Research

November 19, 2013 at 2:16 pm

Taking the Acid Out of Mine Drainage Isn’t the Whole Picture

Ohio University researchers are taking a closer look at a well-established acid-mine drainage treatment method. It’s not enough just to change the chemical pH in the stream, they write. It’s “an unspoken goal” also to restore the biology—the fish and aquatic life—in the watershed.

And that may mean “sacrificing” a section of a stream so that the rest can thrive.

A Hewett Fork site showing iron precipitates.

A Hewett Fork site showing iron precipitates.

Dr. Dina Lopez, Professor and Chair of Geological Sciences, Dr. Kelly Johnson, Associate Professor of Biological Sciences and current holder of the American Electric Power Watershed Professorship, and Geological Sciences graduate students Lisa DeRose and Rebekah Korenowsky were co-authors on “The role of remediation, natural alkalinity sources and physical stream parameters in stream recovery” in the Journal of Environmental Management, along with Dr. Natalie Kruse and co-authors from the Voinovich School of Leadership and Public Affairs Jennifer Bowman and Edward Rankin.

Their test site—Hewett Fork in Raccoon Creek Watershed in Athens and Vinton counties—has been impacted by historic coal mining and has been treated since 2004. Macroinvertebrate and fish communities have begun to recover in Hewett Fork and the watershed, the researchers write, but this is not always the case at treated sites.

“While alkaline addition treatment is a well-established acid-mine drainage treatment method, the mechanisms within a watershed that lead to either stream recovery and treatment success or limited biological recovery are poorly understood,” the authors write. “Acid-mine drainage negatively impacts not only stream chemistry, but also aquatic biology. The ultimate goal of treatment is restoration of the biological community, but that goal is rarely explicit in treatment system design.”

From right: Water collected from the headwaters above the doser (far right). The orange water is from the sacrifice zone, followed by clear water 11 to 12 miles downstream.

From right: Water collected from the headwaters above the doser (far right). The orange water is from the sacrifice zone, followed by clear water 11 to 12 miles downstream.

Treating the Acidity

Mine drainage becomes acidic when previously buried sulfide minerals such as pyrite are exposed to weathering by oxygen and water. The result is metal-rich, sulfur-rich, acidic drainage. Levels of iron, aluminum, manganese and other metals also are elevated. The acidity and metal concentrations are toxic for fish and aquatic macroinvertebrates.

“In traditional acid mine drainage treatment, the goal of treatment is to improve the stream pH to above 6.5 and to maintain net alkaline conditions in the stream. While these goals are valid, the unspoken goal of acid-mine drainage remediation is to restore streams to their pre-mining quality, both chemically and biologically. Many studies have shown that a neutral pH and net-alkaline conditions are not always sufficient to achieve biological recovery,” they write.

“By focusing only on the localized site conditions rather than watershed-wide factors, including stream gradient and natural sources of alkalinity, the potential for stream recovery may be limited. Treatment of acid-mine drainage with alkaline addition helps to recover the water chemistry and alkalinity but at the expense of the generation of considerable amounts of precipitated colloids or minerals (e.g. aluminum and iron hydroxides, sulfates). These sediments cover the stream bed and impact the stream habitat.

“Ideally, alkaline addition treatment systems would be built in sequence with settlement ponds or wetlands for metal retention. In reality, there is often insufficient space and/or funds for pond dredging to include these features in the system. Instead, a section of stream downstream from the treatment system effectively acts as a settlement pond; this is termed the ‘sacrifice zone’ of the stream.”

A sacrifice zone of stream adjacent to a floodplain, they say, can regularly wash the sediment and metals onto the floodplain and and allow downstream recovery to take place. It’s also important to have a natural source of alkalinity downstream, they add.

“Since the stream acts as a part of the treatment system in many watersheds, the physical and chemical condition of the entire stream is key to the success of treatment. Biological recovery can be maximized if remediation designs consider possible multiple sources of acid-mine drainage, additional alkalinity sources along the stream, the flow regime and depositional zones along the stream.”

Hewett Fork subwatershed highlighted in gray.

Hewett Fork subwatershed highlighted in gray.

Hewett Fork’s Recovery

Hewett Fork, a little more than 15 miles long, is the fourth-largest tributary to Raccoon Creek. The glaciers did not reach this far into Southeastern Ohio, so the area consists of sedimentary rock from the Mississippian and Pennsylvanian time periods—sandstone, shale, a little bit of limestone, and three thick beds of coal. One of the two deep mines draining into Hewett Fork was deserted back in 1923, but it’s hydraulically connected to a series of underground mines. The alkaline “doser” treats the discharge from this mine.

Researchers have been studying the mine for almost a decade since the 75-ton calcium oxide doser was installed in 2004. After the doser was installed, the fish and macroinvertebrate communities improved within months. By 2010, biotic quality was good at sites downstream from where Hewett dumps into Raccoon Creek.

“The flowpath of Hewett Fork is divided into three zones based on the aquatic life,” they write. “The first zone is the Impaired Zone and has poor biological quality due to the acid-min drainage and heavy alkaline addition from the Carbondale doser. The Transition Zone is of primary interest and has improved biological diversity. The last zone occurs toward the mouth and sustains good aquatic life and is called the Improved Zone.”

Their research found: “In Hewett Fork, natural alkaline additions downstream are higher than those from the doser. Both, alkaline additions and stream velocity drive sediment and metal deposition. Metal deposition occurs in several patterns; aluminum tends to deposit in regions of low stream velocity, while iron tends to deposit once sufficient alkalinity is added to the system downstream of mining inputs. The majority of metal deposition occurs upstream of the recovered zone.”

Recovery Takes More Than Chemical Measures

“While traditional treatment design has focused on increased alkalinity and pH, this study demonstrates that ecological recovery of an acid-mine drainage treated stream relies on more factors than these simple chemical measures. This study has identified processes that determine transport, deposition and release of aqueous and sediment borne chemicals. Two key mechanisms were identified: alkalinity addition from treatment, groundwater and tributaries and stream velocity….The fate of the contaminants along the stream may influence the biological recovery of the stream.

“In order to achieve ecological recovery in an acid-mine drainage treated stream, several key conditions are necessary: sufficient alkalinity sources downstream from the doser input to supplement the alkalinity generation of the treatment system and a low gradient zone for metal deposition that is downstream of all acid-mine drainage sources. Remediation schemes should consider the whole watershed, tributaries along the water path, and the topographic variations that
determine the areas of deposition and erosion along the stream to achieve biological recovery.”

 

One Comment

  1. Excellent piece on AMD. I fully concur with the authors. In the South African goldfields we are just starting to think about rehabilitation so we have a way to go still. Your work helps us.

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