Lead isotopes in tap water: implications for Pb sources within a municipal water supply system

Applied Geochemistry Applied Geochemistry 20 (2005) 353–365 www.elsevier.com/locate/apgeochem

Lead isotopes in tap water: implications for Pb sources within a municipal water supply system Zhongqi Cheng *, Kenneth A. Foland Department of Geological Sciences, Ohio State University, Columbus, OH 43210, USA Received 22 December 2003; accepted 1 September 2004 Editorial handling by R.S. Harmon

Abstract Residential tap waters were investigated to examine the feasibility of using isotopic ratios to identify dominant sources of water Pb in the Columbus (Ohio, USA) municipal supply system. Overall, both the concentrations, which are generally low (0.1–28 lg/L), and isotopic compositions of tap water Pb show wide variations. This contrasts with the situation for a limited number of available service lines, which exhibit only a limited Pb-isotope variation but contain Pb of two very different types with one significantly more radiogenic than the other. Most tap water samples in contact with Pb service lines have Pb-isotope ratios that are different from the pipe Pb. Furthermore, the Pb isotope compositions of sequentially drawn samples in the same residence generally are similar, but those from separate residences are different, implying dominant Pb sources from domestic plumbing. A separate pilot study at two residences without Pb service lines shows isotopic similarity between water and solders in each house, further suggesting that the major Pb sources are domestic in these cases and dominated by Pb from solder joints. Although complicated by the broad range of overall Pb-isotope variations observed and limited by sample availability, the results suggest that Pb isotopes can be used effectively to constrain the sources of Pb in tap waters, especially for individual houses where multiple source candidates can be identified.
2004 Elsevier Ltd. All rights reserved.

1. Introduction Lead is one of the most important toxic inorganic constituents in drinking water in the US and many other countries (e.g., Nriagu, 1978; Briskin and Marcus, 1990; US EPA, 1991). Tap water from municipal supply systems is the source of drinking water for a majority of homes in the United States. Addressing Pb sources in tap water from the municipal system sometimes is very * Corresponding author. Tel.: +1 8453658649; fax: +1 8453658154. E-mail address: [email protected] (Z. Cheng).

difficult, especially when the Pb levels are not very high (e.g., <15 lg/L) and multiple potential sources exist. It has been generally known that in a municipal water system, such as the one in the city of Columbus (Ohio, USA), potential Pb source candidates include the treatment plant, water distribution network, and domestic plumbing system (Pocock, 1980; Birden et al., 1985; Neff et al., 1987; Schock and Neff, 1988; Gulson et al., 1994; Kimbrough, 2001). In the absence of pipe Pb, Pb-based solder and brass fittings are known to be dominant Pb sources in public water supply systems (Birden et al., 1985; Kimbrough, 2001). In Columbus, a large number of Pb service lines were installed before World War II

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2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2004.09.003

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and still supply residential water. Case studies in Scotland by Moore (1977) showed that a large amount of Pb could be released from Pb pipes into water under ‘‘soft’’ water conditions. While Pb levels in Columbus water have remained mostly below the US EPA action level of 15 lg/L, most likely due to an effective corrosion control strategy and the ‘‘aging’’ of the pipes, a survey of Pb concentrations in tap water has shown a significant number (5%) of samples between 5 and 15 lg/L and even some exceeding 15 lg/L (Burgess and Niple, 2000). While the relative Pb contribution from Pb service lines versus other sources remains unclear, there is a common assumption that the Pb pipes must be replaced whenever possible. The replacement of a large quantity of pipes in a metropolitan area such as Columbus would be very costly in several ways (Burgess and Niple, 2000). Therefore, a systematic investigation into Pb sources is warranted and, because Pb pipes are quite common in many older cities, the results from this study in Columbus are pertinent to addressing Pb issues in similar water supply systems. The isotopic composition of common Pb offers the potential to identify Pb sources. With 4 stable isotopes – 208Pb, 207Pb, 206Pb and 204Pb (the first 3 of which are radiogenic from Th and U parents) – the isotopic compositions of Pb exhibits wide variations in natural and anthropogenic materials that often can be used as a diagnostic ‘‘signature.’’ While Pb isotopes often have been used as tracers to identify environmental Pb sources (e.g., Flegal et al., 1993; Graney et al., 1995; Erel et al., 1997; Planchon et al., 2003; Landmeyer et al., 2003; Toner et al., 2003; Kurkjian and Flegal, 2003), this tracer approach has not been commonly used to delineate drinking water Pb sources. Gulson et al. (1994) first reported significant changes in the isotopic composition of Pb, accompanying decreasing Pb concentrations, for water sequentially collected from the tap after the water sat in the plumbing system overnight. The study demonstrated that Pb in the first flush water was dominantly derived from solder and suggested that there was a change of dominant Pb source, probably from the plumbing system to outside water. In a later study where water samples were collected at hourly intervals, Gulson et al. (1997) showed that there was no change in the isotopic composition of samples, although there were significant changes in Pb concentration from 35 to 52 lg/L. These observations suggested that it may be possible to identify the most important Pb source(s) based upon isotopic composition. The feasibility of using Pb isotopes lies in the assumption that source candidates can be characterized by their different Pb-isotope compositions. In this study, samples of residential tap water were analyzed to determine the concentrations and isotopic compositions of Pb, which were then compared to those for several candidate source materials. The main objective was to investigate the potential of using Pb isotopes

to identify the dominant source(s) in a municipal water supply system and to address the importance of Pb contributions from selected sources, such as Pb service lines and solder joints.

2. Methods 2.1. Sampling 2.1.1. Water samples Most water samples examined were tap waters from cold-water spigots of kitchens of individual residential houses in Columbus. The Columbus Division of Water supplies water from 3 plants: the Hap Cremean Water Plant (HCWP), the Dublin Road Water Plant (DRWP) and the Parsons Avenue Water Plant (PAWP). Fig. 1 shows the locations of water plants, their respective distribution areas and the sampled houses. Most samples were from the ‘‘Columbus Tap Water Collection Project,’’ a project conducted in late-1998 to mid-1999 to survey Pb levels in tap water for 7% of the homes with Pb service lines in Columbus. The sample collection protocol (Burgess and Niple, 2000) is summarized as follows: The plumbing system within a house was first inspected at the meter to see if the upstream pipes were Pb. Where Pb pipes were found, two samples were taken. On the day of sampling, the resident of the house refrained from water usage for approximately 6 h before the first liter of tap water was collected. The first liter of tap water was called the ‘‘first-draw’’ sample. After this sample was taken, the tap was flushed to bring in outside water and another liter water sample was collected as the ‘‘second-draw’’ sample. The water volume flushed was estimated by the length and diameter of the domestic pipelines and empirically checked by water temperature. It was assumed that the second-draw sample represents water from outside of the house and had been in contact with Pb service lines. The first-draw water, however, resided in the inside plumbing and, thus, should bear the effects of the domestic plumbing systems including Pb pipes in the house. In the typical case where no Pb pipes were found in the house, only the second-draw water was collected. The ‘‘Columbus Tap Water Collection Project’’ collected 2042 seconddraw samples and approximately 20 pairs of first- and second-draw samples. All water samples were collected in 1-liter capacity polyethylene bottles with 1 ml of reagent grade HNO3 added for preservation. A subset of samples was selected for the Pb-isotope analysis in this study that included 25 randomly picked second-draw samples, 12 samples selected from the same street, and 8 pairs of first- and second-draw samples (Table 1). Four water samples from the Dublin Road Water Plant were also collected and analyzed one year later (June 2000) to characterize the water supply from that

Z. Cheng, K.A. Foland / Applied Geochemistry 20 (2005) 353–365

355

I-270

25

42

38 10 22 20 37 47 48

Hap Cremean Water Plant service area

HCWP

6 21 54 53 51 49 1 19 5 4 3 43

41

39

6 23

18 2

Dublin Road Water Plant service area

7

DRWP 9

12 7 52 101345 11 9 1124 8 1444

8

16 17 15 46 50 12

Parsons Avenue Water Plant service area

I-270

PAWP Fig. 1. Location map showing where the water and pipe samples were collected and the regions served by different water plants. The 3 water service areas within the region encircled by highway I-270 are shown along with the locations of the 3 water plants themselves (identified by HCWP – Hap Cremean Water Plant, DRWP – Dublin Road Water Plant, PAWP – Parsons Avenue Water Plant) by larger open square, circle, and inverted triangle, respectively. The shaded area indicates the distribution of Pb service lines. Hollow symbols with numbers indicate locations of residential tap water samples while solid diamonds with numbers indicate locations for pipe samples. All numbers correspond to those in Table 1. (Some pipe samples are not shown because the exact location is unknown.) In this and all subsequent diagrams, different hollow symbols reflect the specific water treatment plant that supplied the water: open squares represent water supplied from the Hap Cremean Plant, open circles represent water supplied from Dublin Road Plant, inverted open triangles represent water supplied from Parsons Avenue Plant. Solder samples were collected from residences numbered 41 (house A) and 42 (house B). The solid straight line between samples 22, 20 and 47, 48 indicates the location of the street where samples 26–36 were collected.

plant. The samples were from different stages of water treatment and, therefore, can also be used to monitor the effect of water treatment on Pb. Water and solder samples were also collected in July 2000 from two residential houses, and analyzed in order to evaluate solder of the domestic plumbing system as a Pb source. Both houses are supplied by Hap Cremean

Water Plant, have Cu water supply lines and neither is in an area having Pb service lines. Three cold-water samples were collected from each house with a procedure as follows: After 6 h of no water usage, a 60-ml first-draw sample (‘‘-W1’’) was collected. The spigot was then left open for about 1-min, followed by the collection of a 60-ml second-draw sample (‘‘-W2’’). The tap water was

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Table 1 Concentrations and isotopic compositions of Pb in water samples Sample timeC

Water sourceD

PbE (lg/L)

208

207

206

Randomly picked water samples 1 012350B 06/10/99 2 014053B 04/30/99 3 021463B 11/18/98 4 021803B 01/04/99 5 021932B 12/18/98 6 022560B 11/30/98 7 041888B 04/06/99 8 041984B 03/01/99 9 042185B 03/09/99 10 042201B 02/27/99 11 043241B 03/11/99 12 043309B 04/07/99 13 044617B 03/19/99 14 045857B 02/24/99 15 061158B 05/27/99 16 063860B 06/10/99 17 064725B 07/09/99 18 121115B 04/26/99 19 141509B 02/02/99 20 143311B 01/27/99 21 144318B 01/26/99 22 145544B 02/02/99 23 214571B 05/10/99 24 235930B 04/10/99 25 853659B 03/02/99

7:00 a.m. 6:40 p.m. 6:00 p.m. 8:00 a.m. 5:00 p.m. 10:00 a.m. 7:40 a.m. 2:40 p.m. 7:20 a.m. 5:40 a.m. 7:00 p.m. 6:00 a.m. 7:00 a.m. 2:00 p.m. 6:00 p.m. 5:00 p.m. 7:20 p.m. 7:20 a.m. 7:20 a.m. 7:40 a.m. 8:20 a.m. 5:20 p.m. 6:00 p.m. 9:00 a.m. 7:00 a.m.

HC DR/HC HC HC HC HC DR PA/DR/HC DR DR DR DR DR DR PA/DR/HC PA/HC/DR PA/HC DR/HC HC HC HC HC DR/HC DR HC

0.16 0.18 0.26 0.28 0.29 0.07 0.24 10.7 12.2 0.11 0.98 0.70 0.49 0.49 2.45 0.48 1.57 0.51 0.44 0.93 27.6 0.75 0.12 0.09 0.33

1.9800(9) 2.0287(3) 2.0614(10) 2.0748(9) 2.1020(8) 2.0145(12) 2.0313(16) 2.0543(20) 1.9319(5) 2.0513(6) 2.0068(5) 2.0409(8) 2.1095(9) 2.0397(2) 2.0390(3) 1.9783(9) 2.0496(2) 2.0159(6) 1.9700(5) 1.9771(7) 2.0989(2) 2.0607(4) 2.0559(8) 2.0593(7) 2.0131(9)

0.7999(2) 0.8268(2) 0.8429(2) 0.8620(2) 0.8598(2) 0.8131(3) 0.8245(3) 0.8386(4) 0.7743(1) 0.8276(3) 0.8168(1) 0.8314(2) 0.8587(2) 0.8314(1) 0.8318(1) 0.7944(2) 0.8384(1) 0.8171(2) 0.7950(1) 0.7969(1) 0.8649(1) 0.8408(1) 0.8424(2) 0.8399(3) 0.8145(2)

19.66(4) 18.92(5) 18.55(6) 18.01(4) 18.03(4) 19.52(8) 18.79(60) 18.70(2) 20.41(1) 18.86(1) 19.20(2) 18.84(6) 18.14(2) 18.82(1) 18.82(2) 19.87(5) 18.52(3) 19.27(3) 19.89(3) 19.74(2) 17.98(1) 18.55(2) 18.53(3) 18.88(17) 19.24(2)

Water samples from the same street 26 141294B 01/28/99 27 141424B 02/11/99 28 142528B 02/01/99 29 142654B 01/14/99 30 143123B 01/27/99 31 143579B 01/21/99 32 143738B 02/08/99 33 145369B 01/15/99 34 146002B 01/27/99 35 146754B 01/14/99 36 146768B 01/14/99 37 146901B 02/08/99

6:00 4:20 2:00 8:00 3:40 7:40 9:00 1:40 7:20 3:40 6:00 5:40

HC HC HC HC HC HC HC HC HC HC HC HC

0.07 0.65 6.4 3.85 2.64 0.78 2.99 1.55 1.00 0.70 0.69 0.37

2.0299(35) 2.0587(11) 2.0762(11) 2.0606(6) 1.9227(12) 2.0020(6) 1.9595(6) 1.9213(4) 2.0610(5) 1.9226(15) 2.0638(4) 1.9281(8)

0.8262(8) 0.8372(2) 0.8420(2) 0.8400(1) 0.7551(2) 0.8101(2) 0.7790(2) 0.7560(1) 0.8389(1) 0.7546(3) 0.8370(5) 0.7594(1)

18.86(10) 18.69(6) 18.60(2) 18.54(1) 20.99(4) 19.38(3) 20.25(3) 20.91(4) 18.60(3) 20.83(6) 18.71(8) 20.74(4)

20.9 2.10 0.20 0.19 1.40 0.34 0.20 0.10 20.8 0.26 1.09 1.57 1.50 1.06

2.0697(1) 2.0650(1) 2.0612(15) 2.0377(6) 2.0207(3) 2.0246(1) 2.0324(2) 2.0855(4) 1.9975(3) 2.0172(2) 2.0777(5) 2.0769(2) 1.9000(11) 1.8924(6)

0.8424(1) 0.8383(5) 0.8398(4) 0.8296(4) 0.8212(1) 0.8238(1) 0.8291(1) 0.8599(3) 0.8085(1) 0.8173(1) 0.8535(1) 0.8542(1) 0.7498(5) 0.7434(1)

18.52(1) 18.61(1) 18.59(4) 18.78(6) 19.06(2) 19.01(4) 18.86(4) 18.00(6) 19.50(4) 19.20(4) 18.26(3) 18.20(1) 21.06(1) 21.34(3)

Sample Loc. No.A

Field no.B

Sample dateC

a.m. p.m. p.m. a.m. p.m. a.m. a.m. p.m. a.m. p.m. a.m. a.m.

Water sample ‘‘pairs’’ (first-draw and second-draw water samples) 38a 141257A 01/12/99 6:00 a.m. HC 38b 141257B 01/12/99 1:00 p.m. HC 39a 216503A 05/27/99 6:00 a.m. DR 39b 216503B 05/27/99 6:20 a.m. DR 49a 146746A 01/20/99 4:00 p.m. HC 49b 146746B 01/20/99 4:20 p.m. HC 50a 062510A 07/09/99 5:40 a.m. PA/HC 50b 062510B 07/09/99 5:50 a.m. PA/HC 51a 145875A 12/30/98 4:00 p.m. HC 51b 145875B 12/30/98 4:10 p.m. HC 52a 046450A 03/10/99 3:20 p.m. DR 52b 046450B 03/10/99 3:30 p.m. DR 53a 142770A 01/20/99 7:00 a.m. HC 53b 142770B 01/20/99 7:20 a.m. HC

Pb/206PbF

Pb/206PbF

Pb/204PbF

Z. Cheng, K.A. Foland / Applied Geochemistry 20 (2005) 353–365

357

Table 1 (continued) Sample Loc. No.A

Field no.B

Sample dateC

Sample timeC

Water sourceD

PbE (lg/L)

208

Pb/206PbF

207

Pb/206PbF

206

Pb/204PbF

54a 54b

146563A 146563B

12/30/98 12/30/98

6:20 p.m. 6:40 p.m.

HC HC

0.50 0.71

1.9562(2) 2.0731(4)

0.7965(1) 0.8445(2)

19.62(2) 18.44(4)

Water samples from the Dublin Road Water Plant 40a Inlet 06/20/00 1 p.m. 40b Acol 06/20/00 1 p.m. 40c Asof 06/20/00 1 p.m. 40d Afil 06/20/00 1 p.m.

DR DR DR DR

0.108 0.06 0.017 0.034

2.0496(20) 2.0320(10) 2.0299(16) 2.0431(42)

0.8398(15) 0.8282(7) 0.8298(7) 0.8421(22)

-

Water samples collected from two residential houses 41a A-W1 07/15/00 7:00 a.m. 41b A-W2 07/15/00 7:03 a.m. 41c A-W3 07/15/00 11:00 a.m. 41d A-HW 07/15/00 11:05 a.m. 42a B-W1 07/15/00 6:00 a.m. 42b B-W2 07/15/00 6:03 a.m. 42c B-W3 07/15/00 6:10 a.m. 42d B-HW 07/15/00 6:03 a.m.

HC HC HC HC HC HC HC HC

0.45 0.48 0.16 0.25 0.79 0.078 0.064 0.057

2.0109(3) 2.0181(3) 2.0292(7) 2.0042(5) 2.0644(2) 2.0739(8) 2.0754(9) 2.0602(10)

0.8171(2) 0.8209(1) 0.8262(4) 0.8143(3) 0.8534(1) 0.8563(5) 0.8564(6) 0.8497(3)

19.12(4) 19.04(1) 18.72(9) 19.22(7) 18.23(2) 18.31(24) 18.31(6)

A Sample location number. The locations of residences where water samples were collected are shown in Fig. 1. ‘‘a’’ and ‘‘b’’ indicate different samples collected from the same residential house. Samples 40a, 40b, 40c and 40d were from the Dublin Road Water Plant. B Samples 012350B to 146563B were collected during the Columbus Division of Water ‘‘Tap Water Sample Collection Project.’’ Among them, samples 012350B to 853659B were picked randomly from all the water samples collected, samples 141294B to 146901B were from the same street (Royal Forest Blvd, Columbus, OH), and samples 141257A to 146563B were 8 pairs of ‘‘first-draw’’ and ‘‘second-draw’’ water samples. Samples ‘‘Inlet’’ to ‘‘Afil’’ were water from different stages of water treatment, with ‘‘Inlet’’ represent plant inlet, ‘‘Afil’’ represents plant effluent. Samples A-W1 to A-HW were collected in residential house A; samples B-CW1 to B-HW were from residential house B; -W1, -W2, -W3 were cold water samples, while -HW were hot water samples. C Collection time. The dates and time are when the samples were collected from kitchen tap. Normally within 1–2 days, 1 ml of concentrated HNO3 was added for preservation. D Water plant which supplied water for the residential house where sample was collected. Individual residential houses were located on a street map and compared to the water service area map provided by the Columbus Division of Water, to estimate water sources (Fig. 1). Those samples from the boundary area of two or more water plant supply areas may have water from one or the other or a mixture, as indicated by the water sources given. Here ‘‘DR’’ represents water from the Dublin Road Water Plant, ‘‘HC’’ represents water from the Hap Cremean Water Plant, and ‘‘PA’’ represents water from the ParsonÕs Avenue Water Plant. E Pb concentration is determined by isotope dilution with a 205Pb tracer. Analytical uncertainties (1r) for Pb concentrations are 0.5%. F Uncertainties in the last digit(s), given in parentheses, are two standard deviation of the mean for in-run statistics.

then flushed for about 5 min and a 60-ml ‘‘third-draw’’ sample (‘‘-W3’’) was collected. The first- and seconddraw samples were estimated to be waters residing in the domestic plumbing system, whereas the ‘‘thirddraw’’ sample was considered to represent water brought in from the outside. A hot water sample (‘‘HW’’) also was collected from each house in order to examine the possible effects of water temperature and the water heating apparatus on Pb. Before sampling, the hot water spigot in the kitchen was flushed for 3 min and then 60 ml of water was collected. The hot water sample represented water that had resided in the hot water tank. 2.1.2. Lead pipes and solders Lead pipes removed from various parts of the cityÕs water supply system were obtained from the Division

of Water. These pipes became available when Pb service lines were replaced during repairs. The surface layer of the Pb pipe sample to be analyzed was scrubbed and scraped to expose a clean surface and a very small piece (typically a few mg) of pipe metal was taken for analysis. For two pipes, multiple sub-samples were taken to check the homogeneity of Pb-isotope composition. Solder samples include those from the domestic plumbing systems of the two residential houses and a commercial electrical solder wire. From house A, multiple solder samples were taken from different parts of the water supply lines to ascertain any variations of Pb-isotope compositions of solders within a single house, and to evaluate their individual contributions to the Pb level of domestic water. All solder samples in house A were from solder joints that were made during original construction about 25 years ago.

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2.2. Analytical methods Because the water samples contain very small amount of Pb, a very low blank level is required. Therefore, the analysis followed stringent low-blank procedures in controlled clean environments. Processing of waters was performed in a clean room where chemical separations of Pb were conducted in a Class-10 hood. Lead pipe and solder samples were processed outside of the controlled, low-blank environment. All reagents and solutions used were prepared by sub-boiling distillation. Water (5–10 g) was spiked with a 205Pb tracer to enable the simultaneous determination of Pb-isotope ratios and Pb concentrations. Lead was separated and purified using a 100-ll Teflon column packed with anion-exchange resin (Bio-Rad AG1 · 8) in 0.55-M HBr medium (Schucker, 1992). For pipe and solder samples, several milligrams were dissolved in hot, concentrated HNO3 and then diluted with water to 100 ml. An aliquot containing about 1–2 lg of Pb was removed, dried, and analyzed for Pb-isotope composition without further treatment. Lead-isotope ratios were measured on a MAT-261A thermal ionization mass spectrometer equipped with 9 Faraday cups and an electron multiplier. Lead (typically 0.5–10 ng for water samples and 100 ng for pipe and solder samples) was loaded with silica gel and phosphoric acid onto a single Re filament. Lead-isotope ratios were measured using static multiple collection of all Pb isotopes and were corrected for isotopic fractionation using an empirical mass-dependent factor determined with the SRM-981 interlaboratory Pb standard. Because similar fractionation factors were observed for measurements with various amounts of SRM-981, an average correction factor of 1.1& per mass unit was applied to the Pb-isotope ratios of all samples. For smaller samples, the minimum uncertainties of the isotopic ratios are 0.07% and 0.035% for 208Pb/206Pb and 207Pb/206Pb ratios, respectively, due to uncertainties in mass fractionation correction, which are larger than in-run statistics. Because of the extremely small abundance of 204Pb, the 206Pb/204Pb ratio bears a much larger minimum uncertainty of 0.24%. For this reason, the 208Pb/206Pb and 207Pb/206Pb ratios are emphasized in the discussion that follows. Several water samples were analyzed in replicate. Determined Pb concentrations agreed within 0.5% and their Pb-isotope ratios were consistent within uncertainties. Lead concentrations measured by isotope dilution in a controlled low-contaminant environment showed generally good agreement with values determined using graphite furnace atomic absorption spectrometry at the Columbus Division of Water Quality Assurance Laboratory. The total procedural Pb blank for water sample was less than 50 pg, whose Pb-isotope composition was likely similar to water Pb.

Therefore, a correction to Pb concentrations and isotope ratios was unnecessary.

3. Results 3.1. Overall variations of Pb concentrations and Pb-isotope ratios The concentrations and isotope ratios of Pb of water samples are given in Table 1. Overall, Pb concentrations in the tap water samples analyzed are low and highly variable, with a mean of 2.15 (±5.15) lg/L and a median value of 0.49 lg/L. The concentration ranges from 0.017 to 28 lg/L, with 29% (n = 19) of samples having more than 1 lg/L Pb. Two of the 8 first-draw samples and one of the 45 second-draw samples exceed the current EPA 15-lg/L limit for Pb content. In addition, 3 samples contain between 5 and 15 lg/L Pb and 13 contain between 1 and 5 lg/L Pb. The Pb-isotope ratios of all water samples analyzed 208 are plotted in Pb/206Pb–207Pb/206Pb, 208 206 204 206 208 Pb/ Pb– Pb/ Pb and Pb/206Pb–Pb diagrams shown in Fig. 2. These Pb-isotope ratios show a large spread: from 1.89 to 2.12 for 208Pb/206Pb, from 0.74 to 0.87 for 207Pb/206Pb and from 18.0 to 21.3 for 206 Pb/204Pb. There is also remarkable variation in both Pb-isotope ratios and Pb concentrations for samples collected from houses along a single street (Fig. 3), which covers virtually the total ranges observed for all samples. Water samples supplied by the different treatment plants show no systematic differences. There is no apparent trend with time and there is no difference between samples collected at different times of day. Four samples obtained directly from the Dublin Road Water Plant have the lowest Pb concentrations (0.017–0.11 lg/L). There was a small reduction of Pb level during the water treatment process at this treatment plant, from 0.11 lg/L at the inlet to 0.03 lg/L at the outlet. The Pb isotopic compositions of all 4 samples are very similar, with a mean 208Pb/206Pb ratio of 2.03 ± 0.01 and 207Pb/206Pb ratio of 0.8350 ± 0.0007. 3.2. First-draw and second-draw samples The Pb concentrations and Pb-isotope ratios for the 8 pairs of first- and second-draw samples are plotted in Fig. 4. Lead concentrations in the 8 first-draw samples vary by two orders of magnitude (0.20–20.9 lg/L) and their isotopic compositions also show a large spread (e.g., 1.90–2.08 for the 208Pb/206Pb ratio). Second-draw samples are less variable in Pb concentration (0.10– 2.10 lg/L), but show virtually the same range of Pb-isotope ratios (1.89–2.09 for 208Pb/206Pb). The first-draw samples generally show higher Pb concentrations than the second-draw ones, whereas

Z. Cheng, K.A. Foland / Applied Geochemistry 20 (2005) 353–365

(a)

(a)

10

Pb (ppb)

10

Pb (ppb)

359

1 0.1

1 0.1

water at DRWP

0.01

204

5.6

0.88

(b)

Pb/206Pb

5.4 5.2 5.0

Pb/206Pb 207

0.80

0.72

4.6

0.84

0.84

0.76

4.8

0.88

(b)

207

Pb/206Pb (x 100)

0.01

1.88 1.92 1.96 2.00 2.04 2.08 2.12 2.16

(c)

208

Pb/206Pb

water at DRWP

Fig. 3. Concentrations and isotopic compositions of Pb in water samples collected from the same street. Lead isotopic data for the pipe samples (solid diamonds) are also shown for comparison.

0.80 0.76 0.72 1.88 1.92 1.96 2.00 2.04 2.08 2.12 2.16 208

Pb/206Pb

Fig. 2. Concentrations and isotopic compositions of Pb in water samples from Columbus. Water samples taken at the Dublin Road Water Plant itself in June 2000 are shown as hexagons; first-draw and second-draw samples are distinguished by fill (gray) for first-draw and open symbols for second draw. For houses A and B only the third (sequential) cold-water sample are shown as second-draw samples, with the first and second sequential cold-water samples treated as ‘‘first draw’’ samples. The different open symbols represent residential tap water supplied by different water plants. Water samples from near the borders of the service areas are shown with the convention of the estimated dominant source (see Fig. 1).

the Pb-isotope ratios can be similar or significantly different between the ‘‘first-draw’’ and ‘‘second-draw’’ samples. For 3 sample pairs (38, 49 and 51), the second-draw sample has a much lower Pb concentration but only slightly different Pb-isotope ratio than the first-draw sample; for pairs 39 and 50, both the firstand second-draw samples have low Pb concentrations, but their Pb-isotope ratios are slightly different; the other 3 pairs (52, 53 and 54), show slightly higher Pb concentrations (by 50%) in the second-draw samples.

3.3. Pipes and solders The Pb-isotope ratios of pipe and solder samples are given in Table 2. Multiple samples from the same pipe have virtually the same isotopic composition; therefore, each pipe appears to be approximately homogeneous. However, each pipe has a different Pb-isotope composition, although some differences are small. The Pb-isotope compositions define two very distinct pipe groups. One group is characterized by higher 208Pb/206Pb and 207 Pb/206Pb ratios (2.08–2.12, and 0.85–0.88, respectively), and lower 206Pb/204Pb ratios (17.79–18.33), indicating that the pipes of this group are made of relatively non-radiogenic Pb. The other group of Pb pipes is significantly more radiogenic, with lower 208Pb/206Pb and 207 Pb/206Pb ratios, and higher 206Pb/204Pb ratios in the ranges of 1.88–1.92, 0.73–0.77, and 20.68–20.80, respectively. There is very limited information about the sources of the pipes (e.g., supplier) in the Columbus system but at least two different sources are evident. The Pb-isotope ratios suggest that the non-radiogenic Pb was from Bunker Hill, Idaho and the radiogenic Pb was from the Tri-State Mining District (Missouri, Kansas and Oklahoma) based on the survey by Rabinowitz (2002). There is probably also a geographic distribution of these two Pb types in Columbus, but it appears that service lines from the same part of the city are not necessarily from the same Pb source. In Fig. 1, pipe samples

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8

0.1

6 39

4 49 54

38

Pb/206Pb

0.88

0.2

51

2 53

207

4.1. Lead levels in Columbus tap water

50

Pb (ppb)

1/206Pb (pmol/g)-1

4. Discussion (a)

52

0.5 1.0

(b)

0.84 0.80 0.76 0.72 1.88 1.92 1.96 2.00 2.04 2.08 2.12 2.16 208

Pb/206Pb

Fig. 4. Comparison of Pb concentrations and 208Pb/206Pb and 207 Pb/206Pb ratios for first- and second-draw samples. For each ‘‘pair,’’ the first-draw sample (filled gray symbols) is connected to the second-draw sample (open symbols). The isotopic data for Pb pipes (solid diamonds) are enclosed by ellipses and are shown for reference. The 208Pb/206Pb–1/206Pb diagram is plotted to show potential mixing relationship.

9, 11 and 12 belong to the more radiogenic Pb type, whereas samples 7, 8, 6 and 10 belong to the non-radiogenic type (see Table 2 for respective Pb-isotope ratios). Solder samples from two different houses and the commercial electric solder have Pb-isotope ratios that are distinct from each other and from the pipes. They have intermediate Pb-isotope ratios between the two types of pipe Pb. This is consistent with a major percentage of Pb used in the US since the 1960s being recycled (Graney et al., 1995). All the multiple solder samples from house A have a very similar isotopic composition, but each of the 7 is slightly but significantly different. 3.4. Water samples from two residential houses The Pb-isotope ratios and concentrations of both water and solder samples in two residential houses are illustrated in Fig. 5. The two houses have very different tap water Pb-isotope compositions although the total range observed within each house is restricted and similar to that of solder within the same house. Concentrations of Pb for cold water are the highest for the firstdraw sample and the lowest for the third-draw sample. For both houses, there are similar trends in Pb-isotope compositions and contents of water with sequential draw. Hot-water samples from both houses generally have lower Pb concentrations and different Pb-isotope compositions than cold-water samples.

Although all the houses studied were supplied by Pb service lines and most houses included other Pb-bearing plumbing materials (e.g., solder, brass fitting and domestic Pb pipes), the overall Pb concentration of the water samples analyzed is much lower than the current 15lg/L action level for drinking water by US EPA. While lower levels of Pb in drinking water has not received as much attention, this situation should change as more and more studies reveal adverse effects from Pb at even very low levels (e.g., Ernhart and Greene, 1990; Bellinger et al., 1991; Canfield et al., 2003; Selevan et al., 2003). The US EPA has established a non-enforceable goal of no Pb in drinking water; therefore, it is advantageous to have the capability to detect and understand Pb sources at lower levels. The low Pb levels in tap water suggest little leaching of Pb into the water, probably due to several factors such as high pH (8.0 for waters from both treatment plants), high hardness (averaging 110 mg/L for the Dublin Road Water Plant, 150 mg/L for the Hap Cremean Water Plant), an effective corrosion control strategy, and the ‘‘aging’’ of the water supply system by which films of deposits develop inside the pipes therefore reduce the contact of water with pipe Pb. The even lower Pb level in hot water from two residences may reflect coprecipitation of Pb with CaCO3 under elevated temperature. 4.2. Lead from water supply The treatment plants received water from the Scioto River, the Big Walnut Creek, and wells extracting groundwater from the Columbus Limestone aquifer. The Pb-isotope ratios of tap water and Dublin Road Water Plant water are significantly different and much less radiogenic than those of the Columbus Limestone, which is the dominant bedrock in the Columbus area. This bedrock has Pb-isotope ratios of 208Pb/206Pb = 0.1 to 1.5, 207Pb/206Pb = 0.1 to 0.6 and 204Pb/206Pb = 0.001 to 0.035 (Cheng et al., 1998; Cheng, 2001). The different Pb-isotope signatures indicate the bedrock limestone contributes little Pb to the overall water supply system. Instead, industrial input and contributions from overlying soil and glacial deposits may be much more important. Compared to samples from residential houses, the Pb levels in water samples from the Dublin Road Water Plant are very low. Although the treatment plant samples were collected significantly later than most residential tap water samples, the plant water Pb level probably has not changed significantly over this interval because of consistent water source and plant operations. Higher

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Table 2 Isotopic compositions of Pb in pipes and solders Sample no.a

208

Pb/206Pbb

207

Pb/206Pbb

206

Pb/204Pbb

Solder samples Solder-1 Solder-2 Solder-3 Solder-4 Solder-5 Solder-6 Solder-7 Solder-8 Solder-9

2.0502(1) 2.1124(1) 2.0098(2) 2.0092(1) 2.0148(1) 2.0008(1) 2.0043(1) 2.0177(15) 2.0153(1)

0.8475(10) 0.8757(10) 0.8135(10) 0.8164(10) 0.8187(10) 0.8114(10) 0.8122(20) 0.8180(30) 0.8176(10)

18.404(7) 17.765(2) 19.733(4) 19.155(3) 19.089(2) 19.301(2) 19.323(12) 19.239(16) 19.214(3)

Lead pipe samples Pipe-01 Pipe-02 Pipe-02B Pipe-02C Pipe-03 Pipe-03B Pipe-04 Pipe-05 Pipe-06 Pipe-07 Pipe-08 Pipe-09 Pipe-10 Pipe-11 Pipe-12

1.9114(1) 1.9143(11) 1.9117(2) 1.9119(1) 1.9105(2) 1.9123(3) 2.1145(2) 2.1290(3) 2.0880(2) 2.0990(1) 2.1145(4) 1.9065(2) 2.0961(4) 1.9180(1) 1.8841(2)

0.76452(3) 0.76625(23) 0.76586(5) 0.76584(2) 0.76402(4) 0.76435(7) 0.86951(4) 0.87235(6) 0.85539(4) 0.86490(1) 0.87197(8) 0.76413(4) 0.85131(8) 0.76770(17) 0.73922(4)

20.773(2) 20.735(13) 20.706(3) 20.706(2) 20.779(3) 20.800(4) 17.902(3) 18.022(3) 18.192(1) 17.910(1) 17.797(3) 20.700(3) 18.327(4) 20.683(9) 21.449(3)

a Pipe samples were from various places in the municipal water supply system of Columbus, Ohio (see Fig. 1). Sample Solder-1 was from residential house B. Solder-2 was a commercial electrical solder wire. Samples Solder-3 to -9 were from various places of the plumbing system of residential house A. The solders contained 50% of Pb. The pipes are 3/4 in. (1.905 cm) in diameter, except for Pipe-12 with a diameter of 1/2 in. (1.27 cm). b Uncertainties in the last digit(s), given in parentheses, are two standard deviation of the mean for in-run statistics.

tap water Pb levels suggest that most Pb was obtained by the water after it left the treatment plant. In addition, the large spread of isotopic ratios for samples supplied by the same treatment plant suggests that there is not likely to be a uniform Pb source, such as the plant supply. Furthermore, assuming that waters from a treatment plant have relatively uniform Pb-isotope composition (as implied from the 4 samples from the Dublin Road Water Plant) and that the water plants are distinct in terms of Pb signature, the similar Pb concentration and isotopic variability for water supplied from the Dublin Road and Hap Cremean plants also indicate that tap water Pb is not controlled by water supply (see Fig. 2). 4.3. Isotopic relationship between tap water Pb and service line Pb Theoretically, the Pb in pipes could be mobilized and released into water due, in part, to the oxidation of ele-

mental Pb to Pb(II). Water in the supply system is constantly refreshed and, thus, carries appreciable O2 that may facilitate the oxidation. The second-draw samples collected in the ‘‘Tap Water Collection Project’’ represent water that was in contact with the Pb service lines for an extended period of time. In Fig. 6, the data for the majority of water samples plot between the two relatively tight clusters for pipe Pb, although some water samples are within the clusters themselves. For a specific water sample, if a type of pipe were the dominant Pb source, the sample should have that type of Pb or at least be very close to the field defined by that Pb type in the plots. Therefore, it is likely that, for most water samples the dominant proportion of Pb came from neither of the two pipe Pb types. In Fig. 6, 3 of the 4 second-draw samples with Pb concentrations above 5 lg/L (samples 8, 9 and 28) fall between the two pipe (‘‘R’’ and ‘‘NR’’) clusters, with the other (sample 21) within the ‘‘NR’’ field. Samples 9 and 28 are relatively close to one of the clusters, which could suggest

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Z. Cheng, K.A. Foland / Applied Geochemistry 20 (2005) 353–365 0.05

(a)

H 3

House B

12

2

0.1

8 House A 3

4

0.2

H 2

1

1

204

Pb/206Pb (x 100)

5.5 5.4 5.3

Pb/206Pb

1.0

1 H

(b)

3

House B

3

House A 1

5.2

2

H

5.1

0.86

207

Pb (ppb)

1/206Pb (pmol/g)-1

16

House B

(c)

1

2

3

H

0.84 House A

0.82

H

1

3

2

0.80 2.00

2.02

2.04

2.06

stalled in two stages (K. Button, personal communication) in the 1940s when only a few Pb suppliers were available; therefore it is also likely that only two sources of Pb were involved, each having distinct Pb-isotope compositions. It is also conceivable, however, that the tap water Pb was generated through variable contributions from two types of pipes, thereby, producing intermediate Pb-isotope ratios with a large spread. The general linear array of the second-draw waters approximately between the ‘‘R’’ and ‘‘NR’’ pipe groups would be consistent with a simple binary mixing of Pb from the two types. The mixing scenario is considered very unlikely if water entering each home goes through one service line. The service line for an individual home is not likely to have more than one type of Pb, therefore it would be expected that one type of pipe Pb would dominate for a given water sample, with waters tending to cluster near one or the other pipe fields (Figs. 3 and 4). The wide range of water Pb-isotope compositions, and the limited number of samples with compositions near the pipes, argue against this supposition. In an attempt to further address this issue, waters from several residences on the same street were ana-

2.08

Pb/206Pb

204

Fig. 5. Pb concentrations and isotopic compositions of water and solder samples from residential houses A and B. Water samples are identified as: ‘‘H’’ – hot water, ‘‘1’’ – first-draw, ‘‘2’’ – second-draw and ‘‘3’’ – third-draw. The third-draw for each series, shown as unfilled open symbols, is for a sample that was presumably drawn from water mains in the supply system. Solder samples are shown as solid triangles. Note that the scales for isotopic ratios are greatly enlarged compared to those of other illustrations that show waters from many residences.

Pb/206Pb (x 100)

208

21

28

5.4

House A

NR House B

5.2 5.0

8

9 R

all water

4.8 4.6 (b)

28

21

e

DRWP

0.84

NR

House A House B

0.80

207

Pb/206Pb

0.88

significant proportions of Pb in these two samples were indeed from pipe Pb. Sample 8 is not close to either of the two clusters. The isotopic data for samples with Pb concentrations between 1 and 5 lg/L (samples 15, 17, 29, 30, 32, 33, 34, 38b, 52b and 53b) do not lie within the fields representing the compositions for pipes. Considering the limited number of Pb service line pipes analyzed, it is also possible that other types of pipe Pb exists in the system. If such sources had intermediate Pb-isotope ratios relative to the two Pb types observed in this study, this would complicate the source fingerprinting for water Pb. However, Rabinowitz (2002) identified 3 distinct Pb-isotope compositions in refined Pb, two of which correspond to those observed in this study for Columbus domestic water. Thus, service lines are not expected to have the rather continuously variable Pb- isotope ratios observed for the water samples. Furthermore, the Pb service lines in Columbus were in-

e

(a)

5.6

0.76

9

8

R

all water

0.72 1.88 1.92 1.96 2.00 2.04 2.08 2.12 2.16 208

Pb/206Pb

Fig. 6. Comparison of the isotopic ratios of Pb for water with those for pipe and solder. All open symbols are water whereas solid ones are pipes or solders (see Fig. 2 for explanation). Only second-draw water samples are shown, with 4 samples containing >5 lg/L Pb indicated by respective sample numbers. The two distinct groups of pipe Pb isotopic compositions are noted as ‘‘R’’ and ‘‘NR’’ as discussed in the text. Solders include a sample of commercial electric solder (‘‘e’’), a pipe joint in house B, and several plumbing solder joints of house A.

Z. Cheng, K.A. Foland / Applied Geochemistry 20 (2005) 353–365

lyzed, assuming a similarity in the makeup of the distribution system. Although it is possible that more than one type of Pb pipe is present along this street, it is likely that all the residences received water that flowed through the same or similar pipes. The idea is that the Pb in waters from these residences should be of similar isotopic composition if the Pb comes from a common pipe source(s). As observed for the full suite of waters, there is remarkable variation in both Pb-isotope ratios and Pb concentrations for this single street, covering virtually the total ranges observed (see Fig. 3). Even the 6 samples with higher Pb levels (>1 lg/L; samples 28, 29, 30, 32, 33, 34) show a wide range of Pb-isotope ratios. These suggest significant, variable contributions of Pb from sources beyond the shared public pipe system. They point to Pb sources in the ‘‘end-of-the-line’’ plumbing systems of the individual houses. In summary, although there may be some tap waters that contain significant Pb contributions from the Pb service lines, their overall contribution appears to be low. All the pipes that were available showed a substantial layer or ‘‘coating’’ (dominantly carbonate deposits?) on the inside wall. The ‘‘coating’’ must have accumulated over many years and should shield pipe Pb from the constantly refreshed water. It may have been critical in preventing pipe Pb from being leached and released into the water. In addition, a Zn-orthophosphate (5:1 PO4 to Zn) corrosion inhibitor is added to Columbus water. This inhibitor serves to facilitate the formation of a thin film (Zn hydroxy-carbonate, etc.) over the pipe surface, thus reducing exposure. 4.4. Lead sources in domestic plumbing systems In a domestic plumbing system several types of materials could release Pb into water. First, in the 8 houses where Pb pipes were observed and first-draw samples were collected, those pipes could be a predominant source for water Pb. Second, before 1987, solders used in plumbing systems contained significant amounts of Pb, and even today solders containing as much as 0.2% Pb are permitted in the US. The Pb-bearing solder could be in direct contact and, therefore, release Pb into water. This release may be influenced by the amount of oxidized Pb, and accelerated by a galvanic action. In addition, some faucet assemblies and fixtures are also problematic sources of Pb, as shown by Schock and Neff (1988) and Gulson et al. (1994). A heating element in an electric hot-water heater could also release Pb into water in a hot-water tank. Other materials, even those with low Pb contents, may contribute to water Pb as well. With a very wide range of possible materials, it was impractical and beyond the scope of this study to distinguish all their Pb-isotope compositions. The influence of domestic plumbing systems on water Pb is clearly shown by comparing the Pb-isotope

363

compositions and Pb concentrations of the first- and second-draw samples. The first-draw samples presumably bear effects of domestic plumbing systems while the second-draw samples would have very short residence time in the house and, thus, have smaller (or be free of) impacts from domestic plumbing systems. In general, the Pb-isotope compositions for first- and second-draw samples are similar, compared to the wide range for tap waters overall. The decrease in Pb content from the first-draw to the second-draw suggests that most Pb comes from sources in the house plumbing network. The similarity of the first- and second-draw samples in Pb-isotope composition indicate nearly the same Pb source(s), suggesting that even the Pb in the second-draw sample also comes from domestic sources. The implication is that some Pb is available from domestic source(s) even when the tap was flushed to get the water from outside of the house, which is consistent with observations by Gulson et al. (1994) and Murphy (1993). This could be due to some water mixing or release of Pb from the faucet when opened or, to the general distribution of Pb within the residential system. 4.5. Isotopic similarity of tap water and solder Pb Compared to the overall spread among waters, the Pb-isotope compositions of 3 cold tap water samples in each house are similar to each other, but differences and trends are also apparent (see Figs. 5 and 6). At house A, the first- and second-draw samples have similar Pb concentrations but the latter have higher 208Pb/206Pb, 207 Pb/206Pb and 204Pb/206Pb ratios. The third-draw sample has higher Pb-isotope ratios, but lower Pb. These values imply contributions from two different Pb sources within the domestic plumbing system, with less control by domestic Pb for the third-draw. The linear arrays on the Pb-isotope diagrams are consistent with the Pb being a mixture of two components. Logically, one Pb source is the house plumbing system and the other may be the supplied water. The Pb concentrations suggest the major source of Pb would be ‘‘local.’’ At house B, the first-draw sample has higher Pb with lower Pbisotope ratios, and the second-draw and third-draw samples are very similar in Pb content and isotopic composition. This probably reflects only one important point source that controlled Pb in the first-draw sample, while the second- and third-draw samples were less influenced by Pb in the house. The lower Pb concentrations in hot water may reflect Pb removal by co-precipitation with carbonates in the hot water tank, or could reflect that most of the Pb was picked up locally in the plumbing between the hot water tank and the tap. Comparisons of Pb-isotope compositions between the solder and the cold water samples reveal strong connections of solder Pb with water Pb. In Fig. 5, the

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first-draw sample is closer to the solder sample(s) than the second-draw sample, which is closer to solder sample(s) than the third-draw sample. The Pb-isotope resemblance and the trends suggest that the Pb that strongly influenced the water Pb composition is likely to be predominantly from these solders. In house A, multiple solder samples are slightly different in their Pb-isotope composition. Ultimately such a difference might be used to infer which solder is responsible for the dominant Pb in a tap water sample.

5. Conclusions This study shows that in the Columbus municipal supply system, tap water Pb concentrations and isotopic compositions are highly variable, while the Pb-isotope compositions of service line Pb are distinct and show very limited variation. Therefore, there is a realistic possibility of using Pb isotopes to trace water Pb sources in a large and complex municipal system. Based upon limited Pb-isotope results, several lines of evidence suggest that the major sources of tap water Pb are the domestic plumbing systems rather than source water (from treatment plants), the distribution network, or the Pb service lines. In several relatively high Pb concentration second-draw samples, pipe Pb could be dominant. It would be very useful to obtain service line samples from these homes in order to look for a match of pipe and water Pb. A strong influence of residential plumbing systems is further suggested by changes in Pb concentrations and Pb-isotope ratios for sequentially drawn samples. Replacing water that has sat in the domestic system with that newly replenished commonly reduces the Pb level. This supports the conclusion of some other studies and demonstrates the wisdom of flushing the tap before taking water for consumption. However, this practice does not appear to eliminate all the Pb introduced locally, based upon isotopic compositions. The limit on Pb levels may well be reduced in the future and the issue of Pb pipes may need to be addressed considering a US EPA Maximum Contaminant Level goal of 0 lg/L in drinking water. The results suggest that, with proper water chemistry and corrosion control strategy, the use of existing widespread Pb pipes may not be a significant contributor of Pb to drinking water. With strategically devised sampling, Pb isotopes should be capable of defining in detail the extent of pipe contribution to the Pb in water supplied to residences. The results also show that Pb sources from individual houses, such as Pb solder, are probably more important issues that should be closely examined. Further Pb isotope investigations based on this study could contribute both to identifying specific Pb sources and to policy decisions as to the need to remove old Pb pipes.

Acknowledgments The authors thank Jeffrey S. Linder for maintaining the mass spectrometer and technical assistance particularly in analyzing small amounts of Pb, and also programming and preparation of graphic illustrations. We thank the Columbus Division of Water and Jeffrey A. Hubbard, Administrator, for cooperation and access to their water samples that made the study possible. In particular, we are indebted to Dr. Kenneth Button for invaluable cooperation and collaboration with this research. Comments and suggestions from Russell S. Harmon, Paul D. Fullager, Rob Ellam, William Manton, and an anonymous reviewer improved the presentation and are gratefully acknowledged. All Pb isotopic analyses were performed in the Radiogenic Isotopes Laboratory of Ohio State University where the mass spectrometer used was funded, in part, by NSF Grant EAR-8419126 and partial technical support was provided by NSF Grant EAR-0137546 (P.I., K.A. Foland).

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