This project evaluated the performance of an ion trap mass spectrometer-membrane interface probe (ITMS-MIP) as a screening tool for obtaining a rapid and cost-effective estimate of subsurface contamination by volatile organic compounds (VOCs).
The ITMS-MIP system is mounted on a site characterization and analysis penetrometer system (SCAPS) cone penetrometer probe that can be pushed to the depth of interest. Subsurface vapor samples are collected through a permeable membrane into a helium carrier gas and transported aboveground for real-time analysis by an ITMS. The MIP can then be pushed to the next depth without retracting the probe. The probe is configured with the membrane located above a standard cone penetrometer soil classification module that contains cone and sleeve stress sensors. The multisensor configuration provides the real-time determination of soil classification and layering prior to the arrival of the membrane, thus enabling the interrogation of subsurface media for VOC contamination based on subsurface geology.
The demonstration was conducted at the Naval Air Station North Island, California, in the area of a light non-aqueous phase liquid plume that was also found to contain free product trichloroethene. VOC analysis results by the ITMS-MIP were generally greater than those from validation sample analysis by U.S. Environmental Protection Agency Method 8260. The ITMS-MIP correctly determined the presence of VOC contamination without false negatives; however, one false positive was noted. The vertical extent of the contamination was determined in 15 days, where 493 discrete samples were analyzed from 28 locations. The average number of analyses was 38 discrete sample interrogations per day at 2 to 4 locations, with a maximum push depth of greater than 15 meters. The cost of operating the SCAPS truck and 4-person crew was approximately $4,500 per day, and the unit cost per sample for collection and analysis using the ITMS-MIP was $228. When compared to estimated costs for drilling conventional monitoring wells at the 28 locations with offsite sample analysis, the cost of using the ITMS-MIP was 25 percent less. The ITMS-MIP was able to sample and analyze multiple samples per location and required one-fourth the time of the conventional method.
The ITMS-MIP performed better in saturated subsurface zones than in vadose zones and also performed better in sandy versus silty or clayey soils. Given these constraints, knowledge of subsurface geology would be helpful in determining whether the ITMS-MIP is appropriate for a specific site. The benefit of its use is the reduced time of sampling and analysis for delineating subsurface contamination. It should be emphasized that the ITMS-MIP system is a screening tool; however, the reduced number of validation samples (as opposed to a full conventional sampling and analysis plan) will result in a more rapid turn-around time and reduced analytical costs. (Project Completed - 2002)