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PUBLIC HEALTH ASSESSMENT

NORTON AIR FORCE BASE
NORTON AIR FORCE BASE, SAN BERNARDINO, CALIFORNIA


Table 2. Population Data: Norton AFB and Surrounding Areas, 19901

  Base West Area Southwest Area Northeast Area Northwest Area County
Total Persons
Total Area (Square Miles)
Persons per square mile
653
2.98
219
3,713
0.83
4,481
1,389 (1,637)
1.40 (1.40) 992
(1,169)
1,850
0.26
7,162
2,847
0.64
4,428
1,418,380
20,061.78
71
% Male
% Female
70.4
29.6
60.9
39.1
49.2 (49.1)
50.8 (50.9)
48.4
51.6
50.3
49.7
50.1
49.9
% While
% Black
% American Indian, Eskimo, or Aleut
% Asian or Pacific Islander
% Other Races
71.7
15.9
0.9
5.4
6.1
53.5
27.1
1.2
8.8
9.3
27.3 (39.2)
34.8 (37.4)
2.2 (2.7)
4.0 (0.9)
31.7 (19.9)
59.1
25.8
1.1
3.7
10.3
60.9
11.0
1.7
3.0
23.5
73.0
8.1
0.9
4.2
13.8
% Hispanic Origin 7.8 32.8 47.9 (28.9) 29.6 41.8 26.7
% Under Age of 10
% Age 65 or Over
10.1
0.3
17.4
2.9
22.6 (21.4)
10.7 (12.3)
26.9
3.6
25.0
6.3
18.9
8.8

1 Data in parentheses from 1980.
Source: Census of Population and Housing, 1990: Summary Tape file 1 (California). Prepared by the Bureau of the Census, Washington, DC, 1991.


Table 3. Housing Data: Norton AFB and Surrounding Areas, 19901

  Base West Area Southwest Area Northeast Area Northwest Area County
Households2
Persons per household
55
3.84
896
2.98
381 (524)
2.98 (3.10)

532
3.48

874
3.26
464,737
2.97
% Households owner-occupied
% Households renter-occupied
1.8
98.2
26.7
73.3
44.4 (49.0)
55.6 (51.0)
45.5
54.5
34.9
65.1
63.3
36.7
% Households mobile homes 0.0 7.4 0.8 (0.6) 0.0 2.4 6.6
Median value, owner-occupied households ($)
Median rent paid, renter-occupied households ($)
NA
650
78,000
438
~75,000 (41,2353)
~340 (1983)
80,400
406
~65,000
~390
129,200
489

1 Data in parentheses from 1980.
2 A household is an occupied housing unit, but does not include group quarters such as military barracks, prisons, and college dormitories.
3 Median value of owner-occupied housing and median rent paid adjusted according to the consumer price index (CPI) for housing for all U.S. cities, 1980; 1990 CPI adjusted median value, owner-occupied and renter-occupied are $58,366 and $265, respectively.

Source: Census of Population and Housing, 1990: Summary Tape File 1 (California). Prepared by Bureau of the Census, Washington DC. 1991.


Table 4. Wells on and in the Vicinity of the Norton AFB TCE Plume Area.

Production Well No. Depth
(feet)
Screened
(ft2)
Base Production Wells
2 818 768 to 818
3 990 480 (top)
5 1,100 840 to 1,1001
11 733 544 to 717
Gage Canal Wells
46-1 415 100 to 393
56-1 1,126 467 to 982
30-1 937 478 to 911
31-1 422 216 to 402
21-1 (destroyed) 474 ND2
29-2 325 122 to 299
29-3 297 54 to 266
6 (new in 1992) ND ND
51-1 710 513 to 708
26-1 390 82 to 270
27-1 396 87 to 386
66-1 808 542 to 752
27-2 418 151 to 406
92-1 1,230 525 to 1,175
92-2 1,190 760 to 1,145
92-3 1,230 730 to 1,210
29-1 303 84 to 279
Wells Located Southwest of Norton AFB
Norman Road 493 218 to 452
Raub No. 2 1188 ND
Raub No. 3 691 534 to 691
Raub No. 4 750 537 to 719
Raub No. 5 804 200 to 800
Raub 6 1,010 540 to 1,010
Raub 8 1,080 525 to 1,035
Warren No. 1 965 556 to 814
Warren No. 2 1,137 94 to 450
Warren No. 3 852 275 to 645
Warren No. 4 1,106 1,008 to 1,088

1 Only base production well located in the TCE plume area.
2 ND - Not determined.


Table 5. Potential Exposure Pathways Associated With Norton AFB1.

Pathway Name Source of Contamination Environmental Medium Point of Exposure Route of Exposure Time of Exposure Exposed Population Comments
Drinking water (Base production wells) TCE--Norton AFB Central Base Area (Monitoring Well 90 area; Building 658; IRP Site 9; and Building 763) Groundwater On-base residents; employees of Norton AFB Ingestion Past:
• Since 1980, TCE was only detected one time (September 1980) above the MCL in one of the four previously used base drinking water wells.
Current and Future:
• No TCE has been detected above MCLs since the one hit in 1980.
Users of the Norton AFB water supply.

Population on-base estimated to be slightly less than 8,000 people in 1984; population approximately 650 people in 1990.

Past:
• Only one of the four previously used base drinking water wells is located in the TCE plume area.
• Historical drinking water well data since 1980 indicates that TCE was only detected above the MCL one time.
Current and Future:
• The only Norton AFB drinking water well located within the contamination plume is screened at 1,100 feet and contamination only extends to a depth of 400 feet. Contamination of this well is unlikely.
• Remedial activities have reduced TCE concentrations in groundwater.
• Groundwater monitoring will continue on-base. If contamination of a drinking water well is detected in the future, the use of the affected well will be discontinued or the well water will be blended with water from uncontaminated sources.

Past, current, and future use of Norton AFB drinking water poses no apparent health hazards.

Drinking water (Riverside water supply) TCE--Norton AFB Central Base Area (Monitoring Well 90 area; Building 658; IRP Site 9; and Building 763) Groundwater Off-base residences; businesses Ingestion Past:
• Since the early 1980s, elevated levels of TCE were detected in the drinking water wells southwest of Norton AFB, particularly the wells screened at depths of less than 400 feet in the aquifer.
Current and Future:
• Raub 5, Warren No. 2, Warren No. 3, Gage 26-1, and Gage 27-1 are the only Riverside drinking water wells to exceed the MCL for TCE since 1991.
Users of Riverside municipal water supply. Past:
• Raub 5 and Warren 2 are the only Riverside wells that had consistent detections of TCE during the period of 1989 to 1991 and they are the only wells with TCE concentrations detected above the MCL. These wells are blended with non-impacted water to reduce contaminant levels.
• The South San Bernardino well, Norman Road, had detections of TCE during the period of 1989 to 1991 above the MCL; this well was deactivated in 1991.
Current and Future:
• Raub 5, Warren No. 2, Warren No. 3, Gage 26-1, and Gage 27-1 are the only Riverside drinking water wells to exceed the MCL for TCE since 1991. Groundwater from these wells is blended with water from other wells to reduce the contaminant levels to those which do not pose a human health hazard.
• Remedial activities for soil and groundwater on base have reduced the contamination levels and the speed at which the TCE plume is migrating toward Riverside drinking water wells.
• Groundwater is monitored and well-head treatment options are available should they become necessary.
• The Air Force is installing a well head treatment facility at Raub 5 as requested by the city of Riverside.

Past, current, and future use of municipal water poses no apparent health hazards.

Drinking water (Riverside water supply)2 Radionuclides--naturally occurring Groundwater Residence Ingestion Past:
• Past groundwater from some city of Riverside drinking water wells located downgradient to Norton AFB indicated high levels of total uranium and gross alpha.
• Contamination levels in the Riverside drinking water wells did not exceed MCLs until 1990.
Current and Future:
• Current groundwater samples indicate radionuclide contaminant levels in the individual Riverside drinking water wells that exceed the MCLs.
Users of Riverside municipal water supply. Past:
• Radionuclides were not detected above drinking water standards in Riverside drinking water wells until 1990; groundwater from drinking water wells was blended with water that does not contain high levels of radionuclides to reduce contaminant levels to those which do not pose a public health hazard.
Current and Future:
• Riverside currently prioritizes well pumping operations from the Bunker Hill groundwater basin to comply with the existing MCLs for gross alpha and uranium; groundwater is also blended to achieve contaminant levels acceptable for human consumption.
• Groundwater is regularly tested and meets regulations.

Past, current, and future use of municipal water poses no apparent health hazards.

Abandoned drinking water wells (South San Bernardino County Water District) TCE--Norton AFB Central Base Area (Monitoring Well 90 area; Building 658; IRP Site 9; and Building 763) Groundwater Residents of the former housing area Ingestion Past:
• There are no historical data for the use of the abandoned wells found in the former housing area. Houses were constructed after 1948 and occupied until approximately 1982, when the Air Force purchased the land.
Residents of the former housing area located at the end of Norton AFB Runway 24.

Approximately 70 housing lots were located in this area; a few additional lots were devoted to businesses for light industrial use.

1980 census data suggests an area-wide average of about 2.75 persons per household for a maximum of about 193 persons potentially exposed.

Past:
• According to Earth Tech (1997a), water to these houses has been supplied by South San Bernardino County Water District, which has since been transferred to The City of San Bernardino.
• No historical monitoring data for these abandoned wells were provided to ATSDR; therefore, data from monitoring wells upgradient to the housing area were used to calculate exposure doses to estimate human effects from exposure to TCE-contaminated drinking water. No adverse health effects are associated with consumption of groundwater from these abandoned wells during the time in which the houses were occupied.

Current and Future:
• The houses were purchased by the Air Force about 1982. The Air Force did not use the wells that were abandoned in the former housing area.
• The Air Force capped or otherwise closed the abandoned wells in 1997 (Earth Tech, 1997e).

Past use of the abandoned wells in the former housing area for drinking water poses no apparent health hazards.

1 Several other exposure pathways have not been included in the table because past, current, or future exposure through the pathway is extremely unlikely because site characterization suggests that the medium (at point of exposure) is/was not contaminated or that exposure was brief or intermittent. Pathways include groundwater exposure via private wells down-gradient of Norton AFB and exposure to on-site soils (Table 1).
2 Also included is the Norman Road Well owned by the defunct South San Bernardino County Water District.


Table 6. Summary of Contaminants in Norton On-Base Drinking Water Wells.

Contaminant Min-Max Duration of Detections Comparison Values Location of Maximum Contamination Detected
TCE ND1 - 6.2 ppb 1979 to 1998 3 ppb (CREG)
5 ppb (MCL)
Well No. 112
1,2 - DCE ND 1980 to 1996 3,000 ppb (EMEG - Intermediate)
70 ppb (MCL)
No Detections
PCE ND - 9.5 ppb 1979 to 1998 0.7 ppb (CREG)
5 ppb (MCL)
Well No. 2A3
Gross Alpha 0.2 - 12.60 pCi/L 1979 to 1996 15 pCi/L (MCL) Well No. 11
Uranium NA - 14.7 pCi/L 1979 to 1996 20 pCi/L (MCL) Well No. 11

1 ND - No detection.
2 One time detection in this South San Bernardino Water District well on September 29, 1980. Forty-two days prior, this sample was 1.7 ppb; subsequent samples were less than 1.7 ppb.
3 Newly installed well owned by the City of San Bernardino Municipal Water Department. Well not currently in use.


Table 7. Summary of Contaminants in Riverside Municipal Drinking Water Wells.

Contaminant

Min-Max

Duration of Detections

Comparison Values

Location of Maximum Contamination Detected

TCE ND1 - 17 ppb2 1979 to 1994 3 ppb (CREG)
5 ppb (MCL)
Warren 2
1,2-DCE ND - 2.90 ppb 1988 to 1996 3,000 ppb (EMEG - Intermediate)
70 ppb (MCL)
Gage 31-1
PCE ND - 2.6 ppb 1979 to 1994 0.7 ppb (CREG)
5 ppb (MCL)
Norman Road3
Gross Alpha ND - 59 pCi/L 1979 to 1994 15 pCi/L (MCL) Gage 29-3
Uranium ND - 55 pCi/L 1979 to 1994 20 pCi/L (MCL) Gage 29-3

1 ND - No detection.
2 Only two wells exceeded the MCL.
3 One time detection. Well owned by the defunct South San Bernardino County Water District. Well deactivated in 1991.


Table 8. Summary of Contaminants in Norton AFB On-Base and Off-Base Monitoring Wells.

Contaminant Min-Max Duration of Detections Comparison Values Location of Maximum Contamination Detected
TCE ND1 - 4,630 ppb2 1992 to 1996 3 ppb (CREG)
5 ppb (MCL)
MW-903

A-level

1,2-DCE ND - 120 ppb 1992 to 1995 3,000 ppb (EMEG - Intermediate)
70 ppb (MCL)
MW-202
PCE ND - 18 ppb 1992 to 1996 0.7 ppb (CREG)
5 ppb (MCL)
MW-246
A-level;
MW-113
B-level
Gross Alpha 2.38 - 456 pCi/L 1994 to 1996 15 pCi/L (MCL) MW-158
Uranium ND - 4,956 pCi/L4 1994 to 1996 20 pCi/L (MCL) MW-158

1 ND - No detection.

2 Maximum TCE concentration during April 1995 sampling round was detected at 940 ppb in the sample from MW-190. Highest concentration observed in the CBA TCE plume since CDM Federal has been conducting groundwater sampling at Norton AFB.

3 MW-90 located in the TCE source area. Maximum contamination only detected one time in May 1993. Well went dry in 1988 and was abandoned in January 1996. Well was replaced by MW 190.

4 One time detection in 1991. Monitoring well resampled and results indicate total uranium levels 40 times lower.


FIGURES

Regional Map Showing Location of Norton AFB
Figure 1. Regional Map Showing Location of Norton AFB

Location of IRP sites
Figure 2. Location of IRP Sites

Norton AFB Abandoned Wells and Excavation Locations
Figure 3. Norton AFB Abandoned Wells and Excavation Locations

Demographic Statistics
Figure 4. Demographic Statistics

ATSDR's Exposure Evaluation Process
Figure 5. ATSDR's Exposure Evaluation Process

Production Wells on and in the Vicinity of Norton AFB
Figure 6. Production Wells on and in the Vicinity of Norton AFB

Hydrostratigraphy of the Bunker Hill Groundwater Basin
Figure 7. Hydrostratigraphy of the Bunker Hill Groundwater Basin

TCE Contaminant Plume - August 1993
Figure 8. TCE Contaminant Plume - August 1993

TCE Contaminant Plume - April 1995
Figure 9. TCE Contaminant Plume - April 1995


APPENDIX A: GLOSSARY

Background Level:
A typical or average level of a chemical in the environment. Background often refers to naturally occurring or uncontaminated levels.


Carcinogen:
Any substance that may produce cancer.


CERCLA:
The Comprehensive Environmental Response, Compensation, and Liability Act of 1980, also known as Superfund. This is the legislation that created ATSDR.


Comparison Values:
Estimated contaminant concentrations in specific media that are not likely to cause adverse health effects, given a standard daily ingestion rate and standard body weight. The comparison values are calculated from the scientific literature available on exposure and health effects.


Concentration:
The amount of one substance dissolved or contained in a given amount of another. For example, sea water contains a higher concentration of salt than fresh water.


Contaminant:
Any substance or material that enters a system where it is not normally found or found in greater concentrations than background levels.


Dose:
The amount of substance to which a person is exposed. Dose often takes body weight into account.


Environmental contamination:
The presence of hazardous substances in the environment. From the public health perspective, environmental contamination is addressed when it potentially affects the health and quality of life of people living and working near the contamination.


Exposure:
Contact with a chemical by swallowing, by breathing, or by direct contact (such as through the skin or eyes). Exposure may be short term (acute) or long term (chronic).


Hazard:
A source of risk that does not necessarily imply potential for occurrence. A hazard produces risk only if an exposure pathway exists, and if exposures create the possibility of adverse consequences.


Ingestion:
Swallowing (such as eating or drinking). Chemicals can get in or on food, drink, utensils, cigarettes, or hands where they can be ingested. After ingestion, chemicals can be absorbed into the blood and distributed throughout the body.


Media:
Soil, water, air, plants, animals, or any other parts of the environment that can contain contaminants.


Minimal Risk Level (MRL):
An MRL is defined as an estimate of daily human exposure to a substance that is likely to be without an appreciable risk of adverse effects (noncancer) over a specified duration of exposure. MRLs are derived when reliable and sufficient data exist to identify the target organ(s) of effect or the most sensitive health effect(s) for a specific duration via a given route of exposure. MRLs are based on noncancer health effects only. MRLs can be derived for acute, intermediate and chronic duration exposures by the inhalation and oral routes.


National Priorities List (NPL):
EPA's listing of sites that have undergone preliminary assessment and site inspection to determine which locations pose an immediate threat to persons living or working near the release. These sites are most in need of cleanup.


No Apparent Public Health Hazard:
The designation given to sites where human exposure to contaminated media is occurring or has occurred in the past, but the exposure is below a level of health hazard.


Plume:
An area of chemicals in a particular medium, such as air or groundwater, moving away from its source in a long band or column. A plume can be a column of smoke from a chimney or chemicals moving with groundwater.


Potentially Exposed:
The condition where valid information, usually analytical environmental data, indicates the presence of contaminant(s) of a public health concern in one or more environmental media contacting humans (e.g., air, drinking water, soil, food chain, surface water), and there is evidence that some of those persons may have an identified route(s) of exposure (e.g., drinking contaminated water, breathing contaminated air, having contact with contaminated soil, or eating contaminated food).


Potential/Indeterminate Public Health Hazard:
The designation given to sites for which no conclusions about public health hazards can be made because data are lacking.


Public Health Assessment:
The evaluation of data and information on the release of hazardous substances into the environment in order to assess any current or future impact on public health, develop health advisories or other recommendations, and identify studies or actions needed to evaluate and mitigate or prevent human health effects; also, the document resulting from that evaluation.


Public Health Hazard:
Sites that pose a public health hazard as the result of long-term exposures to hazardous substances.


Pump and Treat:
A pump and treat system remediates contaminated groundwater by pumping contaminated groundwater out of the aquifer through extraction wells, treating the water via air stripping, and then reinjecting the clean, treated water back into the groundwater aquifer.


Risk:
The probability that something will cause injury, combined with the potential severity of that injury.


Route of Exposure:
The path in which a person may contact a chemical substance. For example, drinking (ingestion) and bathing (skin contact) are two different routes of exposure to contaminants that may be found in water.


Soil Vapor Extraction:
A remediation technology used to treat contaminated soil in situ (in place). This treatment technology uses the contaminant's volatility to separate the contaminant from the soil. SVE is used to treat VOCs and some fuels; the technology will not remove heavy oils, metals, PCBs, or dioxins from soils.


Volatile organic compound (VOC):
Substance containing carbon and different proportions of other elements such as hydrogen, oxygen, fluorine, chlorine, bromine, sulfur, or nitrogen; these substances easily become vapors or gases. A significant number of the VOCs are commonly used as solvents (e.g., paint thinners, lacquer thinner, degreasers, dry cleaning fluids).

APPENDIX B: COMPARISON VALUES

The conclusion that a contaminant exceeds the comparison value does not mean that it will cause adverse health effects. Comparison values represent media-specific contaminant concentrations that are used to select contaminants for further evaluation to determine the possibility of adverse public health effects.

Cancer Risk Evaluation Guides (CREGs)

Estimated contaminant concentrations that would be expected to cause no more than one excess cancer in a million (10-6) persons exposed over a 70-year life span. ATSDR's CREGs are calculated from EPA's cancer potency factors.

Environmental Media Evaluation Guides (EMEGs)

EMEGs are based on ATSDR minimal risk levels (MRLs) and factors in body weight and ingestion rates. An EMEG is an estimate of daily human exposure to a chemical (in mg/kg/day) that is likely to be without noncarcinogenic health effects over a specified duration of exposure to include acute, intermediate, and chronic exposures.

Maximum Contaminant Level (MCL)

The MCL is the drinking water stand established by EPA and enforced by the California Department of Environmental Protection. It is the maximum permissible level of a contaminant in water that is delivered to the free-flowing outlet. MCLs are considered protective of human health over a lifetime (70 years) for individuals consuming 2 liters of water per day.

Reference Media Evaluation Guides (RMEGs)

ATSDR derives RMEGs from EPA's oral reference doses. The RMEG represents the concentration in water or soil at which daily human exposure is unlikely to result in adverse noncarcinogenic effects.


APPENDIX C: ESTIMATES OF HUMAN EXPOSURE DOSE AND DETERMINATION OF HEALTH EFFECTS OF PAST CONSUMPTION OF TCE-CONTAMINATED GROUNDWATER

ATSDR estimated the human exposure dose for past ingestion of groundwater from abandoned wells in the former housing area. The estimated exposure dose was used to determine whether noncancer and cancer effects are a concern for this pathway.

Noncancer Effects

When evaluating noncancer effects, ATSDR uses standard health guidelines, such as ATSDR's Minimal Risk Levels (MRLs), to determine whether adverse effects will occur. An MRL is defined as an estimate of daily human exposure to a chemical that is likely to be without an appreciable risk of deleterious effects (noncarcinogenic) over a specified duration of exposure. In the ATSDR Toxicological Profiles, MRLs are developed for acute, intermediate, and chronic exposure intervals. MRLs for TCE are available for acute (14 days or less) and intermediate (15 to 364 days) exposures only and were used to evaluate potential noncancer effects associated with past exposures in the former housing area. The MRLs were used as benchmark values to predict the likelihood of adverse health effects being associated with the estimated dose for this population. The acute oral MRL for TCE is 0.2 milligrams per kilograms per day (mg/kg/day); the intermediate oral MRL is 0.002 mg/kg/day.

Inadequate data exist for evaluating noncancer effects associated with chronic (365 days or more) exposures to TCE. Chronic oral exposure studies (e.g., drinking water studies) in animals have focused on cancer endpoints, not noncancer effects. When evaluating possible chronic past exposures to TCE in the abandoned wells (i.e., the possible maximum 30-year exposure period), ATSDR considered only cancer.

In general, MRLs are based on levels of exposure reported in animal or human studies and consider the most sensitive outcome observed for oral or inhalation exposures. MRLs are derived from the level at which no effects were observed in the study (i.e., the "no-observed-adverse-effects-level" [NOAEL]) or the lowest level at which effects are observed (i.e., the "lowest-observed-adverse-effects-level" [LOAEL]). To derive the MRL, uncertainty (safety) factors are applied to the NOAELs or LOAELs to account for variation in the human population and the uncertainty involved in extrapolating from animal studies to reflect human exposures.

The study on which the intermediate MRL for TCE was based has been questioned because it contains certain flaws and limitations (e.g., the exact amount of TCE-contaminated water consumed by rats in the study is uncertain) (ATSDR, 1996b). The intermediate MRL for TCE is considered a conservative value because it is based on a LOAEL of 0.18 mg/kg/day for developmental effects in rats, which was the lowest dose in the intermediate oral exposure database. Furthermore, the incidence of the observed effect (cardiac abnormalities) at this lowest dose may be in the range of normal variability (ATSDR, 1996b). No other study in the database reported effects in animals at doses lower than 37 mg/kg/day. Even though the LOAEL of 0.18 mg/kg/day is low compared with effects levels observed in other studies, an uncertainty factor of 10 was required to approximate the NOAEL (an uncertainty factor of 10 was required for extrapolation from an animal dose to a human dose, and an uncertainty factor of 10 was required to account for human variability). Thus, the resulting MRL is conservative (i.e., it represents a dose level likely much lower than that at which adverse health effects would be expected).

Cancer Effects

When evaluating the potential for cancer to occur, ATSDR uses Cancer Potency Factors (CPFs) that define the relationship between exposure doses and the likelihood of an increased risk of developing cancer over a lifetime. The CPFs are developed using data from animal or human studies and often require extrapolation from high exposure doses administered in animal studies to the lower exposure levels typical of human exposure to environmental contaminants. The CPF represents the upper-bound estimate of the probability of developing cancer at a defined level of exposure; therefore, they tend to be very conservative (i.e., overestimate the actual risk) in order to account for a number of uncertainties in the data used in the extrapolation. The link between oral exposure to TCE and cancer in humans is controversial. Available epidemiological studies for TCE (i.e., studies looking at people exposed to TCE and other chlorinated solvents in drinking water) are not conclusive; the data are inadequate to establish a causal link between TCE and cancer, including leukemia (ATSDR, 1995d). Because of these uncertainties, EPA is currently reviewing the weight-of-evidence pertaining to the carcinogenicity of TCE. For screening purposes, ATSDR used a previously derived CPF for TCE of 0.011 (mg/kg/day)-1. This approach provides a conservative evaluation of the likelihood of past exposures to TCE in the abandoned wells being associated with cancer effects.

ATSDR estimated the potential for cancer to occur from ingestion of TCE-contaminated groundwater. The method involved estimating the lifetime cancer risk using the following equation which incorporates the estimated exposure doses and CPF values:

Lifetime Cancer Risk = Estimated exposure dose (mg/kg/day) x CPF (mg/kg/day)-1

Although no risk of cancer is considered acceptable, ATSDR often uses a range of 10-4 to 10-6 estimated lifetime cancer risk (or 1 new case in 10,000 to 1,000,000 exposed persons), based on conservative assumptions about exposure, to determine whether a concern for cancer effects exists. A zero cancer risk is not possible to achieve. This range is consistent with values adopted by EPA for cleaning up hazardous waste sites to a level that does not contribute excess cancer in a population.

Past Consumption of Groundwater from Abandoned Wells in the Former Housing Area

ATSDR used the following equations to estimate potential exposure doses for past ingestion of groundwater from abandoned wells in the former housing area that may have been affected by the TCE plume:

mathematical equations

where:

Conc. = Average contaminant concentration in monitoring well water upgradient to the housing area (depth of 100 to 150 feet): 0.048 ppm
IR = Ingestion rate (liters/day): 2 liters/day for adults; 1 liter/day for children (EPA, 1989, 1995)
EF = Exposure frequency or number of exposure events per year of exposure:
1 event/day x 7 days/week x 50 weeks/year or approximately 350 events/year
ED = Exposure duration or the duration over which exposure occurs: Upper-bound time at one residence = 30 years (EPA, 1989) (also coincides with the approximate time span that housing area was occupied [1952 to 1982])
BW = Body weight (kg): 70 kg for adult; 16 kg for children
AT = Averaging time or the time period over which cumulative exposures are averaged (70 x 350 days/year for cancer risk)

The doses estimated by ATSDR using these equations are presented in the table below.

  Estimated Acute/Intermediate Dose
(mg/kg/day)
Acute MRL
(mg/kg/day)
Intermediate MRL
(mg/kg/day)
Estimated Chronic Dose
(mg/kg/day)
Child 0.003 0.2 0.002 ---
Adult 0.001 0.2 0.002 0.0006

The estimated exposure doses calculated are conservative estimates and may overestimate actual doses received by this population. Actual doses associated with exposure to water in the abandoned wells is expected to be less than estimates presented above, based on the following reasons:

  • The dose is estimated using the average contaminant concentration present in the monitoring wells upgradient of the housing area because actual data from the abandoned wells were unavailable. Although the houses were only occupied until approximately 1982 (Earth Tech, 1997a), monitoring well data were only available beginning in 1992. The average contaminant concentration present in the upgradient monitoring wells in 1992 is likely to be much higher than the concentration present in the abandoned wells prior to 1982.


  • The exposure frequency is extremely conservative, particularly when documentation suggests that the South San Bernardino County Water District supplied water to the houses in this area. The dose is estimated using an exposure frequency of 350 days per year; it is highly unlikely that residents were drinking water from these wells, especially on a daily basis.


  • Similarly, the exposure duration is extremely conservative. The exposure duration used to estimate the exposure dose was 30 years for adults and 6 years for children. While 30 years has been determined as the upper-bound estimate for the length of time a resident could have occupied a house in this area, it may overestimate the length of residency; for instance, residents may have moved out of the area before the development was purchased by the Air Force in 1982, or they may have purchased the house from an original owner years after the housing area was established.

ATSDR compared the doses estimated based on the above-described assumptions and compared them to the MRL to evaluate noncancer effects and applied the CPF to evaluate cancer effects. Based on this exercise, ATSDR concludes the following:

  • The estimated exposure doses for both the child and adult are less than the acute MRL; therefore, adverse noncancer effects are not expected to be associated with any short-term drinking water exposures.


  • The estimated exposure dose for adults (0.001 mg/kg/day) is less than the intermediate MRL value of 0.002 for oral exposure to TCE; therefore, adverse noncancer effects are not expected to be associated with intermediate drinking water exposure.


  • The estimated dose for intermediate exposures in children slightly exceeds the available MRL (i.e., 0.003 versus 0.002 mg/kg/day). Because the increase is so marginal (a factor of 1.5), no adverse effects are expected. As mentioned previously, the MRL is based on a study showing developmental effects in rats at doses of 0.18 mg/kg/day--because conservative uncertainty factors have been applied to derive the MRL value for human health effects and other animal studies do not show effects at this low level, adverse effects would not be expected at the doses estimated for possible past exposures to 0.048 ppm TCE in drinking water.


  • ATSDR calculated a total lifetime cancer risk for ingesting the average TCE concentration found in monitoring wells upgradient of the former housing area. The estimated cancer risk for adults is 6.4 x 10-6 (or 6.4 new cases in 1,000,000 exposed persons); therefore, exposure is not expected to contribute to excess cancers in the population.

In summary, no health hazards are associated with past consumption of TCE-contaminated groundwater from the wells in the former housing area.


APPENDIX D: RESPONSES TO PUBLIC COMMENTS

ATSDR distributed a total of approximately 50 copies of the public comment release of the Norton Air Force Base Public Health Assessment (PHA) (December 1997) to individuals, organizations, or agencies. The 30-day public comment period ended February 15, 1998. A total of three agencies supplied written comments. Most of those comments received by ATSDR suggested minor points of clarification or noted small inconsistencies in the wording used in the text or tables. The identified sections of the assessment were clarified or corrected as necessary.

One commentor submitted new drinking water sampling data. That commentor's points are summarized and ATSDR's reply is given below:

  1. Comment: Recent sampling by the San Bernardino Municipal Water Department has identified TCE/PCE contamination in some of the existing and newly installed drinking water wells on Norton AFB. Does that new information, in any way, result in changes to ATSDR's conclusions or possible subsequent actions?
  2. Response: ATSDR has incorporated the recent water well sampling information in our analysis and made the appropriate changes to the public health assessment on pages 11 - 13 and 22 of the text. The new data was also incorporated in Table 6. Those changes in the text and table do not result in changes in the conclusions reached by ATSDR.

    If, in the future, ATSDR receives water well sampling or other information that suggests that environmental contaminants may exist at levels that might result in adverse human health effects, ATSDR will re-evaluate the conclusions drawn in this report and, if necessary, recommend actions necessary to minimize or eliminate adverse human health effects.

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