PUBLIC HEALTH ASSESSMENT
SHARPE ARMY DEPOT
(a.k.a. DEFENSE DISTRIBUTION DEPOT SAN JOAQUIN, CALIFORNIA--SHARPE)
LATHROP, SAN JOAQUIN COUNTY, CALIFORNIA
Waste Disposal History
Status of Investigation
|OU1||South Balloon Area
(Plumes 1 and 2)
Central Area (Plumes 3, 4/5, and 6)
North Balloon Area
|Waste from aircraft (helicopters) and heavy construction equipment maintenance care and preservation||Groundwater--VOCs||Remediation ongoing. Includes groundwater extraction and treatment (air stripping towers with carbon filters) for VOCs. Air stripping tower effluent must also meet arsenic treatment standards (5 ppb).|
|OU2||Isolated sections of the North and South Balloon Areas||Waste from sandblasting and vehicle maintenance||Soil--lead, chromium, VOCs (primarily TCE and PCE), and pesticides (DDT, DDE, DDD and chlordane)||
|Pathway Name||Source of
|Environmental Medium||Point of Exposure||Route of Exposure||Exposed Population||Comment|
Potential Exposure Pathways
|Sharpe wells||VOCs--Sharpe or other sources
Arsenic--Not known with certainty, possibly naturally occurring
|Groundwater||Sharpe wells||Ingestion, dermal contact, inhalation||Sharpe residents and workers
(estimated 1,400 people)
|Current and Future:
Arsenic is present in Sharpe supply wells at levels above ATSDR's CREG screening value, but below the CA and EPA MCLs.
ATSDR determined that consumption of potable well water is unlikely to result in adverse health effects.
VOCs and arsenic were detected at levels above ATSDR's CREGs, but below the CA and EPA MCLs.
ATSDR determined that past consumption of potable well water is unlikely to result in adverse health effects.
|Private off-site wells||PCE--Sharpe||Groundwater||Private residences and businesses||Ingestion, dermal contact, and inhalation||Private residents and business owners (estimated (120 to 400 people)||
Current and Future:
|On-site surface soil||Sharpe||Soil||On-site areas||Dermal contact||Sharpe workers; on-site residents
(estimated 100 to 1,400 people)
|Past, Current, and Future:
Elevated lead, chromium, and VOCs contaminate on-site soil. Workers in the contaminated areas should not suffer health effects because they will most likely be protected and/or have only brief, infrequent contact with soil.
Only very low levels of lead were detected in the residential area soil. Children playing in this area are not at increased risk of lead-related health effects.
|Off-site surface soil||Arsenic--Sharpe or other sources||Off-site surface soil||Off-site property||Dermal contact||Area residents (estimated 120 to 160 people)||Past, Current, and Future:
Arsenic was detected in the off-site soil at levels above the CREG and slightly above regional values but below EPA clean-up levels.
ATSDR does not expect arsenic to be appreciably absorbed through the skin to pose a health hazard.
|Area grown/raised crops and cattle||Arsenic--Sharpe or other sources||Crops and cattle||Off-site property||Ingestion||Consumers of crops and beef (estimated 120 to 5,000 people)||Past, Current, and Future:
Arsenic has been detected in soil. Crops and cattle most likely are not accumulating arsenic from soil at levels associated with health hazards to consumers.
ATSDR determined that consumption of area grown crops or raised beef is not likely to result in adverse health effects.
|Contaminant||Maximum Contaminant Concentration (ppb)||Plume|| Comparison Value
5 EPA and CA MCL
5 EPA and CA MCL
|Lead||12.3||ND||ND||ND||15 EPA Action Level|
50 EPA MCL
|Nitrate||37,200||19,600.0||5,600||ND||10,000 EPA MCL|
Key: CREG = cancer risk evaluation guide for 10-6 excess
cancer risk; EMEG = environmental media evaluation guide; LTHA = lifetime health
advisory for drinking water; MCL = maximum contaminant level; ND = not detected.
|Maximum Contaminant Concentration (ppb)|| Comparison Value
50 EPA MCL
|Nitrate||17,700||6,660||2,990||ND||10,000 EPA MCL|
|Key:||CREG = cancer risk evaluation guide 1 x 10-6 excess
EMEG = environmental media evaluation guide
LTHA = lifetime health advisory
MCL = maximum contaminant level
ND = not detected
Note: Data presented only for metals, pesticides, and nitrate concentrations exceeding comparison values.
|Comparison Value (ppb)|
5 EPA and CA MCL
50 EPA MCL
|Key:||CREG = cancer risk evaluation guide for 1 x 10-6 excess
EMEG = environmental media evaluation guide
LTHA = lifetime health advisory for drinking water
ND = not detected
|Number of Detections/ Number of Samples||Number of Detections Greater than the CREG|| Comparison Values
|PW 19-Light Industrial||TCE||2.75||3.1||ND||2/37||1||3 CREG
|PW20-Light Industrial||TCE||0.62||1.6||1.1||17/41||0||3 CREG
|PW21-Private Residence||TCE||ND||ND||ND||0/32||0||3 CREG
0.5 CA MCL
1 Results from 1997 Quarter 2 private-well monitoring.
Key: CREG= cancer risk evaluation guide for 1 x 10-6 excess
cancer risk, MCL = EPA and CA maximum contaminant level (unless otherwise noted)
|Contaminant||Contaminant Concentration (ppm)||Comparison Value (ppm)|
|North Balloon Area||Central Area||South Balloon Area|
|Lead||5,750||3,720||27,500||130 CDTSC PRG|
Source: ESE, 1994b
|Key:||CREG = cancer risk evaluation guide for 1 x 10-6 excess
RMEG = reference dose media evaluation guide
CDTSC PRG = California Department of Toxic Substances Control preliminary remediation guide
ND = not detected
NA = not analyzed
|Contaminant||Off-Site Surface Soil||Comparison Value
|Number of Detections/
Number of Samples
|Arsenic||0.86 - 22.6||32/35||0.5 CREG)|
|Nitrates||1.07 - 731.0||12/12||80,000 RMEG|
Source: ESE, 1994a
|Key:||CREG = cancer risk evaluation guide for 1 x 10-6 excess
risk of cancer
EMEG = environmental media evaluation guide
RMEG = reference dose media evaluation guide for nonpica child
ND = not detected
NOTE: The source of arsenic contamination has not been identified and may be naturally occurring or associated with local agricultural practices.
|Produce|| Uptake Factor
(mg chemical/kg dry weight plant)
per (mg chemical/kg dry weight soil)
| Estimated Arsenic Concentration
in Edible Portion of Plant (ppm) 1
Source: EPA, 1989
1 Estimated arsenic concentration in the edible portion of the
plant = Uptake Factor x Concentration (ppm) where Concentration =
Maximum arsenic concentration in soil: 22.6 ppm.
Figure 1. Area Map
Figure 2. Site Map
Figure 3. Demographics in a 1-Mile Buffer Around Sharpe
Figure 4. ATSDR's Exposure Evaluation Process
Figure 5. VOC Groundwater Plumes
Figure 6. North Balloon Area Potable Well Locations
The conclusion that a contaminant exceeds the comparison value does not mean that it will cause adverse health effects, rather they 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)
CREGS are estimated contaminant concentrations that would be expected to cause no more than one excess cancer in a million (10-6) persons exposed over lifetime. ATSDR's CREGs are calculated from EPA's cancer potency factors (CPFs).
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 standard established by EPA. It is the maximum permissible level of a contaminant in water that is delivered to the free-flowing outlet. MCLs are considered protective of public 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.
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.
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.
Any substance or material that enters a system (e.g., the environment, human body, food) where it is not normally found.
Referring to the skin. Dermal absorption means absorption through skin.
The presence of hazardous substances in the environment. From a 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.
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).
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.
Soil, water, air, plants, animals, or any other parts of the environment that can contain contaminants.
National Priorities List (NPL)
The Environmental Protection Agency's (EPA) listing of sites that have undergone preliminary assessment and site inspection to determine which locations pose immediate threats to persons living or working near the release. These sites are in need of cleanup.
Parts per billion (ppb)
A common basis of reporting water quality analysis.
Parts per million (ppm)
A common basis of reporting soil analysis.
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 the evaluation.
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.
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 have an identified route(s) of exposure (e.g., drinking contaminated water, breathing contaminated air, having contact with contaminated soil, eating contaminated food).
In risk assessment, the probability that something will cause injury, combined with the potential severity of that injury.
Route of Exposure
The way 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.
Volatile Organic Compounds (VOCs)
Substances 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 (paint thinners, lacquer thinner, degreasers, and dry-cleaning fluids).
- Plume 1
- Plume 1 spans across the eastern and central portion of the South Balloon Area, where extensive disposal operations occurred. VOCs have been found in the A, B, and C zones, but not in the D zone. In general, VOC concentrations in plume 1 have been decreasing because of the effectiveness of the groundwater treatment system installed in 1987. Treatment has been most effective in the shallowest A zone. An area in the C zone down gradient from the main body of the plume, however, shows slightly increasing TCE concentrations. The increasing concentrations may result from movement of a previously undetected portion of the main plume or from a separate, smaller plume in the area. Since this detection, a zone C extraction well has been installed to capture contaminants in this area.
- VOCs associated with plume 2 have been detected in the A and B zones. VOC concentrations in zone A appear to be stable, while concentrations in zone B appear to be more erratic, varying from non detected to 15 ppb. Sampling data from 1996 indicate a decreasing trend in contaminant concentrations.
- VOCs in plume 3 have been detected in the A and B zones. The main part of plume 3 has probably moved off site but at concentrations below the EPA and CA MCL. Varied VOC concentrations have been detected up to 319 ppb in the A zone over time. Pumping from nearby agricultural wells may have influenced contaminant migration in this area in the past. The agricultural wells have since been closed (1991). VOC levels appear to be declining and no significant ongoing source has been reported.
- Unlike other plumes, the highest concentrations in plume 4/5 have been located in the deeper C aquifer zone rather than in the shallower A and B zones. TCE-contaminated groundwater is moving deeper as it moves from east to west. This is believed to be the effect from past pumping of agricultural wells.
- Plume 6 contains the highest TCE concentrations at Sharpe, where TCE has been detected in the A, B, and C zones. The contamination is believed to be related to waste liquids used as fuel during fire training activities. Plume 6 has moved off site, but TCE is migrating more slowly in zones A and C than in zone B.
- Significant levels of VOC contamination are present in plume 7/8 within the south-central and western portions of the North Balloon Area. Most contamination is attributed to a small isolated area of VOC contaminated soil. Plume 7 contains predominantly TCE at levels between 20 ppb and 3,000 ppb; the wide variation in concentrations is believed to have been in response to gradients created by the former pumping of Sharpe Well 2. Plume 8 contains TCE but also high concentrations of PCE. The source of PCE contamination is unknown.
ATSDR typically evaluates the public health implications of exposure by considering the contaminant's chemical class, concentrations of the contaminants to which people may have been exposed, and how often and how long exposure to those contaminants occurred. Health effects are also related to individual characteristics such as age, gender, and nutritional status that influence how a chemical might be absorbed, metabolized, and eliminated by the body. Together, these factors help influence the individual's physiological response to chemical contaminant exposure and potential noncarcinogenic and carcinogenic health outcomes.
ATSDR used the following equation to estimate an exposure dose for ingestion
|Estimated exposure dose =||
Conc. x IR x EF x ED
BW x AT
|Conc.||=||Maximum concentration in the water (ppm).|
|IR||=||Ingestion rate: 2 liters per day.|
|EF||=||Exposure frequency or number of exposure events per year of exposure.|
|ED||=||Exposure duration or the duration over which exposure occurs.|
|BW||=||Body weight: 70 kg.|
|AT||=||Averaging time or the period over which cumulative exposures are averaged.|
When evaluating noncancer effects, ATSDR compares the estimated exposure dose to standard toxicity values, including ATSDR's minimal risk levels (MRLs) and EPA's reference doses (RfDs), to determine whether adverse effects will occur. The chronic MRLs and RfDs are estimates of daily human exposure to a substance that are likely to be without appreciable risk of adverse noncancer effects over a specified duration. The chronic MRLs and RfDs are conservative values, based on the levels of exposure reported in the literature that represent no-observed-adverse-effect levels (NOAELs) or lowest-adverse-effect-levels (LOAELs) for the most sensitive outcome for a given route of exposure (e.g., dermal contact, ingestion). In addition, uncertainty (safety) factors are applied to the NOAELs or LOAELs to account for variation in the human population and uncertainty involved in extrapolating human health effects from animal studies.
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.
ATSDR estimated the potential for cancer to occur using the following equation. The estimated exposure doses and CPF values for the contaminants of concern are incorporated into the equation:
- Lifetime Cancer Risk = Estimated exposure dose (mg/kg/day) x CPF (mg/kg/day)-1
Although no risk of cancer is considered acceptable, it is impossible to achieve a zero cancer
risk. Consequently, 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), on the basis of conservative assumptions
about exposure, to determine whether a concern regarding the risk for cancer effects is valid.
This range is consistent with values adopted by EPA for evaluating the need for remediation of
hazardous waste sites (EPA, 1989).
Estimated Exposure Dose for Consumption of Arsenic in On-Site Potable Well Water
Arsenic was present in the on-site potable wells at levels above ATSDR's CREG of 0.02 ppb, but below the enforceable CA and EPA MCLs of 50 ppb. To determine the health significance of arsenic in the on-site potable water supply, ATSDR estimated an oral exposure dose using very conservative assumptions for on-site residents and on-site workers and Army personnel. ATSDR assumed that a 70-kilogram (kg) resident drinks 2 liters of potable well water at a frequency of 350 days a year (7 days a week for 50 weeks) for a 3-year period (the maximum residential stay) and that on-site workers or Army personnel drink 2 liters of potable water 250 days a year (5 workdays per week for 50 weeks per year) over a 30-year work duration. For both exposure scenarios, ATSDR assumed that the water contains the maximum arsenic concentration detected of 49 ppb.
The resulting estimated exposure doses for residents and workers are 0.0014 mg/kg/day and
0.00096 mg/kg/day, respectively. Although the estimated doses slightly exceed ATSDR's MRL
and EPA's RfD of 0.0003 mg/kg/day for oral chronic exposure, ATSDR does not expect use of
the potable water to cause noncarcinogenic health effects. The estimates are actually several
orders of magnitude lower than the average threshold doses associated with acute (1.0 mg/kg/day
for neurological and gastrointestinal effects) and chronic noncarcinogenic effects (0.014 to 0.08
mg/kg/day) in humans. Furthermore, ATSDR used very conservative assumptions to derive the
estimates, including assuming the water contained the maximum arsenic concentrations over the
period of exposure. The maximum concentration most likely does not reflect the true exposure,
which was probably to lower than the maximum detected concentration over a less than then 30 year time period.
Arsenic has been classified as a human carcinogen by EPA. The lowest exposure level associated with the onset of skin cancer is based on a large Taiwanese study in which people were drinking water containing 170 to 800 ppb arsenic over 45-year exposure period. People developing skin cancer were exposed to an average daily arsenic dose of 0.014 mg/kg/day, an order of magnitude greater than the estimated doses associated with consumption of water drawn from the Sharpe wells. The relevance of the Taiwanese study to skin cancer risk in the United States has been challenged, however, because the risk may have been overestimated (i.e., other arsenic sources were likely present) and the study population may have been more sensitive to arsenic than the general U.S. population (ATSDR, 1993a).
ATSDR also derived lifetime cancer estimates from drinking on-site well for the on-site resident
and worker of 8.98 x 10-5 and 6.16 x 10-4, respectively (or 9 new cancer cases in 100,000 exposed
individuals and 6 new cancer cases in 10,000 exposed individuals). Because the estimate for
residents falls within the range of 10-4 to 10-6, ATSDR does not consider increased risk of cancer
from arsenic a concern for residents consuming on-site potable well water. Although the lifetime
cancer risk estimate for workers slightly exceeds the acceptable range used by ATSDR, ATSDR
does not consider workers to be at increased risk of developing cancer. ATSDR used very
conservative assumptions to derive the estimates, including assuming the water contained the
maximum detected arsenic concentrations for the period of exposure. The maximum detected
concentration most likely does not reflect the true exposure concentration, which was probably
lower than the maximum concentration.
Estimated Exposure Dose for Consumption of Contaminants in Private Well Water
PCE was detected in private wells at a maximum concentration of 6.3 ppb, a level greater than the CREG and slightly greater than the enforceable MCL of 5 ppb. To determine the health effects of ingesting contaminated water, ATSDR assumed that a 70-kilogram person drinks 2 liters of private well water per day containing the maximum PCE concentration (6.3 ppb). It is not known with certainly how long the PCE has been present in the private wells showing positive detections. At least one well was installed in the early 1980s, but PCE was not detected in the well until 1987 when Sharpe began monitoring activities. Presumably, PCE has been present at levels below the maximum level over the 11-year period. As a highly conservative measure, however, ATSDR used EPA's national upper-bound limit at one residence of 30 years for an exposure duration.
ATSDR estimated a daily exposure dose of 0.000173 mg/kg/day. This estimate is approximately 58 times lower than the chronic RfD (0.01 mg/kg/day) and 289 times lower than the acute MRL for oral exposure to PCE. On the basis of this finding, ATSDR does not expect PCE in the private well to pose a public health hazard. Furthermore, animal studies indicate that the central nervous system is most sensitive system or organ to the toxic effects of PCE. The lowest level at which adverse effects on the central nervous system have been observed in experimental animal studies is 500 mg/kg/day; the estimated daily dose is significantly below this threshold.
According to the EPA, a contaminant entering the body through inhalation is approximately equal to the oral route. In applying this generalization to dermal and inhalation exposures, the estimated oral dose is multiplied by three (to incorporate ingestion, inhalation and dermal exposure) to arrive at an overall estimated dose that is still less than the chronic RfD.
PCE has been shown to cause cancer in laboratory animals given large doses. The link between PCE and cancer in humans drinking water is controversial, however. Available studies for PCE are inconclusive and the data are inadequate to establish a link. EPA is currently reviewing the scientific literature pertaining to the carcinogenicity of PCE to determine its cancer classification. As a conservative measure, ATSDR used the current CPF for TCE to estimate the excess lifetime cancer cases resulting from ingestion of water containing the maximum concentration of TCE. ATSDR estimated four new cancer cases per 1,000,000 persons could be expected if people are exposed over an extended period of time (see Table C-2). On the basis of these results, ATSDR concludes that ingestion of PCE at the levels detected in the private well water, or even dermal contact or inhalation of its vapors, is not expected to result in an increased likelihood of developing cancer.
TCE was detected less frequently than PCE in private wells. The maximum TCE concentration detected of 3.1 ppb is just slightly greater than the CREG of 3.0 ppb and less than the enforceable CA MCL of 5 ppb. To determine the health effects from ingesting the maximum detected level of TCE in water, ATSDR applied the same methodology used for PCE in private wells. In using this methodology, ATSDR estimated an exposure dose of 0.0000845 mg/kg/day. In comparing the estimate to ATSDR's intermediate MRL of 0.002 mg/kg/day for intermediate oral exposure (no chronic MRL or RfD has been established for TCE), ATSDR determined that consumption of the maximum level of TCE detected in the private well is unlikely to pose a public health hazard2.
To account for multiple exposure routes, ATSDR assumed that exposures from the dermal and inhalation routes are each equal to the exposure from the oral route. In applying this assumption, ATSDR multiplied the estimated oral dose by three (to incorporate ingestion, inhalation and dermal exposure) to arrive at an overall estimated dose that is still less than the intermediate MRL for TCE.
TCE has been shown to cause cancer in laboratory animals given large doses. The link between TCE and cancer in humans drinking water is controversial, however. Available studies for TCE are inconclusive and the data are inadequate to establish a link. EPA is currently reviewing the scientific literature pertaining to the carcinogenicity of TCE to determine its cancer classification. As a conservative measure, ATSDR used the current CPF for TCE to estimate the excess lifetime cancer cases resulting from ingestion of water containing the maximum concentration of TCE. ATSDR estimated four new cancer cases per 10,000,000 persons could be expected if people are exposed over an extended period of time. On the basis of these results, ATSDR concludes that ingestion of TCE at the levels detected in the private well water, or even dermal contact or inhalation of its vapors, is not expected to result in an increased likelihood of developing cancer.
1,2-DCA was detected in a private well at levels greater than the enforceable CA MCL of 0.5
ppb; in one sampling event, the level was four times the MCLs. Even though the owners of the
well have and are not considering using the well water for drinking, ATSDR evaluated the health
significance from exposure should it occur. In applying the same methodology used for PCE in
private wells, ATSDR derived an exposure dose estimate of 0.000548 mg/kg/day, compared the
estimate to ATSDR's MRL (for intermediate oral exposure) of 0.2 mg/kg/day, and determined
that consumption of the maximum level of 1,2-DCA detected in the private well is unlikely to
pose a public health hazard if people drank water from this well.
EPA has classified 1,2-DCA as a probable human carcinogen; consequently, ATSDR derived an
excess lifetime cancer estimate associated with drinking private well water containing the
maximum detected 1,2-DCA concentration. The estimate of two new cancer cases in one
hundred thousand exposed persons is within the range of 10-4 to 10-6 used by ATSDR; therefore,
ATSDR does not consider increased likelihood of developing cancer from 1,2-DCA a concern
for people consuming private well water. ATSDR strongly emphasizes that no public health
hazard exist because no one is or expected to in the future drink water from the well showing the
elevated 1,2-DCA levels.
Estimated Total Exposure Dose for Private Wells Users
TCE and PCE were detected above their respective CREGs in one private well. ATSDR used a very conservative approach to evaluate the overall public health hazard to these contaminants whereby human exposure was assumed to occur simultaneously to the maximum concentrations of TCE and PCE in private well water over a 30-year exposure period. ATSDR did not consider other contaminants detected below their comparison values. Because comparison values are very conservative, media-specific concentrations that are generally set at levels much lower than levels affecting health, contaminants detected below their comparison values are not likely to contribute significantly to the overall public health hazard.
A measure used to describe the potential for noncarcinogenic health effects is the hazard quotient. For a given chemical, the hazard quotient is the ratio of the estimated exposure dose to the acceptable level (MRL or RfD). To account for exposures that a person may receive from multiple chemicals, a hazard index can be calculated by summing the chemical-specific hazard quotients for TCE and PCE. A hazard index of 1.0 or less indicates that exposure dose is equal to or less than the acceptable levels, and it is considered unlikely that adverse health effects will occur.
The hazard index for ingestion of private well water containing the maximum concentrations of
TCE and PCE is 0.04491. Because the hazard index is less than 1.0, noncancer adverse health
effects are not likely to occur from this pathway.
Evaluating cancer effects from water containing TCE and PCE involved summing the lifetime
cancer risk estimates derived for individual contaminants. ATSDR estimated a total lifetime
excess cancer risk of 4.25 x 10-6 (approximately 4 new cancer cases in 1 million exposed
persons). This estimate falls within the range considered acceptable by ATSDR, thus cancer
effects from the maximum concentration of both TCE and PCE is unlikely to be a concern for private well water drinkers.
ATSDR received the following comments/questions during the public comment period (November and December 1997) for the Defense Distribution Region West Sharpe Army Depot (Sharpe) Public Health Assessment (PHA) (November 1997). For comments that questioned the validity of statements made in the PHA, ATSDR verified or corrected the statements. The list of comments does not include editorial comments concerning such things as word spelling or sentence syntax. ATSDR has not addressed requests for information to be included in the PHA, unless the party who filed the request provided the supporting documentation.
- Comment: How does ATSDR's evaluation process of excess cancer risk from exposure to
private well water compare with the California Regional Water Quality Control Board's
Response: ATSDR and CRWQCB used a similar approach to evaluate excess cancer risk. Both groups derived excess cancer estimates for an individual drinking private well water over an extended period of time. Because there are limits to our ability to assess true cancer risk, both groups used conservative assumptions in deriving cancer risk estimates. Conservative assumptions tend to overestimate the actual magnitude of a health hazard.
Both groups used similar highly conservative assumptions. This included assuming that individuals (adults) drank 2 liters of well water a day containing the maximum contaminant concentration, and that the private wells are the only source of drinking water. The groups differed, however, in their assumption about exposure duration. CRWQCB assumed that people drank private well water over 70 years while ATSDR assumed exposure occurred over 30 years. (ATSDR based this on EPA's upper bound limit of occupancy in a single residence). During our evaluation, ATSDR obtained use pattern data for two of the affected private wells. These data suggest that even the 30-year exposure duration is highly conservative and likely overestimates the time period when people may have been drinking contaminated water. These two wells are located on industrial properties; one well has not been used for drinking water since 1981 and the other well was installed as recently as 1988. Furthermore, it is unlikely that people actually drank water with the highest levels for an extended period. Recent monitoring data for the wells indicate that the contaminant levels have consistently declined since the maximum levels were detected. Fortunately, this information tells us that actual exposure, and therefore the likelihood of developing cancer, is probably much less than estimated by either ATSDR or CRWQCB. Although CRWQCB used a 70-year exposure period, the group acknowledges that actual cancer risk would be lower than estimates if persons were exposed for a shorter period than the 70-year value used to derive the published cancer potencies.
- Comment: CRWQCB has expressed concern that the one-in-10,000 to one-in-1,000,000
(10-4 to 10-6) excess cancer risk range used by ATSDR is not protective of public health.
CRWQCB uses a one-in-a million cancer (10-6) risk value as the point of departure for
acceptable cancer risk. CRWQCB considers risk above this level to be unacceptable.
Response: ATSDR uses the range of 10-4 to 10-6 to evaluate cancer risk estimates. Cancer risk estimates are, in turn, used by ATSDR in combination with the current scientific literature on the carcinogenicity of the contaminant to determine exposure levels of concern. For example, ATSDR estimated four new cancer cases in 1,000,000 persons drinking the well water containing tetrachloroethylene (PCE). While this value exceeds the 10-6 cancer risk range supported by CRWQCB, ATSDR also weighed evidence from current scientific literature that available information is inconclusive to establish a link between PCE and cancer in humans and, because of this, EPA is currently reviewing the carcinogencity classification for PCE. This approach reflects the scientific uncertainties of available literature and therefore, we believe, provides a realistic and responsive approach to public health protection.
In addition, ATSDR also interprets risk values for the potentially exposed population. Under the most conservative scenario, ATSDR estimated two additional cancer cases over a lifetime if 100,000 persons drank water containing 1,2-dichloroethylene (1,2-DCA). Contamination has been detected in only three private wells, two of these wells are not or probably have not been used for drinking water, and only one of these wells actually contained 1,2-DCA. Consequently, very few people, if any, have been exposed. Considering the small risk estimates and the small group of potentially exposed persons, cancer is unlikely to occur in the few individuals who may use these private wells.
- Comment: ATSDR recommends that Sharpe provide an alternative water source if the
MCL is consistently exceeded. The state's position, however, is that an alternative water
supply is required when the water fails to meet all the water quality objectives, in this case
the one-in-a million cancer risk level.
Response: ATSDR's conclusions and recommendations are based on available groundwater/private well monitoring data and exposure information, or conservative assumptions about exposure. If this information should change, ATSDR would reevaluate and modify the recommendation about alternative water supplies as needed for the protection of public health.
- Comment: CRWQCB has not made a determination about the origin of 1,2-DCA as the
public health assessment states. CRWQCB is currently reviewing the 1997 Annual Progress
Report to evaluate additional data on this subject. Until these data are reviewed, no
determination has been made and this statement should be revised.
Response: Thank you. ATSDR has made the appropriate changes to the public health assessment.
1. In drawing conclusions about cancer risk, ATSDR uses a range of 10-4 to 10-6 to evaluate cancer risk estimates. ATSDR uses the cancer risk estimates in combination with current scientific literature about the carcinogenicity of a contaminant. The CRWQCB uses a 10-6 lifetime cancer risk as a point of departure for acceptable cancer risk. Therefore, CRWQCB recommends replacement water supplies when the level of carcinogenic chemicals is detected at or above concentrations representing a 10-6 excess cancer risk.
2. EPA has recently withdraw the intermediate MRL for TCE. ATSDR uses the intermediate MRL only as a screening tool.