The U.S. Geological Survey, in cooperation with the New Mexico Environment Department, is investigating the pre-mining ground-water chemistry at the Molycorp molybdenum mine in the Red River Valley, New Mexico. The primary approach is to determine the processes controlling ground-water chemistry at an unmined, off-site but proximal analog. The Straight Creek catchment, chosen for this purpose, consists of the same Tertiary-age quartz-sericite-pyrite altered andesite and rhyolitic volcanics as the mine site. Straight Creek is about 5 kilometers east of the eastern boundary of the mine site. Both Straight Creek and the mine site are at approximately the same altitude, face south, and have the same climatic conditions.
Thirteen wells in the proximal analog
drainage catchment were sampled for ground-water chemistry. Eleven wells were
installed for this study and two existing wells at the Advanced Waste-Water
Treatment (AWWT) facility were included in this study. Eight wells were sampled
outside the Straight Creek catchment: one each in the Hansen, Hottentot, and La
Bobita debris fans, four in a well cluster in upper Capulin Canyon (three in
alluvial deposits and one in bedrock), and an existing well at the U.S. Forest
Service Questa Ranger Station in Red River alluvial deposits. Two surface waters
from the Hansen Creek catchment and two from the Hottentot drainage catchment
also were sampled for comparison to ground-water compositions. In this report,
these samples are evaluated to determine if the geochemical interpretations from
the Straight Creek ground-water geochemistry could be extended to other ground
waters in the
Total-recoverable major cations and trace metals and dissolved major cations, selected trace metals, anions, alkalinity; and iron-redox species were determined for all surface- and ground-water samples. Rare-earth elements and low-level As, Bi, Mo, Rb, Re, Sb, Se, Te, Th, U, Tl, V, W, Y, and Zr were determined on selected samples. Dissolved organic carbon (DOC), mercury, sulfate stable isotope composition (δ34S and δ18O of sulfate), stable isotope composition of water (δ2H and δ18O of water) were measured for selected samples. Chlorofluorocarbons (CFC) and 3He and 3H were measured for age dating on selected samples.
Linear regressions from the Straight Creek ground-water data were used to compare ground-water chemistry trends in non-Straight Creek ground waters with Straight Creek alluvial ground-water chemistry dilution trends. Most of the solute trends for the ground waters are similar to those for Straight Creek but there are some notable exceptions. In lithologies that contain substantial pyrite mineralization, acid waters form with similar chemistries to those in Straight Creek and all the waters tend to be calcium-sulfate type. Hottentot ground waters contain substantially lower calcium concentrations relative to those in Straight Creek. This anomaly results from the exposure of rhyolite porphyry in the Hottentot scar and weathering zone. The rhyolite contains less calcium than the altered andesites and tuffs in the Straight Creek catchment and probably does not have the abundant gypsum and calcite. The Hansen ground waters have reached gypsum saturation and have similar calcium, magnesium, and beryllium concentrations as Straight Creek ground waters but have lower concentrations of fluoride, manganese, zinc, cobalt, nickel, copper, and lithium. Lower concentrations of elements related to mineralization at Hansen likely reflect the more distal location of Hansen with respect to intrusive centers that provided the heat source for hydrothermal alteration.
The other ground water with water
chemistry trends that are outside the Straight Creek trends was from an alluvial
well from Capulin Canyon (CC2A). Although it had pH values near 6.0 and most
major ions similar to the other
Saturation indices indicate that solubility constraints continue to provide upper limits on some solute concentrations. Siderite, ferrihydrite, calcite, gypsum, rhodochrosite, and barite provide limits for concentrations of Fe(II), Fe(III), Ca, Mn, and Ba, respectively. Beryllium concentrations may be subject to an upper concentration limit by the solubility of Be(OH)2 but these concentrations probably are not reached in the ground waters.
Ground-water isotopic data were
consistent with the meteoric water line estimated for precipitation in the Red
River Valley, indicating that all the ground waters examined in this study were
meteoric, recent in origin, and showed no substantial indication of evaporation.
Tritium-helium-3 and chlorofluorocarbon (CFC) age dating were partially
successful. Generally, dates were consistent with location and depth of wells.
Two samples had good agreement between CFC dates and tritium-helium dates,
whereas a third reflected either substantial mixing with younger or older waters
or complications arising from excess helium-4. The well at La Bobita appeared to
contain a large component of modern water, most likely as a result of mixing
with water from
Explanation of Abbreviations
Purpose and Scope
Physical Description of Study Area
Climate and Vegetation
Water Sample Collection
Ground Water Collection
Surface Water Collection
Quality Assurance and Quality Control
Redox Potentials and Iron Chemistry
Calcium Chemistry and Solubilities of Gypsum, Calcite, and Fluorite
Gypsum and Calcite Solubilities
Fluoride Chemistry and Fluorite Solubility
Alkali Metal Chemistry
Trace Element Chemistry
Dissolved Organic Carbon and Hydrogen Sulfide
Chlorofluorocarbons, Dissolved Gases, and Tritium
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