Des Moines Water Works exceeded allowable federal standards for a substance known as Total Trihalomethanes (TTHM) this spring, but the levels do not pose any immediate health risk to customers, its CEO said late Friday. Des Moines Water Works' violation occurred because of an interaction of chlorine with organic matter in the water system to treat for spring runoff. Some people who drink water containing Trihalomethanes in excess of the standard over many years may experience problems with their liver, kidneys or central nervous system. However, CEO Bill Stowe said the water is safe to consume and customers do not need to use alternative source of drinking water or use additional treatment techniques. Separately, the City of Norwalk has told residents they should boil their water before drinking it or cooking with it, after a Sunday evening water main break.