Using angular resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator Bi2Se3 as a function of water vapor exposure. We find that a surface reaction with water induces a band bending, which shifts the Dirac point deep into the occupied states and creates quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se abstraction, leaving positively charged vacancies at the surface. Because of the presence of water vapor, a similar effect takes place when Bi2Se3 crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the Bi2Se3 band structure.