Use of Portable Oxygen and Carbon Dioxide Detectors to Screen Soil Gas for Subsurface Gasoline Contamination

Abstract

AbstractThe field screening of soil gas CO2 and O2 using portable instruments is shown to be useful for indirectly detecting and delineating subsurface gasoline contamination. Shallow (< 50 cm) soil gas surveys performed at a site of subsurface gasoline contamination in Connecticut indicated a good correlation between elevated CO2, depleted O2, and total hydrocarbon vapor anomalies. CO2 and O2 anomalies were also found to be spatially coincident with the area of known subsurface gasoline leakage. Soil gas CO2 levels ranged from 0.2 to 18.7 volume percent, whereas O2 levels ranged from 20.8 to 0.5 volume percent. The gases exhibited a nearly constant inverse correlation except near the source of contamination. In this area, CO2 generation from anaerobic degradation of hydrocarbons likely caused a decrease in the observed CO2 and O2 correlation. This suggests that the ratio of soil gas CO2 to O2 may be useful to delineate aerobic and anaerobic degradation zones, the latter being in the vicinity of highly contaminated areas. Periodic sampling of seven permanent soil gas probes over a 17‐month period indicated significant seasonal variations in CO2 and O2. When the soil was warmest during the summer, the CO2 reached a maximum and O2 reached a minimum. The opposite was observed during the winter when the soil achieved its minimum temperature. These observations most likely reflect changes in microbial acitivity in response to subsurface temperature variations. Further, hydrocarbon vapor levels were observed to decrease by several orders of magnitude to nondetectable levels during the winter months. Although CO2 and O2 anomalies were also diminished during the winter months, they were still readily detectable. These observations indicate that soil gas surveying for subsurface gasoline leakage may be enhanced by monitoring CO2 and O2.