Abstract

Active and inactive mine sites are challenging to remediate because of their complexity and scale. Regulations meant to achieve environmental restoration at mine sites are equally challenging to apply for the same reasons. The goal of environmental restoration should be to restore contaminated mine sites, as closely as possible, to pre-mining conditions. Metalliferous mine sites in the Western United States are commonly located in hydrothermally altered and mineralized terrain in which pre-mining concentrations of metals were already anomalously high. Typically, those pre-mining concentrations were not measured, but sometimes they can be reconstructed using scientific inference.

Molycorp’s Questa molybdenum mine in the Red River Valley, northern New Mexico, is located near the margin of the Questa caldera in a highly mineralized region. The State of New Mexico requires that ground-water quality standards be met on closure unless it can be shown that potential contaminant concentrations were higher than the standards before mining. No ground water at the mine site had been chemically analyzed before mining. The aim of this investigation, in cooperation with the New Mexico Environment Department (NMED), is to infer the pre-mining ground-water quality by an examination of the geologic, hydrologic, and geochemical controls on ground-water quality in a nearby, or proximal, analog site in the Straight Creek drainage basin. Twenty-seven reports contain details of investigations on the geological, hydrological, and geochemical characteristics of the Red River Valley that are summarized in this report. These studies include mapping of surface mineralogy by Airborne Visible-Infrared Imaging Spectrometry (AVIRIS); compilations of historical surface- and ground- water quality data; synoptic/tracer studies with mass loading and temporal water-quality trends of the Red River; reaction-transport modeling of the Red River; environmental geology of the Red River Valley; lake-sediment chemistry; geomorphology and its effect on ground-water flow; geophysical studies on depth to ground-water table and depth to bedrock; bedrock fractures and their potential influence on ground-water flow; leaching studies of scars and waste-rock piles; mineralogy and mineral chemistry and their effect on ground-water quality; debris-flow hazards; hydrology and water balance for the Red River Valley; ground-water geochemistry of selected wells undisturbed by mining in the Red River Valley; and quality assurance and quality control of water analyses. Studies aimed specifically at the Straight Creek natural-analog site include electrical surveys; high-resolution seismic survey; age-dating with tritium/helium; water budget; ground-water hydrology and geochemistry; and comparison of mineralogy and lithology to that of the mine site.

The highly mineralized and hydrothermally altered volcanic rocks of the Red River Valley contain several percent pyrite in the quartz-sericite-pyrite (QSP) alteration zone, which weather naturally to acid-sulfate surface and ground waters that discharge to the Red River. Weathering of waste-rock piles containing pyrite also contributes acid water that eventually discharges into the Red River. These acid discharges are neutralized by circumneutral-pH, carbonate-buffered surface and ground waters of the Red River. The buffering capacity of the Red River, however, decreases from the town of Red River to the U.S. Geological Survey (USGS) gaging station near Questa. During short, but intense, storm events, the buffering capacity is exceeded and the river becomes acid from the rapid flushing of acidic materials from natural scar areas.

The lithology, mineralogy, elevation, and hydrology of the Straight Creek proximal analog site were found to closely approximate those of the mine site with the exception of the mine site’s Sulphur Gulch catchment. Sulphur Gulch contains three subcatchments—upper Sulphur Gulch, Blind Gulch, and Spring Gulch. Blind and Spring Gulches are largely propylitic zones with negligible pyrite mineralization, and although this lithology is not found at Straight Creek, it is found to the west of the Hansen Creek area where the geology extends to the Spring Gulch catchment. In lower Sulphur Gulch the erosion surface has cut deeper into the hydrothermal-alteration sequence than that found at Straight Creek, exposing the carbonate-fluorite alteration zone. Nevertheless, concentration limits from mineral solubilities and from the highest measured concentrations in ground water undisturbed by mining in the Red River Valley provide a basis for constraining the most likely concentrations for pre-mining conditions at the mine site.

The Straight Creek natural-analog site consists of acid surface drainage derived from the weathering of QSP altered rocks at the headwaters. The surface drainage of pH 2.5–3 disappears into a debris fan and becomes acid alluvial ground water of pH 3–4. Most dissolved constituents behave conservatively during downgradient ground-water transport and are diluted by side-canyon seepage waters and by Red River alluvial ground water at the toe of the debris fan. The main change in chemistry is a transition from strongly oxidizing conditions in the surface drainage (dissolved iron is in the Fe[III] oxidation state) to moderately reducing conditions (dissolved iron is in the Fe[II] oxidation state, without sulfate reduction) in the alluvial ground water. As a result of reduction, substantial amounts of copper are removed from solution and some chromium is removed. A substantial increase in silica concentration is observed between Straight Creek surface drainage and the alluvial ground water. Aluminum and silica appear to coprecipitate once the alluvial ground waters have reached a pH of 4. Bedrock ground waters are circumneutral pH (6–7.8) but also high in iron, manganese, and sulfate. Ferrous iron concentrations in bedrock are limited by siderite saturation, manganese by rhodochrosite saturation, calcium by calcite and gypsum saturation, barium by barite saturation, and fluoride by fluorite saturation.

A water-flow and sulfate-load balance was developed for the reach of the Red River from the Hottentot debris fan to the La Bobita campground. In spite of uncertainties in the data, these estimates indicate that ground waters in the Red River alluvium leaving the La Bobita area are 8 to 9 cubic feet per second and the sulfate flux is approximately 3,000 to 5,000 kilograms per day. Further fluxes of sulfate-laden waters enter the Red River alluvium between La Bobita and the Columbine Park area, but more field work and calculations would be needed to define the ground-water flow and sulfate fluxes in this area.

The origin and geochemical behavior of individual solutes in Straight Creek surface drainage, acid alluvial ground water, and neutral-pH bedrock ground water are compared to solute concentrations and their geochemical behavior in other catchments undisturbed by mining. The same trends apply generally with a few exceptions that can be explained by local mineralized zones that may be suboxic without substantial pyrite oxidation; that is, pyrite (if present) has not been exposed at the ground surface to intense weathering. The results show that acid ground waters derived from scar areas follow regular solute trends demonstrated by linear correlations of elements with sulfate based on the Straight Creek analog trend. The only mineral solubility controls that apply to oxidized acid ground water are hydrous ferric oxide saturation for ferric iron, gypsum for calcium, barite for barium, and aluminum and silica solubility control by an as-yet-unidentified phase when pH values reach 4. Circumneutral-pH anoxic ground water achieves mineral saturation for siderite, rhodochrosite, calcite, fluorite, Al(OH)₃, gypsum, and barite.

The results also show that a wide range of ground-water chemistry is possible over rather short distances. This wide range of solute concentration greatly complicates the objective of obtaining the pre-mining ground-water quality.

The geochemical behavior of solutes that were interpreted for Straight Creek, then applied to other undisturbed catchments, were then applied to catchments on the mine site for pre-mining conditions. The wide range of water chemistry that depends on changes in lithology and alteration, the presence or absence of scar, whether alluvial or bedrock ground water along with the uncertainty that accompanies the data led to the conclusion that a range of concentration for each solute of regulatory concern had to be applied, and that different solute concentrations had to be applied to bedrock than to alluvial ground waters, and that different concentrations had to be used for different parts of the same catchment if the lithology changed markedly. Different ranges of solute concentrations had to be estimated for each different watershed on the mine site because the geology changed markedly from one catchment to another. In spite of these complications, there is little doubt that under natural, pre-mining conditions, several elements of regulatory concern must have exceeded New Mexico ground-water quality standards by as much as tenfold in several locations both at the mine site and along the Red River Valley between the towns of Red River and Questa. The most common exceedances were for iron, manganese, sulfate, and fluoride. Manganese exceeded New Mexico ground-water standards by as much as 250 times. This exceedance is caused by the dissolution of rhodochrosite, MnCO₃, and manganiferous calcite, (Ca,Mn)CO₃, that are common accessory minerals from hydrothermal alteration.

Revised March 18, 2009

First posted May 2, 2008

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