Entry by JoeDerhake
This blog is a re-post. Math is the same, but I corrected a typo and I added some clarifications (see italics).
What do you do when you have groundwater sampling that shows solvent contamination and you want to infer a soil gas concentration, but you don’t have soil gas testing data? This situation comes up a lot.
Maybe you are doing a Phase I ESA and know that the property across the street has high solvent concentrations in groundwater – hence the question, what are the highest soil gas concentrations that could partition from the saturated zone to the vadose zone?
Predicting the relationship between soil gas concentrations and groundwater concentrations is relatively easy in a controlled environment. You can use Henry’s Law, which is a ratio of the concentration of a substance in air to its concentration in water. Of course, you can only go so far in the open environment of vadose zone soils with differing temperatures and pressures, etc.; but it is helpful to make a couple of assumptions to predict a range of reasonable soil gas concentrations.
Henry’s Law Constant (unitless) is defined as:
H’ = C air / C water
H’ = Henry’s Law Constant (dimensionless)
C air = concentration of compound in air (volumetric basis)
C water = concentration of compound in water (volumetric basis)
Therefore, C air = C water * H’
Henry’s Law Constants are the partition coefficients for various substances for water to air partitioning. Henry’s Constants were determined in laboratory experiments published by the U.S. EPA and also by John Washington in the journal Ground Water. You can view Henry’s Constants for various contaminants here.
Let’s assume that we have 100 µg/L PCE in a groundwater plume. What is the equilibrium soil gas concentration at 25 °C and standard pressure? Henry’s Constant (H) for PCE at 25 °C is 0.0.0174 atm-m3/mol (U.S. EPA Henry’s Law Constant Calculator website).
According to Environmental Forensics Principles & Applications (link below), you can convert H to H’ (Henry’s Law Constant unitless) via the Ideal Gas Law:
Henry’s Law Constant (unitless) H’ = H/(RT)
where R = ideal gas constant (8.20575E-5 atm-m3/mol-K)
T = temperature (Kelvin; 25 oC = 298.15 oK)
Henry’s Law Constant PCE (unitless) H’ = 0.0174/(8.20575E-5 X 298.15) = 0.711
Cair = Cwater x H’ = 100 µg/L X 0.711 = 71.1 µg/L
I reposted the blog because Dr. Peter Woodman correctly points out that ppbv is not the same as µg/L for atmospheric concentrations of soil gas and my first post implied that they were the same. Henry’s Law applies to partials pressures at the parts per billion level on a molar basis (ppbv). To use Henry’s Law on a water concentration in µg/L, you would first convert to parts per billion by volume (ppbv), by dividing by the Molecular Weight of PCE (165.83 g/mol), and then covert back to µg/L in gas by multiplying by the Molecular Weight — the molecular Weight Calculations to ppbv and from ppb cancel out.
The mathematical results in the first and second post are the same.
These two links provide additional support for these methods:
So, 71.1 µg/L is the worst-case on-site scenario for PCE groundwater concentration equivalents likely to produce atmospheric concentrations of PCE in soil gas; however, in reality this concentration is highly unlikely because Henry’s constant assumes that equilibrium was reached (with vigorous blending in a laboratory beaker, etc.). This form of mixing doesn’t occur in the subsurface, so the actual soil gas concentrations tend to be much lower.
You could also calculate a range of soil gas concentration equivalents, by changing the soil temperature, which gives you a new Henry’s Constant coefficient value (H’) to determine the worst-case scenarios in winter and summer. This could be particularly useful in screening out an off-site vapor intrusion source of concern.
How else could you use Henry’s Law?
Maybe you did a Phase II Subsurface Investigation and measured significant concentrations of soil gas – is this from an on-site source or could it be from the off-site groundwater plume?
You may find that you can associate lower concentrations of a chemical in soil gas based on vapor partitioning from groundwater, but not higher soil gas concentrations. Well, where is that line? Henry’s Law can be useful in determining that the soil gas concentration is too high to be coming from the groundwater (but may be from the soil, an active leak, or a preferential vapor migration pathway).
Be careful with this calculation as the groundwater in contact with vadose zone is NOT a closed system and temperature and pressure, etc. vary. This calculation will not predict the precise value of atmospheric soil gas concentrations; rather, help frame the possible range of atmospheric soil gas concentrations that could be detected above a groundwater plume.
Henry's Law, Henry's Law Constant, Henry's Constant, Henry's Coefficient, soil gas testing, groundwater sampling, Phase II Subsurface Investigation