To the best of my knowledge, there is no way to specify it in Gaussian explicitly. But you can try to calculate the dielectric constant (known as relative permittivity) of your solvent mixture and specify it using SCRF(Read) option for PCM model. For SMD model input is much broader.
Honestly, I assume that the most reasonable option is to neglect presence of water and proceed only with ACN which is recognizable by Gaussian.