As discussed in Chapters 4 and 5, breaches in physical and hydraulic integrity can lead to the influx of contaminants across pipe walls, through breaks, and via cross connections. These external contamination events can act as a source of inoculum, introduce nutrients and sediments, or decrease disinfectant concentrations within the distribution system, resulting in a degradation of water quality. Even in the absence of external contamination, however, there are situations where water quality is degraded due to transformations that take place within piping, tanks, and premise plumbing. Most measurements of water quality taken within the distribution system cannot differentiate between the deterioration caused by externally vs. internally derived sources. For example, decreases in disinfectant concentrations with travel time through the distribution system could be the result of demand from an external contamination event or it could be due to disinfectant reactions with pipe walls and natural organic matter remaining after treatment.
This chapter deals with the various internal processes or events occurring within a distribution system that lead to degradation of water quality, the consequences of those processes, methods for detecting the loss of water quality, operational procedures for preventing these events, and finally, how to restore water quality integrity if it is lost. In many cases, the detection methods and recovery remedies are similar to those discussed in previous chapters.
