Simulating exposures to deliberate intrusions into water distribution systems

A well-known network solver (EPANET) and a novel water-use generator (PRPsym) were linked in Monte Carlo computer experiments to simulate a deliberate biochemical assault on a municipal drinking-water distribution system. The attack was modeled as a steady 6-hour injection delivering 3,600 g of a soluble conservative contaminant to a single node on the main line in a small town. Migration of the contaminant plume was tracked for 55 hours throughout the pipe network, and the cumulative mass loading was computed at four target nodes strategically located on looping links and dead-end branches. This exercise was repeated for 1,000 independent trials to establish a baseline distribution of consumer dose exposures at the target nodes. A battery of simulation experiments was then performed to examine the sensitivity of the nodal load distributions to various system characteristics and water-use patterns. Results show that variability in the total mass load received at a node can be apportioned between the variability in the water-use volume and variability in the mean delivered concentration. Overall, however, the operation of the network storage tank had the greatest influence on the nodal mass loadings. This study demonstrates that Monte Carlo techniques are a useful tool for simulating the dynamic performance of a municipal drinking-water supply system, provided that a calibrated model of realistic network operations is available.