MEMORANDUM Metcalf&Eddu

FILE: 004609-0018-010-008 DATE: November 2, 1993

TO: D. Simone OFFICE: Wakefield

FROM: D. Murray, J. Young, and J. Best COMPANY: Inc.

SUBJECT: Data Useability of Ambient Air SUMMA Canister Samples at the Rose Hill Landfill Site

Assessment of Inhalation Risks

The draft RI risk assessment evaluates risk from inhalation of air by using a box model to predict exposures of a visitor of a landfill. The draft RI stated that assessment of risks to a resident from indoor air exposures was to be completed in the final RI report. Laboratory-analyzed SUMMA canister data, recently received from the EPA Emergency Response Team (July and August, 1993) can be used to supplement the draft RI risk assessment. However, there is a data useability issue and there are still some data gaps that preclude predicting risks as accurately as might be possible.

The following air data are not available, and represent data gaps: (1) laboratory-quality data from the surface of the landfill during near-calm conditions and (2) reliable vinyl chloride data from outdoor air in the area of the residences. This memo highlights these data gaps, and states how risks might nevertheless be assessed in the final RI if no further air data become available.

Air Above the Landfill Surface

No 24-hour SUMMA canister samples were collected from the surface of the landfill. The 8-hour samples were dominated by relatively high winds, and no vinyl chloride was detected. Because more conservative calm conditions were not measured, actual measurements of air concentrations are inadequate to evaluate risks to a site visitor. Therefore, the revised risk assessment would simply update the evaluation of risks to site visitors from inhalation as modeled in the draft RI, which used a simple box model for dispersion on the landfill surface. The landfill gas generation estimate was revised downwards by a factor of about 20 during the evaluation of gas migration barrier systems (M&E, May 1993); incorporating this updated emission estimate would lead to an

estimated risk to a regular site visitor of about 1 x 10"3. There do not appear to be any representative SUMMA canister data to re-evaluate ambient air exposures on the site.

Ambient Air at Residences

M&E has conducted a preliminary review of the July 1993 analytical data, without conducting a formal validation. The data review shows low recoveries for vinyl chloride in the matrix spike (MS) samples. The percent recoveries for vinyl chloride from the SUMMA data range as low as 3.2% in soil gas samples and as low as 9.1 % in ambient air samples. EPA Region I guidelines for data validation require all non-detected results with less than 10% MS or MSD recovery to be rejected. As a result, the SUMMA data for vinyl chloride would continue to be qualified as approximate with a "J" flag, representing an estimated value, for detected values. Non-detected values would be rejected in data validation. Because the matrix spike data shows low recovery, it may be estimated that vinyl chloride concentrations in ambient air at the landfill could have been an order of magnitude higher than the values reported. Because the vinyl chloride data would not be rejected, this data may be used to evaluate offsite risks to residents from ambient air exposure. However, the uncertainty associated with the measured vinyl chloride concentrations would need to be stressed in the risk assessment report.

In terms of including a significant period with low wind velocity, the 24-hour SUMMA sample collected between the hours of 6 PM on 5-26-93 and 6 PM on 5-27-93 was the most representative sample of ambient air which might have volatile contaminants. During the night, the wind was near calm for 10 hours. During the hours of slowest air movement, according to site meteorological data, the slight wind was from the northeast. For this 24-hour period, vinyl chloride was detected at all receptors sampled. The maximum vinyl chloride concentration of 4.1 ug/m3 was detected at receptor #8, which is south of the landfill, and probably downwind of source areas when the wind was weakest. The other samples were taken during periods of time dominated by windy conditions, including the 8-hour sample taken on 5-27-93, which was within the time period of the 24-hour sample discussed above. Vinyl chloride was not detected in the corresponding 5-27-93 8-hour sample; however, the meteorological data for this 8-hour period indicates that the wind was fairly strong from the northwest for about 6 of the 8 hours. The 4.1 ug/m3 value may be the most appropriate on which to base the human health risk assessment because much of the sampling occurred during relatively high winds, because periods of relative calm are of greatest concern for landfill gas, and because EPA Region I is generally interested in maximum site contaminant concentrations.

We are not recommending using the results of the Point, Area, and Line (PAL) model for the risk assessment, because the model predicts ambient concentrations a few orders of magnitude below what was actually measured at the site. Therefore, the field measurements appear to disprove modeling results. Perhaps this was because the model could not predict the concentrations present under near-calm conditions, due to a regulatory requirement that during periods of calm (< 1 m/sec), a 1 m/sec wind from the prior wind direction be assumed, which in this case happened to be from the northwest.

Another possible reason is that the Tedlar bags used for the Trace Atmospheric Gas Analyzer (TAGA) may have allowed loss of vinyl chloride. In any case, actual field measurements are generally preferable, where available, to modeled concentrations, for risk assessment purposes.

For the revised risk assessment, we would assume that the data collected at receptor #8, for the 24-hour period showing the maximum vinyl chloride concentration in air, is most representative of ambient air concentrations on an average day at whichever residence is most affected by landfill gas emissions. Depending on the exposure assumptions that are selected, we would anticipate the estimated risk from vinyl chloride alone to be up to 1.4 x 10"*; we would add that this estimate could underestimate risks by an order of magnitude due to the low recoveries identified with the matrix spikes. Risks from benzene or other compounds could add to this estimate, depending on the results of SUMMA canister data validation.

Use of this data is further supported by a similar vinyl chloride concentration in ambient air collected on March 11, 1993 at 278 Rose Hill Road, during the February and March SUMMA canister sampling event.

Air Inside Residences

In February and March 1993, a limited number of SUMMA canister samples were collected from inside residents' homes in the area. The concentrations detected within the homes in February and March compare with ambient air concentrations in the May 1993 sampling. By extrapolation from methane measured in the basements of these homes, it has been estimated that risks from vinyl chloride from within the homes may be significant (see, for example, Mary-Beth Smuts' memo of March 2, 1993). Concentrations of vinyl chloride in air from samples collected in February and March inside the residences could be used to calculate risk. Alternatively, the outdoor samples from May 1993 could be used to calculate risk from indoor exposures using the assumption that indoor air concentrations are the same as outdoor concentrations.

APPENDIX E-5

SOIL VAPOR EMISSIONS CALCULATIONS

The purpose of this appendix is to present emission rates for each of the components found in the Rose Hill Regional landfill generated gases and to document the approach used to calculate the rate of soil vapor emissions. The calculation of emission rates takes into consideration both molecular diffusion and convection. Emission rates due to molecular diffusion from each of the disposal areas (solid waste, bulky waste and sewage sludge) are presented in Tables E-ll through E-13; emission rates due to convection from each of the disposal areas are presented in Tables E-14 through E-16. Molecular diffusion rates are calculated using Pick's 1st Law; convection rates are calculated using Darcy's Law. Both laws are defined below and the source(s) of the variables are provided.

Molecular Diffusion

The flux of gas to the atmosphere caused by molecular diffusion can be predicted by Pick's 1st Law:

F= -Dxn^ x phi dz

where: F = flux of gas through cover material

D = diffusion coefficient of gas components in air at specified temperature and pressure

n,M= gas porosity of the cover material t:

phi= tortuosity of cover material

dC = concentration gradient of gas component below dz cap and at surface of landfill

The emission rate is then calculated by multiplying the flux by the surface area of each disposal area.

The source of each of the variables used in calculating the flux is described in detail below.

*«.

Diffusion Coefficients

The diffusion coefficient for many hazardous compounds arc cited in Table 2-3 of the Superfund Exposure Assessment Manual (U.S. EPA, 1988). Most components found in the landfill gases at the Rose Hill disposal areas were included in the table. For those that were not included on the table, coefficients were calculated using the method suggested in the manual. The method suggests calculating the molecular weight and atomic diffusion volume of the compound not listed on the table using relevant atomic diffusion volumes cited in the document. The diffusion coefficient is calculated by selecting a compound on the table with a similar molecular weight and atomic diffusion volume and multiplying the diffusion coefficient of the compound on the table by the square root of the ratio of the molecular weights (known compound/unknown compound). All diffusion coefficients were then adjusted for temperature assuming field conditions as measured during the collection of gas samples in the impinger system. Since no pressure measurements were obtained in the field and the calculation of the molecular diffusion flux assumes that there is no pressure differential across the soil cover, it was assumed that the diffusion coefficients (expressed in the table at atmospheric pressures) did not require adjustment. Pages 14 through 19 of the Superfund Exposure Assessment Manual (U.S. EPA, 1988) includes the list of diffusion coefficients and the method for determining a coefficient if it not included in EPA's Table 2-3; these pages are attached to this appendix for reference.

Porosity

The gas porosity of the cover material is typically calculated from bulk density and particle density, or bulk density and saturation moisture measurements for a soil. These date were not collected on the cover material from the Rose Hill disposal areas. The data that were collected on the cover material included hydraulic permeability, grain size and moisture content. Several empirical formulas have been developed to determine porosity based on permeability. Using the Bretjinski formula (de Marsily, 1986) for sandy soils and hydraulic conductivities obtained on five cover material samples, porosities were calculated; values ranged between 5.6 and 8.2, which is very low for sandy loam. More typical porosities are as follows (Dunn et al, 1980):

well sorted sand or gravel 25-50% > sand & gravel mixed soil 20-35%

glacial till 10-20% silt 35-50% clay 33-60%

Other references include Peck et al, 1974:

mixed-grained sand, loose 0.40 mixed-grained sand, dense 0.30

In the exposure assessment manual, it states that the typical maximum range for hydraulic porosities is 24-62% with 55% typical for dry, non-compacted soils. For compacted soils, it states that the typical porosities is 35%. Based on this information and soil classification based on grain size data, a 40% porosity was selected for the sandy soil cover on the Rose Hill disposal areas.

Tortuosity

Tortuosity is a parameter which is the average length of flow path along the direction of flow. It accounts for the degree of interconnection between the voids. It is most typically measured for a hydraulic fluid. For gases, the tortuosity factor most typically ranges between 1.2 and 1.7. For dry soil, the tortuosity factor can be estimated as the cube root of the gas porosity. MUlington (1977) has calculated tortuosity factors for wet soils, however, this reference could not be obtained in time to develop the emission rates on Tables E-l 1 through E-13. As a result, the tortuosity factor used in the equation was taken to be the cube root of the porosity, or 0.74 (for porosity of 0.40).

Concentration Gradient

The concentration gradient represents the difference in concentration of each gas component between the landfill surface and a location at depth, typically the location directly beneath the landfill cover material. Multiple soil gas points were monitored during the field investigation at a depth of approximately 5 feet below the ground surface. No determination of the depth of the cover material was made at those points, however, based on several observations it was judged that the depth of cover material ranged between 0 and 3 feet. In the calculation of the concentration gradient, it was assumed that the distance over which the gas was diffusing was equal to the sampling depth and the concentration at that depth was the concentration noted during the field observation. It was also assumed that the concentration of each component in the gas was zero at the landfill surface. The error in these assumptions is that the landfill cover (on which the porosity and tortuosity values are based) is not actually 5 feet. However, it is not unreasonable to assume that there is little difference in concentration of each of the components two feet deep into the waste and at a location just below the cover material.

Landfill Area

The landfill area used to calculate a total flux of each gas component coming off the landfill was estimated by digitizing the areas marked on the base map used in the RI. Each disposal area was digitized three times and the area was taken to be the average of those three measurements.

Convection

The flux of gas to the atmosphere caused by convection can be predicted by Darcy's Law:

v = - k d P n,Mdx

where:

v = velocity of gas

k = gas conductivity of the soil cover

dP = pressure difference between the atmosphere dx and a location, x, beneath the cover

n^i. = gas porosity of the cover material

The emission rate is then calculated by multiplying the velocity of the gas escaping the disposal area at the surface by the surface area of the disposal area and the concentration of the individual component (to obtain convective flux of individual gas components).

The source of each of the variables used in calculating the velocity is described in detail below.

Gas Conductivity (k)

The gas conductivity of the cover material is related to the hydraulic conductivity of the cover material by a factor equal to the ratio of the density of water/viscosity of water to the density of landfill gas/viscosity of landfill gas. This calculation assumes that the landfill gas is incompressible. The value for hydraulic conductivity used in this equation is based on an average of three of five values obtained on cover material samples; the average value is 0.26 ft/d. Values for landfill gas density and viscosity were obtained from Farquhar, 1977. The values assume that the gas is composed of 50% methane and 50% carbon dioxide. At 1 atm and 0 °C, the density of landfill gas is 13.2 N/m3, and the viscosity is 1.21 X 10"5 Pa s. The factor was calculated assuming the density and viscosity of water at similar temperatures and pressures. Since the hydraulic conductivity test is conducted at ambient temperatures, the density and viscosity constants for both landfill gas and water should be adjusted to this temperature. This was done as part of a calculation check. Results showed a 30% increase in the factor which was considered to be negligible in relation to the result of the calculation.

Pressure Gradient

The pressure gradient represents the difference in pressure between the atmosphere and the landfill at a specific location, typically directly beneath the cap. Since no pressure measurements were obtained in the field during the Rose Hill RI, an approximation had to be made for this calculation. Multiple references cite that the pressure build-up between the constraining cover layer and the atmosphere ranges between 250 Pa and 3 kPa (Bogner, 1989; Farquhar, 1977; and U.S. EPA, 1989). While the cover at the Rose Hill Regional landfill is not considered to be low permeability and 3 kPa is most reflective of a low permeability cover, a value of 3 kPa was used in the calculation of the velocity. This value was used to make the emission estimate most conservative. Since the pressure gradient needs to be expressed in terms of unit length in the Darcy equation, the pressure gradient is then divided by the density and added to the vertical distance between the surface of the landfill cover and the depth of measurement. The depth of measurement in the equation was assumed to be 5 feet.

It should be noted that the Air/Superfund National Technical Guidance Study Series; Volume n - Estimation of Baseline Air Emissions at Superfund Sites (U.S. EPA, 1989) suggests that for closed landfills with internal gas generation the (Thibodeaux) Convective Add-On Model be used to calculate total emissions due to both molecular diffusion and convective mechanisms. This model combines both Pick's 1st Law and Darcy's Law into one equation and makes several simplifying assumptions. For comparison, this equation was used to calculate the emissions coming off the solid waste landfill for cis-l,2-dichloroethene, a compound found to have a high emission rate with respect to other landfill gas components. Using the Convective Add-On Model, an emission rate of 30,331 mg/s was calculated. This value compared well with the value calculated for convection using Darcy's; the result for this calculation was 30,332 mg/s. In calculating human health risk, the emissions resulting from molecular diffusion and the emissions resulting from convection are be added together to present a worse case scenario.

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