In situ Assessment of the Transport and Microbial Consumption of Oxygen in Ground Water, Cape Cod, Massachusetts

By Richard L. Smith, John Karl Bohlke, Kinga M. Revesz, Tadashi Yoshinari, Paul B. Hatzinger, Cecilia T. Penarrieta, and Deborah A. Repert

ABSTRACT

Oxygen is a key ground-water constituent, controlling both the geochemistry and microbiology of an aquifer. Accordingly, aerobic respiration, the microbial metabolic process that consumes oxygen, is fundamentally important to the overall functioning of the aquifer. However, despite its significance, few studies have directly examined this process in the subsurface. This study has used several different approaches to investigate oxygen consumption on several different scales in parts of a large (> 5 kilometers) plume of dilute sewage contamination in a sand and gravel aquifer on Cape Cod, Mass. First, oxygen concentration profiles and stable isotope ratios were used to infer the net effect of aerobic respiration on the aquifer scale. Second, natural gradient tracer tests were used at an intermediate scale to measure in situ rates of aerobic respiration within different contours of the ground-water oxygen gradient. Third, two different types of laboratory incubations using aquifer core material, potential electron transport activity (ETS) and oxygen uptake activity, were used for small-scale examination of the process. The latter methods yield estimates of rates and kinetic parameters, which can be compared with the tracer test and isotope results. The sum of these approaches views the aquifer within the context of a subsurface ecosystem, integrating the combined effects of the hydrology, geochemistry and microbiology on the process of oxygen consumption.