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. Investigating potential contamination in the border region was best left to the state, they said. Last week, the federal agency denied a petition to review whether a six-mile stretch of the lower river valley qualifies as a Superfund site, a determination it made based largely on data from 2018 and 2019. That data, collected by the U.S. Customs and Border Protection and the International Boundary and Water Commission, found concentrations of hazardous chemicals in water and sediment, but not at levels that exceeded the EPA’s regional screening levels for human health concerns.