Grantee Research Project Results

Final Report: Subsurface In Situ Volatile Organic Contaminant Sampling Using Multiple Sorbent Traps With Rapid On-Site/Off-Site Quantitative Speciation

EPA Contract Number: 68D01015
Title: Subsurface In Situ Volatile Organic Contaminant Sampling Using Multiple Sorbent Traps With Rapid On-Site/Off-Site Quantitative Speciation
Investigators: Dvorak, Michael
Small Business: Dakota Technologies Inc.
EPA Contact: Richards, April
Phase: I
Project Period: April 1, 2001 through September 1, 2001
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2001) RFA Text |  Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)

Description:

The major objectives of this Phase I project were the design and construction of a percussion- deliverable trap valve assembly (TVA) and a custom miniature gas chromatograph (GC). The percussion-deliverable system consists of the TVA and the patented heated Membrane Interface Probe (MIP) (Geoprobe Systems, U.S. Patent 5,639,956). Volatile organic compounds (VOCs) in the soil formation are vaporized at the surface of the hot (120? C) membrane and pass through it into a carrier gas stream. The VOCs then are adsorbed onto the traps of the TVA for later analysis with the miniature GC. This research builds on prior Dakota Technologies, Inc. (DTI) efforts in which GC detectors were adapted for direct-sensing operations in subsurface environments (i.e., "downhole") behind an MIP. Although these systems offer a great deal of utility, the ability to speciate individual compounds is not possible. Current focus on bioremediation technologies makes speciation of VOCs and their associated breakdown products crucial to understanding site contamination and behavior. Percussion delivery of an assembly of traps without fragile and expensive downhole detectors allows for addressing both cost and durability concerns without compromising speciation capability or data density requirements.

Positioning a unit capable of collecting multiple samples on sorbent traps downhole required DTI to: (1) develop analyte traps to preconcentrate VOCs passing through the MIP; (2) heat the traps to launch the analytes onto a GC column as a narrow band; and (3) construct the entire assembly so that it was small enough to fit into a Geoprobe push rod (1.2" in diameter) and durable enough to withstand percussion delivery. It also was necessary to integrate the TVA with a GC for analysis of the VOCs on the traps.

The TVA consists of small packed traps adapted to meet the size requirements imposed by the small-diameter pipe. The traps consist of short lengths (approximately 15 mm) of 1/16-inch tubing packed with Tenax resin attached to the TVA frame. Controlling the gas flow to the MIP membrane and the traps is critical for successful implementation of the TVA. The gas flow to each of the traps can be bypassed independently with the aid of six electronically activated three-way solenoid valves attached to the TVA frame. The control signals to the valves are derived from a remotely activated microprocessor located on the TVA frame. The probe's entire length is 4 feet, including the MIP, and is operated using a 100-foot umbilical cable.

The TVA is supported in the pipe with elastomer rings affixed at either end of the frame to shock-mount the assembly. A Lemo connector is placed at one end of the assembly to allow for quick detachment from the umbilical cable and reattachment to the miniature GC. The miniature GC consists of a custom column (RVM Scientific) in series with a photoionization detector. Control of the GC is conducted with in-house designed electronics and software. The miniature GC occupies a footprint approximately 1/8 the size of standard GCs. It is portable and rugged enough for field deployment.

Various laboratory studies were conducted to characterize the performance of the TVA and the miniature GC, both independently and in concert. The repeatability of retention times and the efficiency with which material is trapped and desorbed were assessed. The project culminated in successful field tests, the first at a site contaminated with chlorinated solvents and benzene, toluene, ethylbenzene, and xylene (BTEX); as well as the final test, percussion delivery of the TVA to a depth of 44 feet.

Summary/Accomplishments (Outputs/Outcomes):

The retention time and peak height stability were studied during an 8-hour period. The MIP, TVA, and miniature GC were operated in series at a constant helium flow rate of 13 mL/min. At the beginning of the experiment, a 1 ppb solution of BTEX was flowed over the MIP at a constant rate. The MIP was allowed to equilibrate to the setpoint temperature (140? C) before any valves were opened. Once equilibrated, carrier gas was directed through trap #1 for 2 minutes. After 2 minutes, trap #1 was bypassed, and the system was again allowed to equilibrate. This process was repeated for all remaining traps on the TVA. Upon completion of the trap loading cycle, the traps were sequentially heated and a chromatogram was generated for each trap. The entire procedure was repeated 8 hours later.

The mean and standard deviation of the retention times and peak heights for all BTEX species are given in Table 1 (note: m, p-xylene co-elute).

Table 1. Average retention times and peak heights.

Compound	Retention Time (sec)	Peak Heights (mV)
Benzene	103 ? 1.5	16.4 ? 1.7
Toluene	161 ? 1.6	19.5 ? 0.6
Ethylbenzene	257 ? 1.4	11.9 ? 2.2
m, p-xylene	272 ? 1.2	19.4 ? 2.8
o-xylene	309 ? 1.6	7.4 ? 1.0

The retention time and peak height precision for all compounds are better than the target figures of 3 and 10 percent, respectively. The target signal-to-noise ratio of 3 was attained or surpassed for all compounds (noise level: 2.4 mV, smallest signal level: 7.4 mV).

Two field tests were conducted with the TVA. First, the TVA was deployed via "static push" (i.e., no percussion) at a local site with both chlorinated solvent and fuel contamination. Each trap was opened over a 4-foot interval, while the MIP temperature was held above the boiling point of water. This procedure effectively integrated the entire depth range (ultimately 23 feet). Chromatograms subsequently were generated for each trap. Contamination was noted in all cases with the major contaminated region noted at or near the water table (approximately 9 feet below ground surface). Several days later, the TVA was percussion-delivered to a depth of 44 feet. The TVA subsequently was analyzed to determine whether the assembly had withstood percussion delivery. A 10 ppm solution of BTEX, tetrachloroethene, and trichloroethene was flowed over the MIP and adsorbed onto three of the six traps. The resulting chromatograms showed good reproducibility of the retention times and peak heights for each of the traps analyzed.

Conclusions:

All of the technical objectives as originally defined in the Phase I proposal were met, although several approaches were modified. The Phase I accomplishments provide a clear technical direction for the Phase II research, which also will better define the path for commercial exploitation. The Phase II proposal will provide complete details on the research plans and technical objectives. The general goals are to increase the number of traps on the TVA for more sample collection per push and to engineer long-term durability into the design. A different valving system will be investigated to determine whether the reconcentration trap module could be eliminated. A Nafion-drying module also will be made an integral part of the TVA frame. The Nafion module will eliminate any concerns about reduction of the chromatographic resolution and/or degradation of the Tenax resin from water. During the Phase I project, DTI developed a miniature GC for use with the TVA for analysis of VOCs adsorbed on the traps. Minor improvements to the miniature GC will be undertaken during the Phase II project, including increasing the analog-to-digital converter resolution from 12 bits to 16 bits and improving the shielding around the detector electrode circuitry.

Several important features of the TVA make it a desirable addition to current field laboratories.
First, because the TVA adsorbs analytes onto various traps downhole, errors due to loss of analyte in the transfer lines are completely eliminated.

Second, with several units on hand, an operator could continue sample collection with one TVA unit while analyzing previously delivered TVAs. Once a site has been thoroughly investigated, the operator will make a more informed decision on where to remove soil cores for offsite analysis, if deemed necessary. This would greatly reduce the cost and amount of investigation-derived waste by focusing on those areas with the greatest contamination.

Third and finally, the TVA and miniature GC will offer a turnkey system for those just beginning MIP work. In addition, those field laboratories currently using the MIP in conjunction with GCs will be quick to embrace the new technology because of the TVA system's relatively low initial investment cost. The ability to interface the TVA with the majority of popular field GCs (currently planned for Phase II work) would reduce the hardware investment required to add the TVA to current field laboratories. The main requirements for operation of the TVA are:
(1) MIP with controller, (2) gas-handling system, (3) 12-volt power supply, (4) computer, and
(5) the TVA. For those already operating MIPs with GCs, the only additional investment will be the 12-volt power supply, the TVA itself, and the interface module.

DTI possesses excellent contacts with all major providers of cone penetrometer services in the United States. In addition, DTI has an exclusive license from Geoprobe to use the MIP for applications in which the chemical sensing is performed downhole. These facts place DTI in an outstanding position for rapid commercial exploitation of the technology described in this report, resulting in better site characterization and ultimately reduced site management costs.

Supplemental Keywords:

gas chromatograph, sorbent traps, percussion delivery, in situ, volatile organic compounds, VOCs, site characterization, BTEX, PCE, TCE, membrane., RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Toxics, Engineering, Chemistry, Monitoring/Modeling, VOCs, Environmental Engineering, Environmental Monitoring, chromatograph, gas chromatography, analyzer, sorbent trap, Volatile Organic Compounds (VOCs)