CHARLESTON, S.C. — Environmental groups suing over the $650 million deepening of the Savannah River shipping channel want a federal judge to rule the project is subject to South Carolina environmental law, saying that will allow a fairer review by state officials.

Environmental groups in two states originally sued saying the deepening work needed a permit under the state's Pollution Control Act. But in papers filed Monday seeking to amend the suit, attorneys noted the U.S. Army Corps of Engineers has said repeatedly the project could proceed without approvals from South Carolina.

The plaintiffs contend a declaration from the court that the project is subject to state law "would allow a fair appraisal of the project's compliance with state standards ... and that a fair appraisal — unclouded by the corps' threats — would result in decisions that reduce the projects' severe environmental impacts."

It said because of the corps' statements, state regulators have been operating under the "mistaken impression" that resistance to the project "is futile and approving the project with meager mitigation is preferable to more aggressive review or conditions."

U.S. District Judge Richard Gergel has given corps and other parties in the suit until next Monday to respond.

Maritime interests want the river deepened to handle larger container ships that will routinely be calling at the Georgia ports when the Panama Canal is enlarged in 2014.

The lawsuit was brought by the Southern Environmental Law Center on behalf of the Savannah Riverkeeper, based in Augusta, Ga., as well as the South Carolina Coastal Conservation League and the South Carolina Wildlife Federation.

The original suit said the corps needs a South Carolina pollution permit because toxic cadmium from river silt will be dumped in a dredge spoils area on the South Carolina side of the river.

The court documents filed Monday note there are two exceptions under which the corps might not be subject to the permitting requirements of the Clean Water Act, which requires federal agencies to adhere to state environmental rules.

One is if information about the discharge is included in an environmental impact statement and that statement is submitted to Congress before the work begins or before the appropriation of money for the project. The second exception is the Secretary of the Army has the authority to maintain navigation.

The plaintiffs say that both the Department of Health and Environmental Control and Gov. Nikki Haley cited the corps' position as part of the rationale for giving state approval.

The document quoted Haley speaking last fall at the Charleston Propeller Club.

"I have in writing, your DHEC board had in writing, a letter from the Corps of Engineers saying we don't need a permit from DHEC to give Georgia their dredging," Haley was quoted as saying. "It's not they didn't need it. They were gonna do it anyway."

In recent weeks, both the Georgia Ports Authority and South Carolina's Savannah River Maritime Commission, the state agency charged with dealing with issues pertaining to the river, have asked to intervene in the lawsuit.

Environmentalists have also sued in South Carolina state court, arguing the water quality permit approved by DHEC last year is illegal because the commission has authority over river activities.