Although federal records show that dispersant use in the Gulf of Mexico has ground to a halt in the weeks since BP PLC affixed a cap to its leaking wellhead, congressional interest in monitoring the controversial oil-spill chemicals is showing no signs of abating.

The Senate Environment and Public Works Committee will hold a joint hearing Wednesday with its oversight panel on the more than 1.8 million gallons of dispersant employed to break up oil from the Gulf leak. The hearing comes as lawmakers weigh new testing requirements and usage limits for the chemical products, raising the stakes on an issue that is already driving new scientific research and debate.

Senators will hear from Edward Overton, a Louisiana State University professor emeritus in environmental science who praised BP's dispersant strategy in a Thursday interview with CNN.

"If anything, the early evidence is that the use of those dispersants saved the shoreline," Overton told the network, drawing a favorable tweet to the media and the public by Nalco, the manufacturer of BP's dispersant of choice. "And that was clearly a good decision."

A contrasting viewpoint may come from Texas Tech University environmental toxicology professor Ron Kendall, who has warned of the unknown risks to seafood posed by massive dispersant use.

While dispersants help prevent oil from affecting shoreline wildlife by breaking up the crude into more easily biodegradable droplets, scientists remain concerned that the chemical products are shifting the risk to larvae by exacerbating the vast subsea plumes now present in the Gulf (Greenwire, July 30). Studies from the region have found dispersed oil particles in the bodies of crab larvae, raising questions about whether the chemicals could alter the leak's effect on the food chain.

Another witness at the hearing, U.S. EPA Assistant Administrator Paul Anastas, in June released a first round of dispersant toxicity studies that the agency conducted after BP resisted its order to abandon its Nalco-made product for a less toxic alternative (Greenwire, May 20). Those June tests found the Corexit 9500 dispersant to be no more acutely toxic than competing products, but EPA has yet to release tests that examine mixtures of dispersant and oil.

Sen. Frank Lautenberg (D-N.J.), a member of the environment panel, introduced a bill last week that mirrors many of the dispersant provisions in the oil-spill response legislation the House passed Friday (Greenwire, July 28).

The House bill includes new rules for dispersants, such as a requirement that manufacturers release the products' chemical ingredients -- which Nalco gave to EPA but waited weeks before publicly sharing -- and a temporary moratorium on their use until EPA concludes a rulemaking on their safe application.

That moratorium was championed by Rep. Jerrold Nadler (D-N.Y.), who has cautioned that dispersant use could result in a long-term public health risk to Gulf residents and workers exposed to the products' toxic components.

"The only thing dispersants seem to do is push the oil below the surface, making it harder to see the damage and determine liability, and making it harder to boom and skim the oil off the surface," Nadler said in a House floor speech Friday. "The only benefit seems to be for PR purposes."

Public debate over the consequences of BP's dispersant use flared anew Saturday when Rep. Ed Markey (D-Mass.), chairman of the House Energy and Commerce's environment panel, released documents cataloguing the Coast Guard's decision to grant the oil company near-daily exemptions to a May 26 federal directive seeking to curb surface dispersant sprays in the Gulf except "in rare cases." While the directive was issued by EPA and the Coast Guard, the latter agency -- represented by the federal on-scene commander -- was tasked with issuing exemptions.

"BP carpet-bombed the ocean with these chemicals, and the Coast Guard allowed them to do it," Markey said in a statement accompanying a strongly worded letter on the issue to Thad Allen, the retired Coast Guard admiral in charge of the Gulf leak response.

Greenwire reported last month that, in the four weeks following the May directive, BP reported spraying 272,000 gallons of surface dispersant. But Markey's staff uncovered discrepancies between those totals, gleaned from public disclosures, and the total dispersant volumes BP referenced in its requests for Coast Guard exemptions.

"It appears to me," Markey wrote to Allen over the weekend, "that the May 26, 2010, directive has become more of a meaningless paperwork exercise."

Schedule: The hearing is Wednesday, Aug. 4, at 10 a.m. in 406 Dirksen.

Witnesses: Paul Anastas, assistant administrator in U.S. EPA's Office of Research and Development; David Westerholm, director of the National Oceanic and Atmospheric Administration's Office of Response and Restoration; Ronald Kendall, professor of environmental toxicology and director of the Institute of Environmental and Human Health at Texas Tech University; David Smith, oceanography professor and associate dean of the Graduate School of Oceanography at the University of Rhode Island; Edward Overton, professor emeritus in the environmental science department at Louisiana State University; and Jackie Savitz, senior scientist and pollution campaign director at Oceana.