Characterizing In-Stream Metal Loading in the Tijuana River Watershed

The Tijuana River Watershed (TRW) is a binational watershed encompassing the urban areas of the Cities of Tijuana and Tecate, Mexico, as well as the Tijuana Estuary, a designated National Estuarine Research Reserve and home to a number of endangered plant and animal species. Over the past few decades, the Mexican portion of the watershed has seen both rapid population growth and increased industrialization from maquiladora facilities located in this border region. Such rapid growth has outpaced the ability of the Mexican infrastructure to meet increasing wastewater treatment requirements, resulting in uncontrolled sewage flows, discharges of industrial effluent, and the subsequent pollution of the Tijuana River, Tijuana Estuary, and Pacific Ocean. Riveles and Gersberg (1999) found that although the dry weather (baseline) flows of the Tijuana River near the U.S./Mexico Border were relatively non-toxic, toxicity increased dramatically during higher flows indicative of the rainy season. The present study was initiated to characterize the temporal loading of heavy metals in surface waters from storm events and base flow conditions within the TRW.From 1998 to present, stormwater sampling was performed using selective grab sampling and auto-sampling equipment at two locations within the TRW. One site is located near the mouth of the Tijuana River near Imperial Beach, California (TJHOLL) and the other is located downstream of the City of Tecate in the Mexican portion of the TRW (TEC). During the 1998-1999, 1999-2000, and 2000-2001 seasons, eight storms were sampled at TJHOLL and five storms were sampled at TEC. Stormwater samples were analyzed for dissolved and particulate forms of copper, lead, and zinc. Baseline flow sampling and analysis were also performed at both sites. Auto-samplers were programmed for the collection of both time-paced grab samples and flow-paced composite samples.In general, metal loading trends could be ascertained from the shape of the storm hydrographs. At TJHOLL, the highest metal concentrations occurred within the first few hours of runoff, and in some cases occurred prior to an observable increase in gradient of a hydrograph. Below Tecate at TEC, relatively high metal concentrations generally persisted near the tail end of storm flows. Although the overall contribution of stormwater flow at TEC is relatively small compared to the overall watershed, elevated metal concentrations exist in base and storm flow conditions.