An effective and adequate response to a contamination event is one of the most important issues that are addressed by SMaRT-OnlineWDN and can save lives of the population in case of a contamination event. As soon as one of the sensors raises an alarm the online source identification algorithm delivers candidate nodes for the contamination source. The worst case node that explains the current alarms(s) is selected as source for an forward calculation of the spread of contaminant. The results shows how large the affected area is at present and how it will develop in the future. In order to make optimal decisions for countermeasures the future spread of contamination is one of the most important information for the decision maker an therefore the basis for all response actions. For that purpose look ahead simulations are carried out that are based on the assumed source locations as well as the prognosis of the future flow velocities that are driven by the demands in the network. With the so calculated contamination maps it is possible to select the population that must be informed first or the valves that have to be closed in order to isolate the contaminant and prohibit its further spreads through the network.