Measuring the Surface air component of Soil Vapor Grab Samples Erik Tyrrell Chem 4101 Fall 2010

Soil vapor grab samples are often collected as a part of environmental assessment and remediation projects. These grab samples are collected by driving a sampling probe into the ground by either a fence post driver or a direct push drilling machine (Geoprobe), opening the bottom of the probe collecting a soil vapor sample in a vacuum canister.

Even with approved procedures involving sealing the top of the bore hole (often with hydrated bentonite). There is still a possibility for the contamination of the sample with surface air.

Problem: Every sample collection point is unique. In order to get an understanding of the extent of surface air contamination in a sample you need to look at that individual sample. Hypothesis: A useful measurement of the proportion of surface air in a soil vapor sample can be taken in a manner that does not make its cost prohibitive.

To determine the proportion of surface air in a soil vapor sample a tracer gas can be added to the surface air. The relative proportion of the tracer gas in the surface air above the sampling point and in the soil gas will give the relative proportion of surface air in the sample. A, B, and C in the above figure illustrate the creation of an enclosures to provide an environment for a stable tracer gas concentration

Procedure Drive the sampling probe to the correct depth for sample collection Prepare the probe for sampling by exposing the end of the probe, attaching tubing, placing a bentonite seal around the annulus of the probe rod and so forth. Prepare the enclosure and fill the enclosure with the tracer gas Measure the tracer gas concentration within the enclosure. Collect the soil vapor sample Connect the thermal conductivity detector to the port used for the collection of the soil vapor sample and measure the concentration of tracer gas Use of helium as a tracer gas Helium was selected for use as a tracer gas because it is inexpensive, it will not react with any of the other species in the vapor sample, and it can be measured economically with a hand held thermal conductivity detector.

Equipment for use of helium as a tracer gas Thermal conductivity detectors function by measuring the cooling of an electrical resistor by the gas passing over it. Helium has an exceptionally large thermal conductivity. Cooling of a resister in the meter can be related to the proportion of helium in the vapor sample

Evaluation of using Helium as a tracer gas with a thermal conductivity detector Advantages : Inexpensive, Helium detectors using thermal conductivity sensors can be rented for $120 per day or $360 per week Easy to use Fast Disadvantages: The vapor sample itself is not tested just the port from which the sample was drawn Manufacturer documents for the Mark Helium Detector Model 9822 claim the LOD is 25 parts per million, and the linear range extends all the way to pure helium gas. Banikowski, Kaczmar, and Hunt (2009) found that the machine under reported high concentrations. The method used for the Helium tracer gas collects a reading from the sampling port after the soil vapor sample has already been collected. Surface air contamination will increase with increasing withdrawal of soil vapor. This will mean that the helium tracer gas method will get a value that is likely higher than the average of the soil vapor sample. This may make the measurement unusable for some purposes.

Procedure Drive the sampling probe to the correct depth for sample collection Prepare the probe for sampling by exposing the end of the probe, attaching tubing, placing a bentonite seal around the annulus of the probe rod and so forth. Prepare the enclosure and fill the enclosure with the tracer gas Collect a vapor sample of the air within the enclosure. Collect the soil vapor sample Analyze both the soil vapor sample and the enclosure sample by GC MS. Add isobutylene standards to calibration standards for the GC-MS. Use of Isobutylene as a tracer gas Isobutylene may be used as a tracer gas for samples. The volatile organic components of soil vapor samples are typically analyzed by GC-MS. Isobutylene is not a frequent component of sol vapor. Levels of isobutylene tracer gas can be measured in the same process as the soil vapor.

Evaluation of using Isobutene as a tracer gas with a thermal conductivity detector Advantages : Accurate GCMS equipment has levels of detection for isobutene in the parts per billion by volume range Tracer gas analysis coincides with soil vapor analysis Disadvantages: More expensive No answer while in the field collecting the sample This method measures the tracer gas levels in the soil vapor sample by using the analysis that is presumably being done on the soil vapor already. This method however requires the analysis of a surface air sample for each soil vapor sample collected thus doubling the number of samples to analyze. The detection at parts per billion levels is excessive to use as a correction for soil vapor concentration of other volatile organic carbon species.

Conclusion: It is certainly possible to measure the quantity of surface air in a soil vapor grab sample. Knowing the quantity of the surface air leakage may sometimes justify the cost in equipment and labor of acquiring this quantity if it is done economically. Method reliability For both the Helium tracer gas method and the isobutene tracer gas method The uncertainty of the final value is a composite if the uncertainty in the surface air measurement and the uncertainty in the soil vapor measurement.

References Ashted technologies Mark Helium Detector Model 9822 product specifications, Banikowski, Jeffrey E.; Kaczmar, Swiatoslav W.; Hunt, John F. (2009), “Field, Validation of Helium as a Tracer Gas During Soil Vapor Sample Collection”, Soil & Sediment Contamination (2009), 18(3), Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air Second Edition, Compendium Method TO-15, Center for Environmental Research Information,Office of Research and Development, U.S. Environmental Protection Agency Cincinnati, OH 45268, January 1999 Guidance for Evaluating Soil Vapor Intrusion in the State of New York, October 2006, Mew Yourk State Department of Health Center for Environmental Health, Bureau of Environmental Exposure Investigation Reynolds, Peter A. (2007) “The Use of Tracer gas in Soil Vapor Intrusion Studies” Proceedings of the Annual International conference on Soils, Sediments, Water and Energy: vol. 12, Article 39 Skoog, Douglas A, Holler James F, Crouch, Stanley R, (2007) “Principles of Instrumental analysis sixth edition” Thompson Brooks/Cole 10 Davis Drive Belmont California Soil gas Sampling PRT system operation Geoprobe Systems incorporated.