KEENE, N.H. (AP) – New Hampshire wants to know the source of salt that is polluting four watersheds along Interstate 93 – especially whether it is from deicing roads in the winter.

The Executive Council will consider contracting Wednesday with Plymouth State University to study the source of salt in the watersheds.

The state can’t widen the highway between Salem and Manchester until it shows it will use the same amount of salt on four lanes each way as it now uses on two lanes.

The four watersheds that exceed state chloride standards are Policy Brook in Salem; Dinsmore Brook in Windham; Beaver Brook in Derry, Londonderry and Chester; and a small tributary on the western side of Canobie Lake in Salem.

Transportation Commissioner Carol Murray says the state believes less than 20 percent of the pollution comes from salting the highway. She said the study also look at salting practices by area cities, towns and businesses.