Dispersion Modeling of Commodity and Structural Fumigation Applications. Rick Reiss, Exponent Presented at Kansas State University May 12 th, 2010

Transcription

1 1 Dispersion Modeling of Commodity and Structural Fumigation Applications Rick Reiss, Exponent Presented at Kansas State University May 12 th, 2010

2 2 Overview of Presentation Risk assessment process for bystander exposure to fumigants Use of dispersion modeling to estimate downwind concentrations Example studies with methyl bromide used to characterize emissions

3 3 Background Fumigants are generally highly volatile Emissions after treatment can lead to downwind exposures to bystanders Regulators are interested in minimizing exposures The solution was to establish buffer zones around applications, which restrict entry for a period of time after the application EPA recently established national buffer zones for most fumigants The principal tool used by EPA was PERFUM

4 4 The Risk Assessment Process Hazard Identification Can the substance cause illness or disease? Dose-Response Assessment What dose is necessary? Risk Characterization What is the risk of disease? Exposure Assessment What levels are people exposed? 4 Source: Modified from NAS 1983 (pg. 21)

5 5 Basis for Buffer Zone Estimation Use of air dispersion modeling to estimate downwind concentrations over range of meteorological conditions Comparison of concentration estimates with toxicity reference concentrations to estimate risk

6 6 Possible Buffer Zone Definitions Whole field buffer zone MOE > 100 5% MOE < 100 Maximum concentration buffer zone

7 7 Factors Influencing Downwind Concentrations Source-specific Application rate Treatment and aeration length Air exchange rates Volatility of fumigant Sealing Location-specific Meteorological conditions Wind speed, wind direction, atmospheric stability Terrain Nearby buildings (downwash)

8 Dispersion Modeling Theory 8

9 Building Downwash 9

10 Building Downwash Another Look 10

11 11 Steps in Modeling Analysis Estimating emission rates using real-world field data Characterization of source of interest Application rate Length of treatment and aeration Air exchange rates Estimating range of downwind concentrations using historical meteorological datasets

12 12 General Methodological Options for Emissions Option #1: Assume a percent release during treatment and aeration Option #2: Assume an air exchange rate during treatment and aeration and use it to calculate the hourly emissions.

13 13 Schematic of Building Air Flow Ventilation Model Q=air flow V=volume C(t)=concentration Q Air Exchange Rate (R) = Q/V

14 14 Dose Response Traditional Approach with Animal Studies Exposure animals for a range of doses and measure chemical-related effects Determine the No Observed Effect Level (NOEL) Apply a 100X uncertainty factor to NOEL to determine the safe dose for risk assessment Includes 10X uncertainty factor to account for uncertainty in extrapolating between animals and humans Includes a 10X uncertainty factor to account for variation in the human population 1

15 15 Dose-response for methyl bromide NOAEL of 40 ppm based on a rabbit study exposures at Days 6-16 of gestation Agenesis (failure to develop) of the gall bladder Fused sternebrae Uncertainty factor of 30 Reference concentration = 1.3 ppm (1300 ppb) over 4 hours

16 Using Field Studies to Estimate Emissions Dispersion Modeling in Reverse 16

17 Review of Methyl Bromide Historical Studies (early 1990s) 17 Location Volume (ft 3 ) Treatment Period (hr) Aeration Period (hr) Watsonville Bakersfield 18, Madera 320, Stanislaus 1,450, Sutter 3,100,000 & 6,800,

18 18 Loss Rates During Treatment in Mid-Sized Warehouse in Madera Building Concentration (ppm) C(t) = 7710 ppm exp(-0.05 h -1 t) R 2 = Time (hr)

19 19 Loss Rates During Aeration in Large Processing Plant in Stanislaus Exhaust Stack Concentration (ppm) y = 3979 ppm exp(-1.7 h -1 t) R 2 = Time (mins)

20 20 Air Exchange Rates from Historical Studies Study Scenario Duration (hrs) ACH (hr) Watsonville A A Bakersfield A Madera T A Stanislaus T A Sutter County T A

21 21 Characteristics of New Methyl Bromide Studies Site A Site B Site C Application Rate (lbs/1000 ft 3 ) (nominal) Initial Concentration 24 g/m ppm 30 g/m ppm 18 g/m ppm Change in Concentration during Treatment -60% -50% -65% Duration to aerate 50% of fumigants 0.5 h (passive) 3.0 h (active and passive) 1.0 h (active)

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24 24 Emission Rates for New Studies Methyl Bromide Concentration (ppm) 1000 y = 5550 e -0.05x R 2 = 0.98 Building airexchange rate is defined by the rate of change in fumigant concentration Time from Treatment (h)

25 25 Emission Rate Estimates for New Studies Site A Site B Site C Air-Exchange Rate hr 17 hr 17 hr Treatment Fumigant mass loss 44 to 70% over 24 hours 55% over 20 hours 62% over 24 hours Aeration Air-Exchange Rate Fumigant mass loss hr (passive) 18 to 78% in 1 hours hr (both active and passive) 0.7 hr (active only) 22 to 86% in 1 hour 63% in 1 hour

26 26 Summary of Measured Air Exchange Rates Treatment Range of hr, consistently across studies Length between 10 minutes and 90 hours Often lose >50% of mass, by 3-5% per hour Aeration Range of hr Larger rates for smaller buildings Typically, similar, but somewhat less, than the rated fan capacity Length between 10 minutes and 3 hours

27 27 Application to Mid-Sized Warehouse in Madera

28 28 Modeled versus Measured Fits for Madera Treatment Period Are Quite Good Measurements AERMOD ISCST Concentration (ppb) Sam pling Location

29 29 Point Comparisons for New Studies Treatment - 3 Sites Summary Aeration - 3 Sites Summary y = 0.26x R 2 = y = 0.86x R 2 = 0.43 Measurements (ppb) Measurements (ppb) Predictions (ppb) Predictions (ppb) Model captures the range of concentrations well, but local wind pattern around building structures makes it difficult to predict the spatial distribution.

30 30 Maximum Concentrations for New Studies Comparison of predicted maximum concentration in the model domain and measured maximum concentration. Site A Site B Site C Treatment Measured (ppb) Predicted (ppb) Aeration Measured (ppb) Predicted (ppb) EPA Level of Concern (4 hrs) = 1300 ppb

31 31 Conclusions Risk assessment with dispersion modeling is a practical method to address bystander exposure and establish buffer zones There is comparatively less data on emissions than for field applications More information would be helpful to refine the risk assessment