We use Born-Oppenheimer Quantum Molecular Dynamics to investigate the reactions of the HfCl4 and H2O ALD precursors with growing HfO2 ALD films. We find various alternative pathways for surface reactions not previously identified. We also find that residual water on metal oxide surfaces from incomplete water purges can play a significant role in the nature of non-ideal ALD surface chemistry and use this insight to explain several experimental observations of the ALD of metal oxides using water as an oxygen source.