If there is a sewage overflow of 550 gallons that does not reach any surface waters, which category does this fall under?

A sewage overflow of 550 gallons that does not reach any surface waters is classified as Category 2. This category refers to discharges containing potentially harmful microorganisms that pose a greater risk to human health than Category 1.

Category 1 typically pertains to sanitary or domestic sewage that is relatively clean and does not pose significant health risks, while Category 2 involves wastewater that may contain pathogens, thus requiring more intensive cleanup procedures to prevent health risks.

Therefore, because the situation describes an overflow that could still harbor potential health risks but does not impact surface waters directly, it aligns with the characteristics outlined for Category 2.