At the time, the island's size was only six acres (2.4 ha), and the whole length of the Detroit River was a prominent coastal marshland.

When the Detroit River became so polluted as to warrant federal intervention, Congress declared Grassy Island and the surrounding area as part of the Wyandotte National Wildlife Refuge in 1961.

[8] This caused a huge buildup of toxic waste around the mouth of the river— the exact location where the sediments came from that were deposited on Grassy Island.

Grassy Island is now contaminated with polychlorinated biphenyl, polycyclic aromatic hydrocarbon, and lethal amounts of harmful heavy metals, such as mercury and lead.

[7][9] While there is no imminent danger to human trespassers because of a thin layer of protective topsoil on Grassy Island, public access is prohibited but not often enforced.

[10] In 1994, the Department of the Interior began working on Grassy Island in order to find remedies for such hazardous sites.