The national average price of retail diesel declined for the fourth consecutive week, dipping 2.5 cents a gallon to $3.81, the Department of Energy reported.

California’s attorney general announced a settlement last week with seven Southern California port terminal operators to resolve the state’s claims that residents near the ports were exposed to dangerous levels of diesel exhaust from the terminals.

American Trucking Associations said the federal government’s proposed 2012-2013 renewable fuel standards could become “a new billion-dollar hidden tax on the trucking industry” because Congress is unlikely to continue underwriting biodiesel’s production costs.

The national average price of diesel fuel declined for a third straight week, dipping 6.2 cents to an almost six-month low of $3.835 a gallon, according to the Department of Energy.

American Trucking Associations has asked the federal government to lower its 2012 and 2013 biodiesel blending requirement under the Renewable Fuel Standard.

The average price of a gallon of U.S. diesel slid 4 cents to $3.897 last week, the Department of Energy reported, while crude oil declined along with equity markets.

The Department of Energy Tuesday lowered its diesel fuel price forecast for this year by 3 cents, to $3.83 a gallon.