  We develop a general scheme of deriving boundary conditions for spin-charge
coupled transport in disordered systems with spin-orbit interactions. To
illustrate the application of the method, we explicitly derive boundary
conditions for spin diffusion in the Rashba model. Due to the surface spin
precession, the boundary conditions are non-trivial and contain terms, which
couple different components of the spin density. We argue that boundary
conditions and the corresponding electric-field-induced spin accumulation
generally depend on the nature of the boundary and therefore the spin Hall
effect in a spin-orbit coupled system can be viewed as a non-universal edge
phenomenon.
