U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C

Stable Isotope Composition of Dissolved O₂ Undergoing Respiration in a Ground-Water Contamination Gradient.

By Kinga Révész, John-Karl Böhlke, Richard L. Smith, and Tadashi Yoshinari.

ABSTRACT

Dissolved oxygen is a key ground-water constituent, controlling both the geochemistry and microbiology of an aquifer. Two methods were developed to analyze isotopes of dissolved O₂: the traditional method, which uses dual inlet mass spectrometry for analyzing CO₂ previously converted from O₂ , and a newly developed method, which uses continuous flow isotope-ratio mass spectrometry for analyzing directly the isotopes of dissolved O₂ . The major differences between the two methods are in the sample size and sample handling in the field and laboratory. Isotope analyses of dissolved O₂ were used to document the occurrence of microbial respiration near the boundary of an anoxic ground-water plume consisting of treated wastewater at the Massachusetts Military Reservation on Cape Cod. The upper boundary of the plume was a mixing zone between contaminated, anoxic plume water and uncontaminated, oxic local recharge water. Concentrations of dissolved oxygen decreased downward near the top of the plume and were inversely correlated with ¹⁸O. Apparent oxygen isotope fractionation factors (e) were -1 to -10 per mil (‰) in the absence of a dilution correction, whereas values as low as -20 ‰ were obtained by adjusting for the effects of dilution.