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A chemical component of a sample to be determined or measured. For example, if the analyte is mercury, the laboratory test will determine the amount of mercury in the sample.

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

Any substance that may produce cancer.

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.

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

Any substance or material that enters a system (the environment, human body, food, etc.) where it is not normally found.

Referring to the skin. Dermal absorption means absorption through the skin.

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 healthperspective, 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).

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.

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.

Breathing. Exposure may occur from inhaling contaminants because they can be deposited in the lungs, taken into the blood, or both.

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 effects(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):
The Environmental Protection Agency (EPA) list of sites that have undergone preliminary assessment and site inspection to determine which locations pose immediate threat to persons living or working near the release. These sites are most in need of cleanup.

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

An area of chemicals in a particular medium, such as air or groundwater, moving awayfrom 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 (i.e., 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 (i.e., drinking contaminated water, breathing contaminated air, having contact with contaminated soil, or eating contaminated food).

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 action 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.

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.

Another name for the Comprehensive Environmental Response, Compensation, andLiability Act of 1980 (CERCLA), which created ATSDR.

Superfund Amendments and Reauthorization Act (SARA):
The 1986 legislation that broadened ATSDR's responsibilities in the areas of public health assessments, establishment and maintenance of toxicologic databases, information dissemination, and medical education.

Vadose Zone:
The zone of aeration (also referred to as the unsaturated zone) in the earth's crust above the groundwater level.

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).


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

Cancer Risk Evaluation Guides (CREGs)

CREGs are estimated contaminant concentrations that would be expected to cause no more thanone excess cancer in a million (10-6) persons exposed over lifetime. ATSDR's CREGs arecalculated 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 andingestion 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.

Maximum Contaminant Level (MCL)

The MCL is the drinking water standard established by EPA. It is the maximum permissible levelof a contaminant in water that is delivered to the free-flowing outlet. MCLs are consideredprotective of public health over a lifetime (70 years) for people consuming 2 liters of water perday.

Reference Media Evaluation Guides (RMEGs)

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


ATSDR estimated the human exposure dose for past ingestion of Schofield drinking well water.The estimated exposure dose was used to determine whether noncancer and cancer effects are aconcern for this pathway.

Noncancer Effects

When evaluating noncancer effects, ATSDR uses standard toxicity values, such as ATSDR'sminimal risk levels (MRLs), to determine whether adverse effects will occur. An MRL is definedas an estimate of daily human exposure to a chemical that is likely to be without an appreciablerisk of deleterious effects (noncarcinogenic) over a specified duration of exposure. In the ATSDRtoxicological profiles, MRLs are developed for acute, intermediate, and chronic exposureintervals. Currently, the only MRLs for TCE are the Intermediate Oral MRL and the Acute OralMRL.

The intermediate MRL is a conservative value because it is based on the levels of exposurereported 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 theNOAELs or LOAELS to account for variation in the human population and uncertainty involvedin extrapolating from animal studies to the human experience.

Hazard is an estimate of the potential that a noncancer effect will occur at the estimate exposuredose for a contaminant. It is calculated as the ratio of the estimated exposure dose (for thecontaminant) divided by the MRL. The hazard index (HI) is the sum of the contaminant-specifichazards for an exposure pathway. Total HI is the sum of the HIs for several pathways, assumingthat simultaneous exposure is occurring via various pathways.

MRLs, in general, have uncertainty spanning an order of magnitude or more. Uncertainty factors(UFs) and modifying factors (MFs) are incorporated into the MRL to reflect the uncertaintiesabout the data and to ensure that the levels are protective of health, even for sensitivepopulations. For chemicals with an MRL that has relatively few uncertainties (less than or equalto a factor of ten), the hazard is typically compared with 1. Conversely, when the MRL for achemical is based on less than adequate data, UFs and MFs are incorporated in the MRL toprovide greater certainty that the value is protective of human health. An MRL that has manyUFs and MFs has a large margin of safety built into it; therefore the hazard can be compared witha less stringent value (greater than 1).

In addition to calculating HI values, ATSDR reviewed the literature-based NOAEL and LOAELvalues to determine possible adverse effects associated with exposure at the doses estimated forthe pathways described above.

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 ofdeveloping cancer over a lifetime. The CPFs are developed using data from animal or humanstudies and often require extrapolation from high exposure doses administered in animal studiesto the lower exposure levels typical of human exposure to environmental contaminants. The CPFrepresents the upper-bound estimate of the probability of developing cancer at a defined level ofexposure; therefore, they tend to be very conservative (i.e., overestimate the actual risk) in orderto account for a number of uncertainties in the data used in the extrapolation.

ATSDR estimated the potential for cancer to occur for the exposure pathway from ingestion ofTCE-contaminated groundwater. The method involved estimating the lifetime cancer risk usingthe 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 10-6estimated lifetime cancer risk (or 1 new case in 10,000 to 1 million exposed persons), based onconservative assumptions about exposure, to determine whether a concern for cancer effectsexists. A zero cancer risk is not possible to achieve. This range is consistent with values adoptedby EPA for cleaning up hazardous waste sites to a level that does not contribute excess cancer ina population.

Past Consumption of Schofield Well Water

ATSDR used the following equation to estimate exposure dose for past ingestion ofSchofield's TCE-contaminated water supply:

Estimated exposure dose equals Conc. times IR times EF times ED divided by BW times AT

Conc. = Maximum concentration in Schofield well water previous to the closure of the wells = 0.030 mg/L for TCE
IR = Ingestion Rate: 2 liters/day for adults; 1 liter/day for children
EF = Exposure frequency or number of exposure events per year of exposure:
   1 event/day x 7 days/week x 52 weeks/year or 365 events/year
ED = Exposure duration or the duration over which exposure occurs = 20 years for adults (the longest period of potential contamination at the supply wells: from the earliest possible year of contamination [1966] to the installation of the stripper [1986]); 6 years for children
BW = Body weight (kg): adult = 70 kg; child (1 to 6 years of age) = 16 kg
AT = Averaging time or the period over which cumulative exposures are averaged (20 or 6 years x 365 days/year for noncancer and 70 years x 365 days/year for cancer).

Estimating Human Exposure Dose and Human Health Effects

  • Using the above equation, ATSDR's estimated past exposure doses for ingestion ofSchofield well water are 0.000857 mg/kg/day and 0.001875 mg/kg/day for adults andchildren, respectively.

  • These estimated exposure doses are less than the intermediate MRL value of 0.002 for oral exposure to TCE; therefore, adverse noncancer effects are not a concern for this pathway.

  • ATSDR calculated a total lifetime cancer risk for ingesting the maximum level of TCE discovered in 1985. The estimated cancer risk for adults is 2.69 x 10-6; therefore, cancer is not considered a concern for this exposure pathway.

Furthermore, the estimated exposure dose calculated here is a conservative estimate. Aconservative exposure dose may overestimate adverse health effects. Actual exposure dose would be less, based on the following reasons:

  • The dose is estimated using the maximum contaminant level present before the wells wereshut down. Four water supply wells are located on Schofield, and water is typicallypumped from more than one pump at a time (Fukuda 1997). Depending on demand, watermay be pumped from any combination of the four wells to storage tanks. Due to theblending of water from the different wells and the volatilization of VOCs that wouldnaturally occur during pumping and storage, the water that reached the tap would contain a TCE concentration less than the maximum level detected at the wellhead.

  • The exposure frequency and duration are extremely conservative. The dose is estimatedassuming people are exposed every day of the year. This is the maximum exposure that anindividual could have, and it is not likely that one would be exposed every day. Forinstance, civilian personnel who work at the installation would typically be exposed only 8 hours per day during a 5-day workweek and not during weekends or on vacation.Similarly, the exposure duration used for estimating dose was 20 years for adults and 6years for children.

Although 20 years has been determined to be the longest possible duration of contaminateddrinking water, it may be an overestimation of the exposure duration; for instance, militarypersonnel and their dependents were typically stationed at the installation for only a few years.


Fukuda, J. 1997. Correspondence with W. Mark Weber, ATSDR, regarding Schofield drinking water wells. July 3, 1997.


The Schofield Army Barracks (Schofield) Public Health Assessment was available for publicreview and comment from October 31, 1997, through November 30, 1997. The Public CommentPeriod was announced in a press release dated October 29, 1997. Copies of the public healthassessment were made available for review at the U.S. Army Garrison, Hawaii Department ofHealth, and the Wahiawa Public Library. The public health assessment was also sent to state andfederal agencies and interested members of the general public.

ATSDR received the following comments/questions for the Schofield Public Health Assessment.The specific comments that were received either identified new information; questioned specificsentences or paragraphs in the text or tables; or suggested additions or corrections to improve theclarity, completeness, or accuracy of a specific sentence or paragraph. Those specific commentswere not repeated by other respondents and are listed separately. This list of specific commentsdoes not include editorial comments concerning word spelling, sentence syntax, format, etc. Ifthe accuracy of a statement was questioned, this statement was verified and corrected. Commentswhich requested that information be added to the document without providing documentedsources of that information were not addressed here.

Comments/Questions on Groundwater Quality and Contamination

  1. Comment: The drinking water supply and irrigation wells of Del Monte's Fresh Produce(Hawaii) Inc.'s Oahu Plantation (Kunia wells) are located near the southern border of theMain Post of Schofield and may have been impacted by groundwater contamination fromSchofield. Due to measurable concentrations of trichloroethylene (TCE), carbontetrachloride, and 1,2 dichloroethylene (all below the corresponding MaximumContaminant Levels [MCLs]) in these wells in 1986, Del Monte installed an air strippingtower, which became operational in 1991. In accordance with the Schofield Operable Unit2 Record of Decision, the Army reimbursed Del Monte for capital costs of the air strippingtower treatment facility and funds the operation and maintenance of the facility. TCE hashistorically been detected below the MCL with the following two exceptions: TCE wasdetected at 7.7 parts per billion (ppb) on October 5, 1993, and at 5.2 ppb on May 1, 1997. Both of these detections were in water collected before the air stripper tower.
  2. Response: ATSDR amended the groundwater section in the Community Health Concerns and Potential Pathways of Exposure section to reflect these TCE detections above the MCL and the installation of an air stripper at the Del Monte Kunia wells. Specifically, changes were made to the conclusions, off-site contamination, past exposure, and current and future exposure segments.

  3. Comment: The summary makes reference to the presence of seven contaminants found in groundwater under Schofield. Only TCE and carbon tetrachloride were found more than once and therefore not considered anomalous; only TCE and carbon tetrachloride should be mentioned in the summary.
  4. Response: Text in the summary was amended.

  5. Comment: On page 12, the public health assessment states that pentachlorophenol wasfound in an off-site well in Waipio, which is in the Honolulu-Pearl Harbor Basal WaterBody, yet in the summary on page 1 it is said to be found in the groundwater beneathSchofield Barracks. This is in error and should be deleted.
  6. Response: Text in the summary was amended. See above comment and response.

  7. Comment: Groundwater at Schofield is monitored on a quarterly basis, not on a continuous basis as stated in the conclusions of the groundwater section.
  8. Response: Text amended.

  9. Comment: Add text to the first conclusion on page 18 to read: Current and futureexposures to contaminants in drinking water are unlikely to occur because air strippersoperating at the wellhead effectively treat the water of the existing wells. Further, anywells drilled in the vicinity to develop water from the same groundwater source fordrinking purposes in the future will be subjected to federal, state, and county regulations to meet safe drinking water standards (MCL).
  10. Response: Text added.

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