The width of the water resonance in a proton NMR spectrum depends critically on the amount of water present. When the concentration of H2O is very low the NMR resonance is very narrow. When the concentration is very high the width is many times greater. The reason for this is that the strong magnetization of the water signal induces currents in the NMR coil which generate magnetic fields which broaden the line. This phenomenon is called radiation damping. The width of the water line is a function of the strength of the water signal which depends on the amount of water, probe tuning, field strength, coil size etc... Radiation damping can also affect the symmetry and phase of the peak. Below are the 500 MHz proton NMR spectra of two samples of H2O / D2O with different concentrations of H2O. In both cases the magnet is well shimmed.