Client:

Molycorp, Inc., Questa Division

Project Description:

Molycorp, Inc. (Molycorp) owns and operates a large tailings facility (240ha) located adjacent to the town of Questa, in northern New Mexico. The facility contains over 1 million tones of tailings from the Questa molybdenum mine, located 3.5 miles east of Questa. It is anticipated that this tailings facility will be operated for at least another 20 years. However, provisions must be made for the eventual reclamation and closure of these tailings impoundments. Under state regulations, Molycorp was required to submit a Revised Closure Plan for the Questa tailings facility by April 1998. One of the key aspects of the Closure Plan was to develop closure measures, which would provide long-term protection of groundwater quality.

In 1997, Molycorp retained Robertson GeoConsultants Inc. to study the potential impact of tailings seepage on the groundwater system and to develop closure measures to mitigate any impacts. A field investigation was carried out to characterize the tailings and the local groundwater system. Geochemical modeling was performed to estimate long-term pore water quality. A three dimensional groundwater flow model was developed to evaluate the fate of tailings seepage in local and regional aquifer systems ("telescopic approach"). Modeling of tailings seepage was also performed to predict the rate of dewatering (leachate flux) over time. The results of these analyses were combined to predict trends of metal and sulphate concentrations after closure in the most vulnerable, perched groundwater system beneath the tailings impoundments.

Robertson GeoConsultants Inc. continues to assist Molycorp in on-going hydrogeological and geochemical investigations required under New Mexico state laws.

Services provided:

• Design and Implementation of an Aquifer Characterization & Monitoring Program;
• Tailings Characterization and Geochemical Modeling (MINTEQA2);
• Development & Calibration of a 2D tailings seepage model (SEEP/W); and
• Development & Calibration of a 3D groundwater flow model (MODFLOW).

Papers:

• Wels, C., Shaw, S., and Royle, M.(2000): "A Case History Of Intrinsic Remediation Of Reactive Tailings Seepage For Questa Mine". Paper presented at the ICARD 2000 Conference, Proceedings Volume 1, pp. 441-458, May 2000, Denver, Colorado.