What do you with a gazillion tons of contaminated soil?

There are three ways of dealing with soils that have been contaminated by years of people and companies dumping toxin crud underground, the old “out of sight, out of mind” mentality. They are:

1. Clean up the soil (often by baking it) and keep it there, which is expensive and not always feasible.
2. Remove the soil and put it in a landfill, replacing it with clean soil
3. Leave it there but wall it off with an underground container so it can’t leach into aquifers.

A huge remediation project of the 15-acre Mohawk Tannery Superfund site has just gotten underway in Nashua. When I worked for the Nashua Telegraph I wrote a bunch of stories about efforts of what to do with that nasty place, which tanned hides from 1924 to 1984, a process that generates a lot of toxic residue. The official NH Department of Environmental Services history includes this telling sentence: “Little is known about the tannery waste treatment and management practices prior to the 1960s” but a lot of it was just buried and the rest flowed into the adjacent Nashua River.

The work will include the construction of massive concrete secant walls extending down to bedrock—creating a containment system that will securely encapsulate more than 90,000 cubic yards of contaminated sludge and soil before being capped and prepared for a $350 million housing development. The vertical walls of the new sludge containment cell will withstand a 500-year flood. In addition to sealing existing contaminants in the large containment cell, located at the site of the tannery’s former disposal lagoons, the project team will also spend the better part of 2026 excavating and relocating various contaminated soils from throughout the site as well as undertaking the removal of underground storage tanks and demolishing the former tannery infrastructure.

In other words, they’re doing both No. 2 and No. 3.

I covered a smaller Superfund site in Milford a decade ago. They did both No. 1 and No. 2 – they baked the contaminated soil to make the volatile compounds “boil out” and ended up replacing a lot of it with clean oil.