Proposed Regulatory Framework for Evaluating the Methane Hazard due to Vapor Intrusion

Transcription

1 VI Specialty Conference Proposed Regulatory Framework for Evaluating the Methane Hazard due to Vapor Intrusion 1

2 Speaker Bart Eklund, CIH Corporation Austin, TX (512)

3 Geyser of CH 4 and Groundwater 3

4 Aftermath of Another Geyser 4

5 5

6 Overview of Topics 1. Methane facts and myths 2. VI evaluations for methane vs those for VOCs 3. Existing regulations for methane 4. Decision matrix for evaluating methane hazard 6

7 Properties of Methane Property Value for Methane Molecular weight 16 Flammable range 5% - 15% Boiling point Water solubility Henry s Law constant -162 C (-259 F) mg/l 37,600 (atm) 28 (dimensionless) Diffusivity in Air (D a ) 0.23 cm 2 /s Viscosity (η) 1.1E-04 g/cm-s 7

8 Methane Myths 1. Methane in a soil matrix can explode 2. Soil gas concentration is correlated with gas production rate 3. Methane will accumulate in soil gas beneath a slab or in indoor air, resulting in higher concs than in underlying soil strata 4. Methane transport through soil is analogous to water transport None of these statements are true 8

9 Methane Facts For an explosion to occur, it requires 5 to 15% methane in air and a flame, spark, or heat source. Most explosion scenarios involve methane flowing under pressure (e.g., burst gas line). Pressure is an important consideration. One atmosphere (atm) of pressure: = 101,300 Pascals (Pa) = 1013 millibars (mbar) = 29.9 inches of mercury ( Hg) = 1033 centimeters of water (cm H 2 O) = 407 inches of water (in. H 2 O)(in. w.c.) = 14.7 psi = 760 mm Hg (Torr) 9

10 Typical VI Evaluations for VOCs vs. CH 4 VOCs Methane Given starting mass No given starting mass Focus on long-term average concentrations Focus on short-term maximum concentrations VOC levels inversely proportional to oxygen Methane levels inversely proportional to oxygen 10

11 Typical VI Evaluations for VOCs vs. CH 4 VOCs Methane Transport via diffusion with advection important near buildings Transport via advection is the main concern Typical attenuation factors are ~10-3 Attenuation factor must be >0.05 to reach 5% indoors 11

12 Concentration vs. Mass Flow Common practice in VI to compare concentrations (e.g., indoor vs. outdoor, indoor vs. soil gas, etc.) and draw conclusions about gas transport Important to realize that concentration is used as a surrogate for mass flow in VI studies For methane, concentration is important, but it must be considered along with other lines of evidence 12

13 Concentration vs. Mass Flow Analogy to point sources Concern is downwind concentrations in breathing zone But concentration within the stack is not regulated, instead it is the mass flow that is regulated (e.g., g/s, lb/hr, TPY). Same concept holds true for methane 13

14 Land Development Regs Few regulations in the US. Southern California has regulations because of thermogenic (fossil) CH 4 City of LA (2004): 1.25%, 2 in. w.c. Orange County (2008): 0.5%, 1.25% Riverside County (2004): 1.5% CA DTSC (2005): 1.25%, 1 psi San Diego County (2002): 1.25% Repealed in 2005 None of the existing regulations are considered to be good templates 14

15 Developing a Decision Matrix Key Issues: 1. Reference existing regulations or develop sitespecific approach 2. Address flammability and explosiveness together or separately 3. How best to address events that are low probability but may have significant consequences 4. Value of determining source(s) of methane 15

16 Factors that impact safety Concentration Volume Safety Pressure 16

17 Factors that impact safety Concentration Safety Volume Pressure 17

18 Decision Matrix (see paper) Shallow Soil- Gas Conc. No Data <0.01% Indoor Air Concentration 0.01 to <1.25% > 1.25% <1.25% to 5% No further action No further action No further action Notify & Evacuate >5% to 30% NFA unless P is >2 H 2 O NFA unless P is >2 H 2 O NFA unless P is >2 H 2 O Notify & Evacuate >30% Collect indoor air data Evaluate on case-bycase basis Evaluate on case-bycase basis Notify & Evacuate 18

19 Degradation of TPH in GW 19

20 Typical Trigger Levels USEPA Landfill Regs Developed for sites with high biogas production (i.e., sites with high CH 4 conc. and high P) 1.25% in indoor air for buildings built atop the biogas source 1.25% in perimeter sentinel wells to identify lateral migration California uses 1.25% in indoor air and 5% in sentinel wells at perimeter of facility These trigger levels may not be meaningful for use with soil gas at sites without high rates of pressure-driven gas flow 20