A second bulkhead collapsed at the Brooklyn, N.Y., Gowanus Canal in late July, two months after the first bulkhead crumbled in May. The U.S. Environmental Protection Agency is preparing to begin clearing the canal of roughly 10-ft-deep toxic sludge at the bottom of the canal. The cause for the collapse has not yet been determined, and the bulkheads were not in the scope of the original $506-million cleanup work. The canal’s bulkheads are 50 to 150 years old, and the EPA is working to reinforce the remaining bulkheads as necessary. The cleanup of the canal, which was once a busy maritime hub and is now a Superfund site, is expected to continue until at least 2022. Engineering firm Langan, which is dealing with the bulkheads, declined to comment.