Part of Radlett was cordoned off on Monday while firefighters wearing protective suits dealt with a chemical spillage.
A member of the public alerted Borehamwood Fire Station at around 5.50pm to a substance leaking from a 40-litre drum, behind a parade of Watling Street shops, near Newberries Car Park.
Four fire engines, including one from the fire service's hazardous materials unit in Hemel Hempstead, arrived at the scene, and firefighters, wearing breathing apparatus and special suits, placed the drum into an over-sized drum and took it away for disposal.
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Leading firefighter Andy Bell, from Borehamwood station, said he believed the drum had been behind the shops for a long time, and had leaked slightly. He added: "We could not identify the substance so we took the most precautions."
The cordon was lifted at around 7pm.