OVER THREE YEARS ago, concern about possibly dangerous organic chemicals in drinking-water supplies all over the country gave a big boost to the Safe Drinking Water Act of 1974. Now, thanks largely to persistent pressure from the Environmental Defense Fund, the federal Environmental Protection Agency has finally proposed the first rules requiring communities to curb these pollutants. If adopted in their current form, the rules could bring about improvements in the water systems serving the District, Northern Virginia and many other metropolitan areas whose water supplies - while not immediately poisonous - are not pristine.

The pollutants at issue here are part of a fairly new, fast-growing group of environmental hazards: those created or compounded by man. One suspect category is trihalomethanes (THMs), a family of organic compounds that includes chloroform, which has caused cancer in test animals and may pose risks for human beings. THMs, ironically, are actually generated at water purification plants through the interaction of naturally created substances with chlorine - which, of course, is added to kill water-borne bacteria. This does not mean that chlorine is unsafe. In fact, the whole purpose of EPA's new rules is to require communities to limit THMs, the undesirable byproduct, to minuscule levels without losing the benefits chlorine provides by warding off typhoid, cholera and other diseases.

The second category of contaminants involved are synthetic organic chemicals, a large group of compounds that get into many water supplies through industrial discharges and spills and the runoff of water from cities and from land exposed to agricultural chemicals. While some of these substances appear to pose definite long-term risks to human health, scientists don't yet know enough to define the dangers precisely. EPA is continuing its research and its efforts to control these pollutants at their sources. Meanwhile, however, the agency wants communities exposed to these compounds to take the precautionary step of installing activated carbon filters, which are the best known means of removing these pollutants from water.

These water-system improvements could raise an average family's water bill perhaps $6 to $10 per year, according to EPA. Even at that price the changes are worthwhile because they can protect millions of Americans against insidious hazards those full impact on human health may not be known for several decades. Granted, the dangers involved may be relatively small. But in such complicated areas of environmental health, precaution is the best public policy - especially when dealing with products such as drinking water, where consumers have little opportunity for individual choice.