By the time Slippery Rock Creek gets to McConnells Mill State Park, it is clear, clean and full of healthy fish.

However, at the creek’s headwaters in northern Butler County, the water can be orange, acidic and inhospitable to life.

This is because the creek and its tributaries, mainly in Washington, Marion, Venango and Cherry townships, are threatened by pollution from abandoned coal mines.

Nearly every coal mine dug before the Surface Mining Control and Reclamation Act was passed by Congress in 1977, poses a threat to nearby waterways.

“At one time it was considered the number one source of pollution,” said Cliff Delholm, an environmental scientist based in Mars. “There are 5,000 miles of streams that are affected in the state and some are so polluted they are lifeless.”

It’s because of this toxic runoff problem that the Slippery Rock Watershed Coalition was founded in 1995.

The coalition, in celebration of its 20th year, held kayak and bus tours this weekend to highlight some of the work it has done and progress it has made in restoring the creek and its tributaries, mainly through passive treatment systems that use plants and natural materials to filter the water.

The coalition does not have 501c3 nonprofit tax status or a board of directors. It instead operates as a collection of volunteers representing private companies, nonprofit organizations and government agencies, Denholm said.

The funding for its work comes from a variety of sources, including state and federal agencies as well as private grants.

The Slippery Rock Watershed is about 420 square miles, including parts of Butler, Venango, Mercer, Lawrence and Beaver counties. Any streams or stormwater runoff in that area ends up in the creek which flows from northern Butler County to its confluence with the Connoquenessing Creek near Ellwood City.

The headwaters area that the coalition primarily focuses on is about 27 square miles.

The primary issue is acid mine drainage, which happens when groundwater gets into abandoned mine shafts. The remnants of coal and surrounding rock, when exposed to water and oxygen, releases sulfuric acid, iron, aluminum, manganese and other heavy metals.

“The problem isn’t actually the coal itself, it’s what is called iron sulfide minerals, like pyrite,” Denholm said.

Aluminum in the water is deadly because it clogs the gills of fish and causes them to suffocate and die. Iron is problematic because it collects on the bottom of the stream and destroys the habitat of insects and algae that fish and other animals need to live.

The presence of iron is the most noticeable because it turns the water an orange, rust color when it is exposed to oxygen.

One of the first projects the coalition was involved in was the installation of a passive treatment system at the Jennings Environmental Education Center in Brady Township, park manager Wil Taylor said.

There is an abandoned mine underneath Jennings that was sealed off in the 1970s, but a flood in 1984 caused the seal to break. What resulted was severe acid mine drainage into Big Run, a tributary to Slippery Rock Creek.

“Big Run was essentially dead in the late 1980s. It had orange staining and goopy white aluminum. It was pretty nasty,” Taylor said.

The park made efforts in 1988 and 1994 to clean up the pollution and mitigate future drainage, but it was not successful.

It was in 1997, with help from the newly formed coalition, that the park installed a passive treatment system that has helped to clean and restore the stream enough for some fish and macroinvertebrates to return, Taylor said.

A passive treatment system directs runoff water into one or sometimes multiple man-made ponds. The water goes into the pond orange and exits into the stream bed clear.

“They are more environmentally friendly (methods) and use of natural materials and natural processes to treat the water. There’s no electricity, it’s all gravity,” Denholm said.

The systems don’t require much periodic maintenance, though the heavy metals from the mines do collect at the bottom of the ponds and must be removed every few years.

The coalition has taken part in installing 18 such systems. One of the biggest systems it has done is along Seaton Creek in Venango Township at what used to be a small town called Erico, built by the Erie Coal Co. in the late 19th century.

Starting in 2003, the coalition came in and removed large above-ground piles of coal refuse and other leftover equipment. It then installed several ponds and beds of limestone, which is alkaline and thus neutralizes the water’s pH level.

The Erico system treats an estimated 260 million gallons of water each year and removes 90 tons of iron each year.

Despite its progress, the work of the group is far from done.

In addition to monitoring and maintaining the passive treatment systems already in place, the group is studying Blacks Creek, which flows into Slippery Rock Creek, to identify areas that need to be cleaned up or could benefit from a new passive treatment system.