A residual is what remains after an event appears to be over.

It may be physical: a metal fragment buried in soil, a contaminant bound to sediment, an abandoned structure, a chemical absorbed into living tissue, or a landscape reshaped by human activity.

But residuals are not necessarily inert.

They can corrode, dissolve, migrate, accumulate, enter food webs, move through water, or become incorporated into bodies and ecosystems. A material left behind decades ago can therefore continue interacting with the present long after the event that produced it has ended.

War leaves residuals. Industry leaves residuals. Development, extraction, pollution, and environmental engineering leave residuals.

Some remain where they were deposited. Others move.

Some become less hazardous with time. Others are transformed into new chemical or biological forms. Some remain hidden until erosion, disturbance, construction, climate change, or human activity brings them back into circulation.

Residuals are the material afterlives of human decisions.

They remind us that an ending on a calendar is not necessarily an ending in the environment.