Geology, Mining, and Water Quality This presentation looks at an issue that is very important in Colorado -- geology, mining, and water quality. Surrounding geology (including natural and human altered rock formations) is important in the water quality of your local watershed. Image description: Mineral Creek watershed in southwestern Colorado has natural and mining-related sources of contamination. In the background of this photo is a large naturally occurring seep that discharges acidic, metal-rich water to a tributary of Mineral Creek. Natural sources of contamination like this one make it difficult to determine appropriate water-quality standards for cleanup actions.

Water takes on the character of the rock it passes over and through, as it flows into streams and ground-water aquifers. The composition, or chemistry, of the water will reflect the geology of the area with which it has been in contact. This generally does not change the water quality significantly enough to be a concern to humans, wildlife, aquatic organisms, or vegetation. In some areas it does. Streams in high, headwater areas can greatly reflect the local geology. If local geology is deleterious, stream water quality can be poor. Why? Stream flows are relatively small within individual tributaries; the chemistry of the “young” water has only been in contact with few types of rock. The blending of water from different types of geology, which can balance water composition, doesn’t happen until further downstream. This is the South Platte River Watershed. In which part of the watershed do you think the water most accurately reflects the geology of the area it passes through?

In Colorado, the headwater areas (largely the Continental Divide) are coincident with the Colorado Mineral Belt. The density of mining along the mineral belt and other ore bearing areas has contributed to water quality degradation in some streams. Primarily, the contaminants are acidity and metals, but may include other problems such as sedimentation.

Water - from rain and snowmelt Acid Mine Drainage Water - from rain and snowmelt + Oxygen - from the air Pyrite - from the mine Reaction = Sulfuric Acid Acid Mine Drainage (AMD) is the term given to the primary means of water quality degradation at mine sites. Essentially, water from rain and snowmelt and oxygen from the air interact with pyrite (fool’s gold), or other sulfide minerals, to form sulfuric acid. Pyrite and other sulfide minerals are often present in ore and associated minerals in mining areas. Where does acid mine drainage come from?

Sources of Acid Mine Drainage (AMD) Mine Effluent Mine Effluent or drainage from the mine workings affects surface and ground water: They serve as collection areas for ground water, increasing residence time in rocks/veins with high metal concentrations. This results in elevation in metal concentrations in the water. Air and water moving through the workings can oxidize the sulfide minerals producing acid. A mine adit (or tunnel) can act like a horizontal well or french drain drawing water into its workings as the water finds the least path of resistance through the mountain and downhill. AMD can also come from waste rock piles, overburden, tailings, etc. containing sulfide minerals associated with the mining activity. Burbank Mine northwest of Silverton, CO. Burbank Mine, San Juan Co.

Mine Dump Mill Tailings Lewis Mine and Mill, San Miguel Co. This is the Lewis Mine and Mill near Telluride, CO in San Miguel County. This photo illustrates additional sources of acid mine drainage. Mine dump material, although not consisting of “high-grade” ore, does contain high concentrations of gangue minerals, minerals that accompany the ore (commonly pyrite, chalcopyrite, bornite). The broken rock increases the surface area exposed to weathering. Mine Tailings. Even though much of the valuable metal is removed during the milling process, mill tailings expose an even larger surface area to weathering. Significant amounts of certain metals can be left in the tails depending on the age and type of the mill. Now its time to look more in depth at the chemistry of AMD. Mill Tailings Lewis Mine and Mill, San Miguel Co.

THE CHEMISTRY of ACID MINE DRAINAGE Reaction 1: FeS2(s) + H2O + 7/2O2 Fe2+ + 2SO4 + 2H+ pyrite water sulfate acid Reaction 2:* Fe2+ + 1/4O2 + H+ Fe3+ + 1/2H2O *Reaction 3: FeS2(s)+ 8H2O + 14Fe3+ 15Fe2+ + 2SO4 + 16H+ Primary Reactions Reaction 1 >> In the presence of water and oxygen, pyrite is oxidized to ferrous iron and sulfate and releases 2 moles of acidity. Reaction 2 >> Ferrous iron is oxidized to ferric iron,consuming 1 mole of acidity. Secondary Reactions Reaction 3 >> This step is catalyzed by the bacteria Acidithiobacillus ferroxidans. Pyrite is again oxidized, this time by the ferric iron ion. This produces 16 moles of acidity. Reaction 4 >> Lastly, ferric iron dissociates water creating more acidity and precipitating the solid iron hydroxide. This is the reaction that results in “yellow boy” the orange precipitate seen in many mine drainages. Reaction 4: Fe3+ + 3H2O Fe(OH)3(s) + 3H+ * catalyzed by bacteria

Background Water Quality There is another factor that needs to be considered in this photo and in water quality investigations related to mining. What is the background water quality of the stream, before it reaches the mine? As mentioned in the beginning of this presentation, water takes on the character of the rock is passes over or through. Fortunately, at this mine site the background water quality is good with a near neutral or slightly alkaline pH. But that is not always the case! Lewis Mine and Mill, San Miguel Co.

THE CHEMISTRY of ACID MINE ROCK DRAINAGE Reaction 1: FeS2(s) + H2O + 7/2O2 Fe2+ + 2SO4 + 2H+ pyrite water sulfate acid Reaction 2:* Fe2+ + 1/4O2 + H+ Fe3+ + 1/2H2O *Reaction 3: FeS2(s)+ 8H2O + 14Fe3+ 15Fe2+ + 2SO4 + 16H+ The chemistry of acid mine drainage can happen anywhere sulfide minerals are present, especially where no alkaline minerals are present to counteract the production of acid. The broader term is Acid Rock Drainage and it is active in many areas of Colorado. Reaction 4: Fe3+ + 3H2O Fe(OH)3(s) + 3H+ * catalyzed by bacteria

Acid Rock Drainage (ARD) sources: Abandoned mines Natural springs and drainages in hydrothermally altered areas The primary sources of acid rock drainage in Colorado are: 1) abandoned mines and natural springs 2) drainages in hydrothermally altered areas. Frequently these two sources occur in the same areas. What is hydrothermal alteration??

A number of areas in Colorado have streams with naturally high concentrations of metals and acidity above mining impacts. This mountain lies in the headwaters of the North Fork South Platte River in Colorado. On the other side of the continental divide (ridge in picture), this area of hydrothermal alteration affects the headwaters of the Snake River. Drainages containing no mines have high acidity with pH= 2.5-3.5 Predominantly iron and sulfate, some copper above stream standards in this area. Red Cone, Park Co.

Ferrosinter deposits are characteristic of springs in hydrothermally altered areas. The smooth rock in the upper left of this picture is actually a spring. Natural acid rock drainage emanating from this spring carries abundant metals that drop out of solution when exposed to oxygen in the atmosphere. The resulting deposit is called a ferrosinter. Ferrosinters are cold water iron precipitate mounds at springs. Composed of predominantly of FeO-FeOH and AlOH precipitates. The streambed below this natural acid rock drainage spring consists of ferricrete. The water quality is poor and the streambed is very hard, eliminating habitat for fish and invertebrate aquatic life. Ferrricretes are river bed (alluvium) or hill slope (colluvium) material cemented by iron precipitates. Iron Creek Lower Spring, Conejos Co.

Slumgullion landslide is in an area of hydrothermally altered rock Slumgullion landslide is in an area of hydrothermally altered rock. Movement and erosion continually expose acid-producing minerals. Drainage from the slide area is very acidic (pH  3.4). Headwall Colorado’s largest landslide is the Slumgullion landslide, southeast of Lake City. This landslide caused the formation of Lake San Cristobal. Drainage along the body of the slide has pH~3.4 and is high in Aluminum, Iron, and Manganese. Slumgullion Landslide near Lake City, Mineral Co.

What can we do about AMD? Passive remediation – utilizes naturally occurring chemical and biological reactions, does not require continuous electrical or chemical inputs or frequent maintenance. Active remediation – requires continuous electrical or chemical inputs ongoing operation maintenance. The goals of reclamation techniques are to neutralize acidity and remove metals in streams. Passive remediation – These methods do not eliminate the cause of the problem. Active remediation – These methods can eliminate the cause of the problem. Acid mine drainage needs water, oxygen, and sulfide minerals to form. To “fix” acid mine drainage, one of the three elements need to be eliminated. The next several slides will show examples of AMD remediation techniques. It is not a comprehensive list of remediation techniques.

Remediation techniques are not “one size fits all!” FYI Remediation techniques are not “one size fits all!” Factors that contribute to selecting appropriate remediation systems include, but are not limited to, cost, amount of drainage, metal content, pH, geography, and climate.

Active Remediation Highly effective at neutralizing acidity Requires long-term commitment and a continuous maintenance budget Often combined with a passive technique to reduce metal contaminants This image is of an active lime treatment of AMD. Active reclamation techniques will always have to be monitored, use electricity to add chemicals to the AMD, and will have to have chemicals refilled on a regular basis to the system to effectively treat acid mine drainage.

Active Remediation – Reverse Osmosis Uses a semi-permeable membrane filtration system and high pressure to separate the solute from the solvent. The metals in the AMD will stay on one side of the membrane and neutral, clean, water moves through the membrane to the other side.

Active Remediation – Alkaline Injections Consists of periodic injection of alkaline material into AMD flow. Active method often used when alkaline material is highly reactive with water. Could use alkaline materials such as quicklime and fly ash. Used to neutralize AMD.

Passive Remediation Initial cost can be high Little long-term maintenance and resources Theoretically, systems should be self-sustaining because they are based upon natural processes The photo is of a passive treatment system at Bell Colliery, Pennsylvania.

Passive Remediation – Aerobic Wetlands The acid mine drainage first flows into a settling pond. There, the water is exposed to oxygen, which oxidizes dissolved iron to a solid form. The solid precipitates out of the water and settles on the bottom of the settling pond. Next the drainage travels into an aerobic wetland. The water depth in the wetland is usually between 6” – 18”. Here more iron precipitates out of the water and forms a plaque on the roots of the wetland plants. This plaque is what helps collect other metals from the acid mine drainage. The living plants themselves do not actually absorb much of the metals. The acid is neutralized by the biological formation of bicarbonate, but wetlands are not ideal for AMD with very low pH. There is often more than one settling pond or wetland to give adequate surface area for the addition of oxygen and adequate time for metals to precipitate and settle out of the mine drainage before it enters surrounding streams.

Passive Treatment – Anaerobic Wetlands 0”-3” water on surface 12” – 24” organic material or compost and limestone Can handle more acidic drainage than aerobic wetlands Aerobic – oxygen is present Anaerobic – oxygen is absent Limestone is either mixed with the organic material or in a layer on the bottom of the wetland. Anaerobic wetlands are more successful at remediating more acidic drainage than aerobic wetlands, as a result of the limestone present. In anaerobic wetlands, water flows horizontally across the system, like in aerobic wetlands. The water depth is less than that of an aerobic wetland, because the drainage needs to make contact with the organic material for this system to be effective. Oxygen is taken out of the AMD by bacteria in the organic matter, making the system anaerobic.

Passive Remediation - Limestone Alkaline minerals such as limestone, help to neutralize the pH. Watch out for armoring – a buildup of iron on the rocks The photo is an image of an open limestone channel. You can see areas of the rock that have turned orange. The AMD pH becomes more neutral as the water dissolves the limestone rocks. As the pH increases metals, such as iron, begin to precipitate out of the water and leave behind the rusty colored precipitate. Armoring is a buildup of iron precipitate (iron hydroxide) on rocks. Armoring occurs when iron hydroxide precipitates out of the water and onto the limestone forming a barrier between the limestone and the water. This results in the water not being neutralized by the limestone. Armoring can happen relatively quickly. Some strategies to minimize armoring include eliminating contact with oxygen (anoxic limestone channels) and placing the channel on a steep slope so fast moving water moves the precipitate downhill.

Passive Remediation - Limestone Anoxic limestone channels prevent armoring Anoxic limestone channel is a method of passive remediation that prevents the formation of iron hydroxide by preventing the AMD from coming in contact with air, but still allows the pH neutralization and the iron to precipitate out of the water. The channel is formed underground, lined with impervious matierial, and capped. Like any reclamation technique, anoxic limestone drainages can be used in combination with other techniques. These drainages are sometimes placed before an aerobic wetland to help increase the AMD pH prior to entering the wetland.

Vertical Flow Reactors (SAPS), Anaerobic Wetlands Vertical flow reactors (aka – SAPS or Successive Alkalinity Producing System) are a type of passive AMD treatment in which AMD flows down (vertical flow) through compost or organic material before it reaches a limestone layer. Passing through the organic matter is important because oxygen is taken out of the AMD by bacteria in the compost. The system is now anaerobic. Then when the AMD comes into contact with the limestone its pH can be neutralize. Not being in the presence of oxygen, the iron will precipitate out of the AMD in a form that will not cause armoring on the limestone. Finally, the drainage flows out of the bottom of the vertical flow reactor.

Passive Remediation – Sulfate Reducing Bacteria This image is from Colorado State University. Sulfate reducing bacteria use a biological process that converts sulfates into sulfides. The sulfides bind to metals and cause them to precipitate out of the water. The pH is also neutralized in this process.

AMD from Red and Bonita Mine near Silverton, CO Sometimes the actions we take to remediate an environmental hazard have unexpected outcomes. The Red and Bonita Mine near Silverton, CO did not have any significant drainage and was considered a “dry mine” until a neighboring mine had a plug installed in it to stop the flow of AMD from that mine. After the plug was installed, Red and Bonita Mine as well as other mines in the area began discharging AMD. AMD from Red and Bonita Mine near Silverton, CO This mine did not start producing acid mine drainage until nearby mines were plugged.

Conclusion The simple formation of acid mine drainage can be a very complicated and resource demanding problem to treat. There are numerous remediation techniques that can be used to treat AMD. Each one comes with its own costs and benefits. Many measures are taken in today’s mining industry to prevent environmental hazards such as acid mine drainage.

Resources This presentation was adapted from the Powerpoint of Matthew A. Sares with the Colorado Geological Survey. "Basic Information." Engineering Technical Support Center. EPA. Web. http://www.epa.gov/ordntrnt/ORD/NRMRL/lrpcd/etsc/basic.html. Ford, K.L. 2003. Passive treatment systems for acid mine drainage. Technical Note 409. BLM/ST/ST-02/001+3596. Bureau of Land Management Web based report available online at http://www.blm.gov/nstc/library/techno2.htm. Fripp, Jon, Paul F. Ziemkiewicz, and Hari Charkavorki. "Acid Mine Drainage Treatment." US AEC, May 2000. Web. http://el.erdc.usace.army.mil/elpubs/pdf/sr14.pdf. Golden, Bruce. “Ask Me About Pyrite.” Western Pennsylvania Coalition for Abandoned Mine Reclamation. Presentation. SAPS & Vertical Flow Reactors. AMRClearinghouse.org. Web. http://www.amrclearinghouse.org/Sub/AMDtreatment/SAPS-VFP'S.htm. “Acid Mine Drainage Remediation.” Colorado State University. http://www.engr.colostate.edu/~apruden/Research%20Group/Projects/Acid%20Mine%20Drainage.htm. Trout Unlimited Assault on San Juan County Colorado. July 2010. http://www.savethesanjuans.com/Save%20the%20San%20Juans%20011611.htm. “Reverse Osmosis.” http://science.howstuffworks.com/reverse-osmosis2.htm.