Bier has been at the company since 1999, holding several executive positions.
Bell joined the company last year as director of accounting. He also has served as interim CFO.
CFO David Sach is set to leave at the end of the month.
Thompson serves as vice president of finance for North America.
Kennedy became interim CFO in July. Before that, he was group director of finance.
Gagnon was CFO at Clean Harbors.