IN-SITU RESPIROMETRY - FIELD METHODS AND IMPLICATIONS FOR HYDROCARBONBIODEGRADATION IN SUBSURFACE SOILS

Using soil ventilation to promote in situ biodegradation is a promisin g technology for remediation of unsaturated, hydrocarbon-contaminated, subsurface soils. Evaluating in situ hydrocarbon biodegradation rates and levels is a critical part of this approach and determining change s in subsurface O-2 and/or CO2 levels is a convenient method for monit oring this process because repetitive, noninvasive measurements can be made. In this study, two rapid held procedures for gas analysis, usin g either liquid- or solid-phase absorption of O-2 and CO2, were compar ed and the implications of these gas measurements for evaluating in si tu hydrocarbon biodegradation considered. Air samples were taken from 5 vadose zone piezometers in a gasoline-contaminated soil under treatm ent by a soil vapor extraction system and one piezometer in a nonconta minated background area. Similar rates and levels of O-2 consumption a nd CO2 production were obtained by both techniques during the 20-d stu dy; thus, the method of choice will depend on factors such as cost, ai r sample volume limitations, or the need for contaminant vapor analyse s in the field. While these techniques were good assays for relative m icrobial activity levels, significant discrepancies between predicted and measured CO2 levels raised questions concerning the validity of ba sing hydrocarbon biodegradation estimates solely on O-2 consumption da ta. To improve the reliability of in situ fuel biodegradation estimate s based on O-2 measurements, the relationship(s) between metabolic act ivities of microbial populations in hydrocarbon-contaminated soils, an d changes in subsurface O-2 and CO2 levels should be clarified.