PUBLIC HEALTH ASSESSMENT
BELTSVILLE AGRICULTURAL RESEARCH CENTER
BELTSVILLE, PRINCE GEORGE'S COUNTY, MARYLAND
Figure 2. AOCs at North Farm, South Farm, and Linkage Farm
Figure 3. AOCs at Central Farm and East Farm
Figure 4. Surface Water Bodies at BARC
Figure 5. ATSDR's Exposure Evaluation Process
Figure 6. BARC Water Supply Wells
ATSDR defines an exposure pathway as having 5 parts:
- Source of Contamination,
- Environmental Media and Transport Mechanism,
- Point of Exposure,
- Route of Exposure, and
- 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.
APPENDIX B: ATSDR'S COMPARISON VALUES
ATSDR comparison values are media-specific concentrations that are considered to be "safe" under default conditions of exposure. They are used as screening values in the preliminary identification of "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 health assessor has selected for further evaluation of potential health effects.
Generally, a chemical is selected as a contaminant of concern because its maximum concentration in air, water, or soil at the site exceeds one of ATSDR's comparison values. However, it cannot be emphasized strongly enough that comparison values are not thresholds of toxicity. While concentrations at or below the relevant comparison value may reasonably be considered safe, it does not automatically follow that any environmental concentration that exceeds a comparison value would be expected to produce adverse health effects. Indeed, the whole purpose behind highly conservative, health-based standards and guidelines is to enable health professionals to recognize and resolve potential public health problems before they become actual health hazards. The probability that adverse health outcomes will actually occur as a result of exposure to environmental contaminants depends on site specific conditions and individual lifestyle and genetic factors that affect the route, magnitude, and duration of actual exposure, and not on environmental concentrations alone.
Described below are the various comparison values that ATSDR uses to select chemicals for further evaluation.
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 persons exposed over a 70-year lifetime according to EPA estimates. As ATSDR's most conservative comparison value, the CREG merits special attention. Note that this does not mean that exposures equivalent to the CREG are actually expected to cause one excess cancer in a million persons exposed over a lifetime. Nor does it mean that every person in an exposed population of one million has a 1-in-a-million chance of developing cancer from the specified exposure. Although ATSDR CREGs continue to be useful devices for screening carcinogenic substances at a site, they cannot be used to predict cancer incidence rates at a site. Furthermore, the exposure assumptions on which EPA'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 in that they are risk-based concentrations derived for carcinogens and non-carcinogens from RfDs and Cancer Slope Factors, respectively, assuming default values for body weight, exposure duration and frequency, 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 water that EPA deems protective of public health (considering the availability and economics of water treatment 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., doses expressed in mg/kg/day) that are unlikely to be associated with any appreciable risk of deleterious noncancer 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's Toxicological Profiles for specific chemicals.
(EPA's) Reference Doses (RfD) are estimates, with safety factors built in, of daily exposure doses to contaminants that are unlikely to cause noncarcinogenic adverse health effects over a lifetime of exposure. 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.
APPENDIX C: CONTAMINANT PERMEATION OF GASKETED DUCTILE IRON PIPE JOINTS
Introduction
The Agency for Toxic Substances and Disease Registry (ATSDR) has been requested by the Washington Suburban Sanitary Commission (WSSC), the public water supply provider, to investigate the potential for pipeline workers and water supply users to be exposed to contaminants via water pipelines located in the Beltsville Agricultural Research Center (BARC). This appendix discusses the calculations and assumptions ATSDR used to assess potential exposures. ATSDR estimated concentrations of contaminants that might enter the subsurface municipal water supply pipes within BARC through gasketed joints. Using conservative assumptions, ATSDR first evaluated concentrations of a potential contaminant that could theoretically be found within pipelines, then calculated the minimum mass transport rate that would yield contaminant concentrations above health-based guidelines. In addition, two different size pipe diameters were used for comparison purposes. This evaluation indicated that the potential exists, under certain circumstances, for some contaminants to reach concentrations exceeding health-based guidelines within water supply pipes.
Methodology
Previous research suggests that all types of jointed pipe used in water distribution systems are susceptible to permeation by volatile organic compounds (VOCs) such as those found in gasoline or common commercial and industrial solvents (Berens 1985). Glaza et al. (1992) suggest that gasketed joints between pipe segments are also a likely point of permeation. Using their calculation for determining pipe water concentrations, ATSDR estimated concentrations of contaminants that could exist within pipelines located in areas of groundwater contamination. For this calculation, the following assumptions were used:
Note: The first three assumptions are based on information provided by WSSC on the construction of its pipelines in the vicinity of BARC (Billingsley 2000). ATSDR did not received information about the construction of BARC water pipelines.
ATSDR used the following equation to calculate pipe water concentration when the mass transport rate through a gasket is known. This equation assumes complete mixing between pipe joints.
Where:
| C | = concentration in micrograms per liter (µg/L) |
| M | = mass transport rate across the gasket in micrograms per day (µg/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 µg/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 µg/d. A concentration value of 2.5 µg/L is obtained for 12-inch pipe under the same conditions and an assumed mass transport rate of 3,000 µg/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.
| Symbol | Units | 8 inch | 12 inch | ||||
| Pipe diameter (i.d.) | inches | 8.0 | 12.0 | ||||
| Pipe circumference | inches | 25.1 | 37.7 | ||||
| Pipe volume per foot | gallons | 2.6 | 5.9 | ||||
| Pipe cross section | (Ac) | sq. feet | 0.35 | 0.79 | |||
| Mass flow rate | (M) | µg/d | 2000.0 | 3004.0 | |||
| Pipe length | (Lp) | feet | 18.0 | 18.0 | |||
| Stagnation time | (Ts) | days | 0.33 | 0.33 | |||
| C= | 3.71 | µg/L | 2.48 | µg/L | |||
| If C = | 5.00 | µg/L | |||||
| M = | 2692.79 | 6058.77 |
A large plume of PCE and its daughter products (including TCE) originates from the W. P. Ballard site, north and west of BARC's Linkage Farm. Each of these contaminants has a maximum contaminant level (MCL) of 5 µg/L. In the literature ATSDR obtained regarding contaminant permeation of rubber gaskets, mass transport rates of PCE and TCE through rubber gaskets were not reported. The values used in the calculations above are conservative values selected from research by others; however, these values have no experimental basis. To overcome this, the above equation was modified to calculate the minimum mass transport rate that would result in a concentration exceeding the MCL for PCE and TCE. Equation (2) shows the modified equation (1):
Where:
C = 5 µg/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 µg/d for benzene to 80,000 µg/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 µg/d and 6,100 µg/d could result in concentrations exceeding 5 µg/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.
Discussion
The work of others has shown that rubber gaskets, especially SBR, can be permeated by common contaminants in the liquid or gas phase (Glaza et al. 1992). The experimentally-determined mass transport rates through gasket material vary widely depending on the type of rubber and the specific contaminant. To substantiate conclusions drawn from this evaluation, it is recommended that mass transport 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 theoretical concentration within water pipes can be determined. ATSDR calculated the average concentration within the pipe between two gaskets by assuming complete mixing occurs between gaskets and that the water in the pipe stagnates for a long period, such as overnight. Mass transport through the gasket is assumed based on a driving force that carries the contaminant from a higher concentration outside 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 that concentrations will exceed the MCL decreases. This appears to be due to pipe volume increasing faster than the estimated mass transport rate. The effects of pipe size (i.e., gasket size) on the mass transport rate have been assumed to vary proportionally with pipe circumference. If this assumption is valid, pipelines with smaller diameters in contaminated areas will accumulate higher concentrations 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, depending on the environment and the conditions to which the pipe/joint will be exposed. SBR or plain rubber joints are recommended for common uses of fresh water, salt water, or sanitary sewage. Other types of 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 others has shown that SBR gaskets allow the highest mass transport rates, whereas other types of gaskets such 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 µg/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 material has become saturated with the contaminant by first being exposed to a high concentration of the specific contaminant. Once saturated, the steady state release of the contaminant was measured and reported as the mass transport rate through the gasket. The concentrations found within the groundwater in the vicinity of the BARC site may or may not cause the gasket material to become saturated. The rate at which SBR type gaskets adsorb a specific contaminant and the concentration of 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" pipeline within the plume of contaminated groundwater in the BIC. This is the equivalent of over 10,000 gallons of potentially contaminated drinking water that may or may not be entirely flushed out of the system each day. Flow characteristics and mixing regimes within the pipeline will have substantial effects on the end-of-pipe concentrations that reach water supply users. ATSDR did not receive any information about the proximity of BARC water pipelines to groundwater contamination.
ATSDR recommends that BARC, WSSC, and MDE work together to identify areas in which contaminants may contact small diameter pipelines and where cross contamination may occur. In addition, ATSDR recommends that water samples be taken from any water supply tap within the BIC and other areas where WSSC or BARC water pipes that are 8" or smaller contact contaminated groundwater. ATSDR is available to evaluate the results of any such sampling.
References
ACIPCO. 2000. American Cast Iron Pipe Company Web Site, visited October 2000.
http://www.acipco.com 
.
Berens, A.R. 1985. Prediction of Organic Chemical Permeation Through PVC Pipe. Journal AWWA. 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), Beltsville Agricultural 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: RESPONSE TO COMMENTS
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
