Publications—Scientific Investigation Report


By  Philip L. Verplanck, R. Blaine McCleskey, and D. Kirk Nordstrom

Scientific Investigations Report 2006-5028

This report is also available as a pdf.


    As part of a multi-year project to infer the pre-mining ground-water quality at  Molycorp’s Questa mine site, surface-water samples of the Red River , some of its tributaries, seeps, and snow samples were collected for analysis of inorganic solutes and of water and sulfate stable isotopes in selected samples. The primary aim of this study was to document diel, storm event, and seasonal variations in water chemistry for the Red River and similar variations in water chemistry for Straight Creek, a natural analog site similar in topography, hydrology, and geology to the mine site for inferring pre-mining water-quality conditions.

Red River water samples collected between 2000 and 2004 show that the largest variations in water chemistry occur during late summer rainstorms, often monsoonal in nature. Within hours, discharge of the Red River increased from 8 to 102 cubic feet per second and pH decreased from 7.80 to 4.83. The highest concentrations of metals (iron, aluminum, zinc, manganese) and sulfate also occur during such events. Low-pH and high-solute concentrations during rainstorm runoff are derived primarily from alteration “scar” areas of naturally high mineralization combined with steep topography that exposes continually altered rock because erosion is too rapid for vegetative growth.

The year 2002 was one of the driest on record, and Red River discharge reflected the low seasonal snow pack. No snowmelt peak appeared in the hydrograph record, and a late summer storm produced the highest flow for the year. Snowmelt was closer to normal during 2003 and demonstrated the dilution effect of snowmelt on water chemistry. Two diel sampling events were conducted for the Red River, one during low flow and the other during high flow, at two locations, at the Red River gaging station and just upstream from Molycorp’s mill site. No discernible diel trends were observed except for dissolved zinc and manganese at the upstream site during low flow.

Straight Creek drainage water was sampled periodically from 2001 to 2004 at the down stream end of surface drainage near the point at which it disappeared into the debris fan. This water has a minimal range in pH (2.7 to 3.2) but a substantial concentration range in many solutes; for example, sulfate concentrations varied from 525 to 2,660 mg/L. Many elements covary with sulfate suggesting that dilution is the primary control of the range in solute concentrations. A transect of water samples higher in the scar area were collected in October of 2003. They had a lower range in pH (2.44 to 3.05) and higher solute concentrations than those collected periodically from lower in the catchment. Water isotopes for the upper transect samples indicated slight evaporation, and in part, may account for the higher solute concentrations. Drainage waters also were collected from Hottentot, Junebug, Hansen, Little Hansen, and Goat Hill Gulch drainages. Most constituents from other scar drainage waters showed ranges of concentration similar to those of the Straight Creek waters. An exception was water collected from Goat Hill Gulch, which has some of the highest concentrations of any surface-water sample collected but also contained waste-rock leachates.





    Purpose and Scope 

    Physical Description of the Study Area 


    Climate and Vegetation 




    Sampling Sites 

    Water-Chemistry Sampling

    Laboratory Methods

    Quality Assurance and Quality Control 

Water Chemistry 

    Red River Water Chemistry, 2000-2003

    Red River Questa Ranger Station Gage Site Water Chemistry, 1965-2003 

    Red River Storm Events , 2001-2003 

    Red River Diel Variations, 2003

    Summary of Red River Water Quality

    Straight Creek Water Chemistry, 2001-2003

        Rare Earth Elements

        Stable Isotopes 

        Mineralogical Controls on Water Chemistry 

        Summary of Straight Creek Water Quality

    Seeps and Tributaries, 2001-2004

        Stable Isotopes



References Cited

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