ESTCP Research on Optimization of Vapor Intrusion Mitigation Systems in Large Military Buildings

Transcription

1 ESTCP Research on Optimization of Vapor Intrusion Mitigation Systems in Large Military Buildings Todd McAlary, Ph.D., P.Eng., P.G., CUT Principal and Practice Leader Vapor Intrusion Services Geosyntec Consultants, Inc. USEPA Vapor Intrusion Workshop at the AEHS Conference San Diego, CA, 23 March 2015

2 Research Goals Improve energy efficiency of mitigation system Develop new design and diagnostic tools Demonstrate and validate Scientific review Regulatory acceptance Publish protocols 2

3 Case Study: Building 205, Raritan, NJ 160 ft Suction point 400 ft Building area = 67,000 ft 2 Area per vent pipe = 2,480 ft 2 Radius = 28 ft 3

4 SSD System (2005) 9 HS-2000 High Suction Fans Connected to 3 suction points each Total system flow ~500 scfm 4

5 Mass Flux Conceptualization Extracted Flux Ideally, F 2 ~ F 1 and F 3 = C IA x Q build F 3 < IASL x Q build Upward Diffusive Flux Capillary Fringe Groundwater Flow

6 Subsurface Conceptualization Key Variables: Flow, vacuum, permeability, thickness, porosity Hantush-Jacob Leaky Aquifer Model (1955) 6

7 Vent-Pipe Monitoring Data Thermal anemometer flow rate in scfm 30-day Waterloo Membrane Sampler - VOC concentrations in µg/m³ 1 hour Durridge RAD 7 - Radon concentrations in Bq/m 3 Area with TCE vapors near SSSL 7

8 8 Extracted Mass Flux Fan ID Fan Q (scfm) Fan Q (m 3 /min) TCE Conc. (ug/m 3 ) Mass Flux (g/d) Proportion of MF HSF % HSF % HSF % HSF % HSF % HSF % HSF % HSF % HSF % Average [TCE] less than industrial IASL Total MF (g/d) How Much Mass Flux poses a problem? C IA = MF/Q build Q build = V x AER Threshold MF = 2.6 g/d 8

9 9 Pneumatic Testing: Specific Capacity Measure flow (Q) and vacuum (ΔP) at each vent-pipe (<2 min) Q/ΔP = Specific capacity (depends on permeability) Specific Capacity (scfm/in.h 2 O) ~30X range in values across 27 vent-pipes

10 Pneumatic Testing: Static ΔP vs r Measure steady vacuum at sub-slab probes, takes <30 minutes Area with ΔP > 6 Pa (ASTM Spec) Subfloor Vacuum in Pascals Note: this is with only Fan #3 running do we need 9 fans? 10

11 Vacuum inches of water Pneumatic Testing: Transient ΔP vs t Conducted tests at 11 locations near fans 1, 3, 5, 7 and ,000 1,500 2,000 2,500 3,000 3,500 4,000 4, recovery drawdown -0.8 Slower than average response (~1-hr) because of low permeability time (s) 11

12 12 Pneumatic Analysis: Hantush-Jacob T r/b The fit between measured vacuum and the Hantush-Jacob Model (blue curve) is usually nearly perfect

13 Vacuum versus distance: Velocity versus distance: Travel time for sub-slab gas from a given distance: Proportion of total flow originating below the floor: Bulk gas permeability of the floor and Qsoil: Qw Vacuum K0(r/B) 2 π T Qw 1 v( r) K 1(r/B) 2 π b n B Q( r) Q w K = T b B 2 r B Bonus Information K (r/b) 1 Q soil = K i A T = transmissivity [L 2 /t] B = leakance [L] Q w =volumetric flow rate from well (ft 3 /day) r = distance from extraction point (ft) b = thickness of fill layer (ft) n = fill layer porosity (ft 3 /ft 3 ) K 1 = Modified Bessel function of first order K 0 = Modified Bessel function of zero order K = bulk gas conductivity of the floor slab [L/t] b = floor slab thickness [L], easily measured i = pressure gradient across the floor slab A = Area of the building All of these calculations can be done in a spreadsheet (thanks Hantush and Jacob!) 13

14 Vacuum (Pa) Distance from Extraction Point (ft) 14 Vacuum vs Radial Distance Qw Vacuum K0(r/B) 2 π T

15 15 Sub-Slab Velocity: 1) Inter-well Tracer Test Inject helium into a sub-slab probe, then let it flow Monitor Helium in a nearby vent-pipe, get a breakthrough curve t 1 t 2 t 3

16 16 Sub-Slab Velocity: 1) Inter-well test results Travel time = 130 seconds from a radius of 6 ft Travel time = 44 seconds from a radius of 14 ft Travel time = 50 seconds from a radius of 14 ft These tests are quick, simple and very informative using low-cost equipment that is easily rented

17 17 Sub-Slab Velocity: 2) Helium Flood Reverse the flow on a vent-fan and match ΔP and Q Add helium (~2%v/v or so), monitor transport below the slab

18 18 Sub-Slab Velocity: 2) Helium Flood He injected at 2% v/v = 20,000 ppmv Average travel time = arrival of C/Co =0.5 =10,000 ppmv ~100 min at r = 43 ft Projected to about 360 min at a radius of 67 ft

19 Time to Reach Extraction Point (min) Sub-Slab Travel Time: Calibration Inter-well Test Helium Flood Distance from Extraction Point (ft) 19

20 20 Indoor Air Leakance: CO 2 Tracer Test 1) Add CO 2 to indoor air (~2,000 ppmv) 2) Turn on a Fan - measure CO 2 in exhaust CO 2 in Vent exhaust / CO 2 in indoor air = Fraction from Leakage Calculate the cost of lost indoor air

21 21 CO 2 sensors Special thanks to Prof. Jeffrey Siegel and his student Donna Vakalis Lesson Learned: Naturally-occurring (background) CO 2 can be high enough to confound the response Sometimes, that can be a good thing (see next slide)

22 Existing Sub-Floor O 2 and CO 2 Recent data collected below the slab of a different large building O 2 CO 2 Slight increase in O 2 with volume purged Slight decrease in CO 2 with volume purged Both trends are consistent with gradual leakage of air through the floor slab 22

23 Indoor Air Leakance: From H-J Analyses Portion of flow from indoor air via leakance Q( r) Q w r B K (r/b) 1 Portion of flow from below the floor 23

24 HVAC costs per year ~$50,000 for Building 205 SSD system flow is ~2% of total building flow leakance is ~35% Potential saving ~$400/yr Over a 30 year operating period up to $12,000 Electrical power draw ~$5,000/yr 9x reduction = $4,450/yr in savings Over a 30 year operating period - $134,000 in net savings Total potential savings up to $146,000 Cost Assessment (not bad for a weekend of field work) (could have saved $170K more if this approach had been used for the initial design) 24

25 Take-Home Messages We can do better than current standard practice (based on Radon research that is several decades old) Vacuum is not the only metric Several groundwater tools work well for soil gas too Thankfully much faster and less expensive There s still more to learn about non-idealities Preferential pathways Barriers Slab-on-clay It s all part of a bigger and better toolbox 25

26 Study Team Organization Individual Role Geosyntec Consultants, Inc. Todd McAlary Project Director/Principal Investigator Geosyntec Consultants, Inc. William Wertz Principal Investigator Geosyntec Consultants, Inc. Paul Nicholson Principal Investigator WPB Enterprises, Inc. Bill Brodhead Technical Reviewer Practitioner (Radon) United States EPA Henry Schuver Technical Reviewer EPA Geosyntec Consultants, Inc. Robert Ettinger Technical Reviewer Internal (VOCs) Arizona State University Paul Johnson Technical Reviewer Academic (VOCs) University of Minnesota William Angell Technical Reviewer Academic (Radon) University of Toronto Jeffrey Siegel Technical Reviewer - HVAC US Army Corps of Engineers Sandra Piettro FUDS Manager at Raritan Arsenal, NJ AFCEC Cornell Long DOD Liaison tmcalary@geosyntec.com 26