1918 Harry Wolff, a chemical engineer, starts working at a chemical plant in Ridgewood, Queens. Around this time, the plant begins producing rare-earth metals for heavy industry, a process that can generate radioactive waste.

1919 A fire at the plant causes an explosion. The building sustains little damage.

1920 The chemist Alcan Hirsch incorporates Hirsch Laboratories at the site. He also helps to found the Molybdenum Corporation, known later as Molycorp.

November 15, 1923 The Ridgewood plant is transferred to Harry Wolff and his business partner, Max Alport. The firm is reincorporated as the Wolff-Alport Chemical Company.

November 25, 1923 Ten days after the company’s reincorporation, a gas buildup causes another explosion at the plant.

1942 The Manhattan Project, the United States’ nuclear-weapon research-and-development program, begins.

1946 The successor to the Manhattan Project, the Atomic Energy Commission (A.E.C.), identifies Wolff-Alport as a source of radioactive thorium, a byproduct of rare-earths production.

1947 The A.E.C. begins buying thorium from Wolff-Alport. Previously, the company had simply dumped their thorium into the sewer or buried it.

1954 Wolff-Alport sells its last batch of thorium to the A.E.C. The company ceases operations.

April 15, 1955 A nuclear weapon using uranium-233 derived from thorium is tested in Nevada.

October 2, 1968 A reactor powered by thorium-derived uranium-233 goes online at the Oak Ridge National Laboratory, in Tennessee.

1974 The Formerly Utilized Sites Remedial Action Program ( FUSRAP ), a Department of Energy initiative to manage the cleanup of environmental contamination caused by A.E.C. activity, begins.

1980 The Superfund program begins, granting increased authority and funding to federal agencies for cleaning up pollution and contamination.

1987 The Department of Energy notifies New York City about contamination at the Wolff-Alport site, but reports that it is not eligible for FUSRAP because thorium was not the company’s main product.

1988 New York City and the Environmental Protection Agency survey the site and determine that radiation levels are below regulatory limits.

2000 City and state departments survey the site again and report that radiation levels are below regulatory limits.

2007 Another joint survey finds that radiation at the site exceeds normal background levels and regulatory limits.

2010 A comprehensive follow-up survey shows elevated radiation levels.

2013 The E.P.A. installs lead shielding over radioactive hot spots at the site.

December 11, 2013 The E.P.A. officially proposes adding the site to the Superfund list. An E.P.A. official says that it will “very likely” be approved.