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Intro Map
Figure 1. Intro Map

Private Well Locations
Figure 2. Private Well Locations

Soil Gas Well Locations
Figure 3. Soil Gas Well Locations

APPENDIX B: Quality Assurance and ATSDR Methodology

Quality Assurance

In preparing this report, ATSDR relied on the information provided in the referenced documentsand by contacts with the Texas Natural Resources Conservation Commission, Texas Department ofHealth, San Antonio Metropolitan Health District, Environmental Protection Agency, UnitedStates Geological Survey, community members, and Kelly Air Force Base. ATSDR assumes thatadequate quality assurance and control measures were taken during chain-of-custody, laboratoryprocedures, and data reporting. The validity of the analyses and conclusions drawn in thisdocument are determined by the availability and reliability of the information.

Human Exposure Pathway Evaluation and the use of Comparison Values

ATSDR assesses a site by evaluating the level of exposure in potential or completed exposurepathways. An exposure pathway is the way chemicals may enter a person's body to cause a healtheffect. It includes all the steps between the release of a chemical and the population exposed: (1) achemical release source, (2) chemical movement, (3) a place where people can come into contactwith the chemical, (4) a route of human exposure, and (5) a population that could be exposed. Inthis assessment, ATSDR evaluates chemicals in the soil, air, and groundwater that people living innearby residences may consume or come into contact with.

Data evaluators use comparison values (CVs), which are screening tools used to evaluateenvironmental data that is relevant to the exposure pathways. Comparison values areconcentrations of contaminants that are considered to be safe levels of exposure. Comparisonvalues used in this document include EPA's Region III risk-based concentration. Comparisonvalues are derived from available health guidelines, such as ATSDR's minimal risk levels andEPA's cancer slope factor.

The derivation of a comparison value uses conservative exposure assumptions, resulting in valuesthat are much lower than exposure concentrations observed to cause adverse health effects; thus,insuring the comparison values are protective of public health in essentially all exposure situations.That is, if the concentrations in the exposure medium are less than the CV, the exposures are not ofhealth concern and no further analysis of the pathway is required. However, while concentrationsbelow the comparison value are not expected to lead to any observable health effect, it should notbe inferred that a concentration greater than the comparison value will necessarily lead to adverseeffects. Depending on site-specific environmental exposure factors (for example, duration ofexposure) and activities of people that result in exposure (time spent in area of contamination),exposure to levels above the comparison value may or may not lead to a health effect. Therefore,ATSDR's comparison values are not used to predict the occurrence of adverse health effects.

The comparison values used in this evaluation are defined as follows: The CREG is aconcentration at which excess cancer risk is not likely to exceed one case of cancer in a millionpersons exposed over a lifetime. The CREG is a very conservative CV that is used to estimatecancer risk. Exposure to a concentration equal to or less than the CREG is defined as aninsignificant risk and is an acceptable level of exposure over a lifetime. The risk from exposure isnot considered as a significant risk unless the exposure concentration is approximately 10 times theCREG and exposure occurs over several years. The EMEG is a concentration at which dailyexposure for a lifetime is unlikely to result in adverse noncancerous effects.

Selecting Contaminants of Concern

Contaminants of concern (COCs) are the site-specific chemical substances that the health assessorselects for further evaluation of potential health effects. Identifying contaminants of concern is aprocess that requires the assessor to examine contaminant concentrations at the site, the quality ofenvironmental sampling data, and the potential for human exposure. A thorough review of each ofthese issues is required to accurately select COCs in the site-specific human exposure pathway.The following text describes the selection process.

In the first step of the COC selection process, the maximum contaminant concentrations arecompared directly to health comparison values. ATSDR considers site-specific exposure factors toensure selection of appropriate health comparison values. If the maximum concentration reportedfor a chemical was less than the health comparison value, ATSDR concluded that exposure to thatchemical was not of public health concern; therefore, no further data review was required for thatchemical. However, if the maximum concentration was greater than the health comparison value,the chemical was selected for additional data review. In addition, any chemicals detected that didnot have relevant health comparison values were also selected for additional data review.

Comparison values have not been developed for some contaminants, and, based on new scientificinformation other comparison values may be determined to be inappropriate for the specific type ofexposure. In those cases, the contaminants are included as contaminants of concern if currentscientific information indicates exposure to those contaminants may be of public health concern.

The next step of the process requires a more in-depth review of data for each of the contaminants selected. Factors used in the selection of the COCs included the number of samples with detections above the minimum detection limit, the number of samples with detections above an acute or chronic health comparison value, and the potential for exposure at the monitoring location.

APPENDIX C: Estimated Exposure Dose and Cancer Risk for On-site Soil

  • ATSDR selected an exposure scenario for incidental ingestion of soil at East Kelly asrepresenting the greatest potential exposure to soil and its potential contaminant concentration if migration off-site occurs. Although ingestion of maximum concentrations of chemicals detected may be unlikely, it represents the greatest potential exposure to chemicals in the soil.

  • ATSDR screened the maximum contaminant concentrations by comparison to the EPA's 1997Region III Risk-Based Concentration (RBC) Tables for residential soil. All contaminants that exceeded this comparison value were selected for further evaluation. Maximumconcentrations were used to calculate an exposure dose. Risk-based screening levels forcarcinogens were based on combined childhood and adult exposure. Risk-based screeninglevels represent theoretical risks and should not be viewed as predictors of adverse health effects; the actual risk may be zero1.

  • ATSDR assumed the same ingestion rates as recommended for incidental soil ingestion of 200 milligrams (mg)/day for children and 100 mg/day for adults. ATSDR used standard body weights for children (15 kilograms[kg]) and adolescents/adults (70 kg).

  • The exposure frequency was assumed to be constant since the scenario is targeting possible residential areas. Durations were assumed for children, 0-6 years and adolescents/adults, 7-30 years.

  • The carcinogenic potency slope oral taken from RBC Tables were as follows:
Benzo(a)pyrene: 7.30E+00 risk per mg/kg/day
Dibenzo(a,h)anthracene: 7.30E+00 risk per mg/kg/day
Benzo(a)anthracene: 7.30E-01 risk per mg/kg/day
Benzo(b)fluoranthene: 7.30E-01 risk per mg/kg/day

The results are presented in Table 2.

APPENDIX D: Soil Gas and Estimated Risk

Table 3.

East Kelly Soil Gas Sampling Results Compared to RBCs and Estimated Indoor Air Cancer Risk
Chemical Maximum Soil Gas Concentration µg/m3 RBC µg/m3 Maximum Estimated Indoor Air Concentration1 µg/m3 J&E Estimated Cancer Risk2 ATSDR Estimated Cancer Risk3
1,1,2-Trichloro-1,1,2-trifluoroethane 275 31,000      
cis-1,2-Dichloroethene 344 37 0.0059 Not classifiable4 Not classifiable4
1,1,1-Trichloroethane 9.3 1000      
1,1-Dichloroethane 20.6 520      
Benzene 7.3 0.22 0.00013 4.9E-10 1.08E-08
Ethylbenzene 10.4 1000      
Methylene Chloride 286 3.8 0.0054 1.9E-8 2.54E-08
Tetrachloroethene (PCE) 14,230 3.1 0.23 5.8E-8 1.34E-06
Toluene 165 420      
Trichloroethene (TCE) 618 1 0.011 7.9E-9 1.89E-07
Xylenes 29.9 310      
1,2,4-Trichlorobenzene 179 210      
Hexachlorobutadiene 265 0.081 0.0037 3.5E-8 8.17E-07
Styrene 8.1 1000      
Vinyl Chloride ND        
1 Indoor air concentrations were derived using EPA's Johnson & Ettinger model for subsurface vapor intrusion into buildings
2,3Cancer risks were derived using both EPA's Johnson & Ettinger model and ATSDR's Public Health Assessment Guidance Manual for chemicals detected above RBC
4 Not classifiable as to human carcinogenicity based on no data on humans or animals
ppbv = parts per billion volume
g/m3 = micrograms per cubic meter
RBC = 1997 Risk-based concentration
ND = Not detected
Shading indicates 1997 RBC ambient air comparison value exceedence

Estimated Exposure Dose and Cancer Risk from Soil Gas to Indoor Air

Johnson and Ettinger Model Assumptions

  • Cancer risk estimates were calculated using the Johnson and Ettinger model for carcinogenic chemicals that exceeded their comparison value. The default time for carcinogens is 70 years; the default residential exposure duration is 30 years; the default exposure frequency is 365 days per year; the default hazard quotient for noncarcinogens is 1. Default reference doses and carcinogenic potency factor slopes in the model are from the Integrated Risk Information System and Health Effects Assessment Summary Table [1].

  • These assumptions for the Johnson and Ettinger model for subsurface vapor intrusion into buildings were used to calculate the cancer risk in the model. However, the groundwater to soil gas step was eliminated since soil gas concentrations were known.

  • Non-carcinogen hazard quotients were tabulated for each chemical exceedence. Theaccumulative hazard quotient did not exceed 1.

ATSDR Exposure and Cancer Risk Assumptions

  • ATSDR screened the maximum contaminant concentrations by comparison to the EPA's1997 Region III Risk-Based Concentration (RBC) Tables for ambient air. All contaminantsthat exceeded this comparison value were selected for further evaluation. Maximumconcentrations were used to calculate an exposure dose. Risk-based screening levels forcarcinogens were based on combined infant, childhood, and adult exposure.

  • ATSDR assumed the inhalation rates of 3.8 cubic meters (m3)/day for infants, 15 m3/day for children, and 23 m3/day for adults [2]. ATSDR used standard body weights for infants (15 kilograms[kg]), children (42.5 kg), and adults (70 kg).

  • The exposure frequency was assumed to be constant at since the scenario is targeting possible residential areas. Durations were assumed for infants, 0-1 years; children, 1-10 years; adults, 11-30 years.

The carcinogenic potency slope inhalation taken from 1997 RBC Tables are as follows:
Benzene: 2.90E-02 risk per mg/kg/day
Methylene Chloride: 1.64E-03 risk per mg/kg/day
Tetrachloroethene (PCE): 2.03E-03 risk per mg/kg/day
Trichloroethene (TCE): 6.00E-03 risk per mg/kg/day
Hexachlorobutadiene: 7.70E-02 risk per mg/kg/day
  1. US Environmental Protection Agency. 1997. User's Guide for the Johnson and Ettinger (1991) Model for Subsurface Vapor Intrusion into Buildings. Washington, DC.

  2. ATSDR. 1993. Public Health Guidance Manual. Lewis Publishers, Boca Raton, FL.

APPENDIX E: Exposure Pathway Table

Table 1.

Pathway Name Source Medium Route of Exposure Exposure Elements Exposure Activities Chemicals of Concern
Point of Exposure Receptor Population Time
Off-site Soil Gas Contaminated Shallow Groundwater Soil Gas Inhalation Off-site Residents p, c, f Indoor Activities VOCs
Key: p = past; c = current; f = future;

1. US Environmental Protection Agency. 1986. Guidelines for Carcenogenic Substances. FR 51:3392-34006.

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