The modern security environment is marked with significant changes and unpredictability. This is true both for natural threats caused most often by forces of nature and for threats to civilization such as technical disasters or failures. The fact that these threats may be synergistic not only complicates efforts that are mainly intended to mitigate them but also makes it difficult to determine their direct cause. The article presents a hypothetical model of the spread of dangerous substances released during a disaster involving a tanker carrying 20,000 kg of anhydrous ammonia by road, by rail, and by water in the territory of the Municipality of the City of Szczecin. A 2016 rail disaster involving two derailed tankers carrying ammonia is proof of the likelihood of such an event. Great amounts of dangerous substances are transported through Szczecin’s densely populated areas, most often to and from Zakłady Chemiczne Police S.A. (Police Chemical Plant), which causes a risk to the life and health of persons on the premises. The method of situation assessment in chemical, biological, and radiological contaminations (ATP 45 (Delta)) and the Promień IT system were used to forecast the hypothetical development of contamination. The results were compared with those of the ALOHA program. The author assumes that the conclusions from this paper will be used in the assessment of the likelihood of threat categories (KZ-3) and in the crisis response procedure (PRK-15), both included in the Municipal Crisis Management Plan.