Pittsburgh Botanic Garden is built on land that was once actively mined for coal. This industrial use, and the ensuing abandonment of the land, created a hazardous landscape that had to be remediated before the site could be opened to the public.
Starting in the 1920s, the area was mined through “room and pillar” mining, a common practice at the time, whereby tunnels were excavated to reach a coal seam. Coal was removed, leaving “pillars” of coal to support the layers of rock above. Overlying layers of rock could collapse into the mines at any time, even after the mine was abandoned.
In the 1940’s, the land was extensively strip-mined. Soil and rock was blasted and pushed to the side, exposing coal for extraction. This process removed all vegetation and created steep “highwalls,” as well as large unsightly spoil piles.
Prior to the Surface Mining Control and Reclamation Act of 1977, coal companies were not obligated to restore mined areas. Once the coal was removed, companies left the land scarred and did little to mitigate this unnatural landscape.
Acid Mine Drainage
One of the first issues encountered at the Pittsburgh Botanic Garden was the presence of acid mine drainage. Acid mine drainage forms when air and water come into contact with exposed sulfurous rocks in coal mines, forming acidic water often with dissolved toxic metals. This acidic water pollutes streams and ponds downstream of the mine, and persists even after the mine is abandoned.
Acid mine drainage seeping from abandoned mines at the Garden polluted various waterbodies on site. A small pond, central to the site, was found to be lifeless with a pH of around 2.9, which is acidic and similar to human gastric juices and dissolved aluminum, which is highly toxic to both plants and animals.
To bring the pond back to life, a drainable limestone bed was installed next to the pond. The acid mine drainage is neutralized as it flows over the limestone bed and the dissolved aluminum reacts with limestone to create aluminum hydroxide, an inert chemical that is captured in a settling basin. Water in the pond and downstream areas are neutral in pH and free of dissolved aluminum; the treated water drains into Chartier’s Creek.
The Lotus Pond is now the centerpiece of the Japanese Garden. A wooden walkway built on top of the limestone bed makes the treatment system nearly invisible. The project was awarded the 2014 Governor’s Award for Environmental Excellence and is a good example of effective public-private partnership.
Two additional water treatment systems have been installed in parts of the property that are not open to the public. Each has a unique design suitable for the unique challenges it is designed to address.