Far infrared spectroscopic studies of electron-density- and magnetic-field-dependenceof manyelectron effects on silicon donor impurities confined in GaAs quantum wells are presented. At low excess electron densities, transitions from D− singlet and triplet states are observed. Temperature- and polarization-dependence measurements show that the relative absorption strengths of various spectroscopic features (D0, D−, and CR) are in qualitative agreement with a statistical calculation in thermal equilibrium in high magnetic fields. At large excess electron densities, the “D−” transition energy shifts to higher energy when electron density is increased, and the magnetic-field dependence of the transition energy exhibits discontinuities in slope at integer filling factors. The spectroscopic features and their relation to excess free carriers and the role of screening and correlation are presented and discussed.