Theme: 09. Vapor Intrusion

Topic Area: 9a. Advances in Vapor Intrusion Investigations

Formatted Title:

Variation in VOC Concentration over Time: Results of Two Years' Measurements

Background/Objectives:

In Denmark the guideline for measuring indoor air concentration of volatile organic compounds (VOCs) is recommended to be a passive method over 14 days in the cold period - October to March. This is in line with the US-EPA 2015 guidelines. The reason for this is the expectation of higher indoor air concentrations during the cold period in contrary to the summer period – thereby the result gives a more solid risk assessment.

Over a period of 2 years (2020-2022) we have measured VOCs in sub-slab soil vapor and indoor air together with a range of other parameters. The measurements were performed at a former drycleaner site, with a shallow groundwater table, 0.3 to 1.5 meter below ground level. The site is contaminated primarily with PCE.

The aim is to measure the variation in VOC concentration over a time period of two years in an unoccupied residence and on the basis of the data assessment when to measure in sub-slab soil vapor and indoor air to give a solid risk assessment. Furthermore, the aim is to investigate which parameters have an impact on the time variation of VOC concentration.

Approach/Activities:

The concentration of PCE, TCE, DCE and vinyl chloride has been measured over a period of two years and is given below:

Sub-slab soil vapor: 11 points with carbon-tubes – active method over 100 minutes. New measurement once a month.
Indoor air: 8 points with ORSA-tubes – passive method over 14 days. New measurement every 14 days.
Sewer system: 2 points in sewer wells and 2 points behind water traps – passive methods over 14 days. New measurement every 14 days.

In addition, there has been logging of radon, differential pressure, temperature, atmospheric pressure, groundwater level and temperature.

Results/Lessons Learned:

The sub-slab soil vapor concentrations of PCE show clear variation over the seasons, with the highest concentrations in summer/autumn and the lowest in winter/spring. The high concentrations in summer/autumn is approximately a factor two to three higher than the concentrations measured in winter/spring. There is a fine correlation between variation in concentrations in the sub-slab soil vapor and the temperature in the groundwater.

The variation in indoor air concentrations follows the same pattern as the concentrations in sub-slab soil vapor with the highest concentrations measured in summer/autumn and the lowest in winter/spring. The high concentration in summer/autumn is approximately a factor 3 higher than the concentration in winter/spring. At our test site the data indicate that the variation in sub-slab soil vapor concentration has a bigger impact on the indoor air concentration than other parameters as differential pressure over the slab or differential temperature outside/inside (stack effect).

The data show that measuring indoor air concentration only in the cold period poses a risk of underestimating the average concentration in indoor air, and hence perform a false risk assessment.

The variation in sub-slab soil vapor concentration over the year makes it important to consider the purpose of the investigation before planning when to perform the sub-slab soil vapor measurements.

Radon concentration in indoor air has been found as a good indicator for variation in VOC concentration over time, and hence a useful tool to optimize sampling strategy.

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Karin Nielsen