For decades, South San Diego County residents have known the Tijuana River as a noxious source of cross-border sewage pollution. This week, business owners in Imperial Beach added a new dimension to the river’s troubled reputation. They called it an economic Grim Reaper. Following a back-channel request from Imperial Beach Mayor Mitch McKay, top-ranking officials from the U.S. Small Business Administration on Tuesday met with a roundtable of l…