Analyses of soil gas compositions and stable and radioactive carbon isotopes in the vadose zone above an alluvial aquifer were conducted at an organic solvent disposal site in southeast Phoenix, AZ. The study investigated the source and movement of carbon dioxide above a plume of organic solvent contamination. Two soil gas monitor wells, each screened and grouted at four discrete depths above the water table, provided sampling access. One well penetrated the uncontaminated vadose zone, the other penetrated a contaminated area now covered with asphalt. Carbon dioxide concentrations in the uncontaminated area range from 1.45% at 8 ft to 3% at 19 ft below land surface. Isotopic evidence suggests root respiration and minor oxidation of organic matter as CO2 sources at this site. Carbon dioxide in soil gas samples from the contaminated area exceeded 15% while O2 levels were as low as 1%. Carbon dioxide concentrations and carbon isotope values are consistent with in situ aerobic biodegradation of the organic pollutants. (Copyright (c) 1990 American Chemical Society.)