Mine Waste Transformation: From Barren Landscape to Green Ecosystem

Turning Mine Waste into Green Pastures: How Innovative Soil Covers are Solving Acid Mine Drainage

Avery Sinclair in Climate & Environment August 2025 • 5 min read.

"Discover how a unique blend of organic materials and limestone is revolutionizing acid mine drainage prevention, transforming toxic landscapes into thriving ecosystems."

For decades, the Curilo uranium deposit in Bulgaria was a hub of intensive industrial activity, focused on leaching uranium and producing uranium-bearing concentrates. However, this activity ceased twenty-five years ago, leaving behind a significant environmental challenge: acid mine drainage (AMD). AMD occurs when rainwater interacts with exposed minerals, creating acidic, metal-rich runoff that can devastate ecosystems and contaminate water sources.

The legacy of uranium mining at Curilo resulted in a landscape prone to spontaneous AMD generation, especially after rainfall. This process was primarily driven by indigenous acidophilic chemolithotrophic bacteria, which thrive in acidic conditions and accelerate the oxidation of pyrite, other sulfides, and uranium-bearing minerals. The result was a continuous outflow of polluted waters containing uranium, radioactive decay products, and various toxic heavy metals.

Efforts to combat AMD at the Curilo deposit have explored various methods, but one approach has shown particularly promising results: the construction of a specific soil cover rich in organic materials and crushed limestone. This method aims to neutralize acidity, inhibit bacterial activity, and ultimately prevent the generation of polluted waters. Let’s dive into how this innovative solution is transforming a former environmental liability into a potential ecological asset.

What Makes This Soil Cover So Effective?

Mine Waste Transformation: From Barren Landscape to Green Ecosystem

The soil cover's success hinges on a carefully engineered composition and strategic construction. The process began by addressing the existing mining waste directly. The surface of a waste dump containing approximately 2600 tons of mining waste, characterized by particle sizes less than 25 mm, was treated. This dump had been constructed in 2003 using mining wastes that hadn't been directly exposed to air and rain, representing an initial effort to contain the problem.

The primary ore mineral in the dump was pyrite, but it also contained a variety of uranium-bearing minerals, including nasturane, torbernite, metatorbernite, pitchblende, metaotunate, and basetite. Non-ferrous metals were present, with chalcopyrite as the main copper-bearing mineral, alongside secondary copper sulfides, sphalerite, galena, and arsenopyrite. The host rock mainly consisted of quartz, with clay minerals and iron hydroxides also present.

Initial Plugging: The top 0.5-meter layer of the dump was plugged to reduce water infiltration.
Limestone and Organic Matter Addition: Crushed limestone was mixed with organic substrates like leaf compost, straw, and beef manure. This combination was incorporated into the ploughed ore to create a neutralizing and biologically active layer.
Clay Soil Cover: A 0.5-meter layer of clay soil enriched with carbonates was placed over the mixture. This layer acted as a barrier to further reduce water infiltration and erosion.
Vegetation: The soil layer was then grassed and planted with herbaceous plants typical of the region. This vegetation further stabilized the soil, promoted evapotranspiration, and added organic matter over time.

While the dump contained its own naturally occurring sulfate-reducing bacteria in deeper layers, the process was augmented by injecting large quantities of liquid mixed cultures of these bacteria, along with nutrients like molasses, ammonium, and phosphate ions, through boreholes into the dump. This injection was performed periodically (usually weekly) to encourage the establishment of thriving anaerobic conditions conducive to sulfate reduction. Within about three months, the ore layers under the soil cover transformed into anoxic biotopes teeming with active sulfate-reducing bacteria and other metabolically connected microorganisms.

A Path to Sustainable Remediation

The construction of organic-rich layers, coupled with pH levels nearing neutral and a positive net neutralization potential, effectively altered the existing conditions within the dump. The resulting cover was characterized by a diverse microflora consisting of typical soil microorganisms. Sulphate-reducing bacteria and metabolically interdependent microorganisms became the primary inhabitants of the organic-rich, anaerobic lower part of the cover. Pollutant concentrations in the dump effluents significantly decreased, often falling below permissible levels for waters intended for agricultural or industrial use (with a few exceptions for iron, manganese, and sulfates in certain samples).

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.4028/www.scientific.net/amr.1130.602, Alternate LINK

Title: Prevention Of The Generation Of Acid Mine Drainage In A Dump Rich-In Pyrite Uranium Ore

Subject: General Engineering

Journal: Advanced Materials Research

Publisher: Trans Tech Publications, Ltd.

Authors: Veneta Groudeva, Ralitca Ilieva, Mihail Iliev, Stoyan Groudev

Published: 2015-11-01

Everything You Need To Know

What is acid mine drainage (AMD) and why is it a problem at the Curilo uranium deposit?

Acid mine drainage (AMD) is acidic, metal-rich runoff that occurs when rainwater interacts with exposed minerals. At the Curilo uranium deposit, this process was triggered by indigenous acidophilic chemolithotrophic bacteria, leading to the oxidation of pyrite, other sulfides, and uranium-bearing minerals. This results in the release of pollutants, including uranium, radioactive decay products, and heavy metals, into the environment, contaminating water sources and harming ecosystems.

What are the key components of the soil cover used at the Curilo uranium mining site, and how do they work together to prevent AMD?

The soil cover consists of several layers designed to combat AMD. It includes a plugged top layer to reduce water infiltration, a mixture of crushed limestone and organic matter (leaf compost, straw, and beef manure) to neutralize acidity and promote biological activity, and a clay soil cover enriched with carbonates to further reduce water infiltration. Finally, vegetation is planted to stabilize the soil, promote evapotranspiration, and add organic matter over time. The addition of sulfate-reducing bacteria further enhances the process by creating anaerobic conditions and transforming harmful sulfates.

How does the engineered soil cover address the issue of acid mine drainage, and what specific materials and techniques are employed?

The engineered soil cover tackles AMD through a multi-pronged approach. The process begins by treating the mining waste with the soil cover. This soil cover includes the addition of crushed limestone to neutralize acidity and organic materials to create a biologically active layer. A clay soil layer further reduces water infiltration, preventing contact with the acid-generating materials below. Furthermore, the introduction of sulfate-reducing bacteria enhances the remediation process by converting harmful sulfates. Finally, vegetation stabilizes the soil and further reduces water infiltration, completing the remediation efforts.

Besides the soil cover, what other biological methods were used to enhance the AMD remediation process at Curilo, and how did they contribute to the solution?

Besides the soil cover, the remediation process at Curilo incorporated the use of sulfate-reducing bacteria. Large quantities of liquid mixed cultures of these bacteria, along with nutrients like molasses, ammonium, and phosphate ions, were injected into the dump through boreholes. This created anoxic biotopes conducive to sulfate reduction. This biological activity significantly decreased pollutant concentrations in the dump effluents.

What were the main pollutants present in the water runoff at the Curilo uranium deposit before remediation, and how successful was the soil cover in reducing their concentrations?

Prior to remediation, the water runoff at the Curilo uranium deposit contained uranium, radioactive decay products, and various toxic heavy metals. After the implementation of the soil cover, pollutant concentrations significantly decreased, often falling below permissible levels for waters intended for agricultural or industrial use. However, in some samples, elevated levels of iron, manganese, and sulfates were still detected, indicating the soil cover's effectiveness was not absolute, but represented a substantial improvement in water quality.