Vapor Intrusion into Large Buildings

Transcription

1 USEPA Workshop: Recent Advances to VI Application and Implementation- A State of the Science Update The 22 nd Annual International Conference on Soil, Water, Energy, and Air San Diego California Vapor Intrusion into Large Buildings David Shea, PE Daniel B. Carr, PE, PG Sanborn, Head & Associates, Inc. Building Trust. Engineering Success.

2 HVAC Basics Fan: moves air, creates +/- pressure Dampers: adjusts air flow through ducts Coils: heat or cool air Mass rate = Q bldg x C indoor = AER x V x C indoor [g/day] Q bldg V, C indoor Large bldg AER: typically 1 to 4/hr

3 Air Handling Units (AHUs) Exhaust Fans

4 Things to Look For: Bldgs w AHUs AHU/Airflow Balance AHU Balance = Outdoor Air-Relief Air Exhaust = + or - HVAC equipment/components in contact with the floor slab Building Wide Plenum w return and outside air (often above ceilings or beneath raised flooring). Areas of dead low AER/ACH Areas of potential low air pressure (mechanical rooms, fan rooms, laboratories, kitchens) Variability of HVAC operations (nightly and weekend turndown, outside air damper position economizers, operator over-rides) 4

5 Positive Pressure Space More air actively supplied (in) than exhausted (out); Pressure is greater in relation to abutting space Air supply Positive or negative are defined in relation to an adjacent space, e.g. outdoors, subslab, abutting room, etc. 5

6 Negative Pressure Space More air actively exiting (out) than entering (in); Pressure is lower in relation to an abutting space Air exhaust

7 Positive AND negative pressure space can exist side-by-side

8 HVAC Operations Variability When system is active, bldg is positive pressure When system is in sleep mode, bldg is neutral Indoor sampling should account for HVAC variability 8

9 Stand-alone AC units Fan Air flow Airflow Neg P area

10 Active positive pressure clean room

11 Vapor intrusion pathway and driver Mitigation by vapor barrier and increase in room pressure C indoor decreased by a factor of 10 Mitigation by subslab depressurization C indoor decreased to ND

12 Using mass flux to link subsurface conditions with indoor air Mass flux estimation methods: Bldg/space mass balance using AER (previous slides) Diffusion across slab Diffusion across vadose zone SSD measure flow and concentration 12

13 Example: Assessment and mitigation of a 330,000 SF industrial bldg 13

14 Soil profiling to estimate mass flux through vadose zone 14

15 Using mass flux as an indicator of likely indoor air levels 15

16 Adjustments to existing HVAC operations: Indoor VOCs decreased to levels consistent with expectations based on increased ACH HVAC Zone ACH Before HVAC Mods [hr -1 ] ACH After HVAC Mods [hr -1 ] Expected Reduction Factor = 1 (ACH before / ACH after ) % PCE Reduction Expected Actual HVAC Zone 16

17 Summary: VI in large buildings is influenced by HVAC design and operation Need to understand the air flow/pressure balance HVAC can have favorable or unfavorable effects Mass flux is a better measure of VI potential than empirical attenuation factor α is unreliable (and likely ultra-conservative) given the wide range of subsurface and bldg conditions Typical regulatory benchmarks for α are not relevant for buildings with AHUs J (µg/m 2 /day) reflects actual mass transfer processes that can be measured/estimated It s the flux that matters - not only concentration in assessing and mitigating VI let s reframe our evaluations in terms of mass flux Questions? David Shea, PE Sanborn, Head & Associates, Inc. dshea@sanbornhead.com 17

18 Extra slides 18

19 Diffusion D e = θ 3.33 D θ 3.33 D a w w a φ 2 K φ 2 H + «Millington Relationship» USEPA, From McWhorter, 1987 D a >>D w = free air and free water diffusion coefficients [L 2 /t] θ a, θ w = volumetric air and water-content (unitless fraction) Φ = soil porosity (unitless fraction); θ a + θ w = Φ K H = Henry s Constant (unitless ratio). 21

20 At steady-state, Mass in from subsurface = mass out through HVAC J * A = C indoor * Q bldg = C indoor * V * (Q bldg / V) J = C indoor * H * ACH Predicting C indoor from mass flux: C indoor = J/(H * ACH) Predicting C subslab from mass flux: C subslab = C indoor * H * ACH * L / D eff 22