Questa baseline and pre-mining ground-water quality investigation. 25. Summary of results and baseline and pre-mining ground-water geochemistry, Red River Valley, Taos County, New Mexico, 2001-2005

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 Gulc