Researchers at Lawrence Livermore National Laboratory have found a good way of determining how much water runoff from streets is polluting the rivers and oceans: following the tracks of the tires.

A chemical used in rubber manufacture and deposited by tires on the roadway is transformed after a while into benzthiazole compounds, according to Richard Spies, Brian Andresen and David Rice, writing in the June 25 issue of Nature.

The scientists measured the concentration of these compounds in the soil near San Francisco Bay and in the bay. Street runoff is believed to be a major pollution problem for bodies of water.

The scientists found that a heavier concentration of the compounds near the highways than far away.

For example, the edge of Highway 80 in Berkeley had about 273 micrograms per kilogram. Within 1,300 feet of the east end of the Oakland Bay Bridge, there were 360 micrograms per kilogram, while farther away from major thoroughfares, concentrations were lower. There were about 50 micrograms in San Pablo Bay and 23 in the sand at Alameda.

It is not known how toxic the benzthiazoles are, but the researchers suggested that the chemicals could make excellent markers of the places in sediment at which street runoff enters a body of water, and in what amounts. This could provide a better measure of how much street runoff -- rather than factory dumping, oil spills or other sources -- contributes to pollution of a river, lake or ocean area.