To make this straight forward.
1) Reputable solder suppliers are REQUIRED to provide dross collection containers.  They make it seem like they are doing you a favor, but they are actually required to do this by law.
2) Dross is worth several dollars (or more) a pound / kilogram, so you want to get a contract going.
3) Storage and compliance is divided into 3 basic categories:  [ Your most like category is last ]

> Very Small Quantity Generator (VSQG): Less than 1 metric ton (2,200 lbs / 1,000 kgs) per year.  If you have any highly hazardous material of any volume this does not apply.  Rule is indefinite storage, but a good rule is to keep it under 2 years.

> Large Quantity Generator (LQG): More than 6 metric tons (13,200 lbs / 6,000 kgs) per year.  In this case, you have to have a specialized plan in place with the Massachusetts DEP.

> Small Quantity Generator (SQG): More than 1 metric ton (2,200 lbs / 1,000 kgs) per year but less than 6 metric tons (13,200 lbs / 6,000 kgs) per year.  In this case, you must have a plan in place to remove hazardous materials within 180 days.
>> Contact your solder bar salesperson for reclaim services.  Ask them for a certificate of salvage.

Lastly, you only have to worry about and interact with the MA DEP unless you have caused a KILL (soil, groundwater, or waterway contamination).