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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Redfield site, Colorado
  5. New York State Temporal Study buildings
  6. Conclusions
  7. Acknowledgments
  8. References

The vapor intrusion impacts associated with the presence of chlorinated volatile organic contaminant plumes in the ground water beneath residential areas in Colorado and New York have been the subject of extensive site investigations and structure sampling efforts. Large data sets of ground water and indoor air monitoring data collected over a decade-long monitoring program at the Redfield, Colorado, site and monthly ground water and structure monitoring data collected over a 19-month period from structures in New York State are analyzed to illustrate the temporal and spatial distributions in the concentration of volatile organic compounds that one may encounter when evaluating the potential for exposures due to vapor intrusion. The analysis of these data demonstrates that although the areal extent of structures impacted by vapor intrusion mirrors the areal extent of chlorinated volatile organic compounds in the ground water, not all structures above the plume will be impacted. It also highlights the fact that measured concentrations of volatile organic compounds in the indoor air and subslab vapor can vary considerably from month to month and season to season. Sampling results from any one location at any given point in time cannot be expected to represent the range of conditions that may exist at neighboring locations or at other times. Recognition of this variability is important when designing sampling plans and risk management programs to address the vapor intrusion pathway.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Redfield site, Colorado
  5. New York State Temporal Study buildings
  6. Conclusions
  7. Acknowledgments
  8. References

Since the U.S. Environmental Protection Agency (USEPA) issued the Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils (USEPA 2002), there has been an increasing awareness that significant spatial and temporal variability in the concentrations of chlorinated volatile organic compounds (CVOCs) in ground water, soil gas, subslab vapor, and indoor air is typical of many vapor intrusion (VI) sites. An analysis of VI sampling results compiled by the USEPA from more than 40 sites across the United States indicates that attenuation factors (indoor vapor concentrations and source vapor concentrations), representing the aggregate influence of geological conditions, spatially and time variable subsurface transport processes, building construction, and building ventilation on the relationship between indoor air quality and ground water quality, typically span a range of three to four orders of magnitude, even at individual sites (Dawson et al. 2007).

Although the plots of estimated ground water vapor source concentration vs. indoor air concentration and subslab vs. indoor air concentration ratios that are depicted in the EPA database illustrate the range of attenuation factors that has been observed at sites, the spatial and temporal relationships between source concentrations and indoor air concentrations are not depicted. In their evaluation of the vapor intrusion pathway at the Raymark Superfund site, DiGuilio and others showed that measured concentrations of CVOCs in subslab exhibited a broad range of spatial and, to a lesser degree, temporal variability between neighboring houses and within individual houses (DiGiulio et al. 2006). Similar variability in subslab CVOC concentrations within and between houses has been observed during vapor intrusion evaluations of several sites in New York State (Wertz and Festa 2007). When developing a strategy for investigating the vapor intrusion pathway at a site, knowledge of the potential for large temporal and spatial variability in subslab concentrations can play a vital role in shaping the investigative approach.

This paper presents data from vapor intrusion sites in Colorado and New York to illustrate the temporal and spatial variability in the concentration of volatile organic compounds that one may encounter when evaluating the potential for exposures due to vapor intrusion. Because the data described herein were obtained from hundreds of houses over a multiyear time span, they provide some unique insights into the pattern of vapor intrusion one may encounter above a CVOC ground water contaminant plume and the potential for subslab and indoor air concentrations to vary through time.

Redfield site, Colorado

  1. Top of page
  2. Abstract
  3. Introduction
  4. Redfield site, Colorado
  5. New York State Temporal Study buildings
  6. Conclusions
  7. Acknowledgments
  8. References

The Redfield site is located in Denver, Colorado, along the southern edge of the Cherry Creek valley, at the “headwaters” of a bedrock channel that runs down the valley sideslope. The alluvial deposits within the channel range from coarse sand and gravels near the base to overlying fine sand, silt, and clay, and are covered by about 10 to 20 feet of silt and clay loess. Near the top of the valley sideslope, ground water generally flows through weathered claystone, siltstone, and fine-grained sandstone bedrock of the Denver Formation (Robson 1987) at depths of approximately 25 to 30 feet below ground surface (bgs). The bedrock channel begins to collect and control ground water flow further down the valley sideslope, with depths to ground water ranging from about 30 to 50 feet bgs. Near the bottom of the valley, ground water discharges from the channel deposits into alluvial terrace deposits at depths ranging from about 30 to 10 feet bgs.

Historical releases of 1,1,1-trichloroethane (TCA), trichloroethylene (TCE), and tetrachloroethylene (PCE) at the former Redfield facility impacted underlying soil, weathered bedrock, and shallow ground water. Investigations conducted between 1998 and 2001 by EnviroGroup Limited found that the resultant CVOC ground water plume extended more than 13,000 feet downgradient of the former Redfield facility and was up to 1200 feet wide and 70 feet deep in certain portions of the site. The principal compound of concern was 1,1-dichloroethene (1,1-DCE), a breakdown product of TCA and, to a lesser extent, TCE. 1,1-DCE concentrations in ground water were approximately 3000 μg/L at the site boundary, decreasing with distance downgradient of the facility.

Investigation procedures

Monitoring wells were typically constructed of 2-inch I.D. schedule 40 flush jointed, threaded PVC with a screen slot size of 0.020 inch. Screen lengths were typically 10 feet and installed to straddle the water table (and collect samples representative of the upper few feet of the aquifer); although in a few cases the water table fell slightly below or rose slightly above the screened interval. It should be noted that the Redfield site is in a semi-arid region, with an average precipitation of approximately 15.8 inches per year (NWS 2006); therefore, only limited infiltration is available to create a clean water lens over the plume. Ground water samples were collected following standard industry practices and quality control procedures, under the oversight of the Colorado Department of Public Health and Environment (CDPHE), and were analyzed by EPA Method 8260b.

Indoor air samples were collected over a 24-hour period in 6-L Summa® canisters at more than 800 homes overlying or adjacent to the ground water plume to determine the extent of vapor intrusion impacts due to 1,1-DCE. The canisters were dedicated to the project and batch certified clean to the project reporting limits. Samples were collected from the lowest potential living space within the house, away from vents and windows. Houses with basements were sampled in the basement and homes without basements were sampled at a central location on the first floor. The indoor air samples were analyzed in accordance with EPA Toxic Organic Method TO-15 following CDPHE’s Guidance for Analysis of Indoor Air Samples (CDPHE 2000) since October 1998, using a mass spectrometer operated in the selective ion monitoring (SIM) mode with 1,1-DCE reported down to the method detection limit (varying from 0.011 to 0.04 μg/m3).

The single-family residential units in the vicinity of the Redfield ground water plume were constructed predominately between 1950 and 1965 (92.4%). A large majority (94%) are single-story homes ranging from 800 to 1750 square feet in area. Approximately 67.1% have basements, 31.5% have crawl spaces, 0.43% have slab-on-grade construction, and 1% are undefined. Most are heated by natural gas–heated warm air (86.4%), with the majority of the remainder being heated by gas-heated hot water (12.7%). Approximately half of the units have one or more fireplaces. Slightly more than half of the homes have swamp coolers or air conditioners.

CDPHE required the installation of subslab depressurization systems at 387 residences where the initial indoor air concentration exceeded the action level of 0.49 ug/m3 (the action level was increased to 5 μg/m3 in 2004). Periodic indoor air testing was required in mitigated homes (performance monitoring), typically on a quarterly basis for the first year, semi-annually for the second year, and annually thereafter. In addition, periodic indoor air testing was required in select homes around the edges of the mitigated area (verification monitoring), at frequencies ranging from quarterly to annually. Indoor air samples have been collected at annual or greater frequencies in a large number of homes for up to 10 years. Background sources of 1,1-DCE are relatively rare (Kurtz and Folkes 2002); therefore, most indoor air concentrations of 1,1-DCE are likely attributable to vapor intrusion.

Observed correlation between ground water and indoor air concentrations

The estimated boundary of the ground water plume (defined as the 7 μg/L MCL for 1,1-DCE) and the location of homes with 1,1-DCE above the action level of 0.49 ug/m3 in indoor air are essentially coincident at the Redfield site (Figure 1). No vapor intrusion impacts have been observed at distances greater than about one or two houses beyond the estimated extent of the ground water plume. Similar relationships have been observed during extensive vapor intrusion investigations conducted by one of the authors in Endicott, New York, and in Cortlandville, New York. The observed vapor intrusion impacts beyond the estimated plume boundary may be due to lateral diffusion of vapors in the vadose zone, which appears to be generally limited to a distance of about 100 feet (Folkes et al. 2007; Lowell and Eklund 2004), or they may be due to imprecision in the interpolated edge of the plume, or to impacts associated with the presence of 1,1 DCE at concentrations less than 7 μg/L.

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Figure 1. Correlation between extents of ground water plume and vapor intrusion impacts due to 1,1-DCE, Redfield site. •= monitoring well, lines = 1,1-DCE groundwater concentration contour (μg/L), shaded homes = indoor air DCE above 0.49 μg/m3.

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The initial indoor air 1,1-DCE concentrations (prior to mitigation) for each home sampled are plotted vs. interpolated ground water derived soil-gas concentration (ground water concentration multiplied by the Henry’s law constant [HLC] for 1,1-DCE) at the time of indoor air sampling on Figure 2 for 515 indoor air tests conducted between May 1998 and August 2003. The indoor air concentrations are observed to increase with ground water concentration, although the concentrations of 1,1-DCE were very low for a number of homes across the full range of ground water concentrations. Ground water attenuation factors ([1,1-DCE μ/m3] indoor air/[1,1-DCE μ/m3] ground water × HLC)] varied from approximately 10-3 to less than 10-6, consistent with observations at other sites (Hers et al. 2003). Although the plot indicates a positive correlation between ground water and indoor air concentrations, the correlation is weak. Setting aside temporal variability in building ventilation and transport processes over the 5-year period of monitoring, some of this imprecision may be due, in part, to ground water concentrations that are based on interpolation of concentrations between wells separated by distances of several hundred feet (not unusual for a site of this size) and use of paired ground water and indoor air data from sample dates that varied by several weeks to months. It is important to note that ground water concentrations were generally stable over the time period of the indoor air measurements, typically varying by less than a factor of 2 or 3 near the majority of the homes tested. The average distance between the homes included in Figure 2 and the nearest well is 220 feet. Filtering the data to include indoor air data only for homes within 200 feet of a well (average distance 126 feet) does not significantly affect the nature of the plot shown on Figure 2.

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Figure 2. Ground water derived soil gas concentration versus premitigation indoor air concentration of 1,1-DCE, all data 1998–2003.

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We attempted to reduce the imprecision introduced by ground water data interpolation and different sampling dates by selecting a subset of homes that (1) are within approximately 100 feet of wells, (2) have not been mitigated, (3) and have several sets of indoor air and ground water data collected within 90 days of each other over a period of several (2.5 to 7.5) years. Over this long time frame, ground water concentrations for several of these homes varied by approximately an order of magnitude. Plots of paired indoor air and ground water data for the five homes that met these criteria are shown on Figure 3. Again, a positive correlation between ground water and indoor air concentrations is noted (in composite, and for individual homes), with significant data scatter represented by ground water attenuation factors that range from about 10-4 to 10-6. Note that the ground water attenuation factors range by approximately one order of magnitude for several of these homes over time (see later section of paper).

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Figure 3. Ground water derived soil gas concentration versus premitigation indoor air concentration of 1,1-DCE, five homes.

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Other factors that could influence this correlation, or the ground water attenuation factor, include but are not limited to spatial variations in geology, temporal and spatial variability in vadose zone transport processes, depth to ground water, and building construction. We qualitatively examined the potential influence of these factors on the correlation between indoor air and ground water concentrations in a portion of the Redfield site where ground water concentrations and geologic conditions appeared to be relatively consistent, as shown on Figure 4. Indoor air concentrations, largely measured over a 3-month period, between homes within this area varied by up to three orders of magnitude (0.15 to 120 μg/m3), with variations of more than two orders of magnitude occurring between immediately adjacent homes (e.g., 0.15 vs. 21 μg/m3) that were sampled within days of each other. Depth to ground water, vadose zone geology, and variations in ground water concentrations over the time period that indoor air concentrations were measured are provided in Table 1 for the wells shown on Figure 4. Although depths to ground water are fairly consistent both spatially and over time, spatial variations in soil conditions in the capillary fringe and vadose zone may explain, in part, the spatial variation in indoor air concentrations compared to the more consistent (apparently) ground water concentrations.

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Figure 4. Spatial variability in indoor air and ground water 1,1-DCE concentrations in study area, Redfield site.

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Table 1.  Ground Water and Soil Data, MW-35, -36, -38, -41 circa 2001 and 2002.
ParameterMW-35MW-36MW-38MW-41
  1. Note: See Figure 4 for monitoring well locations.

Depth to GW, feet25.5 ± 0.522.0 ± 0.422.2 ± 0.528.2 ± 0.3
Screen depth, feet21.8–31.822.8–32.821.1–26.123.2–33.2
Vadose zone material (top to bottom)–14 feet sandy to clayey silt–14 feet silt to clay–13.5 feet silt and clay–11 feet silt
 –4 feet sand –3.5 feet sand, f-c
–10 feet sand and clay–4 feet clay–2 feet sand, fine to clayey–3 feet silt
–1.5 feet wx siltstone –7 feet m-c sand–2.5 feet sand, f-m
 –4 feet clay
 –4.5 feet sand, f-m
DCE, μg/L(min, max, average)120, 320, 223200, 260, 22458, 250, 13872, 82, 78

The spatial variability in the indoor air data compared to ground water data may also be due in part to differences in building construction and ventilation. Foundation type varied among the homes shown in Figure 4 between full basements, full crawl space, crawl space with basement, crawl space with slab on grade, and trilevel with basement (basement with slab on grade). However, no correlation between building style and indoor air concentration is readily evident on Figure 4. When examined statistically on a site-wide basis, we find a slight trend, with attenuation factors for slab-on-grade buildings being, on average, about half (i.e., twice the attenuation) the average attenuation factor for basements or crawl space homes (Folkes et al. 2004). Nevertheless, the range in attenuation factors calculated for each building type is similar and broad (about three orders of magnitude).

Temporal variability in parameters other than ground water concentration may contribute to the variability in attenuation factors observed between houses, as evidenced by the variation in attenuation factors at several individual homes over time of approximately one order of magnitude in Figure 3. Differences in ventilation (i.e., both depressurization and air exchange rate) may contribute more to spatial variability in the indoor air data than building construction (see below). This variability over time would result in a similar degree of imprecision in the spatial data when based on single indoor air tests conducted over one 24-hour period. The contributions of temporal changes in moisture content in the vadose zone are not known for the Redfield site.

Observed Temporal variations in indoor air concentrations

Indoor air monitoring has been conducted on a quarterly to annual basis at a number of unmitigated homes at the Redfield site, where detectable (but below action level) concentrations of 1,1-DCE have been routinely observed. These homes are generally located along the outside edge of the ground water plume, adjacent to areas with mitigated homes. For the purposes of evaluating the variability of vapor intrusion impacts over time, we selected 45 homes where indoor samples have typically been collected on a quarterly or semiannual basis for a period of approximately 2 to 10 years (average of 7.8 years per property), and where 1,1-DCE was detected above the reporting limit (0.04 to 0.011 μg/m3, depending on the time frame) at least 50% of the time.

The indoor air data set for the 45 monitored homes includes 715 individual indoor air measurements. Approximately 70% of the measurements were above the reporting limit. Values below the reporting limits were replaced with values equal to half the reporting limit. Although the inaccuracy in the standard deviations introduced by this substitution method is recognized, the statistical evaluations presented herein do not appear to benefit from the more rigorous substitution methods (e.g., the Kaplan-Meier method). The individual 1,1-DCE concentrations ranged from below reporting limits (see above) to 1.4 μg/m3. Average annual 1,1-DCE concentration in each of the 45 homes ranged from 0.023 to 0.27 μg/m3 (Figure 5). The data set is limited to this general concentration range because homes with significantly higher 1,1-DCE concentrations were typically mitigated.

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Figure 5. Probability plot of annual average indoor air 1,1-DCE concentrations at 45 unmitigated properties, Redfield site.

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The indoor air data were then normalized by dividing individual concentrations by the average annual concentration in that home, so that the variability in concentrations over time could be compared for all 45 homes on an equal basis. The average annual concentration in each home was calculated by first determining the seasonal arithmetic average 1,1-DCE values (i.e., winter, spring, summer, and fall) for each property. The seasons were defined as the winter months of December through February, the spring months of March through May, the summer months of June through August, and the fall months of September through November. The average annual 1,1-DCE concentration at each property was then calculated as the arithmetic average of the four seasonal values. The plotting position formula used in evaluating seasonal indoor air concentration values was 1 – (rank – 0.326)/(n + 0.348), which is reported to provide a distribution free plotting position (Yu and Huang 2001).

A log-probability plot of the normalized individual 1,1-DCE concentration values (i.e., values divided by the property average annual concentration) is provided in Figure 6, showing the overall log-linear distribution of the normalized observed values. The normalized values range from about 10% of the annual average to about 10 times the annual average concentration of the home being tested. However, most of the time (e.g., plus or minus one standard deviation, or 68% of the data), individual test results are within a factor of 2 or 3 of the average annual concentration.

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Figure 6. Probability plot of indoor air 1,1-DCE concentrations divided by the property average annual concentration, Redfield site.

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The seasonal trends were evaluated by separating the data by season and plotting the distribution of the normalized values for each season, as shown on Figure 7. A clear difference among seasons is observed, with median winter concentrations that were 15% higher than the annual average and median summer concentrations that were 52% lower than the annual average. Spring and fall median concentrations were 23% and 17% lower than the annual average, respectively. Although seasonal trends are observed, it is also important to note that nearly the full range of concentrations is observed in any given season, so that short-term variability can overwhelm any seasonal trend. Compared to the general population of single-family homes on the site, the 45 homes used in the seasonal evaluation have a slightly greater percentage of basements, a greater number of natural gas warm air heating systems, and a slightly smaller percentage of fireplaces.

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Figure 7. Seasonal probability plots of indoor air 1,1-DCE concentrations divided by the property average annual concentration, Redfield site.

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Finally, a plot of the average monthly values divided by the average annual values is shown on Figure 8. Also plotted are monthly values normalized to the annual mean for VOCs measured in apartments in Leipzig, Germany (Rehwagen et al. 2003), normalized monthly values for radon (based on 6-day tests) in England (Camplin and Henshaw 2004), and normalized monthly radon values in homes near Bialystok, northeastern Poland (Karpinska et al. 2004). The normalized trends for both VOCs and radon at all sites, including Redfield, are strikingly similar, with the lowest concentrations (i.e., less than the annual average) occurring from June through September and the highest concentrations occurring from November through April. Concentrations were closest to the annual average in April and October. Based on the group average for these sites, summer concentrations are, on average, about 20% to 40% less than the annual average concentration, and winter concentrations are, on average, about 20% to 50% higher than the annual average. More extreme seasonal variations have been observed in other studies; for example, monthly averages varied by up to a factor of 9 at a site in India (Ramu et al. 1992). Seasonal variations are likely to be related to local climate, ventilation practices (e.g., use of swamp coolers vs. air conditioners), and building construction; therefore, although the Redfield site data compared well with data from other temperate climate sites, including England and Poland, they may not compare as well to sites in more tropical climates (or more severe winter climates).

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Figure 8. Monthly indoor air concentrations divided by average annual concentrations (1,1-DCE, VOCs, and radon).

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New York State Temporal Study buildings

  1. Top of page
  2. Abstract
  3. Introduction
  4. Redfield site, Colorado
  5. New York State Temporal Study buildings
  6. Conclusions
  7. Acknowledgments
  8. References

In 2006, the New York State Department of Environmental Conservation and Department of Health initiated a Temporal Study that included monthly monitoring of subslab air, basement air, and outdoor air in 16 structures at multiple locations in New York State. All air samples were collected as 24-hour composites using 6-L Summa canisters and were analyzed by a New York State Department of Health Environmental Laboratory Approval Program (ELAP) approved laboratory using EPA Method TO-15. Helium tracer gas testing was performed on one probe per house during each sampling event (i.e., each probe was tracer tested once every three events). The structures which were included in the program were structures that had been previously sampled by the agencies during site-specific vapor intrusion investigations. They were selected because previous samples indicated the presence of elevated vapor concentrations (typically, > 50 μg/m3 of TCE or PCE) in the subslab, but minimal (i.e., close to background) concentrations of those compounds in the indoor air. For ethical reasons, homeowners whose structures had elevated indoor air concentrations were offered mitigation systems and were not asked to participate in the study. Therefore, the houses selected for the Temporal Study are not fully representative of the range of indoor/subslab vapor attenuation that one is likely to encounter during a site investigation. Although each structure in the Temporal Study had a unique pattern of temporal and spatial variability in the concentration of subslab CVOCs, the case studies described here are generally representative of the range of variability which was observed.

Houses 1 and 2, Queens, New York

These houses are situated in a densely populated, mixed-use (residential-commercial) neighborhood in the New York City borough of Queens. They are approximately 300 feet downgradient of a former warehouse and dry cleaning supply distribution center that has been identified as the source of a significant release of tetrachloroethene (PCE) to the soil and ground water. Based on descriptions in site reports submitted to the agencies by the state’s environmental contractor (URS), the geology of the uppermost aquifer in the vicinity of the study houses is composed of silty glacial sands and gravels with interbedded lenses of silts and clays. The water table is approximately 65 feet bgs.

The houses were built in the early 1920s. They share a common wall. Each house is approximately 17 feet wide by 35 feet long. Three subslab sampling points were installed at each house. Because the homeowners had tile floors in the basements, they requested that the diameter of the penetrations through the basement floors be as small as possible. Therefore, rather than use permanent probes that require a 3/4 inch hole, the subslab samples were collected through 3/8 inch diameter holes freshly sealed during each sampling event with Sculpey® modeling clay. A Swagelok® stainless steel fitting was used to connect the sample tubing to the 6-L Summa sampling canister.

In addition to the chemical sampling, measurements of differential pressure (indoor/subslab), ground water quality, and atmospheric conditions (temperature, wind speed, barometric pressure, rainfall) were also obtained periodically at or near these houses. The subslab and indoor air data are summarized in Figure 9. The range of concentrations is consistent with past sampling results (April 2006 subslab PCE concentrations were 142 μg/m3 from House 1 and 846 μg/m3 from House 2).

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Figure 9. Measured concentration of PCE in 24-hour composite subslab and indoor air samples, House 1 and House 2. Note that post sampling tracer test in July 2007 indicated significant leakage in the subslab A seal.

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Data Evaluation

Although these houses are similar in many ways and share a common wall, the concentration of PCE in the subslab of House 2 was consistently four to five times greater than in that of House 1. Similar, and even greater differences, in subslab concentrations within and between houses have been observed in other locales (Wertz and Festa 2007; DiGiulio et al. 2006). The indoor air data from both houses were often in the upper quartile of the typical background range for PCE (NYSDOH 2006). Although those data did not exhibit a clear relationship with the measured changes in the subslab concentrations beneath the houses, one cannot rule out vapor intrusion as a potential source of some of the PCE in the indoor air.

The temporal changes in the subslab concentrations from individual sampling points at these and the other houses in the Temporal Study typically varied by less than an order of magnitude over the 18-month course of the study, with no clear trend in the pattern of observed concentrations through time. This range of subslab temporal variability is consistent with the range of temporal variability of the Redfield site indoor air data described earlier. It is worth noting that the subslab data that exhibited the greatest range of temporal variability (House 2, Subslab A) may be biased as a result of subslab seal failures. A postsampling tracer test conducted as part of the July 2007 sampling event indicated significant leakage in the Subslab A seal. Although no tracer tests were performed on Subslab A in March 2007, September 2007, or February 2008, those data may also reflect significant seal leakage. This ambiguity highlights the benefits of using tracer tests when collecting subslab samples.

The lack of any clear temporal trends in the measured subslab concentrations over the course of the study contrasts sharply with the observed changes in the concentration of PCE in a nearby monitoring over the same time period (Figure 10). From February 2007 through February 2008, the measured concentration of PCE in a well approximately 100 feet from the houses dropped from 27,000 to 6,300 μg/L. As observed at the Redfield site, it appears that transport and storage across the 65-foot-thick vadose zone at this Queens, New York, site dampens the short-term impacts of changes in the vapor source.

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Figure 10. Measured concentration of PCE in a monitoring well approximately 60 to 70 feet from House 1 and House 2. The well is screened over a 10-foot interval (59 to 69 feet below ground surface) that spans the water table (∼ 65 feet). Note that the decline in the measured concentration of PCE in the shallow ground water is not reflected in a concomitant decline in subslab concentrations at the nearby houses.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Redfield site, Colorado
  5. New York State Temporal Study buildings
  6. Conclusions
  7. Acknowledgments
  8. References

The case history data presented herein indicate that where dissolved concentrations of CVOCs are the source of vapor intrusion, the overall extent of vapor intrusion impacts generally mirrors the overall extent of ground water impacts. If the nature and extent of the ground water plume have been reasonably well characterized, and are reasonably stable spatially and temporally, one would not expect to find structures beyond the plume that are impacted by vapor intrusion. Conversely, not all structures above the plume may be impacted by vapor intrusion.

Correlations between ground water concentrations (or equivalent soil-gas concentrations based on Henry’s law) and indoor air concentrations at well-characterized sites like Redfield, Cortlandville, and Endicott are likely to be positive but weak, with ground water attenuation factors typically ranging over two orders of magnitude. Factors that vary in space (or between buildings) most likely contribute to this variability, including the imprecision or error introduced by interpolating ground water concentrations between wells at typical monitoring network densities, and the spatial variability in subslab source concentrations that develop above the ground water source due to variations in depth to ground water, geological conditions within the vadose zone, and building construction (Dawson et al. 2007; Weaver and Tillman 2005; Hers et al. 2003). Factors that vary in time, such as soil moisture content, VOC mass storage in the aqueous phase and sorbed phase of the vadose zone, and changes in building pressurization and air exchange rates, are also likely to contribute to the observed variability.

The importance of collecting truly contemporaneous indoor air and ground water samples (to improve correlation precision) is unclear. As illustrated in the Temporal Study houses, and as may be expected based on transport processes, there is likely a time lag between changes in ground water source concentrations and concomitant changes in the indoor air concentrations above the plume. Transport and storage across the vadose zone most likely dampens short-term fluctuations in ground water concentration (Folkes et al. 2006). Therefore, without detailed four-dimensional characterization of subsurface conditions at each building location, it may not be possible to significantly improve the precision of correlations between ground water and indoor air concentrations by more precise timing of sampling events (either contemporaneous sampling or by accounting for time lag). Collection and correlation of data collected over longer periods of time might provide more insight at typical investigation sites.

Short-term and seasonal temporal variability in subslab and indoor air concentrations within individual buildings may have a significant influence on the observed variation in ground water attenuation factors. Up to 10 years of indoor air monitoring data in 45 unmitigated homes at the Redfield site indicate that 1,1-DCE concentrations typically vary by a factor of 2 to 3 about the annual average for each home. Seasonal variations explain a portion of this variability, with winter and summer concentrations that were, on average, about 20% higher and 50% lower than the average annual concentration, respectively. A similar range of temporal variability was observed in subslab concentrations of the Temporal Study homes during that 19-month-long study. The majority of the variability in vapor intrusion impacts over time at the Redfield site appears to be due to nonseasonal factors, which may include short-term variations in air exchange rate and the differential pressure between the indoor and subslab of the building, likely attributable to variation in meteorological (wind, barometric pressure, temperature) and ventilation (exhaust fans, combustion appliances, open windows) factors. This is consistent with the study by Davies and Forward (1970), who observed daily variations in radon concentrations of as much as an order of magnitude. Changes in subsurface moisture conditions could also affect vapor intrusion rates in the short term or seasonally (e.g., Megumi and Mamuro 1974).

Because many factors contribute to vapor intrusion, improving the accuracy and precision of any one factor (such as ground water concentration) will only partly improve the accuracy of our predictions of indoor air concentration at any point in time. For example, sensitivity analyses by Weaver and Tillman (2005) indicated that indoor air concentrations predicted by the USEPA (2004) version of the Johnson and Ettinger model varied by more than an order of magnitude, assuming a reasonable range for the numerous input parameters. Such predictions would likely not account for all the short-term temporal variability seen in indoor air concentrations, nor do they include imprecision in the source (e.g., ground water) concentrations in time and space. Similarly, more accurate measurements of ground water, soil vapor, or subslab concentrations at one point in space are unlikely to increase the accuracy of our predictions if spatial variability is not well represented. It is possible that long-term average indoor air concentrations at any point in time (thus eliminating the effects of temporal variability) could be predicted within order-of-magnitude accuracy at well-characterized sites, but this would have to be evaluated on a case by case basis. It is also likely that reasonable risk management decisions can be made based on subsurface data (in the absence of indoor air measurements), but the inherent order of magnitude or greater imprecision of any such predictions would have to be recognized.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Redfield site, Colorado
  5. New York State Temporal Study buildings
  6. Conclusions
  7. Acknowledgments
  8. References

Krista Anders, New York State Department of Health, played a key role in the design and implementation of the Temporal Study. John Boyd, Thomas Urban, and Jon Sunquist, URS Corporation, collected and organized the Temporal Study data.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Redfield site, Colorado
  5. New York State Temporal Study buildings
  6. Conclusions
  7. Acknowledgments
  8. References
  • [CDPHE] Colorado Department of Public Health and Environment. 2000. Guidance for analysis of indoor air samples. Revised. Denver: CDPHE, Hazardous Materials and Waste Management Division.
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Biographical Sketches
  • 1

    David J. Folkes, PE, corresponding author, is a principal of EnviroGroup Limited in Centennial, Colorado, and the project manager for the Redfield site. He has an M.Sc. in geotechnical engineering from the University of Toronto and more than 30 years of experience in environmental and geotechnical consulting, including more than 10 years focusing on vapor intrusion. He may be reached at EnviroGroup Limited, 7009 S. Potomac Street, Suite 300, Centennial, CO 80112; (303) 790-1340; dfolkes@envirogroup.com.

  • 2

    William E. Wertz, Ph.D., is an engineering geologist with the New York State Department of Environmental Conservation. He can be reached at NYSDEC, 625 Broadway, Albany, NY 12233-7013; (518)402-9814; wewertz@gw.dec.state.ny.us.

  • 3

    Jeffrey P. Kurtz, Ph.D., is a senior project manager with EnviroGroup Limited and the indoor air program manager for the Redfield site. He can be reached at EnviroGroup Limited, 7009 S. Potomac Street, Suite 300, Centennial, CO 80112; (303) 790-1340; jkurtz@envirogroup.com.

  • 4

    Theodore E. Kuehster, PE, is a senior project manager with EnviroGroup Limited. He can be reached at EnviroGroup Limited, 7009 S. Potomac Street, Suite 300, Centennial, CO 80112; (303) 790-1340; tkuehster@envirogroup.com.