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APPENDIX A Evaluate whether a release has transpired:One of the circumstances where passive methods are effective in collecting soil gas samples are in areas with low PASSIVE SOIL GAS METHOD permeability and high moisture setting(i.e.,soil in areas with shallow groundwater due to high water table)where active soil gas samples are more difficult to obtain. Both passive and active soil gas sampling and analyses may need to be performed to When the depth to groundwater is shallow(i.e.,within 5 ft bgs),the capillary fringe provide multiple lines of evidence to evaluate vapor intrusion exposure to organic may prevent sample collection by conventional active methods due to moisture vapors. Passive soil gas sampling is a qualitative method,which consists of the burial preventing soil gas Flow into the probe. Passive soil gas methods can be used to of an adsorbent material into the subsurface soil with subsequent retrieval and determine whether or not volatile compounds have contaminated the soil of current measurement of organic vapors passively amassed onto the absorbent material. Unlike or historic industrial operations along coastal areas with a high water table or to active soil gas sampling,which consists of the physical withdrawal of the soil gas from determine whether or not naphthalene is present in soil gas. Sampling for the subsurface,with passive sampling,there is no forced movement of soil gas into the naphthalene by active methods may be difficult due to absorption or plating of sampling vessel. Instead,as the vapors migrate,the sorbent acts as a sink for the naphthalene onto the tubing and collecting vessels. volatile organic compounds(VOCs)and semi-volatile organic compounds(SVOCs) found in soil gas, Additional advantages over active soil rias methods: Passive soil gas methods measure a mass(e.g.micrograms)of contaminant that has 1. Passive soil gas methods give a time-integrated measurement,and therefore, diffused onto an adsorbent media. Unlike active soil gas methods which yield reduce the uncertainty due to temporal variations. concentration data(e.g.,ug/m3 or ug/L)that can directly be compared to risk-based screening levels or used in predictive models,passive soil gas methods do not generate 2. Passive soil gas methods are frequently capable of detecting and reporting contaminant concentration data, While there are published methods and procedures to compounds present at very low levels not detected by active methods. generate concentration data from a passive sorbent-based sampler in ambient air,there are no published studies demonstrating the applicability of this method to soil gas. This 3. Simplicity of installation and retrieval: Unlike the active sampling method which is because the gas-phase diffusivity in the vadose zone is unknown for the calculation of involves withdrawal of the soil gas(forced extraction)from the subsurface,there a concentration from the adsorbed mass. There are a limited number of field studies is no forced movement of soil gas into the sampling vessel with passive methods. that attempt to calibrate the mass of contaminant collected by passive method to actual Therefore,the seal integrity of the vapor probe does not need to be ensured as in soil gas concentrations and for derivation of a method for this conversion. For this the case of the active method. reason,passive soil gas sampling and analysis is presently not considered to be applicable as a stand-alone method for determination of vapor intrusion risk. Provide Additional Line of Evidence for Vapor Intrusion:Attribution of chemicals in indoor air to vapor intrusion can be a complex and difficult task. Therefore,it is Although passive soil gas methods do not provide contaminant concentration data for important to use multiple lines of evidence to reach decisions based on professional use in predictive modeling and therefore is considered to be a qualitative tool,this judgment. In this regard,alternative sampling methods,in addition to the active soil gas method does have applications in plume characterization and vapor intrusion sampling procedures may be employed especially in certain circumstances as assessments. Potential uses and advantages of this method are as follows: described above. All available data from the multiple lines of evidence collected during the investigation(active and passive)should be used in determining whether vapor Locate contaminant hot spots for later sampling and plume characterization:Passive intrusion is occurring and whether there are potential public health concerns. soil gas can be a viable,cost-effective and simple screening tool to determine areas of contamination. The location of sources and subsurface plumes can be mapped, Attempts should be made to collect active soil gas data from specific locations after the particularly edges of plumes to determine whether contamination is near existing or presence and composition of subsurface chemical constituents in the vadose zone has future buildings. been established by the passive method. Active soil gas sample collection from areas with tighter lithology or in areas with shallow groundwater may need to be collected Locate Dreferentialpathways:Passive methods offer a quick and inexpensive means using a vacuum of less than 100-inches of water column at a rate less than or equal to to find vapor migration pathways into and around buildings and other sub-surface 100 mUmin using canisters to provide a more representative sample(McAlary et al, structures such as sewer and utility corridors. Other preferential pathways for 2009). migration such as geological features including potential spatial variability in the lithology can be identified by use of the passive sampling technique. 12/10/2009 -49- 12/10/2009 -50-