Department of Environmental Health, Boston University School of Public Health, Boston, MA, USA

Department of Epidemiology, Boston University School of Public Health, Boston, MA, USA

Department of Biostatistics, Boston University School of Public Health, Boston, MA, USA

Abstract

Background

From May 1968 through March 1980, vinyl-lined asbestos-cement (VL/AC) water distribution pipes were installed in New England to avoid taste and odor problems associated with asbestos-cement pipes. The vinyl resin was applied to the inner pipe surface in a solution of tetrachloroethylene (perchloroethylene, PCE). Substantial amounts of PCE remained in the liner and subsequently leached into public drinking water supplies.

Methods

Once aware of the leaching problem and prior to remediation (April-November 1980), Massachusetts regulators collected drinking water samples from VL/AC pipes to determine the extent and severity of the PCE contamination. This study compares newly obtained historical records of PCE concentrations in water samples (n = 88) with concentrations estimated using an exposure model employed in epidemiologic studies on the cancer risk associated with PCE-contaminated drinking water. The exposure model was developed by Webler and Brown to estimate the mass of PCE delivered to subjects' residences.

Results

The mean and median measured PCE concentrations in the water samples were 66 and 0.5 μg/L, respectively, and the range extended from non-detectable to 2432 μg/L. The model-generated concentration estimates and water sample concentrations were moderately correlated (Spearman rank correlation coefficient = 0.48, p < 0.0001). Correlations were higher in samples taken at taps and spigots vs. hydrants (ρ = 0.84 vs. 0.34), in areas with simple vs. complex geometry (ρ = 0.51 vs. 0.38), and near pipes installed in 1973–1976 vs. other years (ρ = 0.56 vs. 0.42 for 1968–1972 and 0.37 for 1977–1980). Overall, 24% of the variance in measured PCE concentrations was explained by the model-generated concentration estimates (p < 0.0001). Almost half of the water samples had undetectable concentrations of PCE. Undetectable levels were more common in areas with the earliest installed VL/AC pipes, at the beginning and middle of VL/AC pipes, at hydrants, and in complex pipe configurations.

Conclusion

PCE concentration estimates generated using the Webler-Brown model were moderately correlated with measured water concentrations. The present analysis suggests that the exposure assessment process used in prior epidemiological studies could be improved with more accurate characterization of water flow. This study illustrates one method of validating an exposure model in an epidemiological study when historical measurements are not available.

Background

From May 1968 through March 1980, vinyl-lined asbestos-cement (VL/AC) water pipes were installed in the six New England states to avoid taste and odor problems associated with the asphalt-based lining when the recommended alkalinity level was exceeded

Approximately 660 miles of VL/AC pipes were installed in Massachusetts; a large proportion was installed in the Cape Cod region to replace existing pipe or extend the water distribution system

Two years after the PCE contamination was discovered, the Massachusetts Cancer Registry began operations to monitor cancer incidence in the State, and its initial data reported elevated cancer incidence rates in the Cape Cod region

This analysis compares PCE concentrations in historical pre-remediation drinking water samples with PCE concentrations estimated using the Webler-Brown model. The objectives were to compare the exposure assessment approach used for the epidemiologic studies with independently measured historical data, and to identify characteristics of the water distribution system, exposure estimation process, and water sampling procedure that affected the correlation between the measured and estimated concentrations.

Methods

PCE concentrations in historical drinking water samples

Historical Massachusetts DEP records were reviewed to obtain PCE concentrations in drinking water samples collected in 1980. Sample records were collected for nine Massachusetts towns with VL/AC water distribution pipe (Barnstable, Bourne, Brewster, Chatham, Falmouth, Provincetown, Sandwich, Plymouth, and Wareham). The first seven of these towns were selected because they comprise the geographic site of prior epidemiologic studies. The last two towns were included because they were adjacent to Cape Cod and had a large number of samples available with appropriate documentation.

No written protocol for water sampling was found in DEP files. Additionally, no written records were identified describing the laboratory analysis procedures. The likely equipment used to analyze these samples was a gas chromatograph using heated static head space analysis with a packed column and a Hall electrolytic conductivity detector (Personal communication, Oscar Pancorbo, Director Lawrence Experiment Station, April 2004). This is consistent with various reports

For the current study, it was necessary to select water samples taken before remediation began

Estimated PCE concentrations

The Webler and Brown model developed for our prior epidemiologic studies

In this equation, _{0 }is the initial amount of PCE per unit surface area (μg/m^{2}), _{s}) to the year that the water sample was taken (_{x }is the pipe diameter (meters), and _{x }is the water flow rate (liters/year). Thus, _{0}^{-(T/r) }is the amount of initial PCE remaining in the liner after time

The integral is approximated by summing discrete pipe segments that were designated to implement the model. Each segment ends at a node, defined as the end point of a segment. The model is developed around these segments (_{s}).

Water drawn along a segment is considered removed at the segment node. Thus, the water flow in segment _{s-1.}. Equation (2) gives the estimated micrograms of PCE per liter drinking water at location

Exposure assessments for the present study were conducted by two individuals using Equation (2) and following procedures developed in our prior epidemiological studies

The quantity of water flow (evaluated as loads) was estimated as the number of parcels at and beyond a VL/AC pipe segment. Each parcel was assumed to represent one single-family home, the most common type of residence in the geographic area. Water flow was determined after consulting with water department officials and inspecting features of the distribution network, including pipe diameters and locations of wells and pumping stations. Flow assessments were conducted using simplifying assumptions outlined by Webler and Brown

Data analysis

We conducted descriptive analyses to characterize the measured PCE concentrations among all samples combined and among samples stratified according to characteristics of the water distribution system, and other factors that may affect exposure estimation and water sampling procedure. Samples with undetectable PCE levels were assigned a value of 0.25 μg/L, one-half the laboratory detection limit of 0.5 μg/L

Measured drinking water concentrations were compared to estimated concentrations using Spearman rank correlation coefficients. A linear regression model was also used to quantify the proportion of the variance in the measured concentrations explained by the modeled estimates. The natural logarithm (ln) of measured and estimated PCE concentrations was used in the regression model because the data were skewed with a long upper tail. p-values were used to describe the statistical stability of all parameters.

Comparisons were made among all samples combined, samples stratified according to sampling and location characteristics, and samples with detectable PCE levels. Stratification characteristics were 'town,' 'sampling personnel,' 'season of sampling,' 'water fixture sampled,' 'pipe installation year,' 'complexity of pipe configuration,' 'position along pipe,' 'magnitude of water flow,' and 'housing density' (see below for description of these variables). Lastly, because our prior epidemiologic analyses ^{th }percentile), we examined the measured and estimated PCE concentrations in percentile categories and evaluated the sensitivity and specificity of the estimated concentrations in correctly classifying the lower 50^{th }percentile, upper 50^{th }percentile, and upper 75^{th }percentile of PCE concentrations measured in the water samples.

Town

This variable characterized possible differences in sampling protocols and water distribution characteristics. Towns included Barnstable (n = 7 samples), Bourne (n = 16), Brewster (n = 7), Chatham (n = 6), Falmouth (n = 6), Provincetown (n = 5), Sandwich (n = 9), Plymouth (n = 25), and Wareham (n = 7).

Sampling personnel

This variable captured undocumented differences in the selection of the sampling location and procedure by the person conducting the sampling. Almost all of the samples were collected by two DEP employees (n = 69 for Sampler 1, n = 18 for Sampler 2; n = 1 for Sampler 3).

Sampling season

Most samples were collected in April 1980 (n = 71) shortly after the PCE contamination was publicized. However, some samples were collected in May and the following autumn. As a rough measure of seasonal changes in water temperature, season of sample collection was evaluated. 'Spring' included samples collected in April and May (n = 74) and 'autumn' included samples collected in September and November (n = 14). We hypothesized that PCE leaching rates would be higher in the fall when water temperatures are higher.

Water fixture

This variable captured unknown sampling conditions, including flow intensity during sampling and aeration both before and during sampling. Three types of fixtures were sampled: taps (n = 3), spigots (n = 7), and fire hydrants (n = 18). The former two were combined and represent low flow intensity while the latter represents variable flow intensity. Hydrants were also likely to have long-standing air pockets into which PCE could volatilize and were supplied by spur segments from the main VL/AC pipe that may have contained stagnant water. The kind of the collection point was not specified for 60 samples; these samples were treated as a separate category.

Pipe installation year

VL/AC pipes were installed on Cape Cod from May 1968 through March 1980. Because PCE drinking water levels decreased exponentially following pipe installation, we characterized water samples according to the installation year of the closest VL/AC pipe. Three categories of roughly equal duration were used: 1968–1972 (n = 23), 1973–1976 (n = 32), and 1977–1980 (n = 33).

Complexity of pipe configuration

This variable captured the difficulty in determining the direction of water flow by classifying the pipe configuration in the immediate vicinity of the water sample as 'simple' or 'complex.' The 'simple' category described dead-end pipes that were either directly off a major pipe or close to a water source, thereby ensuring that there was only one possible flow direction, and facilitating the flow rate determination. The 'complex' category described areas with multiple possible flow directions and where it was difficult to determine the area of water demand.

Position along pipe

This variable described the proximity of a sample location to the end of the pipe. An 'end' position was designated for locations within the last 25% of VL/AC pipe (n = 44). Locations within the first 75% of VL/AC pipe were designated as 'beginning/middle' (n = 44).

Magnitude of water flow

This variable categorized the amount of water flowing past the sampling location into 'high,' 'medium,' and 'low' based on the number of loads around a sampling location. The magnitude of flow was considered 'high' when more than 19 homes were served just downstream by the water pipe at the sample location (n = 19). It was considered 'medium' when 3–19 homes were served by the water pipe (n = 33), and 'low' when 1 or 2 homes were served by the water pipe (n = 36). These cutoffs correspond to the tertiles of the loading distribution.

Housing density

Because parcel maps provided by town officials dated from 1988 or later, there was an eight to twenty year gap between VL/AC pipe installation and parcel data used for the model-estimated concentrations. Thus, it is likely that our earlier epidemiological studies

Lastly, we conducted quantitative sensitivity analyses to determine the impact of measurement error on the correlation coefficients. We considered two sources of error based on our knowledge of the water distribution systems and laboratory analysis. The first source stemmed from using a single water sample (taken at a single time point) to characterize fluctuating PCE levels. In reality, up to two-fold fluctuations in the water concentrations were seen in a PCE sampling study that measured concentrations at the same location and time on two consecutive days

Results

The mean and median measured PCE concentrations were 66 μg/L and 0.5, respectively, for all 88 eligible samples combined. Individual sample concentrations ranged from undetectable to 2432 μg/L (Table

PCE concentrations (ug/L) measured in water samples according to characteristics of sampling location and methods

No. of Samples

No. With ND^{a }Level

Percent With ND^{a }Level

Mean^{b}

Median

75^{th }Percentile

Range

All Samples

88

43

49

66

0.5

32

ND-2432

According to:

Complexity of pipe configuration^{c}

Simple

31

13

42

73

20

50

ND-780

Complex

57

30

53

63

ND

22

ND-2432

Magnitude of flow^{c}

High (> 19 homes)

19

11

58

10

ND

20

ND-59

Medium (3–19 homes)

33

22

67

18

ND

20

ND-190

Low (< = 2 homes)

36

10

28

141

18

57

ND-2432

Position along pipe^{c}

End

44

16

36

56

12

37

ND-780

Beginning/Middle

44

27

61

77

ND

25

ND-2432

Overestimated housing density^{c}

Yes

25

11

44

182

6.6

38

ND-2432

No

63

32

51

20

ND

28

ND-235

Season sampled^{c}

Spring

74

42

57

77

ND

38

ND-2432

Autumn

14

1

7

7.7

4.6

11

ND-44

Water fixture sampled^{c}

Tap or spigot

10

3

30

302

32

190

ND-2432

Hydrant

18

7

39

26

21

35

ND-92

Unknown

60

33

55

39

ND

18

ND-780

Sampling personnel^{c}

Sampler 1

69

33

48

38

0.5

31

ND-780

Sampler 2

18

10

56

43

ND

28

ND-350

Sampler 3

1

0

0

2432

---^{d}

---^{d}

---^{d}

Town^{c}

Barnstable

7

7

100

ND

ND

ND

ND-ND

Bourne

16

10

63

57

ND

26

ND-540

Brewster

7

2

29

140

32

100

ND-780

Chatham

6

5

83

4.9

ND

ND

ND-28

Falmouth

6

1

17

47

53

62

ND-75

Provincetown

5

5

100

ND

ND

ND

ND-ND

Sandwich

9

3

33

36

22

59

ND-92

Plymouth

25

7

28

128

3.1

13

ND-2432

Wareham

7

3

43

15

20

28

ND-35

Pipe installation year^{c}

1968–1972

23

16

70

38

ND

1

ND-780

1973–1976

32

15

47

18

4.6

25

ND-100

1977–1980

33

12

36

134

22

59

ND-2432

^{a }ND – Not Detected. The detection limit was 0.5 μg/L.

^{b }Means were calculated using 0.25 μg/L (half the detection limit) for ND samples.

^{c }See text for definitions and further description.

^{d }Analyses were not conducted.

The highest median concentrations were observed in Brewster (32 μg/L) and Falmouth (53 μg/L), and in samples collected in areas with simple pipe configuration (median = 20 μg/L), from taps and spigots (median = 32 μg/L); and along the most recently installed (1977–1980) VL/AC pipes (median = 22 μg/L) (Table

Conversely, undetectable levels were reported in all or nearly all samples collected in Barnstable (100%), Provincetown (100%), and Chatham (83%) (Table

The relationship between measured and estimated PCE concentrations is shown in Figure

Correlation coefficients between measured PCE concentrations in water samples and model-generated estimates

No. of Samples

Spearman Correlation Coefficient

P-Value

All Samples

88

0.48

< 0.001

According to:

Complexity of pipe configuration^{a}

Simple

31

0.51

0.003

Complex

57

0.38

0.003

Magnitude of flow^{a}

High (> 19 homes)

19

0.37

0.1

Medium (3–19 homes)

33

0.30

0.09

Low (< = 2 homes)

36

0.54

0.0007

Position along pipe^{a}

End

44

0.44

0.003

Beginning/Middle

44

0.44

0.003

Overestimated housing density^{a}

Yes

25

0.52

0.008

No

63

0.47

0.0001

Season sampled^{a}

Spring

74

0.50

< 0.0001

Autumn

14

0.58

0.03

Water fixture sampled^{a}

Tap or spigot

10

0.84

0.002

Hydrant

18

0.34

0.2

Unknown

60

0.38

0.003

Sampling personnel^{a}

Sampler 1

69

0.45

0.0001

Sampler 2

18

0.57

0.01

Sampler 3

1

---^{b}

---^{b}

Town^{a}

Barnstable

7

---^{c}

---^{c}

Bourne

16

0.53

0.03

Brewster

7

0.018

1.0

Chatham

6

0.39

0.4

Falmouth

6

0.49

0.3

Provincetown

5

---^{c}

---^{c}

Sandwich

9

0.53

0.1

Plymouth

25

0.56

0.003

Wareham

7

0.48

0.3

Pipe installation year^{a}

1968–1972

23

0.42

0.05

1973–1976

32

0.56

0.0008

1977–1980

33

0.37

0.03

^{a }See text for definitions and further description.

^{b }Analyses were not conducted because of the small sample size.

^{c }Analyses were not conducted because all water samples had undetectable PCE levels.

Log_{e }measured PCE verses log_{e }model estimated PCE concentrations (ug/L)

**Log _{e }measured PCE verses log_{e }model estimated PCE concentrations (ug/L).**

Results of the quantitative sensitivity analysis indicated that the correlation level was robust. The mean Spearman correlation coefficient between the randomly adjusted PCE concentrations in the water samples and point concentration estimates from the model was 0.44 (σ = 0.04, p < 0.0001), and the range extended from 0.29 to 0.53.

The correlation varied according to sampling characteristics. Correlations were higher among samples collected at taps and spigots vs. hydrants (ρ = 0.84 vs. 0.34), and by Sampler 2 vs. Sampler 1 (ρ = 0.57 vs. 0.45). Correlations also varied by factors that may affect exposure estimation: areas with simple vs. complex geometry (ρ = 0.51 vs. 0.38), at low vs. medium and high flow locations (ρ = 0.54 vs. 0.30 and 0.37), and near pipes installed in 1973–1976 vs. earlier and later years (ρ = 0.56 vs. 0.42 for 1968–1972 and 0.37 for 1977–1980). The correlation also varied considerably by town; it was highest in Plymouth (ρ = 0.56) and lowest in Brewster (ρ = 0.02). The lack of correlation in Brewster stemmed from the highest measured PCE concentration in Brewster (780 μg/L) that was predicted to be the town's lowest concentration (82 μg/L). When this location was excluded, the Spearman correlation coefficient for Brewster was 0.64. There was little difference in the correlation according to housing density estimates (ρ = 0.52 vs. 0.47), pipe position (ρ = 0.44 vs. 0.44), and season (ρ = 0.58 vs. 0.50).

When analyses were limited to samples with detectable PCE levels, the highest median concentrations were observed among samples collected in areas with simple pipe configuration (median = 40 μg/L), from taps and spigots (median = 100 μg/L); and along the most recently installed (1977–1980) VL/AC pipes (median = 45 μg/L). This pattern is similar to the entire sample. However, the Spearman correlation coefficient fell to 0.41 (p = 0.005), and the amount of explained variance fell to 19% when the analysis was restricted to these samples. The data were too sparse to stratify the correlations according to the water distribution, exposure estimation, and sampling characteristics.

Table ^{th }and 75^{th }percentile categories were 207 ug/L and 657 ug/L, respectively, among the modeled PCE concentrations while they were 0.5 ug/L and 32 ug/L among the measured PCE concentrations. The large difference in concentration distributions stems mainly from the sizeable number of undetectable levels in the measured samples (Figure ^{th }and upper 75^{th }percentiles of the measured PCE concentrations were 63% and 59%, respectively. The corresponding specificities were 62% and 86%.

Number of samples according to percentile categories^{a }of measured and modeled PCE concentrations

Measured PCE concentration

≤ 50^{th }percentile

> 50–75^{th }percentile

> 75^{th }percentile

Total

Model estimated PCE concentration

≤ 50^{th }percentile

28

10

6

44

> 50–75^{th }percentile

12

7

3

22

> 75^{th }percentile

5

4

13

22

Total

45

21

22

88

^{a }207 μg/L and 657 ug/L were the 50^{th }and 75^{th }percentile modeled PCE concentrations, respectively. 0.5 μg/L and 32 ug/L were the 50^{th }and 75^{th }percentile measured PCE concentrations, respectively.

Discussion

Our study found a moderate, statistically significant correlation between measured and estimated PCE concentrations (Spearman correlation coefficient ρ = 0.48, p < 0.0001). The correlation varied across characteristics of the water sampling procedures; correlations were higher among samples taken at taps and spigots compared to hydrants. Correlations also varied across factors that we hypothesized might affect the accuracy of the estimation procedure; correlations were higher in areas with simple geometry, low flow, and near pipes installed in the earlier years. In contrast, the lowest correlations were observed in areas with complex geometry, and near pipes installed in the most recent years. About 55% of the model estimated and measured concentrations were in identical percentile categories when the data were examined in groupings used in our prior epidemiological studies.

Even though the current analysis was limited to samples taken at VL/AC pipes, only 51% of the samples had detectable PCE levels. The correlation between estimated and measured PCE concentrations was 0.41 (p = 0.005) among these samples. Undetectable PCE levels were more common in areas with the earliest installed VL/AC pipes, at the beginning and middle of VL/AC pipes, at hydrants, in complex pipe configurations, and where housing density estimates were considered more accurate.

These results suggest that (1) the sampling procedures and analytical methods affected the accuracy of the measured PCE concentrations, and (2) the exposure model and assessment process had inaccuracies that depended on the characteristics of the sampling location that, in turn, affected the correlation between measured and predicted concentrations.

Inaccuracies in Measured PCE Concentrations

The historical water samples to measure the PCE concentrations were not collected with the goal of validating the exposure model used in our epidemiologic study, but "to determine quickly the extent and severity" of a public health problem in 1980

Savitz has suggested that a spot measurement is not a gold standard for long-term, cumulative exposures, despite "all the appearances of accuracy" because it reflects "only a single point in time in a fluctuating system..."

Moreover, the laboratory's use of head space analysis may have inconsistently reduced PCE recoveries, thereby reducing the correlation between the measured and estimated concentrations. The head space laboratory analysis, which was done to facilitate timely analysis of hundreds of drinking water samples, relies on the tendency of PCE to volatilize out of water into air. In contrast, the more accurate purge and trap method removes PCE from water by purging the water with an inert gas and then trapping the PCE on a solid sorbent. Duplicate sample analyses conducted by the DEP laboratory suggested that the head space analysis inconsistently underestimated the PCE concentrations. In one set of analyses, the concentration observed using head space analysis was only 20% of that using the purge and trap method (38 and 205 μg/L, respectively), while the concentrations were similar in the second set (160 and 150 μg/L, respectively).

A large proportion of samples had missing data on the water fixture that was sampled. If the remaining results are unbiased, the data suggest that sampling from hydrants may also have introduced error from increased aeration. Hydrant samples had lower measured concentrations (median = 21 ug/L) and one of the lowest correlations (ρ = 0.34, p = 0.2), while tap and spigot samples had the highest measured PCE concentrations (median = 32 ug/L) and the highest correlation with the estimated concentrations (ρ = 0.84, p = 0.002). High flow fixtures such as fire hydrants likely introduced air into water samples, thereby reducing the amount of PCE remaining in the water by air stripping. Fire hydrants may also have had a head space of air, with the loss of PCE along the interface between the water and air. In contrast, taps and spigots were capable of generating the low water flow more suited for characterizing volatile organic compounds, and were less likely to have a high volume head space.

Although all samples were collected along VL/AC pipe, 49% had undetectable PCE levels. In fact, the large difference in concentration distributions stems mainly from the large number of undetectable levels (Figure

Inaccuracies in the Webler Brown model and its implementation

Our prior studies used the Webler-Brown model to estimate cumulative PCE exposure. This model was specifically developed for epidemiological research and not risk assessment. The model used the rate at which PCE leached from the vinyl liner, the surface area of the interior of the pipe, and the loading along the pipe to calculate the RDD, a measure assumed to be roughly proportional to the mass of PCE that entered a home over a specific time period. Simplifying assumptions about the rate and direction of the water flow were needed to implement the model for our epidemiological studies and the present analysis. These simplifications likely decreased the correlation between the estimated and measured concentrations, particularly in areas with complex pipe configurations because water flow direction and magnitude are less predictable in these settings.

Further, while our assumption that every parcel used water at the same rate and that water was constantly flowing was reasonable given that predominant housing on Cape Cod was a single-family dwelling

Variation in the initial amount of PCE in the pipe or inaccuracy in the diffusion rate constant (

Other research evaluating exposure to drinking water contaminants

Many studies have evaluated the validity of models to predict trihalomethanes levels in drinking water following treatment with chlorination [e.g.,

Only a few prior studies have, like us, evaluated historical exposure measures developed for epidemiological research. These results of these studies are similar to ours. For example, Freedman et al. evaluated the validity of using nitrate concentrations in public drinking water supplies from a single year to characterize long-term exposure for a case-control study of non-Hodgkins lymphoma and leukemia

In addition, Ayotte et al. evaluated the validity of a logistic regression model to predict the occurrence of arsenic in ground water for historical exposure assessments among subjects in an epidemiological study of bladder cancer

Lastly, Whitaker et al. examined the validity of a stochastic model to predict exposure to disinfection by-products for a study of adverse birth outcomes

Conclusion

In summary, the Webler-Brown model generated exposure estimates moderately concordant with historically measured PCE data. While these findings are similar to those from other studies of historic exposures, this evaluation suggests that more accurate water flow characterizations would further improve the correlation with historical water data, acknowledging these data are themselves subject to systematic error. Water pipe distribution models are now available to determine flow more accurately than the approximate method we used. The incorporation of more specific load information, such as data on commercial and multi-family use and the year that the sites began to use water, may also increase the accuracy of the flow assessments, an essential part of the Webler-Brown model. This analysis shows how a detailed retrospective examination of historical measurements made for other purposes can suggest further refinements in the model. While this analysis also supports the exposure model used in previous epidemiologic studies, further analyses are currently underway evaluating the impact of the model's inaccuracies on the risk of breast cancer using data from our prior case-control study

List of Abbreviations

_{0s: }initial amount of perchloroethylene per unit surface area for pipe segment _{s: }Diameter of pipe for pipe segments; EPA: United States Environmental Protection Agency; Ln: Natural logarithm; _{s: }Length of pipe segments; ND: Not detected; OR: Odds ratio; PC: Point concentration; PCE: Perchloroethylene or tetrachloroethylene; _{s-1}: Magnitude of pipe water flow in pipe segments; _{x}: Magnitude of pipe water flow at location _{s}: Time of pipe installation for pipe segments; VL/AC: Vinyl-lined asbestos-cement.

Competing interests

Dr. David Ozonoff is Co-editor-in-Chief of

Authors' contributions

LAS carried out a portion of the exposure assessments, conducted DEP file reviews, conducted analyses, and wrote the initial draft of the manuscript. AA conceived the study, participated in its design and coordination, assisted in the analysis, and finalized the manuscript. LG conducted analyses, and helped finalize the manuscript. TW provided technical input to study design, analysis, and modeling. TH provided statistical guidance and review. DO participated in study design, analysis, and manuscript preparation. All authors read and approved the final manuscript.

Appendix A: Derivation of the point concentration estimate

There are two parts to implementing the Webler Brown model: (1) estimating the water flow in pipes and (2) estimating the movement of PCE from the vinyl liner into the flowing water. The estimated concentration of PCE at location _{i}_{x}_{x}, and then integrated along the upstream VL/AC pipe.

To model the PCE leaching rate, _{x}^{® }liner applied to small pieces of aluminum. Webler and Brown fit a first order negative exponent, ^{-T/r}, to these data (_{0}), Webler and Brown estimated the amount of PCE remaining in the Piccotex^{® }liner at time

The flux of PCE from Piccotex was then estimated as the change in PCE per unit surface area over time. Since we are interested in the amount of PCE entering the water, the sign on the flux is positive.

The resulting leaching rate for PCE per unit length of pipe, _{x}

In this equation, _{x }is the pipe diameter. Thus, _{0}^{-(T/r) }is the amount of initial PCE remaining in the liner after time _{x}) and integrated along a pipe to the location of interest,

This integral is approximated by summing discrete pipe segments we designated to implement the model. Each segment ends at a node, defined as the end point of a segment. The model is developed around these segments (_{s}). Lapsed time (_{s}) to the year that the water sample was taken (

Water drawn along a segment (_{s}) was evaluated as removed at the segment node. Thus, the water flow in segment _{s-1}. Equation (A6) gives the estimated micrograms of PCE per liter drinking water at location

The water flow in segment _{s-1}, was estimated as, the flow into a pipe segment minus the water drawn upstream:

The amount of water entering contributing pipe, _{0}, was estimated as the number of homes drawing water along a pipe (_{0}) multiplied by the average household water use (_{z}, is the number of homes along segment _{z}) multiplied by _{z }= 5 and _{z }= _{s-1 }is, therefore, estimated by

Combining equations (A6) and (A8) yields the PCE point concentration estimate for a specific time (

The parameters in (A9) have the following values:

_{0s }– The initial amount of PCE per surface area of Piccotex^{® }liner in pipe segment ^{7 }μg/meter^{2 }^{® }liner application conformed with Johns Manville specifications for the perchloroethylene suspension (30% Piccotex^{® }and 70% PCE), that 6% of PCE remained in the liner at installation, that the liner was uniformly 6.35 × 10^{-3 }meters thick, and that the specific gravity of PCE is 1.624 × 10^{9 }micrograms per cubic meter

_{s }– The day of pipe installation, estimated as one half of the year of installation (_{s }was estimated as 1970.50 years – approximately July 2, 1970)

_{s }– Internal water pipe diameter for pipe segment

_{s }– Water pipe segment

_{0 }– The total number of homes drawing water from contributing VL/AC pipe

_{z }– The number of homes drawing water along water pipe segment

Acknowledgements

The authors would like to acknowledge the assistance of Sarah Rogers when she was at Boston University for assistance in conducting the exposure assessments, and the local water companies and the Massachusetts Department of Environmental Protection for providing us with the PCE measurement data. This study was supported by grant 2P42 ES07381 from the National Institute of Environmental Health Sciences. Its contents are solely the author's responsibility and do not necessarily represent the official views of the NIEHS or the EPA.