The document discusses the relationship between moisture factors such as precipitation, soil moisture, and groundwater levels with chlorinated vapor intrusion in indoor air. Key findings indicate that certain weather conditions can impact vapor concentrations, and there is a complexity in how these factors influence indoor air quality. Recommendations for future data collection and guidance for practitioners on monitoring practices are also provided.

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Indicators, Tracers, and Surrogates of
Chlorinated Vapor Intrusion – Potential for
Rain, Soil Moisture, Water Table, Snow and Ice
March 20, 2018 Presented at EPA Workshop at AEHS
State-of-the-Science for Indicators, Tracers, and Surrogates of
Chlorinated Vapor Intrusion: Supplemental Measurements for
Minimizing the Number of Chemical Indoor Air Samples Needed
Chris Lutes
CH2M is now Jacobs

2. Outline
1. Moisture’s role in the vapor intrusion conceptual site model
2. Physical processes by which easily observable weather factors can
influence vapor intrusion
3. Low cost data sources and measurement equipment available
4. Current guidance document language
5. Are these weather factors good indicators?
6. Suggestions for future data gathering and interim thoughts for
practitioners
2

3. Reprinted with
permission from
Final Report SERDP
Project ER‐1687;
Illangasekare et. All.
Center for
Experimental Study
of Subsurface
Environmental
Process (CESEP)
Conceptual
Site Model

4. Influence of Moist Soil Layers
• Moist soil layers of 60% saturation
between groundwater source directly
below (B) and basement reduce
attenuation factor by about 4x
• Moist layer only at the surface with the
source directly below (D) also reduces
the attenuation factor slightly by
lowering the flow into the building
• But with the shallow (3m) groundwater
source laterally separated the moist
layer at the surface increases the
attenuation factor two orders of
magnitude
Figures 19 and 21 and results from EPA 530‐R‐10‐003 (Abreu)

5. Falling Water Table Can Increase Soil Gas
Concentrations
• Data plotted from Illangasekare (2014) tank study: Falling water table believed
to increase contact between DNAPL and soil gas
• Guo (2015 - dissertation), reported that VOC emissions increased during
declining water table and decreased when water table rising in tank experiments
but effects in field were relatively modest and spatially inconsistent

6. Snow and Ice (EPA Indianapolis Report)
• Snow effects are expected to be
complex because not all snows are
the same (in terms of water content
and air permeability).
• Increasing chloroform and PCE in
indoor air with melting snow packs.
Attributed this potentially to effects
on air permeability or water levels.
• With PCE, there is apparently also
an effect of increasing indoor
concentration with increasing snow
pack depth. Thus, it appears that at
least for PCE this effect is complex.
0
2
4
6
8
10
12
14
16
18
20
0 0.5 1 1.5 2 2.5 3 3.5
PCEconcentration(µg/m3)
Snow Depth on Ground (inches)
Weekly Average Snow Depth vs. PCE Concentration (Radiello)
422 Base South; Mitigation Off or Not Installed Data Only

7. On Line Free Data Sources
Precipitation historical or recent data:
 https://www.wunderground.com/history/
 https://www.ncdc.noaa.gov/cdo-web/search
 https://catalog.data.gov/dataset/u-s-hourly-precipitation-data
 Precipitation data is available for approximately 5,500 US
weather stations
Depth to Water:
 https://cida.usgs.gov/ngwmn/
• 6000 wells
Soil Moisture monitoring
 https://www.drought.gov/drought/soil-moisture-map
 https://www.drought.gov/drought/data-maps-tools/soil-
moisture
Soil Moisture Infiltration Properties
 https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

8. Low Cost Site Specific Data Acquisition
(no endorsement of brands intended, example pricing only)

9. Current Guidance Document Language – Precipitation Timing
• State guidance documents have varying provisions for sampling times based on rainfall
• In some cases requirements are only for weather observation before sampling events (MA, NJ,
NC), but in other cases provisions control when samples are to be taken, which effect the cost of
sampling.
• CA (2015) soil gas sampling should not occur during a significant rain event and should only
occur after five days without a significant rain event. A significant rain event is defined as 1/2 inch
or greater of rainfall during a 24-hour period…Irrigation or watering of soil should stop at least five
days prior to the sampling
• Ohio recommends indoor air and subslab sampling at times when the soil is “Saturated with rain
(1/2” of rain or more within 24 hours)” but calls for avoiding external soil gas sampling for 24 hours
after such a rain event (Ohio, May 2010).
• ITRC (2007) states with regard to indoor air: “Measurements made during or immediately after a
significant rain event (e.g., >1 inch) may not be representative of long-term average conditions.
For other sites, however, frequent rainfall is common, and testing soon after a rain event is both
representative and inevitable.”
• MI (2013) Show that “Site conditions have not been influenced by precipitation prior to sample
collection: •The waiting period will be dependent upon soil type, amount of rain, and previous soil
moisture content (e.g., longer for clays, longer for heavy rains, shorter for coarse sands, etc.).
Information should be provided showing justification of actual time elapsed between rain and
sampling events.”

10. On-Line Information on Rainfall Intensity and Frequency
• https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html
Example from Augusta
Kansas, Note that every year
on average a 3” rain event
occurs in 24 hours
Second example from
Chapel Hill NC – 61 days of
>1” rain in 5 years
https://www.ncdc.noaa.gov/cdo‐
web/datatools

11. Literature – Temporal Prediction using Moisture Related
Variables is Difficult
• Indianapolis: no significant relationship between the measured soil moisture or depth to
water and indoor VOCs in time series analysis. Little evidence of a relationship between rain
events and VOCs.
• Sun Devil Manor; Guo (2015 - dissertation), reported that VOC emissions increased during
declining water table and decreased when water table rising.
• Steinmacher (2009) study at Hill AFB precipitation not significantly associated with
percentage detections of TCE.
• Illangeskare “Vapor intrusion will likely spike in the near term during a rain event due to gas
phase displacement from the initial infiltration front propagation. This observation is
supported on the basis of observations from multiple laboratory experiments, model scenario
simulations, and field-testing”.. “A “washout effect” due to the "cleaner water" of the
infiltration front diluting the "dirtier water" of the vadose was significant in the laboratory
experiment and corroborated by the model, but was not readily observed in the rainfall
scenarios…”.
• Shen (2012): Modeled effects – [VOC] go ↑ then ↓ a er rain event, effects weaker at 1 meter
• Infiltration rates vary from >1 inch per hour to <0.10 inches per hour
http://www.soil.ncsu.edu/certification/manual/slides/chapter5a/img14.html

12. Literature – Spatial Prediction Using Some Moisture
Related Variables Promising
• EPA Residential Database: the AF (normalized indoor air concentration) is 4.6 to 9x
higher for <1.5m water table depth then for 1.5m-3m; the 1.5m-3m depth group is about
2x higher then the 3-5m group, difference at >5m inconsistent.
• Johnston (2013) reanalysis of EPA database negative relationship between attenuation
factor and water table depth; p<0.01; mediated by soil type
• EPA CMS model report “As the source depth increases by a factor of six, the normalized
indoor air concentration decreases by a factor of three.“
• Hill AFB (EA, 2016) study n= 1968 buildings spread over an area approximately 5 miles x
4 miles. Shallow groundwater noted as a primary predictive variable for risk; depths to
groundwater vary from 100 ft down to 10 to 20 feet.
(http://afcec.publicadmin-record.us.af.mil/hcUqrfsGfjQqiPFW56ZmGume3PmymIRpXD2kwgF5_KI1/552693.pdf)
• Many documents caution against the use of default screening approaches with
groundwater more shallow then X. (MI < 3 meters; EPA 2015 <5 ft)

13. Shallow Water Tables are Common (Data from Fan, 2016 and MI DEQ 2016)
Depth in meters
Figure from Fan, Y.,
Li, H., & Miguez‐
Macho, G. (2013).
Global patterns of
groundwater table
depth. Science,
339(6122), 940‐943.
MI DEQ (2016): “over 55
percent of the state is
likely to encounter water
less than 5 feet below the
ground surface and 65
percent of the state is
likely to encounter
groundwater at a depth
of 10 feet below the
ground surface “

14. Conclusions – Vapor Intrusion and Moisture
• There are sound theoretical, mechanistic relationships that connect vapor intrusion
processes to moisture related variables but “the dynamic processes of contaminant
vapor response to both single rain event and seasonal rain events can be observed to be
quite complex” (Shen 2012).
• Cost effective mechanisms monitoring have been developed for many fields
• Much on-line free data is available.
• Many state guidance documents call for considering precipitation in scheduling
• These variables are not good, generalizable, predictor variables for temporal indoor
concentrations, most likely because their influence is dependent on site specific factors.
• Precipitation and soil moisture is only one component of a long chain of events leading to
indoor concentration from VI
• Better correlations between water table depth and spatial VI risk exist
• Do you have other data available data sets and analyses, you can share in the Q&A?

15. www.jacobs.com | worldwideJanuary 11, 2020
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Thank you!
Christopher.Lutes@ch2m.com