A Superfund designation lets the EPA clean up contaminated areas. Noted projects include Love Canal in Niagara Falls, New York and the Upper Columbia River in Washington state.

The push to explore a potential Superfund designation for the Tijuana River Valley hit a snag Wednesday when the U.S. Environmental Protection Agency told San Diego County leaders that the federal agency’s priority is to control the flows of sewage and trash that spill over from Mexico.

"The reality is that things have gotten much worse since then," she said. "Absence of evidence is not evidence of absence."

Evidence keeps mounting, pointing toward a public health emergency in the Tijuana River Valley, where toxic water continues to flow from Mexico. Scientists are preparing to publish new research showing the fumes from this sewage are dangerous.

Federal agency said samples of contaminants from previous years did not exceed its thresholds for public health concerns