Diesel prices remained on the decline in the U.S. this week, while the cost of diesel exhaust fluid (DEF) remained stable for the month of March, according to data tracked by industry sources.

The Energy Information Administration (EIA) said diesel prices declined 1.3 cents this week to a U.S. retail pump average of $3.993 per gallon – a price point 14.9 cents per gallon cheaper than the same week in 2012.

Diesel prices declines in all regions of the country, the agency added, except for the West Coast (up 1.5 cents to $4.116 per gallon) and California (up 5.3 cents to $4.20 per gallon).

However, EIA’s data also indicated that diesel prices are now below the $4 per gallon mark in only four out of nine U.S. regions – the Gulf Coast ($3.915 per gallon), the Rocky Mountains ($3.922), the Lower Atlantic area ($3.958) and the Midwest ($3.97).

The average retail price for gasoline across the U.S. is also down this week by 3.5 cents to $3.645 per gallon – which is a whopping 29.4 cents per gallon cheaper compared to the same week in 2012, EIA noted.

Gasoline prices only increased in one region of the U.S., the agency said; the Rocky Mountains, where prices jumped 2.8 cents to $3.497 per gallon.

In terms of DEF, Chris Goodfellow, emissions analyst with global consulting firm Integer Research, reported that bulk DEF prices largely held between February and March, which includes full TL and LTL bulk deliveries as well as tote refills.

“Agricultural use of urea has been lower than expected in March and this has led to lower than expected pricing,” Goodfellow noted. “Urea is the key component in the manufacture of DEF and stability in raw material costs has had a direct impact on end-user pricing.”

He added that this “lack of volatility” in DEF pump pricing meant that the U.S. average price remained at $2.79 per gallon for the fourth consecutive month, while the national Canadian DEF pump price also held level at CAD 80 cents per liter– roughly equating to $3.16 per gallon in U.S. dollars.

Goodfellow also pointed out that the number of truck stops adding bulk DEF facilities was the lowest since late November 2011, with just 16 locations coming online over the last four weeks – though historically the lowest ebb for such “network expansion” is in the first quarter of the year, he stressed.