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.

Michael Montgomery, director of the EPA’s Superfund and Emergency Management Division, told the Board of Supervisors that “the highest priority” isn’t sampling, but rather plugging the volumes of untreated wastewater and trash that flood the river valley.

"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