Boston — Ever wonder what happens to the tons of salt poured on snow-covered highways - besides nurturing rust colonies on car bellies in the nation's snow belt? Does the salt seep into the ground, get carried into streams and rivers, and eventually run off into the ocean? Or, sometimes, does it even find its way into the water we drink?
The answer is ''yes'' to both questions.
State and federal officials point to an increasing number of cases where application or storage of salt (or chemicals containing sodium) are related to rising sodium levels in public drinking water.
Road salt in the water table poses no problem for the average individual, who eats several hundred milligrams of salt each day in food. But health officials say excessive sodium intake may be linked to a variety of health problems for the small number of persons on low-sodium diets.
Despite these warnings, as well as environmental damage and automobile corrosion, salt is still considered by most states to be the best de-icer. However, some states with ''watershed sensitive areas,'' such as Colorado, use abrasives such as sand, and a bare minimum of road salt.
Common road salt is composed of about 40 percent sodium ions and 60 percent chloride ions. When salt meets water from melted snow or ice, the sodium and chloride ions travel with the water on the surface or in groundwater. The surface water may evaporate or run into rivers and lakes on the way to the ocean. The water that seeps into groundwater may find its way into reservoirs or wells used for private and public water supplies.
In New England, 500,000 tons of road salt are used annually, according to a survey by the Salt Institute of Alexandria, Va. Massachusetts alone uses almost 200,000 tons.
Testing by the Massachusetts Department of Environmental Quality Engineering (DEQE) shows many communities have water supplies with sodium levels above the state's maximum contaminant level of 20 miligrams per liter (mg/L) for drinking water. (The federal level for sodium is between 20 and 250 mg/L.)
By law, public water suppliers in Massachusetts must inform their customers if the water's sodium content exceeds this standard. In Cambridge, Mass., where the level is now 33 mg/L, letters were sent recently to every Cambridge resident explaining the sodium issue in detail.
In the past, the standard was not as clear, says H. Lincoln Harner of the Salt Institute. ''The problem is that when the Massachusetts DEQE first came out with the figures, they weren't explained properly. Everyone receiving the letter asked: 'What . . . is going on? Why is there so much sodium in my water?' ''
Mr. Harner explains that the Salt Institute, representing the salt industry in Washington, ''just tries to put the sodium issue into better perspective. In New England, water sodium content is higher. But there's far less sodium in the water in the Boston area than in a glass of milk.''
Even so, wells in several Massachusetts towns have been closed because of salt levels. And the state Department of Public Works (DPW) is ''being hit with several suits in western Massachusetts'' for sodium content, say a DEQE official. As a result, the DPW has reduced salt application from 400 pounds per mile several years ago to about 300 pounds today.
But this is where the debate between highway safety and health intensifies. Highway officials argue salt must be applied to fresh snow to prevent it from bonding and to keep snow ''mealy.'' Reducing the amount of salt on roads increases the potential for slippery conditions and accidents, they insist.
The answer is for towns to adopt improved management procedures, says Steven Roy, who is responsible for groundwater programs at the Massachusetts DEQE. This , he suggests, includes:
* Mixing salt with sand or chemicals.
* Maintaining road equipment.
* Exploring new alternatives such as calcium magnesium acetate and a chemical defroster added to asphalt.