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

The field screening of soil gas CO 2 and O 2 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 CO 2 , depleted O 2 , and total hydrocarbon vapor anomalies. CO 2 and O 2 anomalies were also found to be spatially coincident with the area of known subsurface gasoline leakage. Soil gas CO 2 levels ranged from 0.2 to 18.7 volume percent, whereas O 2 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, CO 2 generation from anaerobic degradation of hydrocarbons likely caused a decrease in the observed CO 2 and O 2 correlation. This suggests that the ratio of soil gas CO 2 to O 2 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 CO 2 and O 2 . When the soil was warmest during the summer, the CO 2 reached a maximum and O 2 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 CO 2 and O 2 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 CO 2 and O 2 .