The Tao of Sulfate Reducers
This place stunk to high heaven, and I loved it. The rotten egg smell of hydrogen sulfide meant the sulfate-reducing bacteria (SRB) in our remediation system were happy, happy, happy.
SRB are part of the solution and part of the problem in mine waters. They are a quintessential encapsulation of yin and yang: opposing, balanced forces.
The old Leviathan copper and sulfur mine is located up in the pines of the high Sierras in California. No electricity, no buildings. Some combination of truck, four-wheel ATV, and snow machine were needed just to get there every week. What it did have was acid drainage: sulfuric acid with iron, aluminum, copper, nickel and zinc running out of the mine site and into two streams.
The fish started dying in the 1950s when the mine transitioned from an underground mine to an open-pit mine. The mine close in 1962, but the streams were still devoid of fish when the Washoe tribe asked for remediation in 1997.
SRB were a remediation option. If you give them sulfate, which is part of sulfuric acid, they’ll reduce it to a respiratory waste product: sulfide. Sulfide is exactly what metals like iron, copper, and zinc love to bind to, forming fine particulate metal sulfides. The particulates sink, leaving clean water for fish; the precipitate is dredged for disposal.
The University of Nevada Reno, where I was a graduate student, created a unique “semi-passive” bioreactor tailored specifically to SRB. A series of ponds was carved into the mountainside: two rocky ponds for the bacteria, and two where precipitated metal sulfides accumulated. A solar-powered system dripped an alkaline solution to raise stream pH from about 3.5 to 5.5, and dripped in a liquid carbon source. SRB feed on “small” carbons like ethanol.
At the foot of the mountain was a farm, and on that farm they made their own biodiesel for their tractors. The waste product from making biodiesel is mostly methanol with a hint of potassium hydroxide – the perfect SRB food. The sludge of precipitated metals could be used by the farm as a soil amendment. Although our “Grand Plan” of trading mine waste for biodiesel waste in a closed recycling loop never materialized, the bioreactor has been working well since 2003. And fish are returning.
The heroes of the mining remediation story, SRB are the villains in mercury cycling. SRB convert inorganic mercury into organic methyl-mercury (MeHg) which bioaccumulates in fish and poses neurological risks to fish-consumers.
At the historic Malakoff Diggin’s hydraulic mining area, near Nevada City, California, mercury was used in gold extraction in the mid-1800s. That mercury continues to move downstream, collecting in calm pools and potentially depositing in the Sacramento Delta. Anaerobic sediments and wetlands are natural environments where SRB are found. No doubt SRB have something to do with the many fish advisories.
Ironically, SRB are cleaning the water for fish on one mountain in the Sierras, and increasing the toxicity of fish in a different part of the range. The dual nature of sulfate reducers is just one of the complexities to consider in mining remediation.
photo: K. Zamzow
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