Evaluation of Methane Pathway, Risk and Control Rafat Abbasi, P.E., Senior Project Manager Brownfields and Environmental Restoration Program Department of Toxic Substances Control California Environmental Protection Agency 5796 Corporate Avenue Cypress, California John Sepich, P.E., President, Brownfield Subslab 4007 McCullough Avenue, #469 San Antonio, Texas

Agenda Background: Science and Perception Background: Science and Perception Regulatory Perspective Regulatory Perspective Methane: Generation Methane: Generation – Processes that govern generation of methane – VOCs versus Methane – Attenuation factors ASTM approach ASTM approach – Tiered strategy – Sampling – Mitigation Questions and Answers Questions and Answers Rafat Abbasi John Sepich

Background: Science and Perception Although methane even has been highly studied in the context of landfills, coal mines, and oil fields, it is misunderstood Level of awareness is high since historically methane explosions have caused loss of life and property damage Regulations established action levels based solely on concentrations in the subsurface (diffusion only; no consideration for convection) Lack of understanding that methane behaves differently from volatile organic chemicals (VOCs)

Flammability Levels of Methane & Oxygen NOTE—The straight red line illustrates varied mixture ratios of source methane gas (e.g., soil gas;14.1 %v methane) with ambient air (21 %v oxygen) and defines the lowest concentration (14.1 %v) of methane that can be diluted with air to form a flammable (“explosive”) mixture in air. Source: 30 CFR § , MSHA Illustration 27.

Regulatory Perspective Most agencies have prescriptive approach (no CVP) using 25% of LEL (12,500 ppmv) as a threshold; USEPA under RCRA states that methane concentrations not to exceed 25% of LEL in a facility structure Cal EPA finalized a guidance in 2012 that incorporates CVP model in methane evaluation; ASTM form a panel in 2011 to draft a methane standard for evaluation of assessing and interpreting methane hazard and risk and appropriate and urgency of the response

Regulatory Thresholds Source: Eklund

Methane Generation Thermogenic methane Generated at depth under elevated pressure during and following the formation of petroleum (e.g., in oil fields). Biogenic methane Formed at relatively shallow depths by the bacteriological decomposition of organic matter in the soil (e.g., in landfills). It is commonly unpressurized but could be under pressure in municipal landfills

Methane Generation Initial degradation is aerobic After oxygen is gone, denitrification, iron and sulphate reduction may occur After the exhaustion of these processes, methanogenesis (anaerobic methane generation) occurs Leads to 60% methane and 40% carbondioxide

Methane Generation

VOCs versus Methane Adapted from EklundVOCsMethane Mass flux is related to concentration in soil gas Concentration in soil gas is not a good proxy for mass flux Focus on long-term average concentrations Focus on short-term maximum concentrations Typical attenuation factors are Attenuation factor must be >0.05 to reach 5% indoors Transport via diffusion with advection Transport via advection is the main concern Soil gas levels for some VOCs inversely proportional to oxygen levels Soil gas levels for methane inversely proportional to oxygen levels

Attenuation Factors J/E model considered a gold standard for vapor intrusion – Primarily a diffusion model, and does not consider pressurized flow – Attention factor of to – Used for diffusion of toxic and hazardous chemicals – Yield very conservative results – Starting soil gas concentrations of 1,000,000 ppmv and using an attenuation factor of just 10 -2, the theoritical indoor air methane concentration of 10,000 ppmv can be expected (1% by volume and 20% of LEL) MTRAN model MTRAN model – Developed for gas migration under pressure and at high concentrations (single family residential) – Field data collected to correlate soil gas with indoor air concentrations – Data showed attenuation in the range of rather than and 10 -3

Attenuation Factors (cont.) Lessons from Ross Explosion Lessons from Ross Explosion – Thermogenic soil gas into the small clothing store; – Pressures up to 1,000 inches of water (40 psi) were observed; – Pressure may have been exacerbated by rising water table; San Diego Gas Intrusion Study San Diego Gas Intrusion Study – Effects of biogenic methane in ranch Bernardo – Methane tested on hundreds of mass grading projects – Soil methane concentrations of 400,000 ppmv in soil gas – Enacted methane ordinance which was later repealed on the following grounds Gas was not under pressure gas only found in engineered fill soils Gas volume was small Source was natural degradatoin of organic material

Attenuation: Model versus Field Data

C 1 = C 2 *[(AEH*B)/Q] where C 1 : Methane concentration in soil gas entering building through slab, ppmv C 2 : Equilibrium indoor air concentration, ppmv (DTSC default 500 ppmv) AEH: Air exchange rate per hour (DTSC default 0.5 per hour) B : Building volume in cubic centimeters Q: estimated soil gas flux

C 1 = C 2 *[(AEH*B)/Q] where C 1 : Methane concentration in soil gas entering building through slab, ppmv C 2 : Equilibrium indoor air concentration, ppmv (DTSC default 500 ppmv) AEH: Air exchange rate per hour (DTSC default 0.5 per hour) B : Building volume in cubic centimeters Q: estimated soil gas flux

ASTM Approach: Decision Matrix

Tiered Approach: Bottom line

Sampling

Mitigation

Mitigation: Sub-slab Depressurization System

Evaluation of Methane Pathway, Risk and Control Questions and Answers