Temporal Variability of Indoor Air Concentrations under Natural Conditions in a House Overlying a Dilute Chlorinated Solvent Groundwater Plume

Abstract

Current vapor intrusion (VI) pathway assessment heavily weights concentrations from infrequent (monthly-seasonal) 24 h indoor air samples. This study collected a long-term and high-frequency data set that can be used to assess indoor air sampling strategies for answering key pathway assessment questions like: "Is VI occurring?", and "Will VI impacts exceed thresholds of concern?". Indoor air sampling was conducted for 2.5 years at 2-4 h intervals in a house overlying a dilute chlorinated solvent plume (10-50 μg/L TCE). Indoor air concentrations varied by 3 orders of magnitude (<0.01-10 ppbv TCE) with two recurring behaviors. The VI-active behavior, which was prevalent in fall, winter, and spring involved time-varying impacts intermixed with sporadic periods of inactivity; the VI-dormant behavior, which was prevalent in the summer, involved long periods of inactivity with sporadic VI impacts. These data were used to study outcomes of three simple sparse data sampling plans; the probabilities of false-negative and false-positive decisions were dependent on the ratio of the (action level/true mean of the data), the number of exceedances needed, and the sampling strategy. The analysis also suggested a significant potential for poor characterization of long-term mean concentrations with sparse sampling plans. The results point to a need for additional dense data sets and further investigation into the robustness of possible VI assessment paradigms. As this is the first data set of its kind, it is unknown if the results are representative of other VI-sites.

Long-Term Evaluation of the Controlled Pressure Method for Assessment of the Vapor Intrusion Pathway

“…Tables 1 and 2 summarize characteristics of the concentrations and emission rate data, respectively, for the 780 < t < 1045 days period spanning Fall, Winter, and Spring seasons and the beginning of Summer. In these analyses and in Overall, the results indicate that the TCE and radon concentrations and emission rates were relatively consistent with time, both across the 265 days duration of the singleblower/high-speed test and within individual 24 h periods, especially in comparison to the multiple orders of magnitude temporal variability reported by Holton et al 13 As the results in Table 2 show, emission rates were also relatively consistent with time. The maximum 24 h average TCE emission rate is only about 3× greater than the minimum value and both are within about ±50% of the mean value, the maximum real-time TCE emission rate is about 6× the minimum real-time value, and 80% of all real-time and 24 h average TCE concentrations are within about ±50% of their respective mean values.…”

“…Testing took place in preparation for Phase II during 741 days < t < 764 days. Phase I and Phase II results are presented on a timeline consistent with results in Holton et al 13 CPM Test Equipment and Methods. Two identical dualspeed blowers were installed in an upstairs closet and exhausted into the attic, which vents to the atmosphere.…”

“…The subfoundation zone drains to a neighborhood land drain system running across the southern property boundary through a lateral pipe connecting the two (see Figure S1 in Supporting Information (SI)). A house photo appears in Holton et al 13 This house was used by McHugh et al 9 for two short-term proof-of-concept CPM tests.…”

“…The data below come from two study phases. Phase I was about 2.5 years long (−179 days <t < 740 days) and involved monitoring under natural conditions (Holton et al 13 ). Phase II involved monitoring under CPM conditions for about a year, starting at t = 765 ays.…”

“…Having established that CPM results at the test house are relatively insensitive to date and time of testing, we next assess the extent to which CPM tests can be used to anticipate VI impacts under natural conditions. To do so, the 128 < t < 730 day TCE thermal desorption concentration data and SF 6 results presented in Holton et al 13 as well as unpublished radon data collected in that time period were used to calculate TCE and radon emissions under natural conditions (Figure 4). Relevant statistical characteristics of concentrations and emission rates under both natural and CPM conditions are summarized in Tables 1 and 2. Three key conclusions with significance to VI pathway assessment can be drawn from a comparison of the results from natural and CPM conditions.…”