Using stellar evolution models in which atomic diffusion was included self-consistently throughout the evolution of a Population II 0.8 Msolar star (Y=0.2352, Z=0.0001, ?=0.3) from the zero-age main sequence to the end of the core expansion phase on the horizonatal branch (HB), surface-abundance composition is calculated and compared to observations of HB stars in the globular cluster M15. The effect of turbulence is characterized by the outer mass it mixes. It is shown that the overabundances of Fe by a factor of 50-100 observed by Behr in the seven stars of that cluster with Teff>11,000 K require an outer mixed mass of about 10-7 M*. The abundance anomalies of Al, Si, S, Ca, Ti, Cr, and Ni, but not of Mg, are also reasonably well reproduced without any additional adjustment. This constitutes a convincing confirmation of the role of atomic diffusion driven by radiative accelerations in HB stars.