Nitrate (NO3-) accumulation and transport processes in the thick vadose zone affect the evolution of the groundwater NO3- content in intensive agricultural regions. Agricultural land-use change (ALUC), typically accompanied by substantial alterations in nitrogen fertilizer application and irrigation practices, is an important influencing factor. This study evaluated the changes in NO3- accumulation and transport in the deep vadose zone (DVZ, below the root zone), and the groundwater NO3- content associated with ALUC from grain to vegetable fields in the North China Plain (NCP). The ALUC from grain to vegetable resulted in nitrate-nitrogen (NO3--N) accumulation in DVZ increased by 235.5 kg ha-1 m-1 (163.2%) in the piedmont plain and 224.9 kg ha-1 m-1 (102.7%) in the central plain, respectively. This change accelerated downward transport velocity in the DVZ (from 0.81 +/- 0.47 to 0.89 +/- 0.55 m yr-1 in the piedmont plain, and from 0.24 +/- 0.12 to 0.92 +/- 0.12 m yr-1 in the central plain) and increased NO3- leaching fluxes. High transport velocity and leaching fluxes resulted in chemical N-fertilizer entering the aquifer in several areas in the piedmont plain. The impact of the agricultural activity intensity changes, accompanied by the ALUC, on groundwater quantity and quality should be considered in similar regions.