As far as my limited research has found, DDT isn't the boogeyman that people try to make it. People are immune to it, but it is deadly to insects. After WWII we basically fumigated Europe to prevent insect infestations- lice and bed bugs. DDT was actively used in mosquito control in the 1960s. I remember when the "Bug trucks" used to drive through the streets gassing the neighborhood. Many thousands of people die of malaria and other serious diseases, because they are not allowed to use DDT.
In this instance, it is probably safer to let it deteriorate in place than to try to remove it. I do not know if DDT affects marine animals, but why possibly allow it to flow in currents, due to leaking barrels being raised from the ocean floor.
I think this is what you’re looking for. DDT is not a villain.
The Mosquito Killer
By Malcolm Gladwell
https://www.newyorker.com/magazine/2001/07/02/the-mosquito-killer
Way back in the fifties when we lived on Louis the 14th street out near Lake Pontchartrain, my mom used to pull me inside when the mosquito fogger trucks went through the neighborhood.