Storage Cover Test Plot Study for Questa Tailings Facility , New Mexico

Molycorp Inc. owns and operates a large tailings facility located near the town of Questa, New Mexico. Over the last 33 years a total of nearly 100 million tons of tailings from the Questa Molybdenum Mine have been discharged into this facility covering a total surface area of about 260 ha (640 acres) (as of 1997). Molycorp has proposed a store-and-release cover using locally available alluvial material for final closure of the facility. A storage cover test plot study has been initiated to collect site-specific data of cover performance to be used for final design purposes. The test plot study has been designed to meet the following objectives (i) measure climatic conditions at the site; (ii) measure in-situ material properties (characteristic curves); and (iii) calibrate a soil-atmosphere model in order to predict net infiltration. The test plot study consists of two closed lysimeters with a 9” and 24” thick alluvial cover, respectively, an instrumented deep tailings profile (with a 9” thick alluvial cover and existing vegetation) and a fully automated weather station, which monitors all relevant climate parameters (including actual evapotranspiration). The lysimeter test plots were constructed by excavating the tailings, lowering the HDPE tank into the excavation and backfilling it with tailings and cover material. The lysimeters are freedraining and leachate from the test plot is collected and monitored using a tipping bucket. All test plots were instrumented with temperature/suction sensors and moisture content sensors to monitor moisture movement in the cover/tailings profile. This paper describes the design and installation of these test plots and summarizes monitoring results obtained to date. INTRODUCTION Molycorp Inc. owns and operates a large tailings facility located near the town of Questa, New Mexico (Figure 1). Over the last 33 years nearly 100 million tons of tailings from the Questa Molybdenum Mine have been discharged into this facility covering a total surface area of about 260 ha (640 acres) (as of 1997). The tailings originate from a hydrothermally altered molybdenum porphyry deposit of volcanic origin and are produced at the Questa mine, located some 5 miles to the east of the Questa tailings facility. Molycorp proposes to cover the tailings with locally available alluvial material (silty gravel) after final closure of the facility (RGC, 1998). This soil layer would prevent erosion of the tailings, provide a growth medium for re-vegetation and, in conjunction with the underlying tailings, represent a water storage (or “store-and-release”) cover that would reduce infiltration into the deeper tailings profile. Molycorp has implemented a storage cover test plot study at the Questa tailings facility to evaluate net infiltration through the proposed water storage cover (RGC, 2000a). The storage cover test plot study was initiated to collect site-specific data of cover performance for the purpose of final design of the water storage cover. Specifically, the test plot study was designed to meet the following objectives: • measure climatic conditions at the site; • measure in -situ material properties (characteristic curves); and • calibrate a soil-atmosphere model in order to predict net infiltration. It is important to recognize that the primary objective of the test plot study is not to measure cover fluxes per se but instead, to calibrate a soil-atmosphere model for known (measured) boundary conditions. The ultimate goal is to use the calibrated soil-atmosphere model to predict with a high degree of confidence the