Water distribution system deficiencies and the risk of gastrointestinal illness

Distribution systems are vulnerable to intrusion of non-potable water through cracks. This risk is more pronounced when pump failures, power outages or breaks in pipelines cause low or negative pressures in the system (pressure transients). Depending on the number and size of cracks, the frequency and duration of pressure transients, the type and concentration of pathogens near water lines, and the type and concentration of disinfectant residual in the distribution system, the intrusion of non-potable water could plausibly lead to an increased risk of gastrointestinal illness. The objective of this meta-analysis was to assess the impact of distribution system deficiencies on the rates of gastrointestinal illness in consumers of tap water at times when outbreaks are not underway. We reviewed published studies in which the measured exposure was tap water consumption and the reported outcome was gastrointestinal illness. Nine studies were identified from which data were extracted and pooled. Consumers of tap water had a relative risk of 1.28 (1.05-1.58) for gastrointestinal illness compared to consumers of purified water. In studies with evidence of distribution system malfunctioning such as pressure transients or insufficient disinfectant residual, the relative risk from tap versus purified water consumption was 1.39 (1.09-1.78), (AR% = 28, 8-44). Drinking tap water after prolonged episodes of pressure loss versus during normal operation resulted in a relative risk of 5.73 (1.47-22.33), (AR% = 83, 32-96). Positive dose-response relationships were observed with increasing tap water consumption and increasing duration of pressure loss.