Planted along the Broadview Avenue edge of Riverdale Park are 20-odd numbered green posts

Planted along the Broadview Avenue edge of Riverdale Park are 20-odd numbered green posts. What are they?

Riverdale Park was once a verdant homestead owned by John Scadding, manager of Lord Simcoe’s estate. By the turn of the 20th century, however, the site had been converted into a landfill, choked with household waste and ash. The dump closed and was covered over in the ’20s, but the beleaguered landscape faced a new indignity when, in 1962, the DVP was carved through its midst. As compensation, the rest of the space was converted into a park. The old fill wasn’t a health hazard, but there was one problem: volatile methane gas bubbling up as the antique garbage continued its putrefaction. Hence the posts, which are exhaust pipes that gradually (and odourlessly) dispel the methane into the atmosphere.””