This ESTCP project evaluated the performance of the SCAPS Thermal Desorption Sampler (TDS) to determine its range of applications, data quality, reliability, and cost effectiveness as a semi-quantitative screening tool for analyzing volatile organic compounds (VOCs) in soil. The TDS is advanced vertically into the subsurface using a 20-ton truck-mounted Cone Penetrometer Test (CPT) platform to depths up to 50 meters. The TDS secures approximately a 10 cu cm volume of soil in a cylindrical sample chamber where the soil is heated to approximately 110 degrees Celsius. Soil vapor analytes are transported from the TDS sample chamber to the surface via umbilical cable tubing and carrier gas for onsite analysis by an ion trap mass spectrometer (ITMS) located in the SCAPS truck and draft EPA Method 8265. The TDS has the capability to eject soil samples without removing the TDS probe with a conical tip and may be pushed deeper to another interrogation depth where the analysis process is repeated.
The TDS was deployed to depths of 60 feet below ground surface at five Department of Defense (DoD) sites where chlorinated VOCs and BTEX compounds were present. The TDS ITMS system results compared favorably to soil core verification samples collected adjacent to the TDS sample location, preserved with methanol in accordance with SW846 Method 5035, and analyzed at an offsite laboratory by EPA Method 8260B. The in situ TDS ITMS system results were also compared with soil samples that were extracted from adjacent locations, thermally desorbed ex situ, and analyzed using an onsite ITMS. There was good statistical correlation between in situ TDS results and conventional soil laboratory analysis by EPA Method 8260B. Independent evaluation of performance by the California EPA has resulted in formal certification (www.calepa.ca.gov/EnviroTech/).
The SCAPS TDS ITMS Sensor System may be used as a semi-quantitative in situ measurement device to conduct onsite near real-time interrogation of subsurface soil for VOC contamination. Data collected by the TDS ITMS system may also be used to optimize the placement and number of conventional wells needed for site characterization and long-term monitoring of installation restoration activities. The TDS ITMS system reduces costly and time-consuming laboratory analysis during initial site investigations, limits potential personnel exposure to contaminated media, and reduces the amount of investigation-derived waste normally generated during conventional drill and sample activities. Although direct comparison of the costs is difficult, the SCAPS TDS ITMS system is typically expected to provide greater than 28 percent savings when compared to conventional drill, sample, and offsite analysis techniques. In addition to site characterization capabilities, the SCAPS TDS ITMS system also provides site managers with a "certified technology" to monitor remediation activities in near real-time and provides additional savings derived by optimum placement of conventional monitoring wells.
The TDS System has some limitations that include CPT platform accessibility to site, matrix interferences and poor sample recovery in some soil conditions, umbilical transfer line contamination or ITMS overload due to direct sampling in highly contaminated conditions, and impaired TDS deployment in media with cemented sands, gravel, cobbles, boulders, and bedrock. (Project Completed - 2001)