Actually, this technique of cleansing water wasn't new at all when the district made the switch to chloramines in late 1984. Denver had been using chloramines for more than 70 years: St. Louis, 50; and Boston, 40 years. At least 71 water utilities, serving 26 million people were using chloramines before Metropolitan switched back to the method it had started out with in 1941, when the war forced the switch to chlorine.
Then, in the 1970s, federal regulations demanded a level of water quality that as time progressed became more and more difficult for many water agencies to achieve with chlorine alone.
Water on its way to the Southland from Northern California flows through the fertile Sacramento-San Joaquin Delta where it picks up organic materials from riverbeds. These organic compounds are harmless until they reach Southern California combine with chlorine, added to kill water-borne bacteria, and from trihalomethanes (THMs).
THMs weren't detected in drinking water until 1974. They're still not fully understood and no one knows for sure whether they present a definite health threat. But tests with laboratory animals suggest they might, when consumed in large amounts over a long period of time, be linked to cancer. So federal rules limit their presence to 100 parts per billion of drinking water - about one drop per 132 gallons. MWD's water has always been comfortably under the limit.
As Arizona continues to exercise its right to Colorado River water, Metropolitan will have to rely more on supplies from the north-water that contains more of the organic materials that combine with chlorine to create THMs. With continued use of the chlorine-only system, unacceptable THM would have been likely.
Chloramines have some distinct advantages over free chlorine. Most people report chloraminated water tastes better - there's little chlorine flavor or odor. Chloramines don't dissipate as quickly as chlorine, so they continue to disinfect water at the ends of long pipelines where chlorine seldom reaches. Chloraminated water is considered completely safe to drink, to bathe in, to swim in, to cook with, and for all the ordinary uses people make of water every day. It has no effect whatsoever on animals, water plants, crops, soil or any other environmental factor.
But chloramines aren't perfect. There are two problems: they are toxic to fish, and kidney dialysis equipment must be modified to remove them from the water during this medical treatment, just as chlorine had to be removed. There are fairly simple, effective ways to eliminate chloramines.