Mark Kelley Battelle Environmental Technology and Restoration

Transcription

1 !"# Mark Kelley Battelle Environmental Technology and Restoration

2 A recently closed landfill at a military facility is the focus The landfill is a source of methane and VOCs in soil gas, as shown by lab analyses of soil-gas samples collected from soil-gas probes at the landfill A military housing facility is located adjacent to the closed landfill Vapor intrusion into nearby homes is a potential exposure pathway An assessment was needed to determine if landfill gases pose a threat to residents of nearby housing facility via the vapor intrusion pathway

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4 Calculate site specific soil-gas Risk-Based Screening Levels (RBSLs) to compare to measured VOC concentrations in soil-gas Definition: A soil-gas RBSL is the concentration of a compound (VOC) that can exist in soil-gas directly beneath a house without causing unacceptable risk to the occupants of the house if vapor intrusion is occurring. Use the RBSLs to calculate site-specific risks to indoor receptors via vapor intrusion using simple ratio approach (DTSC advocates screening based on risk) Risk = [(Concentrationi/RBSLi)] x 1x10-6 HI = (Concentrationj/RBSLj) DTSC advocates screening based on Risk Use assessment results to determine the need for further action, e.g., Sub-slab or indoor air sampling in homes Corrective action/mitigation

5 $ % & Generic soil-gas screening levels are available, but values vary EPA (2003) CHHSLs (Cal-EPA) ESLs (Cal Water Board) Disagreement between generic screening levels Differences in toxicity data Differences in other input assumptions Calculated values can account for site-specific factors that differ from default conditions used to calculate generic values Exposure parameters Physical parameters Concentration Relative to USEPA Value Comparison of Generic Soil Gas Screeing Levels Benzene Toluene Vinyl Chloride TCE Chemical RBSLs can be used to calculate risks using the Ratio Approach more easily/quickly than forward calculating risks using EPA or DTSC Vapor Intrusion Model (single chemical models) CHHSLs ESLs USEPA

6 # SITE FEATURES Stack Effects 28 VOCs detected in soil gas Elevation (ft msl) Air Streamlines Convection Crack Hypothetical Gas Probe Groundwater Diffusion Crack Diffusion Building Zone of Influence Wind Effects ( ft bgs) Soil Gas Probe Upward Vapor Diffusion Soil-Gas Contamination?? VOCs emanating from waste in closed landfill VOCs are migrating laterally/upward Homes located nearby Thick vadose zone (~150 ft) 60

7 %' Soil texture is fine to coarse sand with gravel Site-specific properties are similar to the default soil parameters for a sand in EPA Vapor Intrusion Model

8 ( ) Four types of military housing unit(s) Duplex Triplex Quadraplex (2 styles) Two story construction No air conditioning, forced heat Slab on grade construction Living area dimensions are the same for all four housing types (~ 1000 ft 2 per unit). Matches default house dimensions in EPA VI model.

9 # # EPA VI Model, Version 2 (April, 2003) to back calculate soil-gas RBSLs Incorporate equation for safe indoor air concentration (C Bldg ) based on specified target risk/hazard quotient Incorporate equation for soil-gas RBSL 4.5 probability functions into the model

10 * C soilgas = C Bldg Johnson and Ettinger (1991)

11 *+," Carcinogens = Risk target AT BW 365d yr 10 3 µ IR EF ED SF g mg Non - carcinogens = HQ target AT BW RfD 365 d yr 10 3 µ IR EF ED g mg EXPOSURE PARAMETERS Receptor Parameter Units Adult 30-yr Adult 10-yr Child 10-yr Exposure Duration (ED) Yrs Exposure Frequency (EF) days/yr Averaging Time for Carcinogens (AT) Yrs Averaging Time for Non-Carcinogens (AT) Yrs Target Risk (TR) unitless 1x10-6 1x10-6 1x10-6 Target Hazard Quotient (HQ) unitless Inhalation Rate (IR) m 3 /d 20** 20** 10** Body Weight (BW) Kg 70** 70** 15** ** Probabilistic Input Parameter

12 --%)-" '%$ Distribution for Inh Rate Adlt/H6 Mean= m 3 /d= % 90% 5% Distribution for Inh Rate Chld/V Mean= m 3 /d= % 90% 5% Inhalation rate, adult (m 3 /kg-d) Gamma Mean = Std Dev = Upper/Lower Limits = 0.195, Inhalation rate, child (m 3 /kg-d) Gamma Mean = Std Dev = Upper/Lower Limits = , 0.8 Source: OEHHA Air Toxics "Hot Spots" Program Risk Assessment Guidelines Part IV (Exposure Assessment and Stochastic Analysis Technical Support Document), October 27, 2000

13 = Q exp D 3 " " " $ $ ( ". L A soil crack eff crack B eff D T AB Q exp QBLT D eff D T AB D + + QBuildingLT Q soil eff crack 7 " $! # ' " ' + ) $ $ (,! & ( 3 4 $ $ ( 5 $ $ ( 6 $ $ ( ( (./ 0 1 (. 2 1, - & # + * * ) eff T Soil! " # $ % && ' Lcrack A B A B Q exp L T D * additional parameters required for Finite Source model. L A soil crack eff crack B 1 Johnson and Ettinger (1991) (infinite source) Soil, building, and chemical properties required to calculate alpha Building and chemical properties are primary input parameters Soil properties affect values of Qsoil and Deff

14 . Mixing Within Building H B = 2.44 m L B = 10 m ER=1 per 2 hrs (0.5 hr) -1 Q building = 33,888 cm 3 /s W B = 10 m (2,033 L/min.) Diffusive Flow Advective Flow Grade L crack =10 cm L F = 15 cm Grade Z crack = 15 cm A B =10mX10m=100m 2 (1x10 6 cm 2 ) L T = cm **= (EPA) A crack = 400 cm 2 (EPA) P = 4 Pa x crack = 4,000 cm r crack = 0.1 cm (EPA) K V (soil parameter) ** Probabilistic Input Parameter

15 --%) Distribution for Crack Ratio Mean = Crack Ratio () Discrete Distribution (DTSC Default Value) (EPA Default Value) Density Dependent parameters that vary as a result of changes in crack ratio Values in Thousandths 90.0% A crack - 5,000 cm 2 (DTSC Default) cm 2 (EPA Default) r crack (DTSC Default) (EPA Default)

16 . SCS Soil Type Parameter Units unitless Deterministic Value Sand Soil bulk density g/cm ** Soil effective permeability, k v cm x10-7 Soil intrinsic permeability, k i cm x10-7 Saturated hydraulic conductivity, K s cm/hr ** Relative air permeability, k rg unitless Van Genuchten shape parameter, M unitless ** Water-filled porosity, m cm 3 /cm Residual water content, r cm 3 /cm ** Total porosity, n ** Probabilistic Input Parameter cm 3 /cm **

17 --%) Distribution for Porosity/F Mean= % 90% 5% Distribution for theta r/g7 Mean= E Values in 10^-3 Default=0.375 Default= % 90% 5% Porosity (T) Normal Distribution Mean = 0.43 Std Dev = 0.06 Upper/Lower Limits = 0.245, Residual Moisture Content (R) Lognormal Distribution Mean = Std Dev = Upper/Lower Limits = , Source: Uncertainty Analyses of Infiltration and Subsurface Flow and Transport for SDMP Sites. NUREG/CR Prepared by Pacific Northwest National Laboratory, Richland, WA

18 --%)-. / Distribution for Ksat cm/sec/b Mean= E Values in 10^-3 5% 90% 5% Distribution for Van Genuchten N/D7 Mean= Default=0.007 Default= % 90% 5% Saturated hydraulic conductivity (Ksat) Beta Distribution Mean = 8.22E-03 Std Dev = 4.39E-03 Upper/Lower Limits = 3.5E-04, Van Genuchten shape parameter (N) Lognormal Distribution Mean = 2.67 Std deviation = Upper/Lower Limits = 1.95, 3.62 Source: NUREG/CR Prepared by Pacific Northwest National Laboratory, Richland, WA

19 "0"12". VLOOK UP TABLE (from EPA 2003 Vapor Intrusion Model) K s (cm/h) α 1 (1/cm) N (unitless) M (unitless) n (cm 3 /cm 3 ) θ r (cm 3 /cm 3 ) Mean Grain Diameter (cm) Bulk Density (g/cm 3 ) θ w (cm 3 /cm 3 ) SCS Soil Name Clay Clay Loam Loam Loamy Sand E Sand Sandy Clay Sandy Clay Loam Silt Silty Clay Silty Clay Loam Silt Loam Sandy Loam =RiskNormal(0.43, 0.06, RiskTruncate(0.245, 0.615), RiskName("Porosity"), RiskCorrmat(NewSoilMatrix,4))

20 "0"123. INTERCALCS TABLE Vadose Enthalpy of Henry's law Henry's law Vapor zone vaporization at constant at constant at viscosity at effective ave. soil ave. soil ave. soil ave. soil diffusion temperature, temperature, temperature, temperature, coefficient, Hv,T S HT S H'T S µt S D eff V (cal/mol) (atm-m 3 /mol) (unitless) (g/cm-s) (cm 2 /s) 8, E E E E-02 EPA (2003) Vapor Intrusion Model Exponent of Infinite Average Crack equivalent source vapor effective foundation indoor flow rate diffusion Area of Peclet attenuation into bldg., coefficient, crack, number, coefficient, Qsoil D c rac k Ac rac k exp(pe f ) α (cm 3 /s) (cm 2 /s) (cm 2 ) (unitless) (unitless) 1.09E E E E E-03 =RiskOutput("ALPHA") + IF(ISERROR(G31),((H21*A21)/(I10*A10))/(((H21*A21)/(D31*A10))+1),(((H21*A21)/(I10*A10))*EXP(( D31*10)/(E31*F31)))/(EXP((D31*10)/(E31*F31))+((H21*A21)/(I10*A10))+((H21*A21)/(D31*A10))*(E XP((D31*10)/(E31*F31))-1)))

21 . ). -- Distribution for Naphthalene Cancer RBSL (10-yr Child) Distribution for Naphthalene Cancer RBSL (30-yr Adult) Relative Frequency X <= % X <= % Mean = Relative Frequency X <= % X <= % Mean = Deterministic RBSL = 10-15% Deterministic RBSL = 45-50% Concentration (µg/m3) Concentration (µg/m3) Deterministic RBSLs fit within the range of probabilistic RBSLs For child receptor, the deterministic RBSLs fall near the low end of the probabilistic distribution (10-20% for all 28 VOCs) For adult (30-yr) receptor, the deterministic RBSLs fall near the middle of the probabilistic distribution (35-55% for all 28 VOCs) Difference between child/adult probably due to difference between point-value and probabilistic inhalation rates A probabilistic RBSL that is protective of 95 percent of the population (i.e., 5th percentile value) would be lower than the deterministic value

22 . ). -- / Distribution for Naphthalene Cancer RBSL (10-yr Child) Distribution for Naphthalene Cancer RBSL (30-yr Adult) Relative Frequency X <= % X <= % Mean = Relative Frequency X <= % X <= % Mean = Calculated Qsoil = 10-20% Calculated Qsoil = 35-55% Default Qsoil = 30-60% 0.15 Default Qsoil = 70-80% Concentration (µg/m3) Concentration (µg/m3) Deterministic RBSLs calculated based on a default Qsoil value (5 L/min) are higher than deterministic RBSLs based on calculated Qsoil (10.7 L/min)

23 % Range = 95 % - 5 % (similar to max-min) Alpha 95%-5% Range is relatively small, suggesting that alpha is not highly sensitive to soil parameters and building parameter that were evaluated in the probabilistic assessment RBSL 95%-5% Range is slightly larger, reflecting added effect of inhalation rate (inhalation rate doesn t affect alpha)!"!!"#!"$ "% "& "! "# 3.2 to to 4.4 Overall, the model was relatively insensitivity to input parameters

24 RBSLs were calculated for all 28 VOCs detected in soil-gas at the landfill VOC concentrations in probes located between the landfill and houses are below the most restrictive deterministic RBSLs 30 yr adult RBSL (carcinogens) 10 yr child RBSL (non-carcinogens) One chemical (PCE) had a maximum concentration that slightly exceeded the 5 th percentile probabilistic RBSL Cumulative risk/hazard was calculated using maximum concentrations from soil gas probes and different RBSLs Cancer risk x 10-6 (default Qsoil) x 10-6 (site-specific Qsoil) x 10-6 based on the 5th percentile probabilistic RBSL Non-cancer hazard (default Qsoil) (site-specific Qsoil) based on the 5th percentile probabilistic RBSL

25 / Regulatory agencies preferred deterministic RBSLs over probabilistic RBSLs because the deterministic values can be verified using existing vapor intrusion models (EPA, DTSC) Probabilistic assessment useful for characterizing the effect of variability in model input parameters (uncertainty analysis) Range of alpha values is smaller than range in RBSLs indicating that RBSLs are more sensitive to inhalation rate than soil and building parameters that were evaluated Probabilistic assessment helps to understand relative protectiveness of deterministic RBSLs and thus increases confidence in these values Child deterministic RBSLs ~ low end to middle Adult deterministic RBSLs ~ middle to high end