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BARC Location
Figure 1. BARC Location

AOCs at North Farm, South Farm, and Linkage Farm
Figure 2. AOCs at North Farm, South Farm, and Linkage Farm

AOCs at Central Farm and East Farm
Figure 3. AOCs at Central Farm and East Farm

Surface Water Bodies at BARC
Figure 4. Surface Water Bodies at BARC

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

BARC Water Supply Wells
Figure 6. BARC Water Supply Wells


How a chemical enters a person's blood after the chemical has been swallowed, has come into contact with the skin, or has been breathed in.

Acute Exposure:
Contact with a chemical that happens once or only for a limited period of time. ATSDR defines acute exposures as those that might last up to 14 days.

Adverse Health Effect:
A change in body function or the structures of cells that can lead to disease or health problems.

The Agency for Toxic Substances and Disease Registry. ATSDR is a federal health agency in Atlanta, Georgia, that deals with hazardous substance and waste site issues. ATSDR gives people information about harmful chemicals in their environment and tells people how to protect themselves from coming into contact with chemicals.

Background Level:
An average or expected amount of a chemical in a specific environment. Or, amounts of chemicals that occur naturally in a specific-environment.

A group of diseases which occur when cells in the body become abnormal and grow, or multiply, out of control.

Any substance shown to cause tumors or cancer in experimental studies.

See Comprehensive Environmental Response, Compensation, and Liability Act.

Chronic Exposure:
A contact with a substance or chemical that happens over a long period of time. ATSDR considers exposures of more than one year to be chronic.

Completed Exposure Pathway:
See Exposure Pathway.

Comparison Value (CVs):
Concentrations or the amount of substances in air, water, food, and soil that are unlikely, upon exposure, to cause adverse health effects. Comparison values are used by health assessors to select which substances and environmental media (air, water, food and soil) need additional evaluation while health concerns or effects are investigated.

Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA):
CERCLA was put into place in 1980. It is also known as Superfund. This act concerns releases of hazardous substances into the environment, and the cleanup of these substances and hazardous waste sites. ATSDR was created by this act and is responsible for looking into the health issues related to hazardous waste sites.

A belief or worry that chemicals in the environment might cause harm to people.

How much or the amount of a substance present in a certain amount of soil, water, air, or food.

See Environmental Contaminant.

Dermal Contact:
A chemical getting onto your skin. (see Route of Exposure).

The amount of a substance to which a person may be exposed, usually on a daily basis. Dose is often explained as "amount of substance(s) per body weight per day".

The amount of time (days, months, years) that a person is exposed to a chemical.

Environmental Contaminant:
A substance (chemical) that gets into a system (person, animal, or the environment) in amounts higher than that found in Background Level, or what would be expected.

Environmental Media:
Usually refers to the air, water, and soil in which chemcials of interest are found. Sometimes refers to the plants and animals that are eaten by humans. Environmental Media is the second part of an Exposure Pathway.

U.S. Environmental Protection Agency (EPA):
The federal agency that develops and enforces environmental laws to protect the environment and the public's health.

The study of the different factors that determine how often, in how many people, and in which people disease will occur.

Coming into contact with a chemical substance.(For the three ways people can come in contact with substances, see Route of Exposure.)

Exposure Pathway:
A description of the way that a chemical moves from its source (where it began) to where and how people can come into contact with (or get exposed to) the chemical.

ATSDR defines an exposure pathway as having 5 parts:

  1. Source of Contamination,
  2. Environmental Media and Transport Mechanism,
  3. Point of Exposure,
  4. Route of Exposure, and
  5. Receptor Population.

When all 5 parts of an exposure pathway are present, it is called a Completed Exposure Pathway. Each of these 5 terms is defined in this Glossary.

How often a person is exposed to a chemical over time; for example, every day, once a week, twice a month.

Hazardous Waste:
Substances that have been released or thrown away into the environment and, under certain conditions, could be harmful to people who come into contact with them.

Health Effect:
ATSDR deals only with Adverse Health Effects (see definition in this Glossary).

Indeterminate Public Health Hazard:
The category is used in Public Health Assessment documents for sites where important information is lacking (missing or has not yet been gathered) about site-related chemical exposures.

Swallowing something, as in eating or drinking. It is a way a chemical can enter your body (See Route of Exposure).

Breathing. It is a way a chemical can enter your body (See Route of Exposure).

National Priorities List:
Part of Superfund, a list kept by USEPA of the most serious, uncontrolled, or abandoned hazardous waste sites in the country. An NPL site needs to be cleaned up or is being looked at to see if people can be exposed to chemicals from the site.

No Apparent Public Health Hazard:
The category is used in ATSDR's Public Health Assessment documents for sites where exposure to site-related chemicals may have occurred in the past or is still occurring but the exposures are not at levels expected to cause adverse health effects.

No Public Health Hazard:
The category is used in ATSDR's public health assessments for sites where there is evidence of an absence of exposure to site-related chemicals.

A line or column of air or water containing chemicals moving from the source to areas further away. A plume can be a column or clouds of smoke from a chimney or contaminated underground water sources or contaminated surface water (such as lakes, ponds and streams).

Point of Exposure:
The place where someone can come into contact with a contaminated environmental medium (air, water, food or soil). For example: the area of a playground that has contaminated dirt, a contaminated spring used for drinking water, the location where fruits or vegetables are grown in contaminated soil, or the backyard area where someone might breathe contaminated air.

A group of people living in a certain area; or the number of people in a certain area.

Public Health Assessment (PHA):
A report or document that looks at chemicals at a hazardous waste site and tells if people could be harmed from coming into contact with those chemicals. The PHA also tells if possible further public health actions are needed.

Public Health Hazard:
The category is used in PHAs for sites that have certain physical features or evidence of chronic, site-related chemical exposure that could result in adverse health effects.

Public Health Hazard Criteria:
PHA categories given to a site which tell whether people could be harmed by conditions present at the site. Each are defined in the Glossary. The categories are:
    - Urgent Public Health Hazard
    - Public Health Hazard
    - Indeterminate Public Health Hazard
    - No Apparent Public Health Hazard
    - No Public Health Hazard

Receptor Population:
People who live or work in the path of one or more chemicals, and who could come into contact with them (See Exposure Pathway).

Route of Exposure:
The way a chemical can get into a person's body. There are three exposure routes:
- breathing (also called inhalation),
- eating or drinking (also called ingestion), and
- or getting something on the skin (also called dermal contact).

Safety Factor:
Also called Uncertainty Factor. When scientists don't have enough information to decide if an exposure will cause harm to people, they use "safety factors" and formulas in place of the information that is not known. These factors and formulas can help determine the amount of a chemical that is not likely to cause harm to people.

Source (of Contamination):
The place where a chemical comes from, such as a landfill, pond, creek, incinerator, tank, or drum. Contaminant source is the first part of an Exposure Pathway.

Superfund Site:
See National Priorities List.

A way to collect information or data from a group of people (population). Surveys can be done by phone, mail, or in person. ATSDR cannot do surveys of more than nine people without approval from the U.S. Department of Health and Human Services.

Harmful. Any substance or chemical can be toxic at a certain dose (amount). The dose is what determines the potential harm of a chemical and whether it would cause someone to get sick.

Abnormal growth of tissue or cells that have formed a lump or mass.

Urgent Public Health Hazard:
This category is used in ATSDR's public health assessments for sites that have certain physical features or evidence of short-term (less than 1 year), site-related chemical exposure that could result in adverse health effects and require quick intervention to stop people from being exposed.


ATSDR comparison values are media-specific concentrations that are considered to be "safe" underdefault conditions of exposure. They are used as screening values in the preliminary identificationof "contaminants of concern" at a site. The latter is, perhaps, an unfortunate term since the word"concern" may be misinterpreted as an implication of "hazard". As ATSDR uses the phrase,however, a "contaminant of concern" is merely a site-specific chemical substance that the healthassessor has selected for further evaluation of potential health effects.

Generally, a chemical is selected as a contaminant of concern because its maximum concentration inair, water, or soil at the site exceeds one of ATSDR's comparison values. However, it cannot beemphasized strongly enough that comparison values are not thresholds of toxicity. Whileconcentrations at or below the relevant comparison value may reasonably be considered safe, it doesnot automatically follow that any environmental concentration that exceeds a comparison valuewould be expected to produce adverse health effects. Indeed, the whole purpose behind highlyconservative, health-based standards and guidelines is to enable health professionals to recognizeand resolve potential public health problems before they become actual health hazards. Theprobability that adverse health outcomes will actually occur as a result of exposure to environmentalcontaminants depends on site specific conditions and individual lifestyle and genetic factors thataffect the route, magnitude, and duration of actual exposure, and not on environmentalconcentrations alone.

Described below are the various comparison values that ATSDR uses to select chemicals for furtherevaluation.

Cancer Risk Evaluation Guides (CREGs) are estimated contaminant concentrations in water,soil, or air that would be expected to cause no more than one excess cancer in a million personsexposed over a 70-year lifetime according to EPA estimates. As ATSDR's most conservativecomparison value, the CREG merits special attention. Note that this does not mean that exposuresequivalent to the CREG are actually expected to cause one excess cancer in a million personsexposed over a lifetime. Nor does it mean that every person in an exposed population of one millionhas a 1-in-a-million chance of developing cancer from the specified exposure. Although ATSDRCREGs continue to be useful devices for screening carcinogenic substances at a site, they cannot beused to predict cancer incidence rates at a site. Furthermore, the exposure assumptions on whichEPA's cancer risk estimates and ATSDR's CREGs are based (i.e., essentially lifetime exposure)seldom apply at contaminated sites.

Environmental Protection Agency (EPA) values are similar to ATSDR's CREGs and EMEGs inthat they are risk-based concentrations derived for carcinogens and non-carcinogens from RfDs andCancer Slope Factors, respectively, assuming default values for body weight, exposure duration andfrequency, etc. Unlike ATSDR values, however, they are available for fish, as well as for water, soil,and air.

Environmental Media Evaluation Guides (EMEGs) are concentrations of a contaminant in water, soil, or air that are unlikely to be associated with any appreciable risk of deleterious noncancer effects over a specified duration of exposure. EMEGs are derived from ATSDR minimal risk levels by factoring in default body weights and ingestion rates. Separate EMEGS are computed for acute (< 14 days), intermediate (15-364 days), and chronic (> 365 days) exposures.

Maximum Contaminant Levels (MCLs) represent contaminant concentrations in drinking waterthat EPA deems protective of public health (considering the availability and economics of watertreatment technology) over a lifetime (70 years) at an exposure rate of 2 liters of water per day.

Minimal Risk Levels (MRLs) are estimates of daily human exposure to a chemical (i.e., dosesexpressed in mg/kg/day) that are unlikely to be associated with any appreciable risk of deleteriousnoncancer effects over a specified duration of exposure. MRLs are derived for acute (< 14 days),intermediate (15-364 days), and chronic (> 365 days) exposures. MRLs are published in ATSDR'sToxicological Profiles for specific chemicals.

(EPA's) Reference Doses (RfD) are estimates, with safety factors built in, of daily exposure dosesto contaminants that are unlikely to cause noncarcinogenic adverse health effects over a lifetime ofexposure. Like ATSDR's MRLs, EPA's RfDs are expressed in mg/kg/day.

Reference Dose Media Evaluation Guides (RMEGs) are concentrations of contaminants in air,water or soil that corresponds to EPA's RfDs for particular contaminants when default values for body weight and intake rates are taken into account.



The Agency for Toxic Substances and Disease Registry (ATSDR) has been requested by theWashington Suburban Sanitary Commission (WSSC), the public water supply provider, toinvestigate the potential for pipeline workers and water supply users to be exposed to contaminantsvia water pipelines located in the Beltsville Agricultural Research Center (BARC). This appendixdiscusses the calculations and assumptions ATSDR used to assess potential exposures. ATSDRestimated concentrations of contaminants that might enter the subsurface municipal water supplypipes within BARC through gasketed joints. Using conservative assumptions, ATSDR firstevaluated concentrations of a potential contaminant that could theoretically be found withinpipelines, then calculated the minimum mass transport rate that would yield contaminantconcentrations above health-based guidelines. In addition, two different size pipe diameters wereused for comparison purposes. This evaluation indicated that the potential exists, under certaincircumstances, for some contaminants to reach concentrations exceeding health-based guidelineswithin water supply pipes.


Previous research suggests that all types of jointed pipe used in water distribution systems aresusceptible to permeation by volatile organic compounds (VOCs) such as those found in gasoline orcommon commercial and industrial solvents (Berens 1985). Glaza et al. (1992) suggest thatgasketed joints between pipe segments are also a likely point of permeation. Using their calculationfor determining pipe water concentrations, ATSDR estimated concentrations of contaminants thatcould exist within pipelines located in areas of groundwater contamination. For this calculation, thefollowing assumptions were used:

  • Water pipe is ductile iron pipe with an 8-inch or 12-inch diameter;

  • Rubber gaskets are made of styrene butadiene rubber (SBR);

  • Pipe length between gaskets is 18 feet; and

  • An 8-hour stagnation period occurs during the night and represents the worst case scenario for pipe water concentrations.
  • Note: The first three assumptions are based on information provided by WSSC on theconstruction of its pipelines in the vicinity of BARC (Billingsley 2000). ATSDR did notreceived information about the construction of BARC water pipelines.

ATSDR used the following equation to calculate pipe water concentration when the mass transportrate through a gasket is known. This equation assumes complete mixing between pipe joints.

C = M Ts / Ac Lp          (1)

C = concentration in micrograms per liter (g/L)
M = mass transport rate across the gasket in micrograms per day (g/d)
Ts = stagnation time in days
Ac = cross sectional area of the pipe in square feet
Lp = length of pipe between joints

A concentration value of 3.7 g/L is obtained for 8-inch pipe, with 8 hours stagnation time, 18 feet between joints, and an assumed mass transport rate of 2,000 g/d. A concentration value of 2.5 g/L is obtained for 12-inch pipe under the same conditions and an assumed mass transport rate of 3,000 g/d. The mass transport rates used in equation (1) were selected within the range of values observed by Glaza et al. for a range of organic chemicals. The purpose of this exercise is to determine if the pipe concentration is close to action levels for tetrachloroethylene (PCE) and trichloroethylene (TCE) at these mass transport rates. The table below shows the values used for the theoretical calculation of pipe water concentrations.

SymbolUnits8 inch12 inch
Pipe diameter (i.d.)inches8.012.0
Pipe circumferenceinches25.137.7
Pipe volume per footgallons2.65.9
Pipe cross section(Ac)sq. feet0.350.79
Mass flow rate (M)g/d2000.03004.0
Pipe length (Lp)feet18.018.0
Stagnation time (Ts)days0.330.33
If C =5.00g/L
M =2692.796058.77

A large plume of PCE and its daughter products (including TCE) originates from the W. P. Ballardsite, north and west of BARC's Linkage Farm. Each of these contaminants has a maximumcontaminant level (MCL) of 5 g/L. In the literature ATSDR obtained regarding contaminantpermeation of rubber gaskets, mass transport rates of PCE and TCE through rubber gaskets were notreported. The values used in the calculations above are conservative values selected from researchby others; however, these values have no experimental basis. To overcome this, the above equationwas modified to calculate the minimum mass transport rate that would result in a concentrationexceeding the MCL for PCE and TCE. Equation (2) shows the modified equation (1):

M = C Ac Lp / Ts          (2)


C = 5 g/L

Further research is needed to obtain a valid mass transport rate through rubber gaskets for PCE and TCE; however, the work of Glaza et al. provides experimentally-determined mass transport rates for benzene and methyl ethyl ketone (MEK). These data can be used to provide a frame of reference for mass transport rates calculated from equation (2) for PCE and TCE. Glaza et al. report values for M ranging from 1,300 g/d for benzene to 80,000 g/d for MEK. ATSDR's calculations suggest that under the set of assumed conditions which could exist at the BARC site, mass transport rates greater than 2,700 g/d and 6,100 g/d could result in concentrations exceeding 5 g/L in 8-inch water pipe and 12-inch water pipe, respectively. The actual mass transport rates through SBR gaskets for PCE and TCE may or may not exceed these values.


The work of others has shown that rubber gaskets, especially SBR, can be permeated by commoncontaminants in the liquid or gas phase (Glaza et al. 1992). The experimentally-determined masstransport rates through gasket material vary widely depending on the type of rubber and the specificcontaminant. To substantiate conclusions drawn from this evaluation, it is recommended that masstransport rates for PCE and TCE through various types of gasket rubber be determined.

Given a known mass transport rate through a gasket and conservative assumptions, the theoreticalconcentration within water pipes can be determined. ATSDR calculated the average concentrationwithin the pipe between two gaskets by assuming complete mixing occurs between gaskets and thatthe water in the pipe stagnates for a long period, such as overnight. Mass transport through thegasket is assumed based on a driving force that carries the contaminant from a higher concentrationoutside of the pipe to a much lower concentration inside the pipe.

From the table above, it can be seen that as the pipe diameter increases, the likelihood thatconcentrations will exceed the MCL decreases. This appears to be due to pipe volume increasingfaster than the estimated mass transport rate. The effects of pipe size (i.e., gasket size) on the masstransport rate have been assumed to vary proportionally with pipe circumference. If this assumptionis valid, pipelines with smaller diameters in contaminated areas will accumulate higherconcentrations of contaminant and potentially pose the greatest risk to water supply users.

Most pipe manufacturers provide guidance on the appropriate gasket material to be used, dependingon the environment and the conditions to which the pipe/joint will be exposed. SBR or plain rubberjoints are recommended for common uses of fresh water, salt water, or sanitary sewage. Other typesof rubber, such as Viton or nitrile, are recommended for pipes exposed to aromatic hydrocarbons,petroleum products, chemicals and solvents (ACIPCO 2000). The work of Glaza et al. and othershas shown that SBR gaskets allow the highest mass transport rates, whereas other types of gasketssuch as Viton and nitrile will provide much greater protection against permeation.

At the BARC site, most gasketed joints are made of SBR type rubber. According to information provided ATSDR by WSSC, these pipelines were constructed prior to contamination existing in the area (Billingsley 2000). The plume of contaminated groundwater begins near the W.P. Ballard property and extends through the Beltsville Industrial Center (BIC) across the B&O Railroad right-of-way and onto the BARC property. WSSC water line maps indicate that a 96" water main runs through the BARC property. This line is reduced to a 12" and subsequently an 8" pipe that feeds the BIC property (Billingsley 2000). Iso-contour plots of the TCE plume show concentrations up to 50 g/L through the BIC and BARC properties; however, this plume encounters the small 8" pipeline only in the BIC (ENTECH 2000). Therefore, water supply users within the BIC have the greatest potential for exposure to elevated PCE and TCE concentrations. Due to the absence of 8" and 12" water pipes located within the plume on the BARC site, it is not expected that water supply users within this portion of BARC will be exposed to levels of PCE or TCE of health concern as a result of permeation through gasketed pipe joints.

Results of mass transport through gaskets reported by Glaza et al. assume that the gasket materialhas become saturated with the contaminant by first being exposed to a high concentration of thespecific contaminant. Once saturated, the steady state release of the contaminant was measured andreported as the mass transport rate through the gasket. The concentrations found within thegroundwater in the vicinity of the BARC site may or may not cause the gasket material to becomesaturated. The rate at which SBR type gaskets adsorb a specific contaminant and the concentrationof the contaminant within the groundwater will determine if the gaskets will become saturated.

According to water line maps provided by WSSC, there are over 4,000 linear feet of 8" pipelinewithin the plume of contaminated groundwater in the BIC. This is the equivalent of over 10,000gallons of potentially contaminated drinking water that may or may not be entirely flushed out of thesystem each day. Flow characteristics and mixing regimes within the pipeline will have substantialeffects on the end-of-pipe concentrations that reach water supply users. ATSDR did not receive anyinformation about the proximity of BARC water pipelines to groundwater contamination.

ATSDR recommends that BARC, WSSC, and MDE work together to identify areas in whichcontaminants may contact small diameter pipelines and where cross contamination may occur. Inaddition, ATSDR recommends that water samples be taken from any water supply tap within theBIC and other areas where WSSC or BARC water pipes that are 8" or smaller contact contaminatedgroundwater. ATSDR is available to evaluate the results of any such sampling.


ACIPCO. 2000. American Cast Iron Pipe Company Web Site, visited October 2000. .

Berens, A.R. 1985. Prediction of Organic Chemical Permeation Through PVC Pipe. JournalAWWA. November 1985.

Billingsley, P. 2000. Communications from Paul Billingsley, WSSC, providing pipe specifications,gasket specifications, and water line maps. October 2, 5, and 24, 2000.

ENTECH, Inc. 2000. Remedial Investigation Report: Biodegradable Site (BARC-06), BeltsvilleAgricultural Research Center, Draft. April 2000.

Glaza, E.C. et al. 1992. Permeation of Organic Contaminants Through Gasketed Pipe Joints.Journal AWWA. July 1992.


Appendix D was not available in electronic format for conversion to HTML at the time of preparation of this document. To obtain a hard copy of the document, please contact:

Agency for Toxic Substances and Disease Registry
Division of Health Assessment and Consultation
Attn: Chief, Program Evaluation, Records, and Information Services Branch E-56
1600 Clifton Road NE, Atlanta, Georgia 30333

Table of Contents The U.S. Government's Official Web PortalDepartment of Health and Human Services
Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta, GA 30341
Contact CDC: 800-232-4636 / TTY: 888-232-6348

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