In Situ Respirometry: Field Methods and Implications for Hydrocarbon Biodegradation in Subsurface Soils

Abstract

AbstractUsing soil ventilation to promote in situ biodegradation is a promising 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 changes in subsurface O2 and/or CO2 levels is a convenient method for monitoring this process because repetitive, noninvasive measurements can be made. In this study, two rapid field procedures for gas analysis, using either liquid‐ or solid‐phase absorption of O2 and CO2, were compared and the implications of these gas measurements for evaluating in situ hydrocarbon biodegradation considered. Air samples were taken from 5 vadose zone piezometers in a gasoline‐contaminated soil under treatment by a soil vapor extraction system and one piezometer in a noncontaminated background area. Similar rates and levels of O2 consumption and CO2 production were obtained by both techniques during the 20‐d study; thus, the method of choice will depend on factors such as cost, air sample volume limitations, or the need for contaminant vapor analyses in the field. While these techniques were good assays for relative microbial activity levels, significant discrepancies between predicted and measured CO2 levels raised questions concerning the validity of basing hydrocarbon biodegradation estimates solely on O2 consumption data. To improve the reliability of in situ fuel biodegradation estimates based on O2 measurements, the relationship(s) between metabolic activities of microbial populations in hydrocarbon‐contaminated soils, and changes in subsurface O2 and CO2 levels should be clarified.