Distribution System Contamination Events: Exposure, Influence, and Sensitivity

This paper presents a two-part investigation of the response of municipal water distribution systems to contamination events. In Part I of the investigation, a contamination event was modeled as a steady 6-h injection of a soluble conservative substance into a single node. The injection was repeated node by node at all 89 nodes in the pipe network. In each case, the fraction of the population at risk of contaminant exposure was estimated at the end of a 72-h simulation period. A dimensionless exposure index (EI) was introduced as a simple global measure of network response, ranging from EI=0 (no consumers are exposed) to EI=100% (all consumers are at risk of exposure). Simulation results were used to construct a zone of influence (ZOI) map, which categorizes network injection nodes on the basis of their potential to expose downstream consumers. In Part II of the investigation, a generalized sensitivity analysis was performed to determine the sensitivity of network response to four dynamic network variables (base demand, storage capacity, injection mass, and injection duration). Latin hypercube sampling was used to set up 1,152 contamination event simulations at two injection nodes. Both nodes were selected on the basis of their exposure potential (one high, one low) as determined from the ZOI map. Based on the Kolmogorov-Smirnov d statistic, exposure levels in the example network were found to be most sensitive to variations in base demand and injection mass. Tank storage capacity was important in certain cases, while injection duration tended to be least important.