For the worst-case and alternative release scenarios of an underground storage tank, should I consider any impact on groundwater, drinking water, or soil?
No. As part of the worst-case and alternative release scenarios, you need to define the offsite impacts to the environment (40 CFR §68.33) by listing the environmental receptors that are within your impact zone. "Environmental receptor" is defined at 40 CFR §68.3 as "natural areas such as national or state parks, forests, or monuments; officially designated wildlife sanctuaries, preserves, refuges, or areas; and Federal wilderness areas" which could be exposed to an accidental release. You only need to list the environmental receptors, not speculate what specific damage could occur as a result of an accidental release. You should, however, consider impacts on groundwater, drinking water or soil in both the accident history for Program 1 eligibility criterion (40 CFR §68.10(b)(1)) and the five-year accident history required as part of the hazard assessment (40 CFR §68.42).
For Program 1 eligibility, you must not have had an accidental release of a regulated substance in the past five years that caused any "response or restoration activities for an exposure of an environmental receptor." An accidental release that led to response or restoration of soil or groundwater of an environmental receptor, such as a park, would make a process ineligible for Program 1. For the hazard assessment five-year accident history, environmental damage is not limited to the defined environmental receptors. Events where there is any known environmental impact of any kind (e.g. fish or animal kills, lawn, shrub, or crop damage) must be included in the history (61 FR 31710; June 20, 1996). Therefore, any known damage to groundwater or soil must be reported in the five-year accident history.