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Albuquerque NM 87112,

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505-293-5945

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N-8581 Vestbygd

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47-76931751 

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Walder, I..F., ²Lichtner, P. and Schuster, P.P. Presented at International Conference on Applied Mineralogy, Gottingen, Germany, 2000

The main environmental problem facing new, operating and abandoned mines is acid rock drainage (ARD).  ARD is caused by sulfides oxidizing and releasing hydrogen ions.   This in turn increases the solubility of most metals.  This paper will discuss: 1) the steps used in characterizing the ARD potential of mine waste systems, and various remediation and immobilization options, and 2) the use of reactive transport modeling to predict possible seepage water quality and to establish a framework for selecting and designing the immobilization option on a particular mine waste system.

    There are may options for remediation and immobilization of mine wastes.  They are used to reduce eolian transport, oxidation rates of sulfides, and transport of water-soluble constituents.  In many cases, seepage treatments are also necessary as part of remediation or in addition to immobilization.  However, prior to selecting and designing an option, a thorough understanding of the mine waste system is required.  This may comprise different analytical testing, such as: mineralogy, chemical analysis (whole rock and acid base accounting), leach tests, reactivity tests, water quality analysis, gas analysis, and water and air permeability.  Once the necessary data has been obtained, modeling can be performed to further understand the geochemical and hydrological processes taking place.  The results can then be used to select and design appropriate remediation/immobilization methods.  Methods include: covers (synthetic or natural soil), waste mixing, chemical additives (neutralization/ inoculation), submerging in water (fjords, lakes, elevating water level), and bactericide.  Seepage treatments include: constructed wetlands, limestone or anoxic limestone drainage, and waste-water treatment systems.

    The reactive transport modeling program, MPATH (Lichtner, 1992), has been used to evaluate the chemical reactions taking place along flow paths within sulfide containing mine-waste material varying in mineralogy and grain size in a semiarid climate.  The flow path in waste material consisting of waste rock occurs both as matrix flow, through particles (boulders), and macro flow, between particles.  Matrix flow is a low-velocity flow, while macro flow is of high velocity.  This results in different reaction times for the two flow paths, and depending upon the primary and secondary mineralogy, different chemistries.  During precipitation events, rainfalls, the macro flow may flush out tertiary minerals produced from weathering and oxidation of the primary and secondary minerals, thus leading to poor water quality.   The matrix flow, however, may result in seeps with low flow rates of relatively good water quality between rainfalls.  Therefore, the immobilization design selected, a soil cover, should take this into account.  If the matrix flow results in good water quality, the cover should be constructed to average out peak fluctuations, thus avoiding heavy flushing.  Nutrients may be added to the cover, such that sulfate reducing bacteria can reduce sulfate to sulfide and, thereby, precipitate sulfide minerals within the mine-waste material.

The Mansfield Canyon Mining District, Coronado National Forest, Arizona, was actively mined in the late 1800’s and early 1900’s, with minor activity until 1953. The main products were lead, silver, copper, zinc, and gold occurring in polymetallic quartz veins. The mining district contains numerous adits and waste dumps, some of which generate metal-loaded acid.  Samples of seeps, waste dumps and bedrock have been collected for various analyses: metal concentrations, Acid Base Accounting (ABA), and water quality to evaluate the extent of oxidation and metal release from the start of mining until present, the potential impact, and reclamation options.  Although the waste-rock piles within the district are small (2,000-10,000 tons), they have a high sulfide content with little or no neutralizing capacity.  It is planned, therefore, to move the waste piles either to a constructed repository site within the canyon or to a copper leach facility to be used as an acid generator.