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Vapor Intrusion Fundamentals 39 – Henry’s Constants

By: Martin (Mort) Schmidt, CPG

In the 19th century, English chemist William Henry experimented with the solution of gases in water.  He found that vapors escaping from water exerted varying amounts of pressure, depending on the chemical, its concentration, and the water temperature.  If you go to the chemical properties (Chem Props) sheet on US EPA’s VISL Calculator, you’ll see several columns with Henry’s constants for each chemical.  The value in column L provides Henry’s constants in terms of pressure, at the default groundwater temperature of 25 degrees Celsius (77 degrees Fahrenheit).  That’s the groundwater temperature in Miami, Florida, but according to the Ohio EPA’s VI guidance, the appropriate temperature for Ohio is 11 degrees Celsius (52 degrees F), which results in much lower VI.  Groundwater temperature normally reflects the average annual temperature, so one normally uses the same setting any time of year.

The VISL calculator uses groundwater temperature, solubility, and other factors to calculate Henry’s constant for each chemical, and uses it to calculate the groundwater VISL.  You’ll notice that when you change the temperature setting in the VISL sheet, the groundwater VISLs change, but the soil gas VISLs do not.  That’s because due to Henry’s constants, vapors emanate from water at a higher concentration if it’s warm, but subslab vapor attenuation, which is mostly the dilution of soil gas by indoor air, has no direct link to temperature.

Several other columns in the spreadsheet are devoted to Henry’s constants, but the values in Column R are the most useful, as they provide Henry’s constants at the desired temperature, as entered on the VISL page.  They also provide Henry’s constants in the dimensionless form, which are more useful than pressure values, for our purposes.  The dimensionless Henry’s constant is the ratio of a chemical’s concentration in air, relative to its concentration in adjacent water, assuming equilibrium.  For example, if you set the groundwater temperature on the VISL sheet to 11 degrees C, the dimensionless Henry’s Constant used to calculate the groundwater VISL (column R) for benzene is 2.27E-01 (0.227).  So if a liter of water at 11 degrees C contains1,000,000 molecules of benzene, a liter of air directly above it would contain 227,000 molecules of benzene.  Therefore, it seems that you couldn’t get high vapor concentrations from low-level groundwater contamination, but there’s a catch – water concentrations are typically reported in micrograms/liter (ug/L), while air concentrations are reported in micrograms per cubic meter (ug/m3), and 1 m3 equals 1,000 liters.  Consequently, a benzene concentration in groundwater at the maximum contaminant level (MCL) of 5 ug/L, 11 degrees C, would have a vapor concentration in adjacent soil gas of 5 x 0.227 x 1,000, which equals 1,135 ug/m3.  Of course, those vapors would undergo further attenuation on their way to indoor air, but it’s possible to get high soil-gas concentrations from relatively low groundwater concentrations.  This is a concern in areas with shallow groundwater (<5 feet), because contaminated groundwater in the capillary zone can potentially seep into the building and cause high vapor concentrations.

The VISL calculator applies Henry’s constants and does the groundwater-to-soil-gas calculations  for us, but an understanding of Henry’s constants is useful for other applications.  Cox-Colvin recently evaluated a site with contamination in groundwater and overlying soil gas.  A calculation using Henry’s constants showed that the vapor concentrations in soil-gas from the site were too high to come from known groundwater, which meant that either more highly contaminated groundwater or some unknown soil contamination existed nearby.

It’s also useful to understand the relationship between Henry’s constants and data quality in groundwater samples.  People often assume that a small bubble in a groundwater seriously reduces data quality.  Many believe that all of the volatile organic compounds (VOCs) in a sample bottle migrate out of the water and into the bubble, but not so.  If the Henry’s constant is less than 1, which is true of many compounds, especially chlorinated compounds, the amount of vapors in the bubble is relatively small, and the loss is negligible, and often not measurable.

You probably won’t use Henry’s constants often, but if you do, the VISL calculator has all of the chemicals you’re likely to need, and it makes the temperature adjustments for you.

In next month’s issue of Focus on the Environment, we’ll discuss three problematic compounds: trichloroethene (TCE), benzene, and chloroform.

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