Acid Lakesfrom Lignite Mines Dan Henderson
Lignite Brown/soft coal . Used for steam electric power generation. Mined in open pits. Production declining in most countries.
____ Where Germany (Lusatia) Russia United States (SE) Australia Greece
___ _____ ____ Acid Mining Lakes (AML) • Artificial Lakes form in open pits when mining ceases. • Groundwater recharge • Stream inputs • Natural Lakes receive Acid Mine Drainage (AMD) from nearby mines. • pH as low as 2 recorded in some lakes.
___ ___ _______ Acid Mine Drainage Outflow of acidic water from mines. High Concentrations of sulfate and iron Result from pyrite (iron sulfide) and other sulfides from mine tailings oxidizing in water. Bacteria found in rock may promote acidification (acidophiles). Vary depending on local geology.
Effects on Lake Chemistry • Metals are more soluble at lower pHs. • High concentrations of dissolved Fe. • Light Attenuation • High concentrations of Al. • May limit P • May also increase levels of Mn, Zn, As, and Pb. • Depend on acid neutralizing capacity.
Effects on Vegetation • Low pH Limits species diversity and richness. • Many plants cannot tolerate high metal concentrations. • Tolerant species may thrive. • Vegetation may increase surface concentrations of metals by evapotranspiration.
Phytoplankton • Development not directly related with pH decrease. • Decrease related to limitations of inorganic carbon, phosphorus, and underwater light.
Effects on other Biota • Limits lake food for birds and wildlife that find food in lake. • Low pH decreases fish diversity and richness. • Insect larvae and crustaceans not likely to be found below pH of 4. • Usually enough silica still present for diatom development.
Why treat AMLs • Ecosystem Health • Wildlife • Recreation • Aesthetics
Main Treatment Methods • Preventative measures best approach. • Altering mine site hydrology. • Sulfate reducing bacteria. • Application of organic wastes. • Application of alkaline materials (Limestone). • Wetlands.
Sulfate reducing bacteria • Good method for treatment. • Require anaerobic conditions which may be limited in well mixed lakes. • Increase alkalinity and help eliminate Fe and sulfate. • Most are heterotrophs which require and may be limited by organic carbon.
Biobags (Organic Wastes) • Bags of organic material placed into lakes. • Form anoxic microbial reaction compartments. • Increase organic carbon and P concentrations which promote sulfate reducing bacteria. • Increase primary production by increasing nutrients. (Eutrophication) • Use local organic material.
Application of Limestone and/or Phosphorus • Limestone increases pH levels needed for biological growth. • Reduces metal concentrations by making them less soluble. • Phosphorus is frequently limited in AMLs. • Phosphorus promotes phytoplankton development and biological growth.
Wetlands • Used extensively in last two decades to remove metals from AMD. • Precipitation of sulfides. • Roots of wetland plants release oxygen and form Fe plaque taking other metals with it. • Biological Uptake.
More treatment methods • Inundation • Bacteriacides for acidophiles • Coating pyrite to prevent oxidation • More needed
Progression • Some lakes may take decades to recover. • Allochthonous alkalinity of river water will promote progress. • Bottom of food chain must be established first to support the rest. • Ultimate goal is to establish fish populations.
Results • All methods have been shown to work. • Best methods are very site specific. • A combination is usually the best approach. • Certain species found to recover much faster than others. • Neutralized AMLs have much greater species richness.
Importance • Treatment of AMLs may produce circum-neutral lakes much faster than if left alone. • May prevent the loss of diversity. • Treated lakes may be used for recreation and promote ecosystem health.
Research Needed • A low percentage of AMLs have been studied. • Recovery methods can be expensive and time consuming. • AMLs continue to grow in number even though mining is in decline. • Many effects of AMD may yet to be identified and understood.
Conclusion • Raising pH to near circum-neutral level is how most AMLs are treated, but this is not sufficient to meet all ecological demands. • Effects on ecosystems, treatment processes, and recovery times all vary greatly. • Lake acidification is a worldwide problem and limnologists are needed to continue to refine, identify, and implement new strategies for remediation.