This study was performed to evaluate the potential damage caused by a chemical accident through the analysis of off-site consequence (OCA) and the presence of ammonia residues in crops. The assessment of OCA was carried out for worst-case and alternative-case scenarios. To determine the ammonia residues in crops, samples were collected from both sweetpotato (SW) and wormwood (WW) crops in 9 separate groups within the affected area, and the ammonium levels were quantified using ion chromatography. As a result of the OCA, for the worst- and alternative-case scenarios, the impact distances were estimated at 1.6 km and 666 m from the accident site, respectively. In the impacted zone, SW samples exhibited ammonia concentrations ranging from 69.6 to 218.0 mg/kg, which were up to 16.9 times higher than those in the unaffected areas. Meanwhile, WW samples in both the worst- and alternative-case scenarios showed lower ammonia, with concentrations varying from 62.2 to 77.8 mg/kg and 49.0 to 67.8 mg/kg, respectively. These results indicate that the ammonium residue in crops outside the affected area is at least 49 mg/kg due to the inherent ammonium in plants, the application of nitrogen fertilizer, and various nitrogen sources. Thus, it may not be possible to predict the actual impact range accurately. However, the simultaneous assessment of OCA and the residue analysis offers valuable insights into the initial contamination levels of the accident-affected area.