40 CFR 264.193(c)(4) requires
Sloped or otherwise designed or operated to drain and remove liquids resulting from leaks, spills, or precipitation. Spilled or leaked waste and accumulated precipitation must be removed from the secondary containment system within 24 hours, or in as timely a manner as is possible to prevent harm to human health and the environment, if the owner or operator can demonstrate to the Regional Administrator that removal of the released waste or accumulated precipitation cannot be accomplished within 24 hours.
Your safety person is correct in saying that the regulations require it to be pumped dry. The practical solution is to try to work with the regulating authority to deal with the sump issue. The main thing they'll want is to minimize the volume of liquids remaining. How much liquids would be remiaing in the sump when it's shut off? If it's a significant amount, you may need to revise the sump design to reduce this volume.
There's several items that will be a concern for the regulator and possibly your safety person:
1. How much liquids could be remaining?
2. How were the secondary containment calculations determined and was the sump volume included?
3. What is the chemical compatibility of the RCRA wastes and the secondary containment systen. (This may be a critical factor, if you have a coating that may be deteriorated after several days of exposure to the wastes.)
4. How will the secondary containment system (e.g., coating) in the sump be inspected if there are liquids present?
5. How is the facility maintaining the secondary containment system of the rest of the tank system?
6. Is there a schedule that will work for both the facility and the regulating agency for the removal of the liquids in the sump (e.g., weekly, bi-weekly, monthly)