The Tijuana River watershed between southern California and northern Mexico has been the epicenter of an environmental and public health crisis for more than two decades. Since October 2023 alone, more than 31 billion gallons of raw sewage and polluted water have flowed into the U.S. and, eventually, the Pacific Ocean. Emergency response procurements from the U.S. Section of the International Boundary and Water Commission, the treaty-defined agency charged with managing binational water issues, have been issued in reaction to the crisis since 2023—including expansion of the South Bay International Wastewater Treatment Plant in San Diego County by a design-build team of Stantec and PCL Construction. The pollutants fouling water and air in county communities such as Nestor and Imperial Beach are experiencing water-to-air transfer of hydrogen sulfide—a toxic gas and sewage tracer.…