Agricultural intensification in the humid tropics increasingly relies on fertilizers and pesticides, raising concerns about their mobility, persistence, and environmental risks. This study investigated the fate of nitrate, phosphate, potassium, and lambda-cyhalothrin in irrigated farmlands of Rivers State, Nigeria, through integrated field monitoring, spectrophotometric and GC–MS analyses, and transport modeling. A randomized complete block design (nine plots: control, fertilizer-only, and fertilizer + pesticide treatments) was established, with irrigation (25–30 mm weekly) and seasonal rainfall (~210 mm) providing hydrological drivers. Nitrate concentrations increased rapidly after fertilizer application, peaking at 38.6 ± 4.5 mg/L by day 7 and declining to 15.2 ± 2.8 mg/L by day 28, indicating strong leaching potential. Orthophosphate remained largely sorbed to soils (0.8–2.5 mg/L in water), while potassium showed higher mobility in sandy soils than in clay-rich plots. Lambda-cyhalothrin residues declined from 0.42 ± 0.05 mg/kg in soils (day 1) to 0.09 ± 0.02 mg/kg (day 28), with irrigation water concentrations peaking at 0.15 ± 0.03 mg/L and crop tissues containing trace residues below FAO/WHO limits. Transport modeling reproduced breakthrough curves with high accuracy (R² = 0.87–0.92), estimating pore-water velocities of 0.12–0.18 cm/s and dispersion coefficients of 0.65–1.02 cm²/s. These findings reveal nitrate as highly mobile, phosphate as strongly retained, potassium as soil-type dependent, and lambda-cyhalothrin as moderately persistent. The results underscore the urgent need for precision fertilization, soil-type–specific nutrient management, and integrated pest management strategies to minimize agrochemical losses, protect water quality, and ensure long-term agro-ecosystem sustainability in the Niger Delta.

 Keywords: Nitrate leaching, Phosphate retention, pesticide persistence, transport modeling, Niger Delta