Workers renovating one of Washington DC’s most historically symbolic sites in a project ordered by Donald Trump may be risking their safety as they race to finish on time for the US’s 250th anniversary celebrations, a union monitoring the site has warned.

Trade union scrutiny has focused on the reflecting pool on the US capital’s National Mall – scene of Martin Luther King’s 1963 “I have a dream speech” – after it was drained of water and fenced off from the public to allow contractors the chance to upgrade it by 4 July.

The pool, a Washington landmark since it was dug in 1922, is currently the site of frenetic repair activity, its usual watery surface occupied instead by vehicle and work equipment. Tourists visiting the area have found their view obscured by black tarpaulin.

Leaks and algae blooms have for decades dogged the 2,000ft pool, which sits between the Lincoln and George Washington memorial monuments, turning its water green and confounding previous expensive government-commissioned repair schemes, including one commissioned by Barack Obama’s administration.