World's Most Acidic Waters are Found Near Redding, California.

The most acidic waters ever measured are percolating through an underground mine near Redding, Calif., according to scientists with the US Geological Survey.

Hot acid solutions, more concentrated than battery acid, are dripping off colorful mineral stalactites in the abandoned copper and zinc mine at Iron Mountain, a northern California "Superfund" site that is undergoing remediation by the US Environmental Protection Agency (EPA).

Although a water-treatment plant at Iron Mountain has reduced the amount of copper and zinc leaching from Iron Mountain by 80 to 90 percent since 1994, some acid waters from the mine site still make their way to Spring Creek, a tributary to the Sacramento River, a few miles upstream from Redding, according to Kirk Nordstrom and Charlie Alpers, USGS geochemists who have been sampling and analyzing the Iron Mountain drainage waters since the early 1970s.

Spring Creek enters the Sacramento River system in the Spring Creek arm of Keswick Reservoir, about 2 miles upstream of Keswick Dam. The City of Redding is located about 4 miles downstream of Keswick Dam. Although the City of Redding gets its drinking water from the Sacramento River, the metals from Iron Mountain do not pose a significant threat to this water supply because of dilution effects and the water-treatment plant. Fish are much more sensitive than humans to trace metals such as copper, zinc, and cadmium, so water-quality criteria for aquatic life that are enforced by the Regional Water Quality Control Board and the EPA are much more stringent than for drinking water for these metals. In the event of an uncontrolled release of metal-rich drainage from Iron Mountain, the City of Redding has a contingency plan to switch to alternate ground-water sources.

The two USGS scientists Nordstrom and Alpers, in cooperation with colleagues from the University of Waterloo, in Ontario, Canada, have developed a new method for measuring negative pH values in highly concentrated acid mine waters. The most acidic waters ever recorded were sampled by the USGS scientists in 1990 and 1991, but it took several years for them to find a way to accurately quantify the extreme acidity and to have the concept of negative pH accepted by the scientific community.

Chemistry textbooks usually describe pH values as ranging from 0 to 14, with 7 being neutral. Values of pH near 0 are highly acidic, becoming less acidic and more alkaline at the higher numbers. Because pH is measured on a logarithmic scale, each declining unit represents 10 times more acidity. Alpers said several of the drip-water samples at Iron Mountain had pH values below zero, indicating hydrogen ion activities greater than one. The lowest pH found at the site was -3.6.

Alpers and other members of the underground sampling team had to contend with dangerous conditions in the mine workings. The temperature was more than 120 degrees Fahrenheit, humidity approached 100 percent, random rock falls from the 40-foot ceiling were a common occurrence, and extremely acid water was dripping everywhere. Huge stalacite-stalagmite structures, composed of rhomboclase and other iron sulfate minerals, spanned from floor to ceiling in the 40-foot high mine opening, where the most acidic waters were found. Rhomboclase, a rare mineral, is a solid form of sulfuric acid and ferric iron sulfate.

Some additional information is provided to put the hydrology of the Iron Mountain site in context. The drip rate of the negative pH waters found in the Richmond Mine is on the order of less than one gallon per minute, whereas the overall flow rate of the discharge from the Richmond Mine ranges from 20 to more than 1,000 gallons per minute and the pH of the overall discharge ranges from about 0-1. All of the water from Richmond Mine is currently piped to the water-treatment plant, along with water from another mine tunnel on the north side of Iron Mountain (the Lawson tunnel, which drains the Hornet Mine, with pH 1-2) and ground water from an area on the south side of Iron Mountain (the Old Mine and Number Eight Mine area, with pH 2-3). The acidic, metal-rich waters that currently leaves the site untreated come from waste piles and seepage on both the north (Boulder Creek) and south (Slickrock Creek) sides of Iron Mountain. The EPA is planning to implement additional site remediation during 2001 that will dam and treat the acidic water in the Slickrock Creek drainage, bringing the total proportion of metals removed from the continuing acidic drainage to greater than 95 percent for both copper and zinc.

Iron Mountain is known geologically as a "massive sulfide" ore deposit. The deposit was mined intermittently from the 1860s to the early 1960s for its copper, zinc, gold, and silver, and for the mineral pyrite, which was used to produce sulfuric acid. This type of deposit has high concentrations of metals and sulfur and typically produces the worst acid mine drainage.

Such high concentrations of acid and metals can have severe environmental effects when they enter a river system, because they may kill all aquatic life except microorganisms, for up to several miles downstream. Species that are potentially harmed by the metal-rich acid drainage from Iron Mountain include winter-run chinook salmon, a federally listed endangered species that spawns in the Sacramento River near Redding, as well as steelhead trout and spring-run Chinook salmon, which are listed as threatened species by the US Fish and Wildlife Service.

The USGS reports on Iron Mountain were prepared in cooperation with the EPA, and are being used by the EPA and its contractors in progressing with the "cleanup" of the site. Additional information on USGS's research on the geochemistry and hydrogeology of Iron Mountain can be accessed on the World Wide Web at http://mine-drainage.usgs.gov/mine/sum/ and http://www.pnas.org/cgi/content/abstract/96/7/3455.

Copies of the article "Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California," Environmental Science & Technology, v. 34, p. 254-258, by Darrell Kirk Nordstrom, Charles N. Alpers, Carol J. Ptacek, and David W. Blowes may be obtained at http://pubs.acs.org/hotartcl/est/2000/research/es990646v_rev.html or in printed form (with color cover photo) from:

D. Kirk Nordstrom,
US Geological Survey,
3215 Marine Street, Boulder, CO 80303.

For more information, please contact:

Michael Shulters, District Chief, US Geological Survey,
Placer Hall, 6000 J Street,
Sacramento, CA 95819-6129,
telephone 916-278-3026,
or
Michael Reddy, Regional Research Hydrologist,
Mailstop 418, Denver Federal Center, P.O. Box 25046,
Denver, CO 80225-0046,
telephone: 303-236-5021.

Links:

• US Environmental Protection Agency (EPA)
• USGS Mine Drainage Group
• Environmental Science and Technology Article
• Iron Mountain Photos
• Remediating Iron Mountain

As the Nation's largest water, earth and biological science and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to the sound conservation, economic and physical development of the Nation's natural resources, and enhance the quality of life by monitoring water, biological, energy, and mineral resources.

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