  Using angle resolved photoemission spectroscopy we studied the evolution of
the surface electronic structure of the topological insulator
$\text{Bi}_2\text{Se}_3$ as a function of water vapor exposure. We find that a
surface reaction with water induces a band bending shifting the Dirac point
deep into the occupied states and creating 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. Due to the
presence of water vapor, a similar effect takes place when
$\text{Bi}_2\text{Se}_3$ crystals are left in vacuum or cleaved in air, which
likely explains the aging effect observed in the $\text{Bi}_2\text{Se}_3$ band
structure.
