Low and Negative Pressures in Distribution Systems: Do They Actually Result in Intrusion?

The magnitude of low and negative pressures induced by the closure and repair of a transmission main in a distribution system (DS) was investigated along with the potential for intrusion and changes in water quality associated with these low/negative pressures. Field equipment included high-speed pressure transient data loggers, on-line turbidimeters and grab samples that were analyzed for total coliform bacteria, E. coli, aerobic spore forming bacteria, residual chlorine, temperature and conductivity. Results of the monitoring showed that the closure of the transmission main was the cause of significant and sustained negative pressures in the DS under study. However, the water quality monitoring did not reveal significant water contamination. The specific conditions of the system under study and the inability to detect contamination in chlorinated systems with the current analytical methods could explain such results.