The potential for the release of volatile organic compounds (VOCs) to our natural environment is pervasive. However, the ability to accurately measure and predict VOC soil vapor concentrations is still limited. A polyethylene (PE) quantitative passive sampler using performance reference compounds and deployed via a hand driven probe is proposed as a solution. Additionally, a 1D diffusion mass transfer model was developed in MATLAB to predict the mass uptake into the PE sampler over time. The model was then implemented to investigate the effects of PE size and deployment time on the detection limit of BTEX compounds. Preliminary testing of the deployment probe indicates that a design to secure the PE around the outside of a driven rod must include a protective cover over the PE during insertion. A perforated pipe design is suggested. After deployment and recovery, the PE is extracted into water. The extraction water is then analyzed by direct aqueous injection to GC/FID. The minimum concentration detectable in soil vapors, by this PE passive sampling method, was determined to be the product of the target compound's air-water partitioning coefficient and the analytical detection limit. Assuming a 5 ng/mL analytical detection limit, the minimum soil vapor detection limit for toluene was approximately 1.25 mg/m 3. This limit would be similar for all BTEX compound and is above sub-slab vapor intrusion screening levels for the more toxic compounds such as benzene. This indicates that direct aqueous injection provides insufficient sensitivity and that purge and trap concentrations of VOCs is likely needed. It was also determined that a PE sampler, with dimensions as small as 5"x5/8"x0.0005", could theoretically reach 10 mg/m 3 sensitivity within a 1 h deployment time. This result suggests potential applications of the sampler for rapid and accurate site characterization of BTEX compounds.
Passive sampling has been used as a qualitative and semi-quantitative method in detecting volatile organic compound (VOCs) concentrations in soil vapors or water. Passive sampling for soil vapor takes an absorptive material and places it underground for a period of time to allow the VOCs to diffuse into the absorptive materials. In this report, I use low density polyethylene (PE) as the absorptive material and determine two key parameters for passive sampling: the PE-water partition coefficient (Kpew) and diffusion coefficient in PE (Dpe). These two parameters help passive sampling to transition from a qualitative method to a quantitative method. The report describes the steps used to carry out the experiments, gives the results for several specific VOCs, and makes an attempt to draw more general conclusions on how to estimate these two parameters according to some other well-known properties.
The feasibility of polyethylene (PE) as a passive sampler for quantitative analysis of volatile organic compounds (VOCs) was analysed in this work by means of a benchtop testing. A benchtop physical model was setup, which consisted of a jar of glass beads or sand, containing a known mass of toluene as the compound of concern (COC). A beaker of water was placed in the physical model as a second form of measurement of toluene concentration in the air. The concentration of toluene in the air of the physical model was measured using the PE passive sampler and compared to results found by measurement toluene in water in the beaker. The PE-inferred vapour concentrations were consistent with the measurements in the water. With benzene, toluene, ethylbenzene and o-xylene (BTEX) selected to be quantified in the actual soil, both the PE passive sampler and the water-based measurement showed inconsistency in contrast to previous experiments with glass beads and sand. This inconsistency could probably be due to the presence of biodegradation. Nonetheless, if proved consistent in future, PE passive sampling can also be used to estimate the concentrations of compounds based on molecular weight in absence of known literature values of required parameters.
Obtaining defensible and conservative estimates of the nature, extent and concentration of volatile organic compounds (VOCs) in the vadose zone is extremely important when formulating the conceptual model of the site, when performing risk assessments, for estimating contaminant mass, for assessing remedial alternatives, for selecting target areas for cleanup, and/or for making no further action/investigation decisions. Studies have shown that soil gas analytical results provide both a more complete indication of the VOCs present and a higher estimate of their respective concentrations in the vadose zone than the analysis of soil samples alone. For the past several years deep downhole (to 30+ meters) soil gas sampling and analysis has been performed by various consultants during remedial investigations (RIs) and remedial actions (RAs) of the vadose zone at McClellan Air Force Base. A number of these soil gas results have been confirmed by the concurrent collection and analysis (for VOCs) of soil samples (preserved by either refrigeration to 4 degrees centigrade or refrigeration combined with methanol preservation). The use of this VOC sampling and analysis strategy has resulted in the optimization of VOC sampling and analysis procedures, in a better understanding of the relationship between the concentration of VOCs in soil gas and in the soil, and in a more accurate and comprehensive conceptual model for VOC contamination in the vadose zone. The paper will present the methodologies used by the various consultants for sample collection, preservation, and in the analysis of soil gas and soil samples, present the results of a focused QC study on soil gas sampling and analysis, and discuss the correlation between the soil gas and soil matrix analytical results. The current and future strategies for the sampling, analysis, and estimation of VOCs in the vadose zone during RIs and RAs will also be presented.
This thesis documents a demonstration/validation of passive diffusive samplers for assessing soil vapor, indoor air and outdoor air concentrations of volatile organic compounds (VOCs) at sites with potential human health risks attributable to subsurface vapor intrusion to indoor air. The study was funded by the United States (U.S.) Department of Defense (DoD) and the U.S. Department of the Navy (DoN). The passive samplers tested included: SKC Ultra and Ultra II, Radiello®, Waterloo Membrane Sampler (WMS), Automated Thermal Desorption (ATD) tubes, and 3M OVM 3500. The program included laboratory testing under controlled conditions for 10 VOCs (including chlorinated ethenes, ethanes, and methanes, as well as aromatic and aliphatic hydrocarbons), spanning a range of properties and including some compounds expected to pose challenges (naphthalene, methyl ethyl ketone). Laboratory tests were performed under conditions of different temperature (17 to 30 oC), relative humidity (30 to 90 % RH), face velocity (0.014 to 0.41 m/s), concentration (1 to 100 parts per billion by volume [ppbv]) and sample duration (1 to 7 days). These conditions were selected to challenge the samplers across a range of conditions likely to be encountered in indoor and outdoor air field sampling programs. A second set of laboratory tests were also conducted at 1, 10 and 100 parts per million by volume (ppmv) to evaluate concentrations of interest for soil vapor monitoring using the same 10 VOCs and constant conditions (80% RH, 30 min exposure, 22 oC). Inter-laboratory testing was performed to assess the variability attributable to the differences between several laboratories used in this study. The program also included field testing of indoor air, outdoor air, sub-slab vapor and deeper soil vapor at several DoD facilities. Indoor and outdoor air samples were collected over durations of 3 to 7 days, and Summa canister samples were collected over the same durations as the passive samples for comparison. Subslab and soil vapor samples were collected with durations ranging from 10 min to 12 days, at depths of about 15 cm (immediately below floor slabs), 1.2 m and 3.7 m. Passive samplers were employed with uptake rates ranging from about 0.05 to almost 100 mL/min and analysis by both thermal desorption and solvent extraction. Mathematical modeling was performed to provide theoretical insight into the potential behavior of passive samplers in the subsurface, and to help select those with uptake rates that would minimize the risk of a negative bias from the starvation effect (which occurs when a passive sampler with a high uptake rate removes VOC vapors from the surroundings faster than they are replenished, resulting in biased concentrations). A flow-through cell apparatus was tested as an option for sampling existing sub-surface probes that are too small to accommodate a passive sampler or sampling a slip-stream of a high-velocity gas (e.g., vent-pipes of mitigation systems). The results of this demonstration show that all of the passive samplers provided data that met the performance criteria for accuracy and precision (relative percent difference less than 45 % for indoor air or 50% for soil vapor compared to conventional active samples and a coefficient of variation less than 30%) under some or most conditions. Exceptions were generally attributable to one or more of five possible causes: poor retention of analytes by the sorbent in the sampler; poor recovery of the analytes from the sorbent; starvation effects, uncertainty in the uptake rate for the specific combination of sampler/compound/conditions, or blank contamination. High (or positive) biases were less common than low biases, and attributed either to blank contamination, or to uncertainty in the uptake rates. Most of the passive samplers provided highly reproducible results throughout the demonstrations. This is encouraging because the accuracy can be established using occasional inter-method verification samples (e.g., conventional samples collected beside the passive samples for the same duration), and the field-calibrated uptake rates will be appropriate for other passive samples collected under similar conditions. Furthermore, this research demonstrated for the first time that passive samplers can be used to quantify soil vapor concentrations with accuracy and precision comparable to conventional methods. Passive samplers are generally easier to use than conventional methods (Summa canisters and active ATD tubes) and minimal training is required for most applications. A modest increase in effort is needed to select the appropriate sampler, sorbent and sample duration for the site-specific chemicals of concern and desired reporting limits compared to Summa canisters and EPA Method TO-15. As the number of samples in a given program increases, the initial cost of sampling design becomes a smaller fraction of the overall total cost, and the passive samplers gain a significant cost advantage because of the simplicity of the sampling protocols and reduced shipping charges.
