Methane-concentration and methane-isotope data for ground water and soil gas in the Animas River valley, Colorado and New Mexico, 1990-91

Abstract

Important shallow water supplies in alluvium and Tertiary sedimentary rocks overlying Cretaceous gas-bearing formations in the San Juan Basin in Colorado and New Mexico locally are contaminated by methane. Recent development of coal-bed methane from the Cretaceous Fruitland Formation has caused public concern about the possibility of increasing contamination of shallow ground-water supplies. In July 1990, the U.S. Geological Survey, in cooperation with the Colorado Oil and Gas Conservation Commission, La Plata County, and the Southern Ute Tribal Council, Colorado, began a study in the Animas River Valley between Durango, Colorado, and Aztec, New Mexico. The purpose of this study is to map the occurrence of methane in shallow ground water and to identify possible sources of and migration pathways for methane. This report addresses the data-collection phase of the study. This report presents data collected by the U.S. Geological Survey mostly during August 1990-May 1991. These data consist of: (1) Records of water wells and gas wells; (2) water-quality data, including methane concentrations, from wells and springs in the Animas River Valley; (3) concentrations of methane in soil gas near water wells and springs and adjacent to gas-well casings within about one-half mile of the Animas River Valley; and (4) molecular composition and methane-isotope data for gas samples collected from ground-water headspace, soil, and gas-well production casings. Water-quality data consist of onsite and laboratory measurements of concentrations of methane and other dissolved constituents in water samples collected from wells and springs. Seventy-one wells and 1 spring in Colorado were sampled, and 132 wells and 1 spring in New Mexico were sampled. Onsite measurements were made for determination of specific conductance, pH, temperature, and dissolvedmethane concentrations. At 68 selected sites, additional water samples were collected for onsite determination of alkalinity and laboratory determinations of calcium, magnesium, sodium, potassium, sulfate, chloride, bromide, silica, iron, and manganese concentrations. Onsite methaneconcentration measurements of soil-gas and water samples were made with a portable organic gas analyzer equipped with a chromatographic column. Dissolved-methane concentrations were determined by measuring the concentration of methane in headspace gas. Soil-gas concentrations were measured near 192 ground-water sites and adjacent to the casings of 352 gas wells. Gas was collected from 16 water samples, from 3 soil seeps in open fields, from 10 soil columns adjacent to gas-well casings and 1 soil column adjacent to a cathodic-protection well, and from 30 gas wells. These samples were analyzed for molecular composition and carbon-13 content of methane. Seventeen of these samples were selected for analysis of deuterium content of methane.