Major accidents involving flammable substances can lead to significant environmental damage. The operators of chemical warehouses—in order to prevent and mitigate harmful environmental impacts—based on fire prevention strategies should apply “firewater pollution prevention” (FPP) measures. The identification of affected warehouses already in operation is an important law enforcement task. Therefore, the authors—based on the assessment of firewater run-off scenarios—propose a simple and easy-to-use dangerous establishment identification procedure and methodology based on event tree analysis and indexing preliminary risk analysis approaches. Two independent expert groups validated—in the case of 10 facilities—the index components of the approach. The testing of the applicability of the approach took place in parallel with the analyses of the Hungarian operator’s practice. The research results—covering the inspection of 24 facilities—can assist the operators in the effective and unified implementation of FPP measures. In the case of 14 facilities, it was necessary to introduce FPP measures, which highlight the need to improve the law enforcement compliance of identified operators. The investigation results can also contribute to increases in the fire and environmental safety performance of chemical warehouses, which ensures a higher level of environmental protection and people’s health near chemical warehouses.