try digging a hole filling it wit water then put all the tapes in it fill it back in then put it back the way it was wen u started i think if they stay there a while they will b destroyed but put them some where where it will take a long time for any one to dig there or see if they float if not u could throw them in like a big lake it will b a long time b4 they turn up again but u would have to go way far out there tho
Degaussing is the most reliable way to destroy the data on the cartridges. Sounds like you are going to have to approach your employer and indicate that this is a “special project” which requires extra incentives to complete and specify that it will take several months to do so. The other option would be to look up a company called Iron Mountain (if in your area) and see if they offer a service where they’d pick up the cartridges and degauss them for a fixed fee.