Contamination（污染） of water supplies is usually due to poor sanitation（卫生） close to water sources, sewage（污水） disposal（丢掉） into the sources themselves, leakage（漏） of sewage（污水） into distribution（分发） systems or contamination（污染） with industrial or farm waste. Even if a piped water supply is safe at its source, it is not always safe by the time it reaches the tap（轻叩）. Intermittent（间歇的） tap（轻叩） water supplies should be regarded（把…看作） as particularly suspect. Travellers on short trips to areas with water supplies of uncertain quality should avoid drinking tap water or untreated（ 未治疗的） water from any other source. It is best to keep two hot drinks, bottled or canned drinks of well-known brand names. International standards of water treatment are usually followed at bottling plants.