John Wandke - This page created 11/15/98

Acid Mine Drainage-The Environmental Disaster at Summitville Mine, CO

History
Geologic Backround
Environmental Concerns
Downstream Effects
Reclamation Efforts
 
 
Aerial photo of Summitville mine site (From:USGS bulletin 2220)

History

    The Summitville gold mine is situated about 10 km to the northwest of Platoro, Colorado in the southern San Juan mountains, at an elevation of 11,500 feet.  Gold was first discovered at Summitville in 1870, and during the years between 1870 and 1900, significant quantities of gold were mined out of underground workings.  In 1903, the Reynolds adit was constructed as a means of drainage for the underground workings and also as a haul tunnel, which could be used to transport ore.  Throughout the middle of the century, the Summitville gold deposit was mined sporadically and there was even some consideration for the development of the copper resources present there.  In the early 1980's, the cyanide leaching method was developed for extracting gold out of lower quality ore and it became common practice at numerous gold mines.  From 1985 to 1992, SCMI (Summitville Consolidated Mining Company Inc.), a division of Galactic Resources of Canada, used cyanide heap leach techniques at the Summitville mine.  They created an open pit, mined ore out of it, crushed it and spread it out on a lined leach pad.  The ore was then sprayed with a weak cyanide solution, which leached through the ore and was then collected in order to obtain the pure gold.  SCMI had problems with leakage of cyanide water through the liner shortly after the leach pad was installed and initiated plans for water treatment.  Low pH water was also draining from the heap leach, which results in inefficient gold extraction from the ore.  The site remediation plans never were carried out by SCMI because they declared bankruptcy and abandoned the mine site in 1991.  Cyanide heap leaching finally stopped in the early spring of 1992.  Shortly after Summitville was abandoned by SCMI, the EPA (Environmental Protection Agency) added it to the list of superfund sites, which are extremely damaged areas that require immediate attention.  The EPA then began studies and reclamation efforts on the areas of environmental concern at Summitville, which include the open pit, the heap leach pad, and the Cropsy Creek waste piles.
 

Geologic Background

    The Summitville gold mine is part of the South Mountain volcanic dome and gold is present at the site because of the volcanic history of the area.  Early in the geologic history of Summitville, a series of eruptions occurred, which eventually resulted in the formation of a caldera.  After the formation of the caldera, eruptions and intrusions continued to occur along with severe hydrothermal alteration of rocks.  The principal type of hydrothermal alteration at Summitville was acid sulfate alteration (Gray et al., 1994).  During the active volcanic period, volcanic gases, such as sulfur dioxide (SO2) and hydrogen sulfide (H2S), combined with water to form sulfuric acid (H2SO4).  This highly acidic, hydrothermal water then circulated through fractures and reacted with the quartz latite rocks of the volcanic dome, expending their capability to buffer acids.  Metals present in the hydrothermal fluid were deposited with the pH increase and contact with cooler waters, resulting in a valuable ore deposit.  Gold, silver, and copper were deposited along with other elements such as sulfur, arsenic, selenium, and mercury.  These other metals are typically closely associated with precious metals like gold, silver, and copper.  At Summitville, the main gold bearing rocks (vuggy silica, quartz alunite, and quartz kaolinite) also happen to be the principal sulfide bearing rocks.  These rock qualities were grossly overlooked by SCMI, and are the reason why acid mine drainage problems are severe at the Summitville site.
 

Environmental Concerns

    There are several areas at the Summitville site that are of particular concern:  the open pit, the heap leach pad, waste rock piles, and the system of underground fractures that control groundwater movement.  Short term environmental concerns include the presence of cyanide and metal rich solutions in the leach pad, while the long term environmental concern is acid mine drainage.  The acid mine drainage has the most potential to affect water quality in the downstream agricultural and wetland areas of the San Luis Valley.
    Acid mine drainage is a term that refers to waters that have leached through mine sites and have dangerously low pH values and high metal contents.  These waters take on these qualities because of the presence and oxidation of sulfide bearing rocks, primarily pyrite (FeS2).  Pyrite chemically weathers upon contact with oxygenated water to form sulfuric acid.  Obviously, this is a natural reaction and has been occurring at Summitville ever since its formation.  However, the creation of an open pit and mine workings greatly increases the exposure of sulfide bearing rocks, which allows more water to come in contact with sulfides and oxidize them.  Summitville is especially vulnerable to the environmental danger of acid mine drainage because all of the rocks there have lost their capability to buffer acids due to acid sulfide hydrothermal alteration.  In fact, the rocks of Summitville contain less than 5% sulfides, but because no carbonates are present to counteract the acid waters that have passed through the ore body, the pH remains very low (Gray et al., 1994).
   The main environmental problem with the open pit area is acid mine drainage and metal rich waters escaping from seeps around the pit area and out of the Reynolds adit, which is beneath the pit.  During the years of 1990-1993, measurements of the quality of water draining out of the Reynolds adit revealed a pH of 2.7 - 3.2 and SO4 levels of 1920 mg/L - 4510mg/L (Flohr et al., 1995).
Reynolds adit (USGS bulletin 2220)
Another problem with the open pit is that evaporation of ponded water in the pit can significantly concentrate the water, increasing its metal content and rendering it more harmful to downstream environments.  The formation of soluble secondary salts due to the evaporation of pit waters is also a concern.  The most prevalent of these salts is chalcanthite (CuSO4 5H20), which is a product of sulfide oxidation, and it is thought to make a significant contribution to the acidity and metal content in drainage waters (Gray et al., 1994).  Because chalcanthite is a soluble salt, it can be dissolved easily upon contact with water.  This poses a threat to the downstream environment because pulses of water with very high metal concentrations occur during periods of intense runoff.  Figure 1 shows the increases of Cu levels in the water downstream from Summitville in coordination with the seasonal spring snow melt.
Figure 1 (USGS bulletin 2220)
    The heap leach pad is a source for short term environmental concerns, which includes the presence of cyanide and leakage of any remaining cyanide solution that was added to low sulfide, low grade, silicified ore.  The presence of cyanide is a short term concern because it deteriorates relatively quickly and no longer poses a threat on water quality.  Although the sulfide content of rocks in the leach pad is low compared to the open pit, it is still a cause for concern.
    Waste rock piles that were created at the Summitville site are a significant source of acid waters, particularly because of their location.  SCMI created the Cropsy waste pile in Cropsy creek and attempted to divert the creek and create a french drain system for the waste pile.  Not too surprisingly, this system failed and spring and surface water penetrated the pile, producing waters with a pH less than 3 and high levels of iron, aluminum, copper, zinc, arsenic, and lead (Gray et al., 1994).  Cropsy creek then carried these waters to the Alamosa river via the Wightman fork.
    Another environmental problem is the fact that the South mountain volcanic dome is extensively fractured and the path of groundwater seems to be determined by the orientation of the fractures.  The presence of such fractures makes it clear that the water quality downstream from this area was affected long before mining because water has been circulating through these cracks and reacting with sulfide rocks.  Numerous springs with thick iron deposits (ferricrete), which indicate passage through the ore body, reinforces this assumption.  Mining affected this underground fracture system and consequently changed the behavior of the groundwater.  Underground mine workings diverted water and caused it to flow along paths of high sulfide content, such as the Reynolds adit.  The alteration of ground water flow upon the construction of tunnels is visible in the disappearance of springs that were flowing freely prior to mining(Gray et al., 1994).
 

Downstream Effects

    The principal downstream effect of acid and metal rich water draining from the Summitville site relates to agriculture in the San Luis valley.  The Alamosa river, which receives water from Summitville, flows into the San Luis valley and is captured in Terrace reservoir.  Water from Terrace reservoir is then used to irrigate barley and alfalfa crops.  Barley is used in beer production and the alfalfa is mostly used to feed sheep and cattle.  The extent to which metals from Summitville affect the soil in these areas is of some concern and studies of sediment from the Wightman fork suggest that arsenic, chromium, copper, lead, nickel, manganese, and zinc are present in the water.  The pH of the Alamosa river is probably affected by Summitville acid mine drainage because it has a pH of 5.6 - 6.8 while the Rio Grande has a pH of 7.6 - 9.2.  USGS studies of barley and alfalfa in the San Luis valley show that copper concentrations are higher in plants that were irrigated with Alamosa river water compared to plants irrigated with Rio Grande water (King, 1995).  However, the metal levels in these crops is still well below the level that is reportedly toxic to cattle, which indicates that these metals are not large enough in quantity to have a dangerous effect on the bulk soil composition (King, 1995).
Cu in soil (USGS bulletin2220)
    A second area that could potentially feel effects of Summitville acid mine drainage is the Alamosa National Wildlife Refuge.  This area is a refuge for migrating birds, such as ducks and the endangered whooping crane, and toxic levels of certain metals like selenium can cause severe birth defects.  A USGS study of the water in the refuge shows that the pH is alkaline and the metal content is generally low.  However, the area of the wetlands that is directly influenced by the Alamosa river displays a higher accumulation rate of metals like chromium, copper, nickel, vanadium, and zinc.  Plants in that region contain relatively large quantities of copper and zinc (King, 1995).  From this information, the wetlands do not appear to be in immediate danger, but the efficiency and success of remediation efforts at Summitville will play an important role in the future preservation and quality of the wildlife refuge.
(map from USGS bulletin 2220)

Reclamation Efforts

    Because of the severity of the environmental problems at the Summitville site, the 1995 estimate for the total reclamation costs was 100 - 120 million dollars (King, 1995).  However, to date, the estimated amount of money already spent is about 125 million dollars, and closure of the reclamation process is far from finished (oral communication, Ken Klco, 11/15/98). Remediation efforts are funded by the EPA, but the process has been overseen by the Colorado Department of Health.  Remediation focuses on specifically the Cropsy waste dump and the open pit, the heap leach pad, the Reynolds and Chandler adits, and revegitation and drainage control.
    There are several topsoil stockpiles at the Summitville mine, which were employed in the reclamation of the open pit, the Cropsy waste dump, and the heap leach pad.  The soil has a very low pH, which is characteristic of virtually every material present at the site, and was treated with large quantities of lime in order to neutralize it.  The Cropsy waste pile was moved out of the Cropsy creek drainage and placed back into the open pit.  Before the waste material was put back into the pit, the bottom of the pit was lined with the kaolinite ore in order to create a layer of low permeability.  This clay layer would prevent water from passing through the ore in the pit.  The filled pit was then capped with more of this clay, and finally it was contoured.  The area in the Cropsy creek drainage, which had contained the waste material, was covered with soils treated with lime and enriched with compost.  A layer of hydromulch, which is a fine, wood fiber based mulch that is sprayed onto the ground, fortified with an inorganic fertilizer and grass seed was finally applied in order to encourage revegitation of the area (oral communication, Ken Klco, 11/15/98).
    Remediation procedures at the heap leach pad include water treatment and sealing off the pad from infiltrating water.  Water draining out of the bottom of the leach pad was circulated through a water treatment facility containing hydrogen peroxide.  This served to easily oxidize the cyanide, which removes it from the water.  The water was then recirculated back through the leach pad until a desirable water quality was reached.  After the removal of any remaining cyanide, the leach pad was covered with a membrane constructed out of woven plastic and bentonite, which isolated the heap leach material from outside water sources.  The leach pad was then covered with a thermal layer of the leach pad material in order to protect the bentonite membrane from frost damage, and finally it was covered with topsoil and seeded (oral communication, Ken Klco, 11/15/98).
    In order to stop the drainage of acid, metal bearing waters from the ore body, both the Reynolds and the Chandler adits were plugged.  This eventually resulted in an increase in the water table of around 300 feet, which is approximately equivalent to its pre-mining level (oral communication, Ken Klco, 11/15/98).  This rise in the water table resulted in the leakage of the Chandler adit and the reactivation of numerous springs in and around the mine, which had been inactive since pre-mining times.  These ferricrete seeps are rich in metals, especially iron, and have low pH, which indicates that groundwater freely moves through the highly fractured rocks of the Summitville dome ore body.  There is good evidence that such seeps had poor water quality before the area was mined because of the thick iron deposits.  As discussed earlier, the geology and mineralogy of the Summitville dome is not conducive to good water quality.  However, the construction of underground adits could have contributed further to the metal content and low pH of the groundwater.  Because of the long period of inactivity of these springs, they are somewhat difficult to detect and monitor, but such springs should give an indication of the rate of water quality improvement over time.
    Although the adits are plugged, approximately 100 gallons per minute still escapes from the Reynolds adit.  This water immediately flows into a water treatment plant where it is treated with hydrated lime.  This treatment results in a pH increase, and as a result the metals in solution drop out and accumulate as a sludge, which can be filtered out of the water.  This system greatly improves the quality of the water, but it costs around 2 million dollar per year to operate (oral communication, Ken Klco, 11/15/98).
    A few future reclamation plans at Summitville include the regrading of a high gradient area in the central part of the mine site called the Chandler groin, the creation of rip rap lined storm drainage ditches, and continued revegitation work.  The EPA hopes to see a gradual increase in water quality over the next few years as a result of their remediation efforts, but because of the immensity of the ore body and severity of acid mine drainage potential still in the rocks, any water improvement will be very slow and spending of millions of dollars will continue.  Fortunately, evaluation of the geologic setting is now required before the development of ore bodies can proceed. This practice will prevent severe acid mine drainage situations such as Summitville from happening again.
 

References

Flohr, M. J. K., Dillenburg, R. G., Nord, G. L., and Plumlee, G. S., 1995, Secondary Minerology of
    Altered Rocks, Summitville Mine, Colorado: U.S. Geological Survey, open file report 95-808.

Gray, J. E., Coolbaugh, M. F., Plumlee, G. S., and Atkinson, W. W., 1994, Environmental Geology
    of the Summitville mine:  Economic Geology, v. 89.

Gray, J. E., Coolbaugh, M. F., and Plumlee, G. S., 1993, Geologic Framework and Environmental
    Geology of the Summitville, Colorado Acid-Sulfate Mineral Deposit:  U.S. Geological Survey,
    open file report 93-677.

King, T. V. V., 1995, Environmental Considerations of Active and Abandoned Mine Lands, Lessons
    From Summitville, CO:  U.S. Geological Survey Bulletin, 2220.

Plumlee, G. S., and Edelman, P., 1995, An Update on USGS Studies of the Summitville Mine and
    It's Downstream Effects:  U.S. Geological Survey open file report 95-23.