Radon Entry into Buildings Driven by Atmospheric Pressure

In the context of radon entry into houses, indoor−outdoor pressure differences .... flow; details of the sensor and its calibration are described by...
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Environ. Sci. Technol. 1997, 31, 1742-1748

Radon Entry into Buildings Driven by Atmospheric Pressure Fluctuations ALLEN L. ROBINSON AND RICHARD G. SEXTRO* Energy and Environment Division, E. O. Lawrence Berkeley National Laboratory, Berkeley, California 94720

To examine the effects of atmospheric pressure fluctuations on radon entry into houses, we report measurements of soil-gas and advective radon entry made using an experimental basement. Based on these measurements, we quantify the contribution of atmospheric pressure fluctuations, steady indoor-outdoor pressure differences, and molecular diffusion to the long-term radon entry rate into the experimental basement. In the absence of a steady indooroutdoor pressure difference, atmospheric pressure fluctuations at the study site induce a radon entry rate 1.5 times greater than that due to molecular diffusion. A steady indoor-outdoor pressure difference reduces the contribution of atmospheric pressure fluctuations to the longterm radon entry rate. For sustained indoor-outdoor pressure differences with a magnitude greater than 1.5 Pa, atmospheric pressure fluctuations have essentially no effect on the time-averaged radon entry rate into the experimental structure. The results of this study demonstrate that under certain conditions, such as periods during which indoor-outdoor pressure differences are small, atmospheric pressure fluctuations will contribute measurably to the total radon entry rate into a building, potentially doubling indoor concentrations. However, in absolute terms, atmospheric pressure fluctuations drive approximately the same amount of entry as molecular diffusion and, therefore, will probably not cause houses to have long-term, elevated indoor radon concentrations.

Introduction Advective flow of radon-laden soil gas is the dominant transport mechanism for radon into most houses with elevated indoor radon concentrations (1). This advective entry is commonly associated with small (0-5 Pa) but sustained indoor-outdoor pressure differences created by temperature effects, wind interaction with the building shell, and the operation of heating, ventilation, and air-conditioning (HVAC) systems (1). In the context of radon entry into houses, indoor-outdoor pressure differences are the pressure differences, accounting for the effects of hydrostatics, between the ambient atmosphere at the soil surface and the indoor air at the mouth of an opening between the building and the soil. However, several field studies have reported higher than expected indoor radon concentrations during periods when these pressure differences were small (2-5). Two of these field studies hypothesized that the elevated indoor radon concentrations were caused by radon entry induced by atmospheric pressure fluctuations (2, 5). Recent theoretical (6-8) and experimental studies (9, 10) suggest that atmospheric pressure fluctuations induce transient soil-gas and radon entry into buildings without the * Corresponding author fax: 510-486-6658; e-mail [email protected].

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indoor-outdoor pressure differences commonly associated with advective radon entry. Atmospheric pressure fluctuations create an oscillating gas flow between the interior of a building and the underlying soil through the compression and expansion of the soil gas underneath the building foundation. Although, over the long-term, the net gas flow between a building and the soil caused by atmospheric pressure fluctuations is zero, these fluctuations drive radon entry because soil-gas radon concentrations are typically 3 orders of magnitude larger than the radon concentrations of indoor air. Thus, falling atmospheric pressure draws more radon into a building than is carried out by the gas flow induced by rising atmospheric pressure, creating a net radon entry rate. Although progress has been made on the development of this conceptual model to describe how atmospheric pressure fluctuations cause advective radon entry, no experimental data indicating the relative importance of natural changes in atmospheric pressure for driving radon entry have ever been reported. The goal of this paper is to determine the relative importance of atmospheric pressure fluctuations as a mechanism for driving radon entry into an experimental basement structure. The experiments reported here examined the interaction of atmospheric pressure fluctuations and steady indoor-outdoor pressure differences on the advective radon entry rate. Based on these results, we quantify the contribution of atmospheric pressure fluctuations, steady indooroutdoor pressure differences, and molecular diffusion to the long-term radon entry rate into the experimental structure.

Experimental Methods Experiments were conducted in a controlled experimental basement. In this section, we first describe the experimental facility and the structure instrumentation. We then describe the experimental procedures used to determine the contribution of molecular diffusion, steady indoor-outdoor pressure differences, and atmospheric pressure fluctuations to the long-term radon entry rate. Structure and Site Description. The measurements reported in this study were made using a highly instrumented experimental structure that was designed and constructed to study soil-gas and radon entry into buildings in a wellcharacterized setting (11-13). To ensure that the results obtained from these experiments can be applied to real houses, this structure has several features in common with actual houses. The structure’s concrete walls and floor are 15 cm thick, a common footer design was employed, and the floor of the basement is located ∼1.9 m below grade. A schematic diagram of the structure and the surrounding soil is shown in Figure 1. The structure is a single chamber with interior dimensions of 2.0 × 3.2 m and a height of 2.0 m; only about 0.1 m of the walls extend above grade. The floor slab rests on a 0.1 m thick, high-permeability gravel layer. The soil at the experimental site has been extensively characterized (12, 14, 15). Table 1 reports the measured permeability of the gravel, backfill, and undisturbed soil at the structure site. The backfill region, shown in Figure 1, was excavated during construction of the structure. After construction, it was carefully refilled and compacted in an attempt to minimize the disturbance of the native soil environment; however, as Table 1 indicates, the backfill region has a lower permeability than the undisturbed soil. Table 2 summarizes measurements of the air-filled porosity, radon emanation fraction, and radium content of the soil at the structure site. All openings between the structure interior and the soil are sealed except for a 3.8-cm-diameter hole in the center of the structure floor. Although this hole is not geometrically

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FIGURE 1. Schematic of experimental structure and flow sensor. A CRM is a continuous radon monitor. The structure and backfill are drawn to scale. The flow sensor and the hole in the center of the structure floor are drawn at a scale approximately four times larger than that of the structure drawing.

TABLE 1. Averages of Measured Soil and Gravel Permeability at Structure Site soil region

permeability (m2)

undisturbeda

3.0 × 10-11 (h); 1.8 × 10-11 (v) 3.5 × 10-12 2.0 × 10-8

backfillb gravelc

a Horizontal permeability (h) based on measured permeability at 3.5-m length scale; vertical permeability (v) based on measured ratio of vertical to horizontal permeability (14). b The average of single-point measurements taken around the basement structure (12). c Based on laboratory measurements using a vertical column filled with a sample of the gravel used below the basement structure (11).

TABLE 2. Averages of Measured Air-Filled Porosity, Radon Emanation Fraction, and Radium Content of Soil at Structure Site depth of layer (m)

soil-grain radium densitya contentb (kg m-3) (Bq kg-1)

0-1.6 2.8 × 103 1.6-2.2 2.8 × 103

30 30

2.2-5 2.8 × 103 5.0-8.5 2.8 × 103

30 30

air-filled porosityc

emanation fractionb

0.45 approximately linear decrease from 0.45 to 0.25 0.25 0.25 (inferred)d

0.31 0.45 0.31 0.31

a Ref 24. b Ref 15. c Based on gravimetric analysis (25) of soil cores taken by Flexser et al. (15). d We have extrapolated the measured profile to 8.5 m, the measured depth of the water table below the soil surface.

representative of the perimeter cracks that exist in many houses (16), it is similar to the gaps frequently found around plumbing and utility penetrations through a building foundation (17). These experiments require such an opening in combination with a high-permeability subslab gravel layer to enable atmospheric pressure fluctuations to generate soilgas velocities greater than the detection limit of our flow sensor. Previous experiments have shown that, as long as the opening in the structure floor does not provide significant resistance to flow, the advective radon entry rate into the structure only weakly depends on the geometry of this opening due to the presence of a high-permeability subslab gravel layer (13). Therefore, the measured radon entry rate through the hole is representative of the flow between the structure and the underlying soil for a wide range of opening configurations.

Instrumentation. The atmospheric pressure was measured at 5-s intervals using a high-resolution pressure transducer (Paroscientific Model 1015a) connected to an outdoor omnidirectional static pressure tap located ∼3 m from the structure. The response time, accuracy, and resolution of this pressure transducer are 1 s, ( 5 Pa, and 0.1 Pa, respectively. The pressure difference between the interior of the structure and the static pressure tap was measured at 30-s intervals using a differential pressure transducer (Validyne Model DP103). The gas flow rate through the entry hole in the center of the structure floor was measured at 5-s intervals using the flow sensor shown in Figure 1. This sampling interval is much shorter than the ∼2-min characteristic response time of the soil gas at the structure site to changes in atmospheric pressure (10). The sensor incorporates two omnidirectional hot-film velocity transducers (TSI Model 8470) mounted in a U-shaped tube (1.9 cm i.d., ∼30 cm high) and can measure the magnitude and direction of gas flow as small as 0.15 L min-1. Two velocity transducers are required to determine the direction of the soil-gas flow; details of the sensor and its calibration are described by Robinson (18). The response time, accuracy, and resolution of the flow sensor are 2 s, 5% of reading, and 0.02 L min-1, respectively. For the range of flows considered in this study, the pressure drop in the flow sensor tube varies linearly with flow rate and was measured in the laboratory to be 0.14 Pa min L-1. Continuous radon monitors (CRMs) were used to measure radon concentrations in three locations. As indicated in Figure 1, one CRM sampled air from the top of the U-shaped flow sensor tube, another CRM sampled soil gas from the subslab gravel layer at a location ∼15 cm from the entry hole in the center of the structure floor. From these locations, gas samples were drawn at a constant flow rate of 66 cm3 min-1, passed through a 33 cm3 scintillation cell, and then exhausted to the outdoors. To reduce the potential effects of 220Rn on the measurements, the samples were drawn through an 11m-long tube, which, at these flow rates, provided a 3-min delay before the sample gas arrived in the scintillation cell. Counts from each CRM were recorded at 1-min intervals and interpreted using the algorithm described by Busigin et al. (19). These low-detection-volume, low-flow CRMs were designed to minimize the effects of sampling on the flows into and out of the structure and yet maximize the temporal resolution of our measurements. The 66 cm3 min-1 CRM sampling flow rate is much smaller than the 500-1000 cm3 min-1 flow rates in the sensor tube caused by typical atmospheric pressure fluctuations. The high radon concentration of the soil gas underneath the structure, ∼105 Bq m-3, enables us to achieve acceptable levels of statistical uncertainty despite the small size of the scintillation cell. An unmodified CRM (scintillation cell volume of 100 cm3 and a sample flow rate of ∼200 cm3 min-1) was used to measure the radon concentration of the air inside the structure. To allow accurate sampling of the structure radon concentration from a single location, an oscillating fan continually mixed the structure air. The indoor radon concentration was determined by analyzing the counts from the structure CRM using the method described by Thomas and Countess (20). Measurements of Radon Entry Rate. Radon enters the experimental structure by advection of soil gas through the hole in the center of the structure floor and by molecular diffusion through the structure’s concrete walls and floor. The total radon entry rate into the structure, ST(t) (Bq s-1), is

ST(t) ) SA(t) + SD

(1)

where SA(t) is the advective radon entry rate as a function of

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time and SD is the diffusive entry rate through the concrete walls and floor. Both atmospheric pressure fluctuations and steady indoor-outdoor pressure differences induce advective radon transport. Experiments were conducted to make time series measurements of SA(t) as a function of steady indoor-outdoor pressure difference. For each experiment, a pump and a computer-controlled mass-flow controller was used to establish a steady indoor-outdoor pressure difference, henceforth referred to as ∆P. In addition to the imposed ∆P, the pressure of the interior of the structure was allowed to fluctuate with the natural variations in atmospheric pressure. Two 1.25-cm-diameter holes in one wall of the access hatch permitted the pressure in the structure to respond faster than 0.1 s to changes in atmospheric pressure. This time scale is comparable to the measured response times of the interior of real houses to changes in atmospheric pressure (21). Because changes in atmospheric pressure occur simultaneously inside the structure and at the soil surface, these changes do not affect ∆P. For those experiments conducted at neutral pressure conditions (∆P ) 0 Pa), the interior of the structure was pressurized to offset the slight ∆P, ∼ -0.15 Pa, created by the thermal stack effect. This effect is created by the 5-10 °C temperature difference between the soil gas and the air inside the structure. We calculated SA(t) using the measured gas flow rate through the flow sensor tube, Q(t) (m3 s-1), and the measured radon concentration of the air inside the flow sensor tube, C(t) (Bq m-3):

SA(t) ) Q(t)C(t)

(2)

When the magnitude of C(t) falls below the detection limit of the CRM used to monitor the radon concentration of gas inside the flow sensor tube, we assume that the radon concentration of the air inside the sensor tube is equal to the measured indoor radon concentration. This occurs when structure air is forced out through the sensor tube and into the soil by rising atmospheric pressure. The detection limit associated with our measurement of C(t), typically 7500 Bq m-3, is similar in magnitude to the measured indoor radon concentration, but it is much smaller than the ∼105 Bq m-3 radon concentration of the soil gas underneath the structure floor slab. Most of the detection limit is due to counting statistics and to the 1-min counting interval used in the analysis of the flow sensor CRM data. The diffusive radon entry rate into the experimental structure, SD, is 0.1 Bq s-1. This value was determined by averaging radon flux measurements from various locations on the inside surface of the structure’s concrete walls and floor (22). The flux measurements were made under neutral pressure conditions, ∆P ) 0 Pa. Because of the small crosssectional area of the entry hole in the center of the structure floor, we neglect the contribution of the diffusive flux through this hole to the total radon entry rate. For this study, we approximate the diffusive radon entry rate into the structure as constant, independent of the advective radon entry rate, because the difference between the indoor radon concentration and that of the surrounding soil gas was relatively invariant. Radon measurements made in the subslab gravel layer indicate that the radon concentration of the soil gas underneath the structure floor slab was essentially constant except for a small region surrounding the entry hole. In addition, the variation in the measured indoor radon concentration was small relative to the concentration difference across the concrete walls and floor. Radon Entry Driven by Atmospheric Pressure Fluctuations. Since both atmospheric pressure fluctuations and ∆P drive advective radon entry, we must determine the contribution each of these mechanisms to the total advective radon entry rate. We calculate the time-averaged contribution of

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FIGURE 2. Measured gas flow rate through the 3.8-cm-diameter hole in the center of the structure floor as a function of steady indoor-outdoor pressure difference. The solid line is the linear regression, the slope of which represents the inverse of the resistance of the soil-structure system to flow. atmospheric pressure fluctuations, SAP (Bq s-1), to the total advective radon entry rate using

SAP ) Sh A - SSS(∆P)

(3)

where Sh A is the measured advective radon entry rate averaged over a time period T:

Sh A )

1 T

∫ S (t) dt T

0

A

(4)

and SSS(∆P) is the entry rate driven by a steady indooroutdoor pressure difference (∆P). For a given value of ∆P, we determine SSS(∆P) using

SSS(∆P) ) QSS(∆P)CSS

(5)

where QSS(∆P) (m3 s-1) is the soil-gas entry rate caused by ∆P, and CSS is the radon concentration of the undisturbed soil gas underneath the structure floor slab, ∼96 000 Bq m-3. CSS is the measured radon concentration of the gas inside the flow sensor when the interior of the structure is depressurized a steady 5 Pa, a value at which atmospheric pressure fluctuations are not observed to cause any low-concentration indoor air to flow out of the structure and into the soil. In addition, this value of ∆P is not high enough to produce air flow rates from the soil surface through the soil sufficient to dilute the radon concentration of the soil gas next to the structure. We calculate QSS(∆P) using

QSS(∆P) )

∆P R

(6)

where R is the resistance of the soil-structure system to steadystate soil-gas entry (Pa min L-1). We determined R by measuring the gas flow rate through the entry hole in the center of the structure floor for a range of ∆P. A linear regression of these measurements, shown in Figure 2, yields the resistance of the soil-structure system, 2.9 Pa min L-1. The measurements shown in Figure 2 were made with the holes in the structure access hatch sealed to prevent atmospheric pressure fluctuations from inducing soil-gas flow. Long-Term, Time-Averaged Radon Entry Rates. To examine the importance of atmospheric pressure fluctuations as a mechanism for driving radon entry, we calculated the time-average advective radon entry rate into the experimental structure as a function of ∆P. Each average contained at least 7 days of measurements. Since more than 99% of the

total power of the time rate of change of the atmospheric pressure spectrum occurs at frequencies greater than 1 day-1 (8), this average entry rate should represent the effect of typical atmospheric pressure fluctuations on the long-term radon entry rate into the experimental structure. For this analysis, data from each experiment were first broken into 24-h time blocks. To minimize the influence of temporal variations in the indoor-outdoor pressure difference on the radon entry rate, we discarded time blocks in which the standard deviation of the measured indooroutdoor pressure difference exceeded 0.5 Pa. Large fluctuations in indoor-outdoor pressure typically occurred during storms or other periods with high winds (wind speeds greater than 5 m s-1). For the remaining time blocks, we calculated the 24-h average advective radon entry rate and the 24-h average indoor-outdoor pressure difference. Time blocks with similar indoor-outdoor pressure differences, within (0.15 Pa, were grouped together and averaged to estimate the long-term advective radon entry rate as function of ∆P. The total, long-term radon entry rate is the sum of SD and the long-term advective radon entry rate.

Results and Discussion In this section, we first present time series measurements of radon entry to examine the relationship between atmospheric pressure fluctuations and the advective radon entry rate into the experimental structure. Next, we discuss the effects of soil-gas dilution caused by the flow of low-concentration indoor air out of the structure and into the soil on the advective radon entry rate. We then time-average our measurements of radon entry to estimate the contribution of atmospheric pressure fluctuations to the long-term radon entry rate into the experimental structure. Finally, we discuss briefly the implications of this work to our understanding of radon entry into buildings. Time Series Measurements of Radon Entry. Time series measurements are shown in Figure 3 to illustrate the relationship between atmospheric pressure fluctuations, the resulting soil-gas flows, the soil-gas radon concentration, and the advective radon entry rate into the experimental structure. These measurements were made under neutral pressure conditions, ∆P ) 0 Pa. Figure 3a shows the measured atmospheric pressure signal for a 2-h period chosen to illustrate the dynamics of the system. Since changes in pressure drive soil-gas flow, the time rate of change of atmospheric pressure, calculated over a 15-s interval, is shown in Figure 3b. The corresponding gas flow rate through the hole in the center of the structure floor, Q(t), is shown in Figure 3c. The measured radon concentration in the Ushaped pipe, C(t), is shown in Figure 3d. The radon entry rate into the structure, calculated using eq 2, is shown in Figure 3e. Summary statistics for this 2-h period, such as the average advective radon entry rate and the volume of gas forced into and out of the structure by the atmospheric pressure fluctuations, are listed in Table 3. Before examining the measurements of radon entry, it is important to understand the relationship between changes in atmospheric pressure and the gas flow rate between the structure interior and the underlying soil because this flow transports the radon into the structure. In this paper, we only outline a couple of important aspects of this relationship, for more details the reader is referred to Robinson et al. (8, 10). A comparison of the calculated time rate of change of atmospheric pressure, shown in Figure 3b, and the measured gas flow rate, shown in Figure 3c, reveals that the soil-gas flow rate follows the time rate of change of atmospheric pressure. Falling atmospheric pressure drives soil-gas entry into the structure; rising atmospheric pressure forces air from inside the structure into the soil. The larger the time rate of change of atmospheric pressure, the larger the gas flow rate into or out of the structure. Over the long-term, the net gas

FIGURE 3. Time series measurements made during neutral pressure conditions (no steady indoor-outdoor pressure difference): (a) measured atmospheric pressure, (b) time-rate-of-change of atmospheric pressure, (c) Q(t), measured gas flow rate, (d) C(t), measured radon concentration in the U-shaped flow sensor, and (e) SA(t), calculated advective radon entry rate. Negative values in Q(t) in panel c indicate flow from the soil into the experimental structure. The dashed line in panel e indicates the time-averaged advective radon entry rate for this 2-h period. flow between a building and the underlying soil driven by atmospheric pressure fluctuations is zero; Table 3 indicates that for the 2-h period shown in Figure 3, changes in atmospheric pressure cause 30 L of soil-gas to enter the structure and 32 L of indoor air to exit the structure. Although over the long-term atmospheric pressure fluctuations do not create a net gas flow rate into the structure, the measurements shown Figure 3 illustrate how the oscillating gas flow created by these fluctuations causes a net radon entry rate into the structure. Figure 3 indicates that falling atmospheric pressure draws high radon concentration soil gas, ∼80 000 Bq m-3, into the structure. In contrast, rising atmospheric pressure forces low radon concentration indoor air, ∼2600 Bq m-3, back into the soil. This large concentration difference causes substantially more radon to be transported into the structure by falling atmospheric pressure than is forced out by rising atmospheric pressure, creating a net radon entry rate without a net gas flow between the structure and the underlying soil. The time-average advective radon entry rate for the 2-h period shown in Figure 3 is 0.23 Bq s-1, which is more than two times the measured diffusive entry rate of 0.1 Bq s-1.

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TABLE 3. 2-h Time-Average Values for Soil-Gas Flow and Radon Entry Measurements Shown in Figures 3-5 figure

∆P (Pa)

QSS (L min-1)

vol of gas in (L)

vol of gas out (L)

av advective radon entry rate (Bq s-1)

no-dilution entry rate (Bq s-1)

indoor Rn concn (Bq m-3)

3 4 5

0 0.9 -0.9

0 0.3 -0.3

32 15 55

30 52 19

0.23 0.02 0.58

0.43 0.21 0.74

2600 1800 4900

FIGURE 4. As in Figure 3 except the measurements were made during a period when the interior of the structure was steadily pressurized 0.9 Pa relative to the ambient atmosphere. The dashed line shown in panel c indicates the gas flow rate out of the structure caused by a 0.9 Pa steady indoor-outdoor pressure difference. The dashed line in panel e indicates the time-averaged advective radon entry rate for this 2-h period.

FIGURE 5. As in Figure 3 except the measurements were made during a period when the interior of the structure was steadily depressurized 0.9 Pa relative to the ambient atmosphere. The dashed line shown in panel c indicates the soil-gas entry rate into the structure caused by a -0.9 Pa steady indoor-outdoor pressure difference. The dashed line in panel e indicates the time-averaged advective radon entry rate for this 2-h period.

Time series measurements are shown in Figures 4 and 5 to illustrate the effect of atmospheric pressure fluctuations in combination with a non-zero ∆P on the advective radon entry rate into the experimental structure. The results shown in Figure 4 are from a period during which the interior of the structure was pressurized, ∆P ) +0.9 Pa; the measurements shown in Figure 5 were made during a period when the interior of the structure was depressurized, ∆P ) -0.9 Pa. Summary statistics for each of these 2-h periods are also listed in Table 3. Again, before examining the measurements of radon entry shown in Figures 4 and 5, it is helpful to understand how atmospheric pressure fluctuations and ∆P interact to determine the gas flow rate between the structure and the underlying soil. Since the equations that govern soil-gas flow

are linear, we can separate the measured gas flow rate into a component caused by ∆P and a component induced by atmospheric pressure fluctuations. The former creates a dc offset in the gas flow rate into the structure. To calculate the magnitude of this offset, we use the measured value of ∆P and eq 6. For example, the 0.9 Pa indoor-outdoor pressure difference measured during the 2-h period shown in Figure 4 creates a 0.32 L min-1 offset in the soil-gas flow rate. The calculated offsets in the gas flow rate created by ∆P are indicated by the dashed lines in Figures 4c and 5c. The timevarying flows due to the atmospheric pressure fluctuations are then added to this offset. The measurements shown in Figure 4 indicate that atmospheric pressure fluctuations drive advective radon entry into the structure even when the interior of the structure is

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slightly pressurized above the ambient atmosphere. Sustained periods of rapidly falling atmospheric pressure overcome the outward gas flow rate created by the slightly positive values of ∆P and draw soil gas into the structure. For the 2-h period shown in Figure 4, atmospheric pressure fluctuations cause a time-averaged advective radon entry rate of 0.02 Bq s-1, which is 20% of the measured diffusive entry rate. If the interior of the structure is slightly depressurized relative to the ambient atmosphere, atmospheric pressure fluctuations also increase the advective radon entry rate into the structure. In effect, the atmospheric pressure fluctuations increase the rate at which soil gas is drawn into the structure in comparison to a period during which the interior of the structure is only steadily depressurized. The average advective radon entry rate for the 2-h period shown in Figure 5 is 0.58 Bq s-1, which is 20% greater than the radon entry rate driven by a -0.9 Pa steady indoor-outdoor pressure difference. Soil-Gas Dilution. In this section, we examine the effects of soil-gas dilution caused by the outflow of low radon concentration indoor air into the soil. This dilution complicates the relationship between the soil-gas entry rate and the corresponding advective radon entry rate. A comparison of the two spikes in the radon entry rate immediately before hour 139 in Figure 3e illustrates this complex relationship. Although these spikes are caused by equivalent soil-gas entry rates, ∼1 L min-1, the second spike in the radon entry rate is much smaller than the first due to the dilution of the soil gas underneath the structure. If there was no dilution of the soil gas underneath the structure, then these two spikes in the radon entry rate should have essentially the same magnitude. The overall effect of soil-gas dilution is to reduce the contribution of atmospheric pressure fluctuations to the timeaveraged, advective radon entry rate. To quantify this reduction, we calculate a hypothetical “no-dilution” radon entry rate for each of the 2-h periods shown in Figures 3-5. The no-dilution entry rate is the product of the rate at which soil-gas is drawn unidirectionally into the structure and the undisturbed radon concentration of the soil gas underneath the structure floor slab, CSS. Comparing the no-dilution entry rate to the actual, time-averaged radon entry rate listed in Table 3 reveals that soil-gas dilution reduces the timeaveraged advective radon entry rate by a factor of 2 under neutral pressure conditions. Pressurizing the interior of the structure dramatically increases the effects of dilution; depressurizing the interior of the structure reduces the effects of dilution. The reduction in the advective radon entry rate caused by soil-gas dilution depends on both the history of the gas flow into and out of the structure and the rate at which the radon concentration underneath the structure is recharged. The gas flow depends on both ∆P and the atmospheric pressure fluctuations. The recharge rate of the soil gas depends on both the production of radon, through the decay of 226Ra in the soil, and the transport of radon through the soil pore space. Since gas flow, radon transport, and radon generation all depend strongly on the soil properties, the effects of soilgas dilution on the advective radon entry rate could be significantly different for houses located in soils with very different properties than those of the soil at the structure site. Long-Term Radon Entry Rate. To examine the effects of atmospheric pressure fluctuations on radon entry for a wide range of environmental conditions, Figure 6 presents the measured long-term radon entry rate into the experimental structure as a function of ∆P. Each data point shown in Figure 6 was determined by time averaging at least 7 days of radon entry measurements, as previously described. Short time series, such as those shown in Figures 3-5, may not accurately indicate the contribution of atmospheric pressure fluctuations to the long-term radon entry rate because occasional, high-

FIGURE 6. (a) Measured long-term radon entry rate as a function of steady indoor-outdoor pressure difference (∆P). Each data point indicates the average of at least 7 days of measurements. (b) Ratio of the long-term radon entry rate driven by atmospheric pressure fluctuations to that due to the combined contribution of molecular diffusion and ∆P. A ratio of 100% indicates that atmospheric pressure fluctuations drive the same amount of radon entry as both diffusion and steady indoor-outdoor outdoor pressure differences, i.e., such fluctuations increase the long-term radon entry rate by a factor of 2. Negative ∆P indicates that the interior of the structure is depressurized relative to the ambient atmosphere. The contribution of steady depressurization of the structure interior to the radon entry rate is estimated based on the resistance of the soil-structure system, as described in the text. Diffusive entry was measured under neutral pressure conditions and is assumed to be independent of indooroutdoor pressure difference. Only diffusion and atmospheric pressure fluctuations drive radon entry when ∆P is greater than or equal to zero. Lines are smooth curves through the data and are intended only for visual guidance. The vertical bars indicate the measurement uncertainties. frequency oscillations in atmospheric pressure can generate large but intermittent soil-gas and radon entry rates. Figure 6a identifies the contribution made by atmospheric pressure fluctuations, ∆P, and molecular diffusion to the total, long-term radon entry rate. Figure 6b shows the ratio of the long-term radon entry rate driven by atmospheric pressure fluctuations to that due to the combined contribution of molecular diffusion and ∆P. The contribution of molecular diffusion is 0.1 Bq s-1 and is assumed to be independent of the effects of soil-gas flow, as previously described. The contribution of ∆P to the long-term radon entry rate is defined by eq 5. As Figure 6a indicates, when the interior of the structure is pressurized above the ambient atmosphere only diffusion and atmospheric pressure fluctuations produce radon entry. When the interior of the structure is depressurized, the resulting ∆P, the atmospheric pressure fluctuations, and diffusion all contribute to the long-term radon entry rate. Figure 6 demonstrates that, in the presence of small values of ∆P, atmospheric pressure fluctuations measurably con-

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tribute to the long-term radon entry rate into the experimental structure. When ∆P ) 0 Pa, the total radon entry rate into the structure is 0.25 Bq s-1, of which 0.1 Bq s-1 is due to diffusion and the remaining 0.15 Bq s-1 is caused by atmospheric pressure fluctuations. This represents a 150% increase in long-term radon entry due to atmospheric pressure fluctuations, as shown in Figure 6b. Small indoor-outdoor pressure differences commonly occur during the summer when indoor-outdoor temperature differences are small and winds are light. Steady indoor-outdoor pressure differences reduce the contribution of atmospheric pressure fluctuations to the longterm radon entry rate. When the magnitude of ∆P is greater than 1.5 Pa, atmospheric pressure fluctuations have a negligible effect on the long-term radon entry rate into the experimental structure. In real houses, indoor-outdoor pressure differences exceeding this value are common during the winter heating season or on windy days. Figure 6a indicates that as the value of ∆P becomes more negative the radon entry rate increases. For ∆P values beyond -1.0 Pa, advective entry caused by this underpressure overwhelms the contribution of molecular diffusion and atmospheric pressure fluctuations to the long-term radon entry rate. This finding is consistent with the existing conceptual model for radon entry into houses which states that advective radon entry caused by steady indoor-outdoor pressure differences is the dominant transport mechanism of radon into houses with elevated indoor concentrations (1). Implications. This study demonstrates that atmospheric pressure fluctuations can induce measurable advective radon entry rates when the magnitude of the indoor-outdoor pressure differences are less than 1.5 Pa. Therefore, analyses that only consider the effects of molecular diffusion and steady indoor-outdoor pressure differences will underpredict the total radon entry rate into a building. In addition, the results of this study have implications on the transport of other soilgas contaminants into buildings. The effects of atmospheric pressure fluctuations on longterm radon entry in actual houses will depend on the soil properties and the building structural configuration. This study only examined the combination of soil properties listed in Tables 1 and 2 and the structural configuration of our experimental basement. Several theoretical studies have considered the effects of atmospheric pressure fluctuations for a wider range of building and soil types (7, 8). Ultimately, the importance of atmospheric pressure fluctuations as a mechanism for driving radon entry must be judged in terms of its impact on the indoor radon concentration. In relative terms, the results of this study suggest that, under certain conditions, atmospheric pressure fluctuations can significantly increase indoor concentrations. For example, noting that steady-state indoor radon concentrations vary linearly with the radon entry rate when the building ventilation rate is constant (23), Figure 6a indicates that atmospheric pressure fluctuations will more than double indoor concentrations in experimental structure when ∆P ) 0 Pa. Consequently, radon entry driven by atmospheric pressure fluctuations is a plausible explanation for the higher than expected indoor radon concentration observed when indoor-outdoor pressure differences were small (2-5). However, in absolute terms, the results of this study and the previous theoretical analyses (7, 8) suggest that atmospheric pressure fluctuations drive approximately the same amount of entry as molecular diffusion; thus, the entry rates produced by these fluctuations will not lead to long-term, elevated indoor radon concentrations.

by the Director, Office of Energy Research, Office of Health and Environmental Research, Environmental Sciences Division, and by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology of the U.S. Department of Energy (DOE), under Contract DE-AC03763SF00098.

Acknowledgments

ES960715V

The authors thank J. Daisey, K. Garbesi, and P. Price for reviewing a draft of this manuscript. This work was supported

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Received for review August 19, 1996. Revised manuscript received January 23, 1997. Accepted February 3, 1997.X

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Abstract published in Advance ACS Abstracts, April 1, 1997.