepting contaminated groundwater and limiting plume expansion beyond the source zone. However, it posed a progressive contamination risk to irrigation canal water quality. Mass balance analysis showed that cadmium and cobalt continued to accumulate in the aquifer without reaching steady-state conditions, whereas lead, nickel, and chromium approached dynamic equilibrium due to the balance between source loading and canal interception. These results highlight the urgent need for source water treatment before aquaculture use to prevent long-term degradation of groundwater and irrigation canals.