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The Fundamentals of Vapor Intrusion – Part 6. Petroleum versus Chlorinated Compounds, Part 2.
In last month’s Focus on the Environment, we discussed the differences between petroleum hydrocarbons (PHCs) and chlorinated volatile organic compounds (CVOCs) during vapor intrusion (VI). In this issue, we’ll continue the discussion…..
First of all, the elements chlorine, fluorine, bromine, and iodine are all halogens. During manufacturing, halogens may be added to volatile organic compounds (VOCs) to make them less flammable and slower to evaporate. We’ll follow the common practice of using the term “chlorinated VOC” (CVOC) as an all-inclusive term to cover various halogenated VOCs (HVOCs), that may or may not contain chlorine.
Picking up where we left off last month, we can make the following generalizations between CVOCs and PHCS as they pertain to VI:
CVOCs sink, PHCs float. That’s because the halogens are far heavier than the hydrogen atoms they replace. Consequently, the screened interval for a monitor well with significant PHCs (e.g., 0.1% of the solubility limit), should bracket the water level, so that any floating product will show up in samples. The good news is, that while it’s never good practice to connect separate aquifers (water-conducting layers) with a well screen, if a well does “bridge” two aquifers, light non-aqueous phase liquids (LNAPLs) generally can’t go down into the lower aquifer the way dense non-aqueous phase liquids (DNAPLs) can. On the other hand, PHCs in the pure, LNAPL state, can enter basements and sumps at flammable concentrations in areas of shallow groundwater. Vapor concentrations from PHCs are apt to be higher than from CVOCs, since pure PHCs form NAPLs and can float into a building, while CVOCs form DNAPLs, which tend to sink to the bottom of the aquifer. Notice, however, that in the dissolved state, CVOCs and PHCs neither sink nor float, and they go wherever groundwater goes.
CVOCs break down slowly, PHCs break down rapidly. This is ultimately the main difference between them, and the reason that PHC vapor intrusion (PVI) may be subject to different guidance. In rare cases, CVOCs might break down faster, but overall, PHCs break down much faster.
CVOCs break down better with low oxygen, PHCs break down better with high oxygen. Fire is really just rapid oxidation, and since CVOCs are designed to be nonflammable, it’s no surprise that they don’t readily oxidize in soil and groundwater. The more highly chlorinated compounds, especially tetrachloroethene (perchloroethylene or PCE) and trichloroethene (TCE), can persist for decades in high oxygen settings with very little degradation. Less highly chlorinated compounds, such as vinyl chloride, and of course PHCs, break down rapidly in the presence of oxygen, largely due to the activity of microorganisms. Unfortunately, this means that for PCE to break down into TCE, dichloroethene (DCE), vinyl chloride, and finally ethene, the conditions have to change or the contaminants have to migrate from low to high oxygen settings.
CVOCs don’t burn, PHCs and their byproducts (methane) can burn and explode. Gaseous combustion only occurs in the presence of oxygen when the gas concentration is between its lower explosive limit (LEL) and upper explosive limit (UEL). Even methane, which is highly flammable, has an LEL of approximately 5%. This is thousands of times higher than the concentrations generally associated with VI, so explosions from soil gas are rare, but they occur. CVOCs have little or no flammability, while PHCs are, obviously, explosive. But another, perhaps greater risk, is the explosion of methane. PHCs break down fastest in the presence of oxygen, but they can break down without it, and in the process generate methane. This is one of the reasons that when Cox-Colvin measures subslab soil gas, we take readings on LEL and oxygen. We’ve consistently seen somewhat reduced oxygen with zero LEL at CVOC sites, and near-zero oxygen with high LEL at PHC sites. Both CVOC and PHC vapors are typically mitigated with radon-type systems, but PHC vapors may require intrinsically safe motors and metal piping to reduce explosion risk.
CVOCs can travel long distances without breaking down, PHCs break down closer to their source. EPA’s 2013 Final Guidance for Assessing and Mitigating the Vapor Intrusion Pathway (EPA VI Guide) specifies a VI area of inclusion, or “footprint”, of 100 ft, laterally, from any detection of subsurface CVOCs – without regard to CVOCs versus PHCs. In contrast, EPA’s Guidance for Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites (EPA Petroleum VI Guide) states that, for PHCs, “All buildings directly over the contamination, whether LNAPL or the dissolved phase, are considered to be within the lateral inclusion zone”. But there is no lateral exclusion zone between a building and subsurface PHCs.
The American Association for Testing of Materials (ASTM) VI standard E2600-10 specifies a distance of 100 feet for “nonpetroleum hydrocarbons” (CVOCs), 100 feet for PHCs in the LNAPL state, and 30 feet for PHCs in the dissolved state. Interestingly, ASTM defines PHCs by their chemical behavior and readiness to break down, whereas EPA defines them by their association with fuels. In any case, the reason for smaller exclusion zones for PHCs boils down to their tendency to break down over short horizontal distances.
As to vertical distances, EPA’s VI guidance says that no VOCs can be excluded from consideration for VI, or “screened out”, on the basis of depth. The PVI guidance says that “In most situations, EPA finds the vertical separation distance of 5.4 feet from dissolved sources and 13.5 feet for LNAPL sources adequate to eliminate the potential for PVI”. This can lead to interesting arguments (or at least heated arguments!) for benzene, whose toxicity ranks it among the worst VI offenders, but whose origin is apt to be associated with fuels.
CVOCs are better for Johnson & Ettinger (J&E) modeling than PHCs. The J&E model has been the go-to model for estimating indoor vapor concentrations from subsurface sources, and it has long been available at EPA’s VI website. But the J&E model (or more accurately, EPA’s spreadsheet version of the J&E algorithm), does not take chemical breakdown into account. Subsequently, the standard J&E model is unpopular among the hydrocarbon crowd, as it typically overestimates PVI by orders of magnitude. Alternative models are available, but the favorite one is a modified form of the J&E model that includes a component for chemical breakdown.
In next month’s issue of Focus on the Environment, we’ll discuss the differences between VI guidance and OSHA for evaluating indoor air.