UNIVERSITY PARK, Pa. — As Penn State researchers stood on the banks of Scalp Level Run, an acid mine drainage (AMD)-polluted stream in Cambria County, a scientific question formed: How is nature removing toxic metals from the drainage at a rate faster than any other tested waters in the state, under pH conditions deemed too low to do so?

For decades, cleanup efforts have involved raising the pH of AMD before using chemical oxidation to remove iron and other metals. And yet, at Scalp Level Run, the pollutants were being removed at a pH of around 3, and importantly, before entering the stream.

“We initially started this work because of an observation that was intriguing,” said Jennifer Macalady, associate professor of geosciences. “Some of these natural spring sites do a really good job of removing iron so that heavy metals can be treated more effectively, and that’s great because those metals are very toxic. Some AMD treatment methods are not effective because iron coats the treatment bed. Based on observations at Scalp Level Run, we wanted to see if there was a microbial component that was helping facilitate the removal of iron.”

The initial research that began nearly a decade ago sparked an interdisciplinary effort to better understand the important role microbiology plays in mitigating AMD, Pennsylvania’s largest non-point source water pollutant, impacting 2,500 miles of streams, according to the Department of Environmental Protection (DEP).

Understanding AMD

AMD is the acidic metal-rich water formed when water reacts with rock containing sulfur, such as those exposed from abandoned coal mines. When sulfur reacts with air and water it forms sulfuric acid. Rainwater and other drainage then carries the AMD to nearby streams, rivers or lakes, creating environmental risks.

Macalady said AMD is treated using two methods: active or passive mitigation. Active treatment, where drainage is collected and then chemically treated, is expensive and labor intensive. Passive treatment, where drainage is exposed to wetlands or limestone beds, is less costly, and is more commonly used to treat AMD, especially because Pennsylvania has so many points of pollution.

But sometimes these passive treatments fail. Macalady, wondering if the cause for failure could be answered by a better understanding of its ecology, joined a team of researchers to assess these systems.

“We wanted to understand how microbes behave in passive treatment sites and the answer is we still don’t know because they’re complicated,” Macalady said. “But we have made some important discoveries.”

Because Scalp Level Run did the best job of mitigating AMD on its own, Macalady figured assessing the ecology there might offer the strongest clues.

“Using microbiology, we have started to investigate these systems to find out if we can improve them, Macalady said. “Our main focus is not building a better treatment system but providing info to people who do.” 

At Scalp Level Run and other polluted waters across the state, the team began looking at the water’s microbiology but also other factors such as pH, iron levels and turbidity to see how physical, biological and chemical factors impact AMD mitigation. Knowing these relationships, Macalady said, could be key to designing better systems.

One takeaway so far is that pH level determines which microbes are present.

“At Scalp Level Run, there was no change in which microbes were present and that was initially very surprising but it allowed us to confirm a hypothesis, which was that pH is the main driver of which AMD microorganisms succeed,” Macalady said.

At areas such as Brubaker Run the pH shifted, as did the types of microbes.

“An application of that finding would be if you’re going to design an acid mine treatment system and you know that the pH is going to change you should expect a variety of unrelated microbial species to be involved in the job of cleaning up the waste,” Macalady said.

Science in action

In a lab at Penn State, a team of researchers led by William Burgos, professor of civil and environmental engineering, got a break.

They enriched microbial communities from Scalp Level Run and Brubaker Run and fed equal concentrations of iron to each while controlling the pH and water flow rate. They expected that Scalp Level’s microbes would continue to dominate. Luckily, they were wrong.

“What was proven instead was that diverse microbial communities that exhibit diverse kinetics in the field converge to similar kinetics when the hydrodynamics and the geochemical conditions are held constant,” Burgos said. “That’s a good thing from an engineering standpoint because we won’t have to make these passive treatment systems site-specific.”

Together Macalady and Burgos’s results mean that engineered systems could rely on existing microbes in acidic wastewater for mitigating most of the state’s AMD sources. Burgos’s group has since focused on other variables, such as increasing the surface area for acid-neutralizing bacteria to colonize, as a means of improving AMD treatment.

Burgos first learned about mother nature’s ability to treat AMD after touring DEP sites across the state. They measured pH at the point it surfaces and contrasted it with drainage after it passed through the so-called "kill zone," the rust-stained soil where trees and other vegetation were destroyed by the pollution.

The results were stark. Much of the iron and acidity was being removed naturally.

“One of the first things the DEP would often do is build their treatment system right in the kill zone,” Burgos said. “So we were able to convey the message to them — and this was an important one — don’t bulldoze the kill zone because the kill zone was giving you a remarkable amount of treatment.”

Now, when possible, treatment systems are installed after the kill zone. For especially problematic areas, engineered terraces, which increase the contact time microbes have with the AMD, are added.

Solution spans disciplines

Christy Grettenberger, now a post-doctoral researcher in the Department of Earth and Planetary Sciences at University of California, Davis, spent years wading through Pennsylvania waters, studying AMD en route to earning her doctorate in ecology under Macalady’s guidance.

Like Macalady, she has a background in geosciences and ecology, and was drawn to the project because it crossed disciplines in an effort to combat a daunting environmental issue. Grettenberger has published several research papers on AMD at Scalp Level Run.

The project offered her two things: the ability to study microbes in a simplified environment due to life-restricting pH levels and the chance to work within a team capable of putting science into action.

“I like the applied portion of science because I can conduct research and work with engineers tasked with building something that uses that science,” Grettenberger said. “I can work with watershed groups or others to implement it. It’s really getting to see your science put into action in a way that you could never do by yourself, which results in making a difference in the environment and hopefully in the state budget.”

The why in science

Macalady grew up in Colorado but her parents shared stories with her about a problem that’s plagued Pennsylvania for generations.

“There are two things that come together nicely in this project for me, one is that both of my parents grew up in this area and dealing with AMD issues was part of their childhood,” Macalady said. “The other reason is it’s such a beautiful system for learning about the interactions between microbes and minerals. It’s really a nice combination for me. I feel like I’m doing something to help with a problem that has been a problem for generations.”

Burgos said he’s compelled to tackle the state’s AMD problem.

“Penn State is the land-grant university of Pennsylvania, which has more than 2,500 miles of streams that have some sort of negative impact from AMD. It’s the state’s No. 1 water quality problem. I feel a personal obligation to address such a major issue for the state’s future.”