Iron Mountain Mine near Redding in northern California was really a group of at least ten mines. Operating from approximately 1879-1962, Iron Mountain, in its heyday, was the tenth largest copper production site in the world. Iron Mountain also supplied silver, iron, gold, and zinc. Pyrite (iron sulfide) was mined as well as a source of sulfur for use in munitions, fertilizers, and in petroleum refining.
Portal to the Richmond Mine, Iron Mountain, CA. Photo by: Kirk Nordstrom, 1990.
AMD can have devastating effects on aquatic life once it enters a river system. Copper-laden acidic waters from Iron Mountain, for instance, were causing fish kills at least as early as 1899. AMD can also make waters unusable for purposes such as human consumption. Because of the environmental tolls, in 1983, Iron Mountain became one of the U.S.’s first “Superfund” sites or sites on the Environmental Protection Agency’s National Priority List under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) regulations.
Site remediation at Iron Mountain has included partial capping of the mountain to decrease the infiltration of precipitation and snowmelt and thus acid drainage generation. Acid mine waters from various locations on the mountain are also sent to a wastewater treatment plant where they are neutralized and metals are removed.
USGS researchers have been studying Iron Mountain since the 1970s both to aid in the remediation of the site and also as an extreme ecosystem providing some unique opportunities to study hydrogeochemical and microbial processes with parallels, in some cases, to a Martian environment. Some findings of CWEST researchers are described below. In addition, much more extensive information can be found at the USGS California Science Center Iron Mountain website (http://ca.water.usgs.gov/projects/iron_mountain/).
The Most Acidic Waters in the World
In the early 1990s, USGS geochemist and CWEST researcher Kirk Nordstrom and USGS researcher Charles Alpers descended into the tunnels of Richmond Mine amidst temperatures sometimes as high as 120 °F and humidity near 100%. There they thought they might find water drippings with pHs that were literally off the scale. Values for pH are typically described as ranging from 0 to 14. However, because the scientists had already found waters with pHs less than 1, they thought there might be waters with pHs that were actually negative mixing with higher pH waters.
Although the researchers collected drippings from various Richmond mine locations between 1990 and 1991, the answer as to just how acidic the samples were, was not immediately forthcoming. The scientists first had to come up with a method to accurately measure the extreme acidity. Over the course of several years they came up with a method for measuring pHs less than one that involved applying what is known as the Pitzer ion interaction theory to sulfuric acid and then using a set of standardized sulfuric acid solutions for calibration.
In the end, Nordstrom and his colleagues found that some of their samples did indeed have negative pHs - in one case as low as -3.6. Because pH is on a logarithmic scale, this is exceptionally acidic. They also found that the negative pHs were attributable to a combination of pyrite oxidation and the evaporative concentration of hydrogen ions in a higher temperature environment. The below zero pHs of the drippings collected at Richmond Mine make them the most acidic water observed anywhere in the world. However, negative pH waters are not flowing out of the Richmond portal. Instead, the negative pH waters quickly mix with waters of higher pHs so that the overall pH range from the water discharged from the mine is a still low but not negative, 0.2 to 1.5.
Some of the most acidic waters in the world, Richmond Mine, Iron Mountain, California.
References
Alpers, C.N., Nordstrom, D.K., & Spitzley, J. (2003). Extreme acid mine drainage from a pyritic massive sulfide deposit, The Iron Mountain end-member, in Jambor, J.L., Blowes, D.W., and Ritchie, A.I.M., eds., Environmental Aspects of Mine Wastes: Mineralogical Association of Canada, v. 31, p.407-430.
Nordstrom, D. K., & Alpers, C. N. (1999). Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California. Proceedings of the National Academy of Sciences, 96(7), 3455-3462. DOI: 10.1073/pnas.96.7.3455
Nordstrom, D. K., Alpers, C. N., Ptacek, C. J., & Blowes, D. W. (2000). Negative pH and extremely acidic mine waters from Iron Mountain, California. Environmental Science & Technology, 34(2), 254-258. DOI: 10.1021/es990646v
U.S. Geological Survey News Room. World’s Most Acidic Waters Are Found Near Redding, CA. March 23, 2000. Downloaded from: http://www.usgs.gov/newsroom/article.asp?ID=639.
A matter of pipe scale
As noted above, AMD from different locations on Iron Mountain is transported to a water treatment plant where the water is neutralized and metals are removed. This takes place through two main underground pipelines, one draining the Old Mine #8 workings (also known as pump station PW3) and the second draining the Richmond and Hornet mine portals. In the PW3 pipeline, despite a travel time less than one hour, minerals precipitate out causing scale to build up. Because this can end up significantly clogging the pipeline and result in occasional AMD spillage, PW3 is cleaned out approximately every 2-4 years. Given that the pipeline is 3.4 km long, this can be a costly and time-consuming undertaking. USGS scientists and CWEST researchers, Kate Campbell, Kirk Nordstrom (and Alex Blum), and some other USGS colleagues set out to investigate why exactly the pipe scale was occurring and how its formation could be minimized.
They collected scale and AMD samples at several locations along the length of the pipeline. The scale was analyzed using a combination of X-ray diffraction, a series of wet chemical extractions, and a scanning electron microscope. Chemical and microbial processes in the AMD were investigated using a combination of laboratory batch-scale experiments and the geochemical models, WATEQ4F and PHREEQC.
The researchers found that the scale was the result of the microbial oxidation of Fe(II), which ended up resulting in the precipitation of hydrous Fe(III) oxides in the pipeline - primarily schwertmannite with some goethite. The AMD in the pipeline does not completely fill the pipeline and the resulting turbulent flow could contribute to the exposure of AMD to oxygen, facilitating Fe(II) oxidation.
Schwertmannite and goethite become more soluble at lower pHs. This may be why the Richmond/ Hornet pipeline, which carries AMD with pH less than 1, has never had scale problems in contrast to the PW3 pipeline, which transports AMD with pHs typically ranging from 2.5 to 3. This pH control on solubility may provide one possible solution to the scaling problem. Even a relatively small pH decrease from 2.6 to 2.3 can inhibit precipitate formation. Mixing in 5-10% by volume of Richmond/ Hornet portal pipeline AMD with that from the PW3 pipeline could potentially lower the pH sufficiently to prevent scaling.
References
Campbell, K. M., Alpers, C. N., Nordstrom, D. K., Blum, A., and Williams, A. “Characterization and Remediation of Iron(III) Oxide-Rich Scale in a Pipeline Carrying Acid Mine Drainage at Iron Mountain Mine, California, USA.” in Reliable Mine Water Technology, edited by A. Brown, L. Figueroa and C. Wolkersdorfer, Publication Printers, Denver, p. 287-294, 2013.
U.S. Geological Survey California Water Science Center. Pipe Scale Studies at Iron Mountain Mines. Downloaded from: http://ca.water.usgs.gov/projects/iron_mountain/pipescale.html on May 22, 2014.