Environ. Sci. Technol. 1996, 30, 2914-2920
Effectiveness of a Passive Subslab Ventilation System in Reducing Radon Concentrations in a Home DIANA J. HOLFORD* AND HARLEY D. FREEMAN Pacific Northwest National Laboratory, Richland, Washington 99352
The effectiveness of a passive subslab ventilation system in reducing radon concentrations in an occupied home was investigated by measuring radon concentrations and pressure differentials during a 1-year period when a passive subslab ventilation system was being cycled on and off. Radon concentrations in the house were 30% lower during periods when the stack was open to the atmosphere. This effect was most pronounced when the home was unoccupied and during the winter and spring months. Furnace use and wind speed were the best predictors of transient changes in basement radon concentrations, whether the stack was open or closed. Pressure differential measurements show that subslab depressurization occurs when the stack is open during the winter and spring months due to bouyancy-driven air flow up the stack, but not during the summer. Numerical simulations of gas flow and radon transport into the house from the surrounding soil were calibrated to observed pressure differentials and radon concentrations. The model predicts that peak radon concentrations caused by furnace use will be reduced by flow out of the stack. However, the model is unable to account for the reduction in average radon concentrations observed while the stack is open in the winter.
Introduction Objective. In 1993, the Washington State Department of Health initiated a study of passive subslab ventilation system (“passive stack”) effectiveness in new homes. The primary objective of the study was to establish whether passive stacks, as they are being installed in residential construction, have an effect on indoor radon levels in houses in the Spokane, WA, and Post Falls, ID, areas (1). The passive stacks in 23 houses were alternately opened and closed in 2-week cycles from February to November 1994. Average radon concentrations in the basement of most houses were significantly lower when the passive stack systems were * Corresponding author telephone: (509) 372-6132; fax: (509) 3726328; e-mail address: dj
[email protected].
2914 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 10, 1996
FIGURE 1. Effectiveness of passive stack in lowering radon concentrations in basements of 21 homes.
operating (Figure 1). As a continuation of that study, radon concentrations, pressure differentials, temperature differentials, and wind velocities were collected in the house where the largest (72%) reduction in radon concentrations was observed (site 006). The objective of this paper is to determine, using a numerical model, whether it was possible for the reduction in radon concentrations observed in this home to be caused by the reduction in subslab-basement pressure differentials observed when the passive stack was in operation. Background. Subslab ventilation is an effective and common method of reducing indoor radon concentrations in houses and basements (2-4). Since 1991, Washington State’s Ventilation and Indoor Quality Code has had radonresistive construction requirements specifying installation of either passive subslab ventilation systems or “alternate” active subslab ventilation (5). The active system commonly consists of a loop of 4-in. flexible slotted drain tube laid on native soil and hardpiped through the roof with a permanently wired fan (6). Active stack systems cost $800-2500 to install and $40300 to operate annually; the indoor radon reduction efficiency is up to 98% (3). The passive system consists of a soil-gas retarder membrane laid under the basement slab on top of a 10-cm layer of coarse aggregate (Figure 2). A continuous sealed pipe runs from a T-joint connected to a 5-foot length of perforated pipe laid horizontally in the aggregate, through the heated portion of the house, to a point at least 1 ft above the roof of the house. The sealed pipe is referred to as a “stack”. In our study house, a slide gate valve allowed the stack to be opened or closed. Buoyancy-driven air flow up the warm stack, analogous to the flow up a chimney, causes a small depressurization in the gravel at the base of the stack and inhibits pressure-driven radon entry through the slab floor (7, 8). The passive system costs the same to install as an active system (minus the cost of the exhaust fan), but costs nothing to operate. The indoor radon reduction efficiency is typically 30-70% (3). S0013-936X(95)00790-5 This article not subject to U.S. copyright. Published 1996 by the American Chemical Society.
FIGURE 2. Passive stack installation.
Methods House Data Collection. As a part of the original passive stack study, radon concentrations were measured in the site 006 house from February 23 to November 15, 1994, and from May 17 to July 21, 1995, using Rad Elec E-PERM electret ion chambers. The radon detectors were hung from the basement ceiling approximately 2.3 m above the slab. The passive stack was opened and closed repeatedly for periods ranging between 7 and 71 days. The E-perm detectors provide average concentrations for the periods that the stack was either open or closed. Prior to the first radon measurement taken on February 23, 1994, the site 006 house was under construction and unoccupied. The house was occupied on July 14, 1994. Radon concentrations in the basement were also measured at 15-min intervals using a BPA/DOE/LBL continuous radon monitor, Model CRM1 (9). The monitors were calibrated at the Environmental Protection Agency’s (EPA) Las Vegas facility by personnel form the EPA and Faytek Inc. of Spokane, WA. The radon concentrations were measured 0.25 m above the basement slab at a point 0.5 m from the passive stack. Radon concentrations were collected as a continuation of the Passive Stack Study by employees of Faytek Inc. Continuous radon measurements were collected from July 14. 1994 to April 14, 1995, when the radon detector malfunctioned. No continuous radon measurements were made after April 14, 1995. Radon measurements from the E-Perm and continuous radon monitor agreed to within 10%. A number of environmental variables were monitored in each house using Campbell Scientific CR10 data loggers. These data loggers are capable of monitoring a variety of sensor types (e.g., thermocouples, voltages, pulses) and periodically storing the information in internal memory for later retrieval. The data were retrieved with a portable computer every 2-4 weeks. All temperature data were compensated by the data logger while the pressure data were stored as raw voltage values and converted to engineering units using separate calibration curves for each sensor. Temperature was monitored at four locations in each house using type T thermocouple wire. Temperature
was monitored at the base of the stack, under the basement concrete slab, in the radon stack at the first floor ceiling level, and in the stack at the roof line. Pressure differentials were measured using Setra Model 264 pressure sensors with a full-scale range of (0.25 in. of H2O. Pressure differentials were measured (1) between the subslab gravel (at a point 2.0 m from the stack base) and the basement, (2) inside the stack between a point at the base and a point at the roof of the house, and (3) between the base of the stack and the basement. The measurement points designated as “basement” were within 0.5 m of the stack. Temperature and pressure differential measurements were collected between January 17 and May 17, 1995, and between July 12 and August 22, 1995. A power sensor was attached to the furnace fan to monitor furnace usage. Wind speed was measured by an anemometer positioned on the roof just above the stack orifice. Wind speed and furnace usage measurements were made between July 14, 1994, and August 22, 1995. Soil samples were collected at several depths before the houses were constructed. Sieve analyses were used to determine sand, silt, and clay fractions; the soil can be classified as a sandy loam. Radium contents were determined by sealing 250 g of soil in an aluminum can for 30 days and then using γ-spectroscopy to detect radon and associated daughter products. Other than furnace usage, the activities of the occupants of the house were not monitored. Some activities that could have affected the measurements taken are the opening and closing of windows and doors, the frequency of rainfall and lawn watering, and the installation of landscaping. Because the monitoring location was in a part of the basement used only for storage, the occupants did not often come near the monitoring equipment. Numerical Modeling. The transport of radon through soil is governed by the radium content of the soil as well as its transport properties: the diffusion coefficient, permeability, and porosity. These properties are important because radon gas moves by advection with the bulk flow of liquid and gas in the soil as well as by diffusion through liquid and gas in the soil pores. Transport of radon into a home is affected by these same soil properties as well as the presence of small cracks, joints, and holes in the concrete slabs and walls (10). The radon concentration in a home is a function of the surface area in contact with soil, the volume of the house, and the ventilation rate. A finiteelement model, Rn3D, has been developed that simulates liquid flow, gas flow, and both advective and diffusive radon transport in porous media (11). The governing equation for radon transport was developed using Fick’s law of diffusion and the continuity equation (11):
[
]
∂θw ∂Cg ∂Cg ∂θg ∂ (Dg + Dwκ) Cg + Cw + [θg + θwκ] ) ∂t ∂t ∂t ∂z ∂z ∂Cg - (λ + λv)[θgCg + θwCw] + R (1) (vg + vwκ) ∂z where θg is the gas-filled porosity, θw is the water-filled porosity, Cg is the gas-phase concentration of radon, Cw is the water-phase concentration of radon, κ is ∂Cw/∂Cg, i.e., Henry’s law distribution coefficient, Dg is the gas-phase diffusion coefficient of radon, Dw is the water-phase diffusion coefficient of radon, vg is the velocity of gas phase, vw is the velocity of water phase, λ is the radioactive decay
VOL. 30, NO. 10, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2915
TABLE 1
Model Parameters Used in Simulation of Radon Entry into House model parameter
value
basement ventilation rate, λv (h-1) concrete diffusion coefficient, Dg (cm2/s) concrete permeability, kij (cm2) concrete slab and wall thickness (cm) gravel diffusion coefficient, Dg (cm2/s) gravel permeability, kij (cm2) gravel depth (cm) soil diffusion coefficient, Dg (cm2/s) soil permeability, kij (cm2) soil radon emanation, e soil radium content, R (Bq/kg)
0.5 5 × 10-4 1 × 10-12 10 2 × 10-2 1 × 10-3 10 1.8 × 10-2 9.13 × 10-9 0.2 37
ref 15 16 16 17 2 17 18 19 measured
FIGURE 3. House model dimensions, materials, and boundary conditions.
constant (ln 2/half-life of radon), λv is the ventilation rate, and R is the total production rate of radon. The radon production rate is defined as
R ) RFbλE
(2)
where R is the radium content of the soil, Fb is the soil bulk density, and E is the radon emanation coefficient. The governing equation for compressible gas flow in porous medium was developed using Darcy’s law, the continuity equation, and the ideal gas law (11):
[
]
(
[
Mg ∂Pg ∂ krgkijFg ∂Pg + Fggj + F gχ ) RT ∂t ∂xi µg ∂xj
θt (1 - sw)
)]
(3)
where θt is the total porosity; sw is the water-phase saturation; Mg is the molecular weight of the gas phase; R is the ideal gas constant; T is the temperature (K); Fg is the density of the gas phase (Mg/m3) as a function of temperature and pressure; χ ) ∂sg/∂Pg ) ∂sw/∂Pw is the specific moisture capacity; Pg is the pressure of soil gas (kPa); t is the time and xi is the distance, where i and j indicate direction; krg is the relative permeability of the gas phase; kij is the intrinsic permeability of soil (m2/s); µg is the viscosity of gas phase (poise), a function of temperature; gj is gravitational acceleration (9.8 m/s2). Flow was considered to be laminar because the Reynold’s numbers in the porous media were less than 1 (12). The governing equation for liquid flow in porous media was derived using Darcy’s law and the continuity equation and by neglecting the compressibility of water (13):
[
(
∂Pw ∂ krwkijFw ∂Pw + Fwgj ) ∂t ∂xi µw ∂xj
θtFwχ
)]
9
rectangular finite element grid with an 123 (horizontal) by 149 (vertical) rectangular finite element grid was needed to accurately render steep concentration and pressure gradients near the model boundaries and at material interfaces. The properties of the porous materials are listed in Table 1. The model coordinate used for comparison to the observed values of radon concentration, 3.8 cm from the stack and 30 cm above the concrete slab, was chosen to approximate the actual position of the radon monitor. A similar criteria was used to select the model coordinates used for comparison to the observed pressure differential between the subslab (2.0 m from the stack base and centered vertically in the 10-cm gravel layer) and the basement (3.8 cm from the stack base and 45 cm above the concrete slab). Transient simulations were performed using 3-h time steps. Input data collected in 15-minute intervals were averaged.
Results and Discussion (4)
where sw is the water-phase saturation; Pw is the pressure of soil water (kPa); krw is the relative permeability of the water phase; µw is the viscosity of water phase (poise), a function of temperature; Fw is the density of water phase (Mg/m3), a function of temperature. A finite element grid with cylindrical coordinates was used to reduce the problem to two dimensions. The domain, materials, and boundary conditions for the simulation are shown in Figure 3. Because large cracks were observed in the basement slab, a 1-cm-wide crack in the concrete was placed at the perimeter of the house. A
2916
FIGURE 4. Average radon concentrations measured in basement of study house during 1994-1995.
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 10, 1996
House Data. Figure 4 shows the radon concentrations measured in the basement of the study house during 19941995. On the average, radon concentrations were lower during periods when the stack was open than when the stack was closed. This effect was even more pronounced during the first four measurement periods, when the house was unoccupied. If we only consider measurements made after the house was occupied, radon concentrations were still significantly lower during periods when the stack was open (37 Bq/m3) than when the stack was closed (74 Bq/ m3). Radon concentrations were generally higher in the summer and fall of 1994 and 1995 than they were during the winter and early spring of 1995. This is in contrast to
FIGURE 5. Radon concentrations in basement of study house during winter and early spring.
FIGURE 7. Relationship between radon concentration in basement and furnace use while stack is closed.
FIGURE 6. Relationship between radon concentration in basement and furnace use while stack is open.
FIGURE 8. Wind speed on roof of house during winter and early spring.
previous work in this region that showed radon concentrations to be 2.5-13 times higher during the winter due to depressurization of the houses relative to the surrounding soil (14). The stack was left open between November 1, 1994, and February 2, 1995, and during this time radon concentrations dropped from 90 to 18 Bq/m3. When the stack was closed for a month between February 2 and March 3, 1995, the average radon increased to 26 Bq/m3. Continuous radon measurements were made from January 17 to April 14, 1995 (Figure 5). Although radon concentrations are very low while the stack is open between March 3 and April 14, concentrations are as high when the stack is open between January 17 and February 2 as they are when the stack is closed between February 2 and March 3. High radon concentrations are positively correlated with furnace use whether the stack is open (Figure 6) or closed (Figure 7). The furnace draws air from the basement, causing air from the surrounding soil to flow into the basement and increasing the radon entry rate. Because radon has a short half-life of 3.8 days, the radon concentration in the basement is very sensitive to changes in the entry rate. The relationship between furnace use and high radon concentrations while the stack is open may explain why the stack has been less effective at reducing radon concentrations since the house has been occupied. High furnace use does not always predict high radon concentrations; however, periods of low radon concentrations are correlated with periods of high or sustained wind (Figure 8). High wind may force fresh air into the basement, which
FIGURE 9. Differential pressure between subslab and basement during winter and early spring.
would decrease the radon entry rate. During the winter, differential pressures between the subslab gravel and the basement (∆Psubslab-basement) are lower and show less variability when the stack is open than when it is closed (Figure 9). The average ∆Psubslab-basement varies from zero to +0.3 Pa between February 15 and March 3, 1995, while the stack is closed. While the stack is open between January 17 and February 2, 1995, and between March 3 and April 12, 1995, the ∆Psubslab-basement averages -0.1 Pa. Pressure differentials inside the stack between a point at the base and a point at the roof of the house are
VOL. 30, NO. 10, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2917
FIGURE 10. Factors affecting differential pressure in the stack when stack is open during winter.
FIGURE 12. Differential pressure between subslab and basement during summer.
FIGURE 11. Factors affecting differential pressure in the stack when stack is open during summer.
FIGURE 13. Comparison between measured and modeled radon concentrations while stack was closed.
positive, indicating that air would be flowing from the subslab gravel up the stack (Figure 10). When the stack is open, air flowing out of the stack is drawn from the subslab gravel, lowering the subslab gravel pressure relative to the basement pressure. The positive correlation between stack length temperature differential and stack pressure differential reflects the fact that, during the heating season, warm air rises in the house due to its lower density, causing air to flow out of the stack. Increases in wind speed also cause an increase in flow out of the stack by decreasing pressure at the top of the stack. The temperature gradient between the house and the outside air is either much lower or reversed in the summer relative to winter (Figure 11), as are the pressure gradients in the stack. As a result, there is no appreciable difference in subslab depressurization when the stack is open or closed during the summer (Figure 12). During the summer, the pressure in the subslab is consistently higher than that of the house (Figure 12), indicating that radon will be continuously drawn from the soil to the basement. When the stack is open, fresh air may be drawn from the outside during the periods when the stack length ∆P is negative (Figure 11), thus diluting the radon concentrations in the subslab and, hence, the basement. Although no continuous radon measurements were available for the summer of 1995, average radon concentrations measured using passive detectors during the summers of 1994 and 1995 showed little difference between periods when the stack was open or closed. This suggests that any diluting effect from the flow into the stack is overwhelmed by the radon entry due
the constant flow of air from the soil to the house of the house. Numerical Modeling. The numerical model was used to determine whether the increase in subslab depressurization observed when the stack was open during the winter was sufficient to reduce radon concentrations in the basement. Because of the sharp reduction in radon concentrations (Figure 5) and the obvious decrease in subslab depressurization (Figure 9) observed when the stack was closed on March 3, 1995, the period between February 22 and March 12, 1995, was chosen for simulation. Because of the observed relationships between radon concentration, furnace use, and wind speed, it was assumed that furnace use resulted in an increase in air flow out of the basement and that high wind speed resulted in an increase in air flow into the basement. Radon concentrations measured between February 24 and March 3, 1995, while the stack was closed were used to calibrate the relationship between furnace use and flow rate out of the basement and the relationship between wind speed and air flow rate into the basement. Air flow in and out of the basement was simulated by varying the flux across the top boundary of the model that intersects the basement. Good agreement between modeled and observed radon concentrations in the basement (Figure 13) was achieved using the following relationship:
2918
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 10, 1996
v ) -2 × 10-6 W + 1 × 10-8 F
(5)
where v is the specific discharge across the model boundary
FIGURE 16. Comparison between measured and modeled radon concentrations while stack was open. FIGURE 14. Comparison between measured and modeled pressure differentials between subslab and basement while stack was closed.
FIGURE 15. Comparison between measured and modeled pressure differentials between subslab and basement while stack was open.
in cm/s, W is the wind speed in m/s, and F is the furnace usage measured in W. The model matches most of the peaks in radon concentration. Both the modeled and measured radon concentrations have an average value of 45 Bq/m3 for this time period. Transient variations in the modeled pressure differentials between the subslab and basement are much smaller than the observed values (Figure 14). The model predicts that transient pressure changes in the house will propagate rapidly through the crack in the slab into the subslab gravel. A multiple linear regression analysis indicated that subslab pressure differentials are most strongly correlated with subslab temperature differentials rather than with furnace use or wind speed. Subslab-basement pressure differentials can be predicted from subslab-basement temperature differentials using the following correlation: ∆Psubslab-basement (Pa) ) 0.55 × 0.72∆Tsubslab-basement (°C). Because Rn3D does not simulate transient heat flow, we were not able to simulate transient changes in subslab pressure differentials, although the average value agrees well. Transient changes in ∆Psubslab-basement are also predicted to be small by the numerical model for the period while the stack is open (Figure 15). Again, ∆Tsubslab-basement is a better predictor of ∆Psubslab-basement. The correlation obtained while the stack was open is ∆Psubslab-basement (Pa) ) 0.05 × 0.32∆Tsubslab-basement (°C). The change in the correlation with ∆Tsubslab-basement after the stack was closed suggests that the average ∆Psubslab-basement decreased by 0.5 Pa due to flow out of the stack.
Equation 5 was also used to simulate the effect of furnace use and wind on flow in and out of the basement March 3 and March 10, 1995, while the stack was open. A flow rate of 1.1 × 10-3 g/s out of the stack predicted a subslab depressurization of -0.5 Pa when the stack is open, which agrees with the observed change in ∆Psubslab-basement that occurred on March 3, 1995. Simulated radon concentrations are greater than observed values (Figure 16). The measured radon concentrations have an average value of 5 Bq/m3 for this time period, while the modeled radon concentrations have an average value of 43 Bq/m3. If the same time period is simulated assuming that the stack is closed, the model predicts that the average radon concentration in the basement would have been only slightly higher, 44 Bq/m3. In the model, the average radon concentration is largely determined by the radon concentration predicted under conditions of steady-state diffusion. Changes in furnace use and wind speed cause variations in radon concentration about this mean value. The model predicts that peak radon concentrations caused by furnace use will be lower with the stack open than with the stack closed. There are several possible reasons that the model is unable to account for the reduction in radon concentrations while the stack is open. First, because periods of low radon concentration are observed while the stack is closed as well as when it is open, there may be other processes reducing radon concentrations in this basement that we have not accounted for. One possible factor not accounted for is the variation in soil moisture due to rainfall and lawn irrigation. Changes in soil moisture would change the amount of radon entering the basement by diffusion. Another factor not accounted for is the variation in air exchange rate due to the opening of doors and windows. The lack of differential pressure measurements between the house and the outside air and the house and the soil surrounding the basement also add uncertainty to these model predictions. There are also limitations in the model itself. First, the relationship between furnace use and wind speed on flux from the basement was estimated. Field measurements of the relationship between furnace use, wind speed, and house ventilation rate would reduce uncertainty in model predictions. Also, the model does not simulate transient heat flow, which would allow better simulation of the transient pressure differentials between the basement and subslab and possibly the soil. Finally, the average radon concentration in the home due to diffusive transport alone was estimated and may have been lower.
VOL. 30, NO. 10, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2919
Acknowledgments We thank Jan Fay and Ray Tekverk of Faytek in Spokane, WA, for helping us gain access to the study home, for providing the continuous radon monitoring devices, and for collecting the radon concentration measurements. This work was funded by the U.S. Department of Energy under Contract DE-AC06-76RLO 1830.
Literature Cited (1) Fay, J. E.; Tekverk, R.; Gerard, T. J. Passive Stack Study. Washington State Department of Health, 1994. (2) Bonnefous, Y. C.; Gadgil, A. J.; Fisk, W. J.; Prill, R. J.; Nematollahi, A. R. Environ. Sci. Technol. 1992, 26, 1752-1759. (3) Henschel, D. B. Radiat. Prot. Dosim. 1994, 56, 21-27. (4) Leovic, K. W.; Sanchez, D. C.; Craig, A. B. Radiat. Prot. Dosim. 1988, 24, 513-517. (5) Washington State Building Code Council. Radon Resistive Construction Standards. In Washington State ventilation and indoor air quality code, 2nd ed.; Washington State Building Code Council, Ed.; State of Washington: 1991; Chapter 5. (6) Fay, J. E.; Tekverk, R.; Gerard, T. J. Washington State passive stack study; Fay, J. E., Tekverk, R., Gerard, T. J., Eds.; Environmental Protection Agency: Research Triangle Park, NC, 1994; Vol. V, pp 4.1-4.10. (7) Schmied, H. Sci. Total Environ. 1985, 45, 195-201. (8) Fisk, W. J.; Prill, R. J.; Wooley, J.; Bonnefous, Y. C.; Gadgil, A. J.; Riley, W. J. Health Phys. 1995, 68, 689-698. (9) Modera, M. P.; Bonnefous, Y. Health Phys. 1993, 64, 291-299.
2920
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 10, 1996
(10) Nazaroff, W. W.; Nero, A. V. Radon and Its Decay Products in Indoor Air; Nazaroff, W. W., Nero, A. V., Eds.; John Wiley: New York, 1988. (11) Holford, D. J. Rn3D: A Finite Element Code For Simulating Gas Flow And Radon Transport In Variably Saturated, Nonisothermal Porous Media: User’s Manual, Version 1.0; Pacific Northwest Laboratory: Richland, WA, 1994. (12) Reddy, T. A.; Gadsby, K. J.; Black, H. E.; Harrje, D. T.; Sextro, R. G. J. Air Waste Manage. Assoc. 1991, 41, 1476-1482. (13) Huyakorn, P. S.; Pinder, G. F. Computational Methods in Subsurface Flow; Academic: San Diego, CA, 1983. (14) Turk, B. H.; Prill, R. J.; Grimsrud, D. T.; Moed, B. A.; Sextro, R. G. J. Air Waste Manage. Assoc. 1990, 40, 498-506. (15) Nero, A. V. Phys. Today 1989, 40, 32-39. (16) Renken, K. J.; Rosenberg, T. Health Phys. 1995, 68, 800-808. (17) Collin, M.; Rasmuson, A. Soil Sci. Soc. Am. J. 1988, 52, 15591565. (18) Mualem, Y. A Catalogue of the Hydraulic Properties of Unsaturated Soils; Technion, Israel Institute of Technology: Haifa, Israel, 1976. (19) Nielson, K. K.; Rogers, V. C. Radon transport properties of soil classes for estimating indoor radon entry; Nielson, K. K., Rogers, V. C., Eds.; Battelle Press: Richland, WA, 1990; Part 1, pp 357372.
Received for review October 25, 1995. Revised manuscript received June 12, 1996. Accepted June 14, 1996.X ES950790Q X
Abstract published in Advance ACS Abstracts, August 1, 1996.
