WASHINGTON -- U.S. Customs and Border Protection (CBP) continues not delay enforcement of a controversial rule requiring carriers to manifest the amount of liquid residue left in tank trailers or cars crossing the border.

According to BulkTransporter.com, Todd Owen of CBP and Homeland Security told participants of the Chemical Sector Security Summit in Baltimore that the agency is still discussing with stakeholders the impact the rule would have on bulk loads crossing from Canada and Mexico.

The rule was originally supposed to go into effect on Sept. 16, 2009 but has since been postponed a couple of times to give industry more time to prepare.

Owen said the agency is still considering methods of quantifying the amount of product that is left in the container and how it should be manifested.

When enforcement begins, it will take some form of radiation to scan tank trailers and tankcars. "We are not looking for scrapings,” he said. “If we can see it, there is a problem.

Previously, residue was exempt from the same reporting requirements as any other good entering the U.S. But the border agency says that allowance is inconsistent with its treatment of other commodities (such as 'petroleum slops' from vessels).

Tankers should also be drained to ensure the "safety and security" of officers who examine them, CBP said.

Although the amount of chemical or fuel left in an empty trailer is usually miniscule, Owen told attendees that the amount of residue left in some rail cars coming from Mexico is what prompted concerns. Some cars being classified as "empty" were one-third full, he said.

He also said that the practice of classifying trailers as 'empty" began when CBP issued an exemption for one company, but it soon became commonplace for all carriers.