A study was performed to determine the source of low concentrations of volatile organic compounds (VOCs) detected in groundwater samples at a solid waste management facility. The affected wells were identified as hydraulically upgradient of an old unlined facility, but downgradient of a new clay-lined landfill. These monitoring wells are close to both sites. Subsurface landfill gas migration was identified after a low permeability cap was installed on the older site. Subsurface gas pressure was monitored to identify horizontal landfill gas migration. Monitoring well headspace gases were evaluated to identify depressed oxygen concentrations and methane because of landfill gas migration into the well. Monitoring well headspace gas VOC concentrations were compared to groundwater VOC concentrations to determine the direction of phase transfer. A ratio above 1.0 of the observed well headspace gas concentration of a VOC to the concentration that would be in equilibrium with the groundwater concentration indicates gas-to-water phase transfer within the well. For the major gas-phase and aqueous-phase VOC, cis-1,2-dichloroethene, gas-to-water phase transfer is clearly indicated from the data for two of the four wells. Fifteen other VOCs were detected in monitoring well headspace gases but not in groundwater samples from the four wells studied. Only one compound in one well was detected in the groundwater sample but not in the headspace gases, and only one compound in one well was detected in both matrices at concentrations that suggested water-to-gas phase transfer. This study suggests that if landfill gas is suspected as the source of detected VOCs, monitoring well construction and stratigraphy are important considerations when attempting to differentiate between groundwater contamination by landfill gas and contamination from other sources.

This paper points out the importance of quality assurance in soil gas surveys through the implementation of quality control measures. A soil gas survey was undertaken at the Cortese Landfill to assess vapor migration. Soil gas samples were analyzed from 54 locations for 15 volatile organic compounds by gas chromatograph (GC) methods. Quality control measures included GC calibration, replicate, spike and blank samples and analysis of split samples by an off site analytical lab. Acetone was detected at many of the soil gas sample locations at unexplainable concentrations relative to concentrations observed in groundwater. A second GC, equipped with a different column, was used to confirm presence of acetone, but at concentrations lower than those estimated using the first column. This indicated that coeluting compounds may have caused false positive results (misidentifications) in the earlier data. This approach to verification of field analytical data quality assurance proved to be beneficial in preventing costly additional sampling and analysis.