Taconite Tailings and Water Quality - A Survey of Existing Data

Abstract

For over 40 years, taconite tailings, a by-product of taconite iron ore processing, has been used in northeastern Minnesota road construction projects as aggregate. The dominant uses have been as fill materials and in bituminous pavements. Over 125 million tons of tailings are produced annually by Minnesota’s iron mining industry (Oreskovich et al., 2007). The Natural Resources Research Institute (NRRI), University of Minnesota Duluth, has been involved in a multiplephase project to evaluate the quality and use of this material. When used as aggregate for bituminous pavements, the taconite tailings grains are encapsulated in the asphalt mix that separates the tailings from contact with water. As fill, taconite tailings can be in contact with water, intermittently, seasonally, or continuously. Do taconite tailings affect water quality? In an effort to evaluate this question, a compilation and review of existing groundwater and surface water chemistry associated with tailings in contact with water has been completed. We evaluate water quality by comparing existing water sample chemical analyses data to published State of Minnesota ground and surface water standards. Data utilized for this study include: Minnesota Department of Natural Resources (DNR) reports from 1989 and 1999, Minnesota Pollution Control Agency (MPCA), Keetac Pollutant Discharge Elimination System (NPDES) permit water sampling data from several mines, and previous investigations completed by the Natural Resources Research Institute (NRRI). Based on our review, the data from water quality and taconite tailings revealed the following findings: 1. Most Minnesota water quality standards are met. The exceptions include arsenic, cobalt, iron, and manganese. Iron and manganese exceed secondary drinking water standards that are based on attributes of the water like taste, odor, and appearance, and not because of health risk issues. Arsenic and cobalt exceed the MPCA’s 2A chronic standard for surface waters of 2 ppb and 2.8 ppb, respectively. These elements do not exceed the drinking water standards or Class 7 surface water standards; 2. Mercury is typically an environmental concern. Based on the NPDES data reviewed, the following information was noted. Chemical analyses completed on surface water collected at three of the mines had the following reported numbers: maximum value 7.24 ng/L, minimum value 0.45 ng/L and a median value of 1 μg/L. Minimum and median reported mercury values meet the most stringent surface water standard, the Great Lakes Initiative, of 1.3 ng/L. Thirty-four water samples were analyzed for total mercury. A total of 678 NPDES water sample data were reviewed. DNR reports do not contain mercury data for water samples. Atmospheric mercury could add to the amount detected by chemical analyses in surface water samples; 3. Iron formation contains arsenic, cobalt, manganese, and iron; 4. Taconite tailings do contain arsenic, cobalt, manganese, and iron. Arsenic occurs at a minimum value of 8.8 mg/kg, maximum value of 39.4 mg/kg, and a median value of 17 mg/kg. Cobalt occurs at a minimum value of 4.4 mg/kg, maximum value of 15.4 mg/kg, and a median value of 7.7 mg/kg. Manganese and iron were not reported as trace metals but were included in whole rock analyses; 5. NRRI completed Toxicity Characterization Leaching procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP) chemical analyses on three samples of taconite tailings. Results indicated that arsenic results ranged from < 2 μg/L to 4.3 μg/L, slightly above the surface water quality chronic standard of 2.0 μg/L for 2A waters; 6. Further evaluation is recommended. Testing on taconite tailings samples, as well as other typical aggregates, should include physical and chemical parameters. Testing on samples of aggregate and water should be done to evaluate all sites by the same methods and current detection limits. Analytes should include: RCRA metals as well as cobalt. Additional testing should include grain size analyses and hydraulic conductivity; and 7. Mechanisms for the potential release of metals into surface water by tailings are dependent on water characteristics such as pH, Eh, time, hydrology, and reduction (redox) potential. Therefore, it is site specific. Additional testing of leachate from taconite tailings is suggested using SPLP test methods and could include pH dependent leaching and liquid to solid (L/S) ratio dependent leaching as described by Jambeck and Greenwood (2007) and Kosson (2002). Data derived from these test methods may produce results more applicable to use of taconite tailings as fill material in contact with wet environments.