The Potential Impacts of the Nogales International Wastewater Treatment Plant on the Santa Cruz River

Abstract

The Nogales International Wastewater Treatment Plant releases treated wastewater from both Nogales, Arizona and Nogales, Sonora, Mexico into the Santa Cruz River. In recent years, the discharged effluent has contained high levels of cadmium and nickel, which exceed the plant's permit standards. Due to the industrial demographic of the region, outdated infrastructure, and differences in sampling schedules of multiple organizations, the treatment facility and the treated effluent is an important area of study. To understand how the treated effluent is affecting the river, data were compiled from existing water quality databases and flow reports from 2008 to 2015. To address how flow quantity has changed during drought periods, effluent flows were compared to historical flood data produced by the USGS. To evaluate water quality issues, water quality reports produced by the International Boundary and Water Commission were examined for past exceedances of constituents. According to flow volumes reported at the U.S.-Mexico border, the majority of the effluent was produced in Nogales, Sonora. Results showed that spikes in effluent flow corresponded with rainfall events. Results also show that rainfall influences the flow volumes from Nogales, Arizona, but there is little impact to flow volumes from Mexico. Although the quality of the effluent generally meets the permitted standards, exceedances did occur. The potential impact of such exceedances on stream water quality was evaluated using measured and simulated data. Although outreach to stakeholders across the border and updated infrastructure has improved the quality of water in the river, there are still many areas to improve upon, including sampling and monitoring schedules. To identify opportunities for improvement, further studies should examine the specific fate of each contaminant present in the effluent.