Of the 177 tanks, the 149 oldest have just a single shell between their contents and the environment. These tanks do not comply with Washington law, which requires that all underground storage tanks have a secondary container. Single-shell tanks pose a greater risk to develop new leaks.
In the late 1980s to mid-90s, much of the liquid was removed in hopes of making them more stable and reducing the risk of leaks. The remaining waste is similar to the consistency of saltcake (crystallized salt structures), wet concrete, or sand.