Indoor Vapor Intrusion from Underlying Groundwater Contamination
Brianne Archer, Supervising Engineer, PE, CPP
Richard Vogl, Principal Hydrogeologist, CHG, CEG, PG
TECH MEMO: How do you know if contaminated groundwater will cause an indoor vapor intrusion issue at your property?
Vapor intrusion (VI) is the general term given to migration of hazardous vapors from any subsurface contaminant source, such as contaminated soil or groundwater, into a building. The vapor-forming chemicals that can potentially provide subsurface sources for vapor intrusion into buildings include, for example, chlorinated hydrocarbons, petroleum hydrocarbons, and other types of both halogenated and non-halogenated volatile organic compounds (VOCs).
In order for a chemical in groundwater to pose a VI risk it must be (i) sufficiently volatile, (ii) present at a sufficient concentration, and (iii) have a higher relative Henry’s Law constant.
Volatility is determined by the vapor pressure at a given temperature, generally determined in a laboratory setting and published by the United States Environmental Protection Agency (US EPA) or other regulatory agencies and institutions. The US EPA Regional Screening Levels (RSLs), published twice a year, report whether a compound is volatile and its recognized vapor pressure.
The concentration of a chemical in groundwater may be converted to the concentration in vapor by applying Henry’s Law which states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid. This proportionality factor, called Henry's Law constant, is also published in the US EPA RSLs. Compounds with low Henry’s Law constants will have lower vapor pressures for a given concentration in water. Conversely, compounds with a high Henry’s Law constant will have higher vapor pressures for a given concentration in water.
The vapors generated from the contaminants in groundwater can travel up through the soil column into an overlying building although they must first move through the partially wet soil layer just above the water table called the capillary fringe.
(Washington State Department of Ecology, “What is Vapor Intrusion?”, https://ecology.wa.gov/Spills-Cleanup/Contamination-cleanup/Cleanup-sites/UW-Tacoma/Vapor-intrusion)
The capillary fringe is the subsurface layer in which groundwater seeps up from a water table by capillary action to fill pore space in the soil above. The capillary fringe plays a large role in limiting vapor intrusion from groundwater by controlling the soil moisture content and oxygen availability. The increased availability of these two components increases the biodegradation and attenuation of the contaminants in soil vapors, thus reducing the contaminant concentrations. The effect of this migration of contaminants through the capillary fringe can significantly decrease the indoor air concentration by decreasing upward diffusion rates of volatile compounds (particularly hydrocarbons) due to enhanced biodegradation at the capillary fringe. (Yao, 2019).
Now, back to our question: How do you know if contaminated groundwater will cause an indoor vapor intrusion issue at your property?
One way to quantify the potential for contaminated groundwater to impact indoor air is to use the US EPA Vapor Intrusion Screening Level (VISL) calculator (https://www.epa.gov/vaporintrusion/vapor-intrusion-screening-level-calculator). In this model, contaminant reduction is taken into account using an attenuation factor to calculate indoor air concentrations from groundwater concentration. To calculate a target groundwater concentration that corresponds to a chemical's target indoor air concentration, you simply divide the target indoor air concentration by an attenuation factor of 0.001 and then convert the vapor concentration to an equivalent groundwater concentration using Henry’s Law.
You could also reverse the calculation to determine the presumed indoor air concentration from VI due to concentrations detected in groundwater in order to compare values to published regulatory screening levels for indoor air published by the California Department of Toxic Substances Control (DTSC) and the US EPA.
The draft Supplemental Guidance for Screening and Evaluation Vapor Intrusion (CalEPA, February 2020) applies a 0.001 attenuation factor for vapors from groundwater. It notes that shallow groundwater plumes are more likely to contribute to vapor intrusion than deeper groundwater plumes; however, the attenuation factor does not take depth to groundwater into account.
A brief example using concentrations of TCE and PCE detected in groundwater is tabulated below. The indoor air concentrations were calculated from the given groundwater concentrations using the 0.001 attenuation factor and then compared against published residential screening levels. It should be noted that these examples assume all contaminants emanate from the contaminated groundwater (with no contribution from contaminated soil or soil gas).
As you can see by the numbers in the table above, ultimately the break point groundwater concentration that would result in a failed residential indoor air screening level is 0.46 ug/L for PCE and 0.48 ug/L for TCE in groundwater. It becomes obvious from this exercise that this is an oversimplistic analysis of the subsurface conditions and an extremely conservative approach ignoring both the depth of the off-gassing groundwater VOC source and the soil type. However, this is an approach a regulatory agency may employ in a default manner without a more thoughtful, scientific approach.
A second way to determine whether groundwater contamination impacts indoor air quality is to use the 2017 updated version of the Johnson & Ettinger (J&E) model which can be found on the US EPA website. Waterstone used this updated model to perform a more in-depth analysis of the VI risk. The J&E model was run a number of times using conservative default parameters to determine the impact of the volatile organic chemical (VOCs) of concern (PCE and TCE), depth to groundwater, and various soil types. Model runs were completed for both PCE and TCE for six different groundwater depths (10 feet, 20 feet, 30 feet, 50 feet, 75 feet, and 100 feet below ground surface) and three different soil types (clay, silt, and sand) to determine the groundwater concentrations of the VOCs which would cause an exceedance of the screening levels for residential indoor air, equivalent to a 1 x 10-6 cancer risk.
Two bar graphs summarizing the results of our calculations are included below to help you better understand whether there is a potential vapor intrusion issue from PCE and TCE groundwater contamination given a certain depth to groundwater and soil type. For example, the Vapor Intrusion by PCE in Groundwater bar chart shows that at 30 feet below ground surface in silty soil, a groundwater concentration of approximately 40 ug/L is sufficient to cause vapor intrusion issues in indoor air.
As can be seen at a glance from these graphs, soil type has the most profound effect on the migration of vapors from groundwater to indoor air (typically an order of magnitude difference) followed by depth below ground surface (in general same magnitude, but still increases with depth).
So “yes”, depending on the circumstances, contaminated groundwater can cause an indoor vapor intrusion issue at your property.
The results of this investigation were eye-opening. In general, the results show that in more porous media, transport of vapors emanating from groundwater through the soil can be a real concern, even at relatively low concentrations of VOCs in groundwater. Depth to groundwater had a much smaller effect; although, as expected, the deeper the groundwater resides, the higher the VOC concentrations needed to impact indoor air through VI. Because of this, it is likely that contaminated shallow groundwater will be looked at by regulators as a source of VOCs to impact indoor air spaces, fuel additional soil gas and indoor air investigations, and further reduce shallow groundwater clean-up levels.
Next Question: Can contamination in soil vapor impact groundwater? Watch out for our next tech memo blog post that addresses this question.
2017 Johnson & Ettinger Model (J&E), Spreadsheet and Documentation, updated September 2017, EPA.GOV
2019, Yijun Yao, Fang Mao, Yuting Xiao, and Jian Luo, “Modeling capillary fringe effect on petroleum vapor intrusion from groundwater contamination”, March 1, Water Research
2020, HERO Note 3, Human Health Risk Assessment (HHRA) Note Number 3, DTSC-modified Screening Levels, California Department of Toxic Substances Control, Human and Ecological Risk Office (HERO), June
2020, US EPA RSLs, United States Environmental Protection Agency (US EPA) Regional Screening Levels (RSLs), May