There is a difference between these two words that ought to be preserved; wastage should not be used as a more dignified alternative to waste. The ordinary meaning of waste is "useless expenditure or consumption, squandering (of money, time, etc.)". The ordinary meaning of wastage is "loss by use, decay, evaporation, leakage, or the like". You may for instance properly say that the daily wastage of a reservoir is so many gallons. But you must not say that a contributory factor is the wastage of water by householders if what you mean is that householders waste it.