This article presents the results of a transboundary water quality monitoring program at the two Nogales area in the Arizona-Sonora border region. The program was carried out jointly in 1990 by U.S. and Mexican institutions. The results show pollution problems due to deficiencies in Nogales, Sonora municipal sewerage system, causing not only sewage spills in several parts of the city but also creating occasional transboundary problems. The results also showed potential illegal dumping of industrial hazardous waste (VOCs) into Nogales' municipal sewerage system. All of the organic compounds found in the sewage samples are solvents frequently used by the border industry. Occasional brakes of pipes spill the pollutants into the Nogales Wash, a water stream that runs parallel to Nogales' main sewerage line. Samples of the municipal water system showed no traces of pollutants. However, two rounds of samples detected concentrations of VOCs in wells used to supply water by trucks to low income neighborhoods in Nogales, Sonora. Ironically, the pollution detected in these wells has a greater impact in low income groups of the city that pay three to four times more per liter of water they consume, than the rest of the inhabitants with clean water from the municipal system.