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APPENDIX C concentration of helium in the sample divided by the concentration of helium in the shroud provides a measure of the proportion of the sample attributable to leakage. QUANTITATIVE LEAK TESTING USING SHROUD AND HELIUM Small leaks may be acceptable,as long as the magnitude of the leak is small compared To ensure valid soil gas samples are collected with no breakthroughof air down the to other unavoidable sources of bias and variability in sampling and analytical data. probe or through leaks in the sampling train,a[racer compound be lied the the Laboratories typically assign a relative percent difference of+/-25%for duplicate Y PP samples,so a leak that comprises less than 5%of the sample is relatively insignificant. base of the probe and near all connections in tracer sampling train. Seal integrity is then confirmed by analyzing collected soil gas samples for the tracer. Gaseous compounds An example sample train for the use of helium as a tracer is shown on Figure C-1. such as helium and sulfur hexafluoride(SF6)using shrouds or liquid compounds such Helium is released into the shroud and a hand-held helium detector is used to monitor as pentane,propanol,or Freons applied to substrates such as towels are commonly and maintain a reasonably steady concentration of helium within the shroud. The used. Gaseous tracers such as helium are considered to be more difficult to use, helium concentration in the shroud should be at least 10%or two orders of magnitude especially to cover leakage in the sampling train. However,helium offers a nice higher than the reporting limit of the laboratory helium analysis or field meter used to • advantage in that leakage can be quantified and is readily measured on site with a field analyze the sample,which will provide sufficient resolution against reporting limits. The meter or by a stationary laboratory after the soil gas sample is collected.Volatile liquid concentration in the shroud will decay over time after the initial helium dose,depending tracers offer logistical simplicity but are more qualitative. on the irregularity of the ground surface,wind speed,etc.,so the concentration in the Volatile leak check compounds(e.g.,Freons)are acceptable for use;however,if a leak shroud should be monitored and adjusted as needed to maintain a reasonable steady state. The monitored helium concentrations in the shroud should be recortled, test compound is kdetected compo in soil vapor samples,the reported contaminant concentration(s)should be considered biased low due to ambient air dilution. Helium screening can be performed in the field by drawing soil gas into a Tedlar°bag Additionally,a small contribution from leak-test compound can necessitate the need for via a lung-box and screening the contents of the Tedlar bag with a helium meter. This sample dilutions prior to analysis,and the resulting reporting limits may no longer meet can be performed in combination with other field screening for gases such as oxygen, the project DQO. In most instances,if the gas probes are properly grouted,air leakage carbon dioxide or methane. This method will identify leaks in real-time,which will allow at the ground surface and probe interface should not occur. Contingency planning is corrections to fittings or seals to be made prior to collection of a sample for laboratory recommended in the event that breakthrough occurs and liquid tracer compounds are analysis. A detail description of a similar Tedlar bag/lung box setup is also described detected in vapor samples. If a leak is detected,a replacement probe can be installed in SW 846 Method 0040(USEPA, 1996). at least 5-feet from the original location,if the problem cannot be corrected. However, probe relocation may not always be feasible due to underground utilities clearance and Helium screening can also be performed using a laboratory analysis of the contents of a access restriction. Under these circumstances the shrouding method that utilizes a SUMMA canister collected under the shroud. This analysis may provide additional tracer gas such as helium should be used. The advantage of helium is that it does not confidence because the helium concentration is measuretl for the identical parcel of gas interfere with analytical instrumentations;therefore,the reporting limits of chemical for which VOC concentrations are measured.For this option,conduct purging and field constituents in question are not compromised by helium. screening,then close the valve between the SUMMA*canister and the lung-box(see An example standard operating procedure SOP Figure B-1),and open the valve on the SUMMA'canister,refresh the helium supply P P g P (SOP)provided by the courtesy of within the shroud and monitor/sustain it during the sample collection interval.The Geosyntec Consultant is described below with a description and diagram of a shroud helium concentration is measured in the canister in the analytical laboratory by modified • setup using helium as a tracer gas. A portable hand held mass spectrometer for helium EPA Method TO-3 following analysis.of VOCs by EPA Method TO-15. Note that if is usedas adeak detection device,which can be rented on a daily basis fora reasonable cost. helium screening is done exclusively from a canister at a stationary laboratory and there is a significant leak,it will not be identified until after the sample has been collected and Helium Tracer Testing: analyzed. Therefore,field screening prior to laboratory confirmation is recommended.It should be noted when using this method that a helium concentration indicating>5% Helium can be used to assess the potential for leaks in the sample train and probe leakage should be clearly noted and discussed in the data validation process. annulus by positioning an enclosure or"shroud"over the probe and sampling train, filling it with a measured amount of helium,and measuring the concentration of helium in samples subsequently drawn from the probe. One of the benefits of this method is that it is reasonably quantitative. The 12/10/2009 -57- 12/10/2009 _58-