355 MONITORING AND MODELING OF WATER QUALITY IN THE TIJUANA RIVER WATERSHED (Project WQ PP96II-10) RICHARD M. GERSBERG C. BROWN SAN DIEGO STATE UNIVERSITY INTRODUCTION The Tijuana River Watershed (TRW) is a binational watershed on the westernmost portion of the US - Mexico border, encompassing much of the City of Tijuana in Mexico and portions of the City and County of San Diego in the US (Figure 1). The basin contains three surface water reservoirs, various flood control works, and a National Estuarine Sanctuary which is home to several endangered species and is protected by the US federal government. For decades, raw sewage from the city of Tijuana, Mexico has flowed into the Tijuana River and across the international border into the Tijuana Estuary. This problem has worsened in recent years with the substantial growth of Tijuana’s population, along with intensive industrial development associated with the maquiladora (in-bond manufacturing and assembly plants) program in Mexico. Unfortunately, due to many factors, an industrial pretreatment program similar to one implemented in the United States, has not been initiated in Mexico. Moreover, in Mexico there is no program equivalent to the U.S. EPA’s National Pollutant Discharge Elimination System (NPDES ) stormwater permitting program, so the threat of chemical contamination of the Tijuana Estuary is pronounced. Additionally, inadequate infrastructure for the collection, treatment, and disposal of sewage originating in Tijuana has long plagued the watershed, as wastewater flows have chronically outpaced the ability of the infrastructure to handle them. These elements yield transboundary and cross-cultural water quality management challenges. Although discharges from the Tijuana River account for only a small percentage of total gauged runoff to the Southern California coastal ocean, it contains the highest concentrations of suspended solids, Cd, Cu, Ni, Pb, Zn, and PCB,s among the eight largest creeks and rivers in Southern California (SCCWRP, 1992). Using flow weighted mean concentrations, SCCRWP (1992) estimated that the mass emission of metals from the Tijuana River for the year 1987-1988 was 7,385 kg for Cr, 16,706 kg for Cu, 39,684 kg for Pb, 46,221 kg for Zn, and 205 kg for Cd. Obviously, significant loading of these toxic heavy metals to the Tijuana Estuary is occurring. To date, however, there has been no data published on the quality of urban stormwater in Mexico.