Lead isotopic analyses of selected soil samples from the USEPA 

Vasquez Blvd.-I-70 study area, Denver, Colorado 

By Daniel M. Unruh1 

Open-File Report 02-321 

2002 

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial 

standards and stratigraphic nomenclature. Any use of trade, product, or firm names is for descriptive purposes 

only and does not imply endorsement by the U.S. Government 

U.S. Department of the Interior 

U.S. Geological Survey 

1

MS 963, Box 25046, Federal Center, Denver, Colorado 80225 

1

Table of Contents 

Chapter Page 

Introduction 5


Analytical Procedures 6


Chemical Procedures 6


Quality Assurance and Quality Control 6


Chain of Custody (COC) 7


Analytical Results and Discussion 7


Lead Isotopic Compositions 7


Sources of Lead 17


Two-Component Mixing 21


Three-Component Mixing 25


Focal versus Adjacent Samples 29


Soils from a Single Property 29


Conclusions 35


Suggestions for Future Work 36


Answers to Questions Posed by USEPA 37


References cited 38


Appendix 1. Standard analytical procedures for lead


isotopic analyses (nitric acid-hydrogen peroxide


digestion) of metal-rich samples. 40


Appendix 2. Quality Assurance and 

Quality Control (QA and QC) 43


Reagent Preparations and Analytical Blanks 43


Mass Spectrometry Calibrations 44


Mass Spectrometry Standards 45


Data Reduction 48


Duplicate analyses 50


Method Determination Limit


(Practical Quantitation Limit) 54


2

List of Figures 

Figure 1. 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb 

diagrams for selected soil samples from the Vasquez Blvd.-I70 

study area, coded by Pb concentrations.
 16 

Figure 2. 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb


diagrams for selected soil samples from the Vasquez Blvd.-I70


study area, coded by As concentrations. 18


Figure 3 As vs. Pb concentrations for selected samples from the


VBI70 study area. 19


Figure 4. 206Pb/204Pb vs. Pb concentration (A) and 206Pb/204Pb vs.


1/Pb concentration for selected soil samples from the VBI70 study area. 22


Figure 5. 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb


diagrams for focal vs. adjacent samples from the VBI70 study area. 30


Figure 6. 206Pb/204Pb vs. 1/Pb concentrations for focal vs. adjacent


samples from VBI70 study area. 31


Figure 7. 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb


diagrams for samples from a single residential property. 33


Figure 8. 206Pb/204Pb vs. 1/Pb concentrations for samples from a single


residential property. 34


Figure A2-1. Percent Standard Error in 206Pb/204Pb vs. 204Pb beam


intensity for seven analyses of NIST standard SRM 981. 57


3

List of Tables 

Table 1. Sample numbers, sample types, and lead and arsenic


concentrations in selected samples from the VBI70 study area.


Table 2. Lead isotopic compositions in acid digestions of selected


soil samples from the VBI70 study area.


Table 3. Sources of lead in residential samples from the VBI70


study area.


Table 4. Two-component mixing parameters for selected soil


samples from VBI70 study area.


Table 5. Three-component mixing calculations for selected soil


samples from the VBI70 study area.


Table A2-1. Lead isotopic analyses of NIST standard SRM 981.


Table A2-2. Lead isotopic analyses of NIST standard SRM 982.


Table A2-3. Lead isotopic compositions from replicate runs of


Sample 3-15642-B, Vasquez Blvd.-I70 Project.


Table A2-4. Lead isotopic data for replicate acid-digestions of


selected soils samples from the Vasquez Blvd.-I70 Project.


Table A2-5. Summary of results for determination of the method


determination limit (practical quantitation limit).


9 

10 

20 

24 

27 

46 

49 

51 

52 

56 

4

Introduction 

Large amounts of arsenic contamination, in excess of 2000 ppm, have been found in 

some residential surface soils in northern Denver, Colorado (USEPA, 1999). Associated with 

the arsenic are elevated levels of metals such as lead, cadmium and mercury. Potential sources 

of this contamination include waste from smelters in the area and commercial herbicide 

containing arsenic trioxide and lead arsenate (USEPA, 1999). As a part of the U.S. 

Environmental Protection Agency’s Vasquez Boulevard-I-70 Project (VBI70 project; USEPA, 

1999), lead isotopic analyses of selected soil samples, smelter waste, and a commercial 

herbicide and a commercial lead arsenate pesticide have been performed in order to constrain 

the possible sources of elevated lead, and by proxy arsenic, found in these residential soils. 

The isotopes of lead can be used as a natural tracer in determining sources of 

contaminants in soils and stream sediments (e.g. Church and others, 1997; Fey and others, 

1999; Unruh and others, 2000). The element lead (Pb) consists of four naturally occurring and 

stable isotopes, 204Pb, 206Pb, 207Pb, and 208Pb. Three of these, 206Pb, 207Pb, and 208Pb are 

radiogenic decay products of naturally occurring 238U, 235U, and 232Th, respectively. Owing to 

variations among the U/Pb and Th/Pb elemental abundance ratios among different types of 

geologic materials, variations in the relative abundances of the Pb isotopes are produced 

among different materials during geologic time. 

In order for the Pb isotope method to be useful for determining possible contaminant 

sources, two important criteria must be met (e.g. Church and others 1997): (1) It must be 

possible to obtain and measure the Pb isotopic compositions in all potential contaminant 

sources and in uncontaminated material and (2) the Pb isotopic compositions of the potential 

contaminant sources must be distinctly different from one another and from uncontaminated 

background. Variations among the Pb isotopic ratios in selected samples can be correlated with 

potential sources of Pb with known isotopic compositions in order to determine the relative 

contribution of Pb from a specific source to the sample. 

The purpose of this study is to use Pb isotopic data obtained from a selected subset of 

samples to determine the Pb isotopic signatures of uncontaminated soils as well as those of 

potential sources of contamination. The data will eventually be used in conjunction with other 

geochemical data to help determine which source or sources of contamination are most likely 

responsible for the elevated arsenic levels in the residential soils. 

5

Analytical Procedures 

Chemical Procedures 

Lead isotopic analyses of natural materials are obtained using anion-exchange, solidliquid chromatography and thermal-ionization solid-source mass spectrometry. Samples were 

received as sieved soils from USEPA. Samples were digested in nitric acid and hydrogen 

peroxide in accordance with Section 7.2 of method 3050B, EPA SW 846, revision 2 

(USEPA, 1992). Ion exchange and mass spectrometry procedures are similar to those 

reported by Tatsumoto and Unruh (1976) and Fey and others (1999) with minor modifications 

to accommodate a larger sample size and the metal-rich nature of the samples. A detailed 

description of the chemical procedures may be found in Appendix 1. 

Samples were loaded onto single rhenium filaments using the conventional silica gelphosphoric acid technique (e.g. Tatsumoto and Unruh, 1976). Mass spectrometry was 

performed using a VG Sector-54, 7-collector, solid source, thermal-ionization mass 

spectrometer operated in “static” mode. Data were acquired with the “Sector 54, version 

3.974” software package provided by the manufacturer of the mass spectrometer. All samples 

and standards were run using collectors L1, H1, H2, and H3 for 204Pb, 206Pb, 207Pb, and 208Pb, 

respectively. 

Quality Assurance and Quality Control (QA and QC) 

Samples were initially submitted by USEPA to the analyst without context. Although 

Pb contents were provided so that the appropriate amount of sample could be processed, 

the nature of the sample (soil, herbicide, smelter waste, etc.) and the arsenic contents in the 

samples remained unknown to the analyst until after the first draft of this report was completed. 

Additional information was then provided so that this report could be completed in a 

comprehensive manner. 

After the first draft of this report was submitted to USEPA for review, fourteen additional 

residential soils were selected for analyses. Samples were received by the analyst in April 

and May of 2001. These samples were selected jointly by USEPA and USGS personnel to 

encompass the complete ranges of lead and arsenic contents within the soils. Included within 

these additional soils were five samples from a single residential property. These were 

selected in order to test the validity of simple mixing models. 

Quality Assurance and control procedures and results are provided in Appendix 2. The 

principal source of analytical uncertainty in lead isotopic analyses is from correction for mass 

fractionation induced during mass spectrometry. Mass fractionation was monitored using NIST 

6

standard SRM-981 (Cantanzaro and others, 1968; Todt and others, 1993). A second NIST 

standard, SRM-982 (Cantanzaro and others, 1968; Todt and others, 1993) was measured as 

a check of the correction procedures (Appendix 2). 

Two Pb blanks were determined during the course of this study, one for the acid and 

hydrogen peroxide digestion procedure alone and one for total chemistry of a 1 ml aliquot from 

the acid digestion (Appendix 2). The Pb blank for the acid digestion was 0.3 ng (10-9 g) and 

that for the total chemistry was 0.09 ng. These values correspond to less than 0.004% and 

0.02% of the lead in the total samples and solution aliquots, respectively. Consequently, no 

blank correction was applied to the isotopic data (see Appendix 2). 

Three types of replicate samples were analyzed; (1) Blind duplicates submitted by 

USEPA, (2) Replicate acid-hydrogen peroxide digestions of the same (digestion replicates), 

and (3) Replicate loads of the same sample from a single chemical procedure (mass 

spectrometry replicates). Results for the replicate analyses are presented in Table 2, and 

Appendix 2 (Tables A2-3, and A2-4). 

Chain of Custody (COC) 

Samples were handled in accordance with U.S. Geological Survey chain-of-custody 

procedures (Murphy and others, 1997). Samples were received from the USGS sample 

control custodian in a sealed box by the analyst and were hand-carried to the laboratory where 

they remained for the duration of the study. The laboratory is secured with a combination lock 

with a combination known only to the analyst. Individual samples were enclosed in bags 

sealed and dated by USEPA. The analyst found all seals to be intact upon receipt. 

The mass spectrometer used to acquire data was secured with two padlocks to lock the 

sample access door. The computer that was used to reduce the data was also secured with a 

lock supplied by the manufacturer that prevented access to the keyboard. Both the mass 

spectrometer and computer were kept locked at all times during the study and all keys were 

retained exclusively by the analyst. 

Analytical Results and Discussion 

Lead Isotopic Compositions 

A total of thirty-four selected samples were analyzed for lead isotopic compositions. 

These included twenty-six residential soils (plus two blind duplicates), one procedural blank, 

two soils from a local smelter, and one sample of smelter waste from the same smelter. Also 

analyzed were a sample of “PAX”, an arsenic trioxide plus lead arsenate herbicide that was 

7

commercially available during the 1950’s through the early 1970’s, and a sample of Acme 

brand lead arsenate pesticide that was also commercially available during this time frame. 

Sample numbers and sample types as well as lead and arsenic concentrations for the 

samples are summarized in Table 1 (USEPA, written communication, 2000, 2001). 

Residential soil samples are listed high-As, intermediate-As or low-As and as either “focal” or 

“adjacent”. Focal samples are those that were collected from a specific property, whereas 

adjacent samples were collected from property adjacent the specific property. Adjacent 

samples all show relatively low As levels (Table 1). 

Lead isotopic data for the samples are shown in Table 2. Each entry represents a 

separate mass spectrometer run. Entries with identical sample numbers are mass 

spectrometry replicates measured from a single processed aliquot of sample. Entries with the 

same sample number but different suffix letters (e.g. 642B-A, 642B-B, 642B-C) are 

digestion replicates obtained from separate acid and hydrogen peroxide digestions of the 

sample. 

Both the raw data and data corrected for mass fractionation are shown. The precision for 

the raw data is given as the percent standard error (1 S.E. ≈ 1 sigma-mean) calculated by the 

on line data-acquisition software. Analytical uncertainties for the corrected data are shown as 

absolute values and are calculated at the 95% confidence interval (C.I., Ludwig, 1994). 

Weighted mean values, also calculated at the 95% C.I., are shown for the mass spectrometry 

replicates and, where appropriate, grand mean values are calculated for each sample from the 

means of the digestion replicates. The mean values for the individual samples are shown in 

bold type in Table 2. 

The lead isotopic data are summarized on conventional 206Pb/204Pb vs. 207Pb/204Pb and 

206Pb/204Pb vs. 208Pb/204Pb diagrams in Figure 1. Each data point represents the mean value 

for an individual sample (Table 2) and data are coded according to their Pb contents (Table 1). 

The data yield moderately well correlated linear arrays with r2 = 0.854 and 0.812 for the 

206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb diagrams, respectively 

(unweighted; r2 is defined by the plotting software, Deltagraph 4.0, as the square of the 

regression correlation coefficient). Also shown by error ellipses in Figure 1 are typical analytical 

uncertainties for a single mass spectrometer run (± 0.09% for 206Pb/204Pb, ±0.13% for 

207Pb/204Pb, and ±0.18% for 208Pb/204Pb). 

Samples with the lowest Pb contents are characterized by radiogenic Pb (206Pb/204Pb > 

18.4) and slightly elevated 208Pb/204Pb relative to the other samples. Residential soils with the 

highest Pb contents form two distinct clusters; one with 206Pb/204Pb > 18.5 and one with 

206Pb/204Pb < 18.0. The data from the remaining residential soils plot between the extremes 

defined by the low-Pb and two high-Pb groups. 

8

Table 1. Sample numbers, sample types, and lead and arsenic concentrations 

in selected samples from the VBI70 study. 

Sample Number EPA Code No. Sample type Location1 As ppm2 Pb ppm3 

3-15639-B 3-15669-R


3-15641-B SC-00006


3-15642-B SC-00010


3-15643-B SC-00011


3-15644-B SC-00017


3-15645-B SC-00018


3-15646-B SC-00023


3-15647-B SC-00025


3-15648-B SC-00027


3-15649-B SC-00039


3-15650-B SC-00046


3-15651-B SC-00046


3-15652-B SC-00048


3-15653-B SC-00065


3-15654-B SC-00065


3-15655-B SC-00069


3-15656-B SC-00084


3-15657-B SC-00104


3-15658-B SC-00099


3-15659-B


Samples received 4/01-5/01


3-13569-B


3-13989-B


3-02246-B


3-10319-B


3-12524-B


3-12820-B


3-01182-B


3-00748-B


3-10740-B


Acme brand Lead Arsenate


High-As residential soil


Intermediate As residential


Smelter Site soil


Low As residential soil


Smelter Site soil


Smelter material


Low As residential soil


Intermediate As residential


Low As residential soil


High-As residential soil


Duplicate


High-As residential soil


Baseline soil


Duplicate


High-As residential soil


Intermediate As residential


High-As residential soil


PAX


Prep. Blank


High-As residential soil


High-As residential soil


Low As residential soil


Low As residential soil


High-As residential soil


Intermediate As residential


Low As residential soil


Intermediate As residential


Low As residential soil


Samples from a single residential property


3-12152-B Low As residential soil


3-12040-B Low As residential soil


3-12039-B Low As residential soil


3-12035-B Low As residential soil


3-12151-B Intermediate As residential


273000 128000 

Focal 8 3 6 1 4 9 0 

Focal 1 7 3 4 2 2 

22400 4 5 2 0 

Adjacent 9 3 9 

27200 5 6 6 0 

7 6 3 0 2 1 6 0 

Focal 2 4 1 2 6 

Focal 1 6 5 1 1 5 

Focal 2 5 9 2 

Focal 1 4 9 0 1 5 6 0 

1 4 9 0 1 5 6 0 

Adjacent 3 5 2 6 1 

Focal 7 1 0 1 

7 1 0 1 

Adjacent 1 5 1 5 1 

Adjacent 2 3 1 7 5 

Focal 1 4 9 0 1 1 4 0 

120000 24300 

n.d. n.d. 

Focal 1 4 9 2 4 0 9 

Focal 9 0 4 3 0 2 

Focal < 1 1 1 6 5 8 

Focal 4 8 1 4 6 9 

Focal 1 1 3 5 7 9 4 

Focal 2 4 2 6 0 7 

Focal 1 4 7 2 0 

Focal 4 0 9 6 3 3 

Focal 2 3 9 8 4 

Focal < 1 1 1 7 3 0 

Focal < 1 1 2 5 6 

Focal 1 5 4 1 3 

Focal 2 6 8 2 4 

Focal 2 2 1 5 4 0 

1

Focal samples are those collected at the site indicted by the sample type. Adjacent samples are those 


collected adjacent to the indicated sample type.


2

 As concentrations provided by USEPA (written communication, 2001).


3

 Pb concentrations provided by USEPA (written communication, 2000; 2001).


9 
Table 2. Lead isotopic compositions in acid digestions of selected soils samples from the Vasquez Blvd.-I70 Project. 

Raw Data1 Corrected for mass fractionatio

Lab No/EPA No.3 Pb ppm4 Type 206Pb/204Pb % S.E. 207Pb/206Pb % S.E. 208Pb/206Pb % S.E. 206Pb/204Pb 207Pb/206Pb 208Pb/206Pb 

Mean 644B A-C 3-15639-B Pb Arsenate 3-15669-B 639B 18.0453 0.0016 0.860891 0.0007 2.10038 0.0010 18.075 ± 0.016 0.86158 ± 0.00037 2.1037 ± 0.0018 639B 18.0497 0.0027 0.860973 0.0010 2.10094 0.0018 18.079 ± 0.016 0.86166 ± 0.00037 2.1042 ± 0.0018 639B 18.0303 0.0020 0.860519 0.0008 2.09888 0.0016 18.060 ± 0.016 0.86120 ± 0.00037 2.1021 ± 0.0018 Mean 128000 18.071 ± 0.009 0.86148 ± 0.00044 2.1033 ± 0.0010 3-15641-B Res. Soil SC-00006 641B 17.8760 0.0030 0.867917 0.0007 2.10752 0.0009 17.906 ± 0.016 0.86861 ± 0.00037 2.1108 ± 0.0018 641B 17.8998 0.0072 0.868367 0.0031 2.10999 0.0064 17.929 ± 0.017 0.86906 ± 0.00038 2.1133 ± 0.0019 641B 17.8656 0.0033 0.867646 0.0008 2.10613 0.0006 17.895 ± 0.016 0.86834 ± 0.00037 2.1094 ± 0.0018 Mean 1490 17.910 ± 0.039 0.86867 ± 0.00083 2.1112 ± 0.0046 3-15642-B Res. Soil SC-00010 642B-A 18.2064 0.0031 0.854041 0.0014 2.09149 0.0024 18.236 ± 0.017 0.85472 ± 0.00037 2.0947 ± 0.0018 642B-A 18.2260 0.0064 0.854359 0.0023 2.09344 0.0053 18.256 ± 0.017 0.85504 ± 0.00037 2.0967 ± 0.0018 642B-A 18.2150 0.0093 0.854019 0.0025 2.09192 0.0045 18.245 ± 0.017 0.85470 ± 0.00037 2.0952 ± 0.0018 642B-A 18.2196 0.0063 0.853997 0.0025 2.09185 0.0054 18.250 ± 0.017 0.85468 ± 0.00037 2.0951 ± 0.0018 642B-A 18.2284 0.0127 0.853709 0.0028 2.09137 0.0044 18.258 ± 0.018 0.85439 ± 0.00038 2.0946 ± 0.0018 Mean 18.249 ± 0.007 0.85471 ± 0.00021 2.0953 ± 0.0008 642B-B 18.1881 0.0041 0.854971 0.0013 2.09284 0.0015 18.218 ± 0.017 0.85565 ± 0.00037 2.0961 ± 0.0018 642B-B 18.2038 0.0043 0.855349 0.0020 2.09467 0.0043 18.234 ± 0.017 0.85603 ± 0.00037 2.0979 ± 0.0018 642B-B 18.1755 0.0023 0.854746 0.0010 2.09143 0.0017 18.205 ± 0.016 0.85543 ± 0.00037 2.0947 ± 0.0018 Mean 18.219 ± 0.031 0.85570 ± 0.00065 2.0962 ± 0.0036 642B-C 18.2174 0.0016 0.854590 0.0005 2.09349 0.0006 18.247 ± 0.016 0.85527 ± 0.00037 2.0968 ± 0.0018 642B-C 18.2150 0.0021 0.854463 0.0009 2.09280 0.0018 18.245 ± 0.017 0.85514 ± 0.00037 2.0961 ± 0.0018 642B-C 18.2080 0.0015 0.854244 0.0006 2.09199 0.0009 18.238 ± 0.016 0.85492 ± 0.00037 2.0952 ± 0.0018 Mean 18.243 ± 0.009 0.85511 ± 0.00021 2.0960 ± 0.0010 Mean 642B A-C 422 18.242 ± 0.029 0.85509 ± 0.00094 2.0956 ± 0.0006 3-15643-B Smelter Soil SC-00011 643B 18.1661 0.0023 0.855456 0.0010 2.09202 0.0020 18.196 ± 0.016 0.85614 ± 0.00037 2.0953 ± 0.0018 643B 18.1539 0.0037 0.855205 0.0011 2.09068 0.0019 18.184 ± 0.017 0.85589 ± 0.00037 2.0939 ± 0.0018 Mean 4520 18.190 ± 0.011 0.85601 ± 0.00026 2.0946 ± 0.0013 3-15644-B Res. Soil SC-00017 644B-A 18.4188 0.0026 0.844483 0.0011 2.09277 0.0014 18.449 ± 0.017 0.84516 ± 0.00037 2.0960 ± 0.0018 644B-A 18.4243 0.0037 0.844582 0.0012 2.09333 0.0027 18.455 ± 0.017 0.84525 ± 0.00037 2.0966 ± 0.0018 Mean 18.452 ± 0.012 0.84521 ± 0.00025 2.0963 ± 0.0013 644B-B 18.4670 0.0026 0.841463 0.0010 2.09140 0.0021 18.497 ± 0.017 0.84213 ± 0.00036 2.0947 ± 0.0018 644B-C 18.5792 0.0062 0.839810 0.0025 2.10998 0.0052 18.610 ± 0.017 0.84048 ± 0.00037 2.1133 ± 0.0019 

39 18.52 ± 0.20 0.8426 ± 0.0061 2.101 ± 0.023 

10 
Table 2. Contd. 

Raw Data1 

Lab No/EPA No.3 Pb ppm4 Type 206Pb/204Pb % S.E. 207Pb/206Pb % S.E. 208Pb/206Pb % S.E. 206Pb/204Pb 207Pb/206Pb 208Pb/206Pb 

Corrected for mass fractionatio

3-15645-B Smelter Soil SC-00018 645B 18.0919 0.0022 0.857832 0.0007 2.09510 0.0009 18.122 ± 0.016 0.85852 ± 0.00037 2.0984 ± 0.0018 645B 18.0911 0.0027 0.857847 0.0007 2.09515 0.0009 18.121 ± 0.016 0.85853 ± 0.00037 2.0984 ± 0.0018 Mean 5660 18.122 ± 0.011 0.85852 ± 0.00026 2.0984 ± 0.0013 3-15646-B Smelter SC-00023 Material 646B 18.0984 0.0022 0.857802 0.0007 2.09641 0.0015 18.128 ± 0.016 0.85849 ± 0.00037 2.0997 ± 0.0018 646B 18.0898 0.0041 0.857609 0.0009 2.09536 0.0016 18.120 ± 0.017 0.85829 ± 0.00037 2.0986 ± 0.0018 646B 18.0876 0.0058 0.857621 0.0011 2.09541 0.0012 18.117 ± 0.017 0.85830 ± 0.00037 2.0987 ± 0.0018 Mean 2160 18.122 ± 0.009 0.85836 ± 0.00021 2.0990 ± 0.0010 3-15647-B Res. Soil SC-00025 647B 18.4147 0.0019 0.845385 0.0007 2.08550 0.0011 18.445 ± 0.017 0.84606 ± 0.00036 2.0887 ± 0.0018 647B 18.4188 0.0034 0.845532 0.0011 2.08635 0.0012 18.449 ± 0.017 0.84621 ± 0.00037 2.0896 ± 0.0018 Mean 126 18.447 ± 0.012 0.84613 ± 0.00025 2.0891 ± 0.0013 3-15648-B Res. Soil SC-00027 648B 115 18.3796 0.0125 0.846401 0.0031 2.08402 0.0059 18.410 ± 0.018 0.84708 ± 0.00037 2.0873 ± 0.0018 3-15649-B Res. Soil SC-00039 649B 18.3648 0.0017 0.848386 0.0006 2.09495 0.0010 18.395 ± 0.017 0.84906 ± 0.00037 2.0982 ± 0.0018 649B 18.3445 0.0034 0.847974 0.0013 2.09255 0.0029 18.375 ± 0.017 0.84865 ± 0.00037 2.0958 ± 0.0018 649B 18.3665 0.0038 0.848447 0.0010 2.09514 0.0010 18.397 ± 0.017 0.84912 ± 0.00037 2.0984 ± 0.0018 Mean 92 18.389 ± 0.024 0.84894 ± 0.00049 2.0975 ± 0.0031 3-15650-B Res. Soil SC-00046 650B 17.7702 0.0018 0.873614 0.0006 2.11319 0.0007 17.800 ± 0.016 0.87431 ± 0.00038 2.1165 ± 0.0018 650B 17.7696 0.0030 0.873548 0.0009 2.11310 0.0013 17.799 ± 0.016 0.87424 ± 0.00038 2.1164 ± 0.0018 650B 17.7741 0.0032 0.873617 0.0012 2.11347 0.0023 17.803 ± 0.016 0.87431 ± 0.00038 2.1168 ± 0.0018 Mean 1560 17.801 ± 0.009 0.87429 ± 0.00021 2.1166 ± 0.0010 3-15651-B Res. Soil SC-00046 DUP 651B 17.7556 0.0025 0.874082 0.0012 2.11399 0.0013 17.785 ± 0.016 0.87478 ± 0.00038 2.1173 ± 0.0018 651B 17.7692 0.0070 0.874419 0.0024 2.11596 0.0055 17.799 ± 0.017 0.87512 ± 0.00038 2.1193 ± 0.0019 651B 17.7534 0.0038 0.873984 0.0011 2.11349 0.0016 17.783 ± 0.016 0.87468 ± 0.00038 2.1168 ± 0.0018 Mean 1560 17.789 ± 0.009 0.87486 ± 0.00022 2.1178 ± 0.0025 3-15652-B Res. Soil SC-00048 652B 18.3237 0.0018 0.849612 0.0007 2.08176 0.0010 18.354 ± 0.017 0.85029 ± 0.00037 2.0850 ± 0.0018 652B 18.3164 0.0024 0.849428 0.0010 2.08098 0.0021 18.347 ± 0.017 0.85010 ± 0.00037 2.0842 ± 0.0018 Mean 261 18.351 ± 0.012 0.85019 ± 0.00026 2.0846 ± 0.0013 

11 
Table 2. Contd. 

Raw Data1 

Lab No/EPA No.3 Pb ppm4 Type 206Pb/204Pb % S.E. 207Pb/206Pb % S.E. 208Pb/206Pb % S.E. 206Pb/204Pb 207Pb/206Pb 208Pb/206Pb 

Corrected for mass fractionatio

3-15653-B Baseline SC-00065 Soil 653B 18.3800 0.0024 0.846355 0.0007 2.08585 0.0009 18.410 ± 0.017 0.84703 ± 0.00037 2.0891 ± 0.0018 653B 18.3816 0.0032 0.846356 0.0010 2.08571 0.0019 18.412 ± 0.017 0.84703 ± 0.00037 2.0890 ± 0.0018 653B 18.3860 0.0030 0.846417 0.0012 2.08631 0.0026 18.416 ± 0.017 0.84709 ± 0.00037 2.0896 ± 0.0018 Mean 101 18.413 ± 0.009 0.84705 ± 0.00020 2.0892 ± 0.0010 3-15654-B Baseline SC-00065 DUP Soil 654B-A 18.8272 0.0048 0.827089 0.0022 2.04638 0.0049 18.858 ± 0.017 0.82775 ± 0.00036 2.0496 ± 0.0018 654B-A 18.8467 0.0029 0.827383 0.0008 2.04795 0.0014 18.878 ± 0.017 0.82804 ± 0.00036 2.0511 ± 0.0018 654B-A 18.8414 0.0026 0.827372 0.0013 2.04786 0.0025 18.872 ± 0.017 0.82803 ± 0.00036 2.0511 ± 0.0018 Mean 18.869 ± 0.010 0.82794 ± 0.00021 2.0506 ± 0.0010 654B-B 18.4285 0.0045 0.844454 0.0012 2.08549 0.0018 18.459 ± 0.017 0.84513 ± 0.00037 2.0887 ± 0.0018 654B-C 18.4232 0.0075 0.845692 0.0014 2.08653 0.0012 18.454 ± 0.017 0.84637 ± 0.00037 2.0898 ± 0.0018 654B-D 18.4659 0.0029 0.843491 0.0011 2.08544 0.0018 18.496 ± 0.017 0.84416 ± 0.00036 2.0887 ± 0.0018 654B-E 18.3547 0.0023 0.847687 0.0008 2.08618 0.0016 18.385 ± 0.017 0.84836 ± 0.00037 2.0894 ± 0.0018 654B-F 18.4291 0.0016 0.844134 0.0008 2.08531 0.0015 18.460 ± 0.017 0.84481 ± 0.00036 2.0886 ± 0.0018 Mean 654B B-F 101 18.451 ± 0.050 0.8458 ± 0.0020 2.0890 ± 0.0008 3-15655-B Res. Soil SC-00069 655B-A 18.2694 0.0018 0.851880 0.0007 2.08647 0.0014 18.300 ± 0.017 0.85256 ± 0.00037 2.0897 ± 0.0018 655B-A 18.2648 0.0017 0.851734 0.0004 2.08594 0.0010 18.295 ± 0.017 0.85241 ± 0.00037 2.0892 ± 0.0018 Mean 18.297 ± 0.011 0.85249 ± 0.00026 2.0894 ± 0.0013 655B-B 18.2823 0.0025 0.850617 0.0006 2.08327 0.0015 18.312 ± 0.017 0.85129 ± 0.00037 2.0865 ± 0.0018 Mean 655B A-B 151 18.303 ± 0.073 0.8519 ± 0.0073 2.088 ± 0.017 3-15656-B Res. Soil SC-00084 656B-A 18.4515 0.0035 0.844073 0.0017 2.06797 0.0035 18.482 ± 0.017 0.84475 ± 0.00037 2.0712 ± 0.0018 656B-A 18.4479 0.0024 0.844073 0.0008 2.06751 0.0018 18.478 ± 0.017 0.84475 ± 0.00036 2.0707 ± 0.0018 656B-A 18.4502 0.0032 0.844119 0.0011 2.06803 0.0011 18.481 ± 0.017 0.84479 ± 0.00036 2.0713 ± 0.0018 Mean 18.480 ± 0.010 0.84476 ± 0.00020 2.0711 ± 0.0010 656B-B 17.9549 0.0021 0.864978 0.0008 2.09423 0.0015 17.985 ± 0.016 0.86567 ± 0.00037 2.0975 ± 0.0018 656B-C 18.5411 0.0042 0.840091 0.0019 2.06893 0.0041 18.572 ± 0.017 0.84076 ± 0.00037 2.0722 ± 0.0018 656B-D 18.3719 0.0029 0.846875 0.0013 2.06935 0.0027 18.402 ± 0.017 0.84755 ± 0.00037 2.0726 ± 0.0018 656B-E 18.3958 0.0036 0.845958 0.0014 2.06725 0.0028 18.426 ± 0.017 0.84663 ± 0.00037 2.0705 ± 0.0018 Mean 656B A, C-E 175 18.47 ± 0.12 0.8449 ± 0.0042 2.0714 ± 0.0007 

12 
Table 2. Contd. 

Raw Data1 Corrected for mass fractionatio

Lab No/EPA No.3 Pb ppm4 Type 206Pb/204Pb % S.E. 207Pb/206Pb % S.E. 208Pb/206Pb % S.E. 206Pb/204Pb 207Pb/206Pb 208Pb/206Pb 

3-15657-B Res. Soil SC-00104 657B-A 17.8771 0.0020 0.868880 0.0007 2.10983 0.0010 17.907 ± 0.016 0.86957 ± 0.00037 2.1131 ± 0.0018 657B-A 17.8647 0.0017 0.868628 0.0006 2.10861 0.0017 17.894 ± 0.016 0.86932 ± 0.00037 2.1119 ± 0.0018 Mean 17.900 ± 0.011 0.86944 ± 0.00026 2.1125 ± 0.0013 657B-B 17.8650 0.0020 0.869417 0.0006 2.11025 0.0009 17.894 ± 0.016 0.87011 ± 0.00037 2.1135 ± 0.0018 Mean 657B A-B 1140 17.898 ± 0.009 0.8698 ± 0.0040 2.1128 ± 0.0010 3-15658-B PAX SC-00099 658B 17.1742 0.0046 0.900857 0.0019 2.14007 0.0042 17.203 ± 0.016 0.90157 ± 0.00039 2.1434 ± 0.0019 658B 17.1802 0.0045 0.900965 0.0012 2.14095 0.0029 17.209 ± 0.016 0.90168 ± 0.00039 2.1443 ± 0.0019 Mean 24300 17.206 ± 0.011 0.90162 ± 0.00027 2.1438 ± 0.0013 3-15659-B prep. blank n.a. 659B 18.0854 0.0036 0.859095 0.0013 2.10784 0.0029 18.115 ± 0.016 0.85978 ± 0.00037 2.1111 ± 0.0018 659B 18.0961 0.0035 0.859319 0.0010 2.10869 0.0020 18.126 ± 0.016 0.86000 ± 0.00037 2.1120 ± 0.0018 Mean n.d. 18.120 ± 0.011 0.85989 ± 0.00026 2.1116 ± 0.0013 

Samples Received 4/01-5/01 

3-13569-B Res. Soil 569B 17.9083 0.0032 0.867068 0.0015 2.10132 0.0028 17.937 ± 0.012 0.86773 ± 0.00038 2.1047 ± 0.0015 15569B 17.8966 0.0031 0.866801 0.0013 2.09968 0.0027 17.925 ± 0.012 0.86747 ± 0.00038 2.1031 ± 0.0015 15569B 17.8991 0.0024 0.866656 0.0011 2.09903 0.0115 17.928 ± 0.011 0.86732 ± 0.00037 2.1025 ± 0.0017 15Mean 409 17.930 ± 0.006 0.86751 ± 0.00021 2.1035 ± 0.0023 15

3-13989-B Res. Soil 989B 18.1118 0.0027 0.857606 0.0009 2.08895 0.0018 18.141 ± 0.012 0.85826 ± 0.00037 2.0924 ± 0.0015 989B 18.1274 0.0038 0.857881 0.0014 2.09051 0.0030 18.156 ± 0.012 0.85854 ± 0.00037 2.0939 ± 0.0015 989B 18.1226 0.0024 0.857770 0.0010 2.08976 0.0019 18.152 ± 0.012 0.85843 ± 0.00037 2.0932 ± 0.0015 Mean 302 18.150 ± 0.014 0.85841 ± 0.00021 2.0932 ± 0.0009 3-12524-B Res. Soil 524B 18.1449 0.0025 0.856547 0.0008 2.08839 0.0019 18.174 ± 0.012 0.85720 ± 0.00037 2.0918 ± 0.0015 524B 18.1597 0.0026 0.856876 0.0007 2.09052 0.0011 18.189 ± 0.012 0.85753 ± 0.00037 2.0939 ± 0.0015 524B 18.1518 0.0022 0.856461 0.0005 2.08830 0.0005 18.181 ± 0.012 0.85712 ± 0.00037 2.0917 ± 0.0015 Mean 794 18.181 ± 0.013 0.85728 ± 0.00021 2.0925 ± 0.0027 3-12820-B Res. Soil 820B 17.9056 0.0036 0.866940 0.0016 2.10109 0.0031 17.934 ± 0.012 0.86761 ± 0.00038 2.1045 ± 0.0015 820B 17.8936 0.0036 0.866585 0.0014 2.09940 0.0024 17.922 ± 0.012 0.86725 ± 0.00038 2.1028 ± 0.0015 820B 17.9225 0.0039 0.867176 0.0014 2.10287 0.0027 17.951 ± 0.012 0.86784 ± 0.00038 2.1063 ± 0.0015 Mean 607 17.936 ± 0.034 0.86757 ± 0.00063 2.1045 ± 0.0042 3-01182-B Res. Soil 182B 18.3017 0.0028 0.849789 0.0013 2.07692 0.0023 18.331 ± 0.012 0.85044 ± 0.00037 2.0803 ± 0.0015 182B 18.3136 0.0035 0.849914 0.0010 2.07790 0.0020 18.343 ± 0.012 0.85057 ± 0.00037 2.0813 ± 0.0015 182B 18.3204 0.0018 0.850153 0.0005 2.07809 0.0007 18.350 ± 0.012 0.85081 ± 0.00037 2.0815 ± 0.0015 Mean 720 18.341 ± 0.020 0.85061 ± 0.00020 2.0810 ± 0.0009 

13 
Table 2. Contd. 

Raw Data1 Corrected for mass fractionatio

Lab No/EPA No.3 Pb ppm4 Type 206Pb/204Pb % S.E. 207Pb/206Pb % S.E. 208Pb/206Pb % S.E. 206Pb/204Pb 207Pb/206Pb 208Pb/206Pb 

3-00748-B Res. Soil 748B 18.2789 0.0028 0.850419 0.0010 2.07145 0.0019 18.308 ± 0.012 0.85107 ± 0.00037 2.0748 ± 0.0015 748B 18.2874 0.0038 0.850664 0.0015 2.07265 0.0029 18.317 ± 0.012 0.85132 ± 0.00037 2.0760 ± 0.0015 748B 18.2928 0.0031 0.850621 0.0008 2.07259 0.0016 18.322 ± 0.012 0.85127 ± 0.00037 2.0760 ± 0.0015 Mean 633 18.316 ± 0.007 0.85122 ± 0.00020 2.0756 ± 0.0009 3-10740-B Res. Soil 740B 18.3013 0.0046 0.848982 0.0022 2.07074 0.0041 18.331 ± 0.012 0.84963 ± 0.00037 2.0741 ± 0.0015 740B 18.3032 0.0057 0.849057 0.0014 2.07117 0.0013 18.333 ± 0.012 0.84971 ± 0.00037 2.0745 ± 0.0015 Mean 984 18.332 ± 0.008 0.84967 ± 0.00025 2.0743 ± 0.0010 3-02246-B Res. Soil 246B 18.5659 0.0041 0.840295 0.0011 2.06094 0.0026 18.596 ± 0.012 0.84094 ± 0.00036 2.0643 ± 0.0015 246B 18.5591 0.0025 0.840132 0.0007 2.05935 0.0009 18.589 ± 0.012 0.84078 ± 0.00036 2.0627 ± 0.0015 Mean 1658 18.593 ± 0.008 0.84086 ± 0.00025 2.0635 ± 0.0058 3-10319-B Res. Soil 319B 18.8051 0.0033 0.829795 0.0014 2.04272 0.0029 18.835 ± 0.012 0.83043 ± 0.00036 2.0461 ± 0.0015 319B 18.8080 0.0029 0.829640 0.0010 2.04198 0.0016 18.838 ± 0.012 0.83028 ± 0.00036 2.0453 ± 0.0015 Mean 1469 18.837 ± 0.008 0.83035 ± 0.00025 2.0457 ± 0.0010 

Samples from a single residential property 

3-12152-B Res. Soil 152B-A 17.5876 0.0025 0.880346 0.0013 2.11644 0.0026 17.616 ± 0.011 0.88102 ± 0.00038 2.1199 ± 0.0015 152B-A 17.5833 0.0038 0.880230 0.0008 2.11612 0.0007 17.611 ± 0.011 0.88091 ± 0.00038 2.1196 ± 0.0015 Mean 17.614 ± 0.008 0.88096 ± 0.00026 2.1198 ± 0.0011 152B-B 17.8703 0.0021 0.867707 0.0008 2.09640 0.0014 17.899 ± 0.011 0.86837 ± 0.00037 2.0998 ± 0.0015 152B-B 17.8691 0.0043 0.867603 0.0011 2.09621 0.0021 17.898 ± 0.012 0.86827 ± 0.00038 2.0996 ± 0.0015 Mean 17.899 ± 0.008 0.86832 ± 0.00026 2.0997 ± 0.0010 152B-C 17.7565 0.0035 0.872904 0.0013 2.10490 0.0027 17.785 ± 0.011 0.87357 ± 0.00038 2.1083 ± 0.0015 152B-C 17.7574 0.0023 0.872922 0.0010 2.10494 0.0019 17.786 ± 0.011 0.87359 ± 0.00038 2.1084 ± 0.0015 Mean 17.786 ± 0.008 0.87358 ± 0.00026 2.1084 ± 0.0011 Mean 152A-C 1730 17.770 ± 0.355 0.87400 ± 0.01573 2.1090 ± 0.0253 3-12151-B Res. Soil 151B 18.1386 0.0037 0.856863 0.0013 2.08485 0.0028 18.168 ± 0.012 0.85752 ± 0.00037 2.0883 ± 0.0015 151B 18.1582 0.0037 0.857266 0.0012 2.08697 0.0023 18.187 ± 0.012 0.85792 ± 0.00037 2.0904 ± 0.0015 151B 18.1378 0.0024 0.856824 0.0009 2.08434 0.0017 18.167 ± 0.012 0.85748 ± 0.00037 2.0877 ± 0.0015 Mean 540 18.174 ± 0.024 0.85764 ± 0.00043 2.0888 ± 0.0031 3-12039-B Res. Soil 039B 18.2487 0.0033 0.852022 0.0017 2.07294 0.0034 18.278 ± 0.012 0.85268 ± 0.00037 2.0763 ± 0.0015 039B 18.2491 0.0022 0.851817 0.0011 2.07196 0.0020 18.278 ± 0.012 0.85247 ± 0.00037 2.0753 ± 0.0015 Mean 413 18.278 ± 0.008 0.85258 ± 0.00026 2.0758 ± 0.0010 3-12035-B Res. Soil 035B 18.0680 0.0028 0.860566 0.0009 2.08797 0.0020 18.097 ± 0.012 0.86123 ± 0.00037 2.0914 ± 0.0015 035B 18.0558 0.0038 0.860315 0.0017 2.08619 0.0032 18.085 ± 0.012 0.86097 ± 0.00037 2.0896 ± 0.0015 035B 18.0648 0.0039 0.860463 0.0011 2.08736 0.0010 18.094 ± 0.012 0.86112 ± 0.00037 2.0908 ± 0.0015 Mean 824 18.092 ± 0.007 0.86111 ± 0.00021 2.0906 ± 0.0009 

14 
Table 2. Contd. 

Raw Data1 

Lab No/EPA No.3 Pb ppm4 Type 206Pb/204Pb % S.E. 207Pb/206Pb % S.E. 208Pb/206Pb % S.E. 206Pb/204Pb 207Pb/206Pb 208Pb/206Pb 

Corrected for mass fractionatio

3-12040-B Res. Soil
040B 18.4208 0.0050 0.845624 0.0020 2.06295 0.0044 18.450 ± 0.012 0.84627 ± 0.00037 2.0663 ± 0.0015 

040B 18.4169 0.0032 0.845433 0.0010 2.06280 0.0016 18.446 ± 0.012 0.84608 ± 0.00037 2.0662 ± 0.0015 

Mean 256 18.448 ± 0.008 0.84617 ± 0.00025 2.0662 ± 0.0010 

1 Uncertainties are in percent and represent one standard error (≈ 1 sigma mean).
2 Corrected for mass fractionation based on analyses of NIST standard SRM 981 (Table A1). Uncertainties are absolute and are at the 95% confidence interval (Ludwig, 1980; 1994).
3 Individual mass spectrometer runs are listed separately under the same lab sample number. Separate acid-digestions of the same sample are listed as "-A" through "-F".
4 Pb concentrations provided by USEPA from acid digestions of splits of the samples.


15 
15.65 

15.60 

208Pb/204Pb 207Pb/204Pb


N 

M 

15.70 

r2 = 0.863 

Single-run 

Uncertainty 

15.55 

15.50 

15.45 

15.40 

17.0 17.5 18.0 18.5 19.0 

206Pb/204Pb


39.0 

38.5 

38.0 

37.5 

N 

M r2 = 0.815 

Single-run 

Uncertainty 

37.0 

36.5 

17.0 17.5 18.0 18.5 19.0 

206Pb/204Pb 

Nominal Range Actual Range 

Pb < 150 ppm (39 - 126 ppm) 

Pb = 150-500 ppm (151-422 ppm) 

Pb = 500-1000 ppm (540-984 ppm) 

Pb > 1000 ppm (1140-1730 ppm) 

Smelter materials 

PAX 

Acme Pb-Arsenate 

3 

M 

N 

Figure 1. 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb 

diagrams for selected soil samples from the Vasquez Blvd.-I70 study area, 

coded by Pb concentrations. Data are from Table 2 and represent the 

mean values for individual samples. Best fit lines (unweighted) through 

the data yield r2 values (square of the regression correlation coefficients) 

of 0.863 and 0.815 for 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 

208Pb/204Pb, respectively. Also shown is the typical uncertainty (95% C.I.) 

for and individual mass spectrometer run. 

16 
Lead isotopic data for the Acme lead arsenate, smelter soils, and smelter waste are 

very similar to one another and to some of the residential soils. On the other hand, the sample 

of PAX has a unique and nonradiogenic (low 206Pb/204Pb) lead isotopic composition with 

206Pb/204Pb = 17.21. 

The Pb isotopic compositions presented in Figure 1 are also shown in Figure 2, but the 

data points are coded based on As concentrations. Low-As samples are generally the most 

radiogenic with 206Pb/204Pb > 18.25 (two exceptions, both from the same residential property). 

High-As samples show two distinct groups; one with 206Pb/204Pb < 17.9, and one with 

206Pb/204Pb = 18.15-18.18. Comparison of Figures 1 and 2 reveals that the soils with high Pb 

and As are characterized by 206Pb/204Pb < 17.9, whereas the samples with radiogenic Pb and 

slightly-elevated 208Pb/204Pb are low in both Pb and As. 

Isotopic compositions in most of the high-As and high-Pb samples are less radiogenic 

than the smelter material, Acme Pb arsenate, or the low-As and low-Pb soils (baseline soils). 

Consequently, these soils must have a component of PAX (or some similar, but as yet 

unidentified material) within them. However, the isotopic data for the three contaminants and the 

residential soils are nearly colinear. Consequently, from the lead isotopic data alone it is not 

possible to determine whether the Pb in the high-As, high-Pb soils is a simple mixture of PAX 

and baseline soil or whether the other contaminants are also present. 

Sources of Lead 

Lead and As concentrations of the residential soils, coded according to their 206Pb/204Pb 

values, are shown in Figure 3. Shown for comparison are data from a selected number of other 

residential soils within the study area that were not analyzed for Pb isotopic compositions. 

Uncontaminated or least-contaminated samples, represented by the cluster of points with low 

As and Pb, have 208Pb/204Pb = 18.3->18.5, although the majority of the samples have 

208Pb/204Pb = 18.4-18.5. The samples highest in As (>1200 ppm) are characterized by 

208Pb/204Pb < 18.0, even though Pb contents vary from 400 to over 1500 ppm. As 

mentioned above, samples with the highest Pb contents fall into two groups; one with 

208Pb/204Pb > 18.5 and one with 208Pb/204Pb < 18.0. Both isotopic groups are found among 

low-As-residential soils (lower right in Figure 3). These results indicate that there are at least two 

high-Pb, low-As contaminants present in these soils. 

Lead isotopic compositions, represented by 206Pb/204Pb, are shown as a function of Pb 

concentration in Figure 4. Six lead sources are represented as summarized in Table 3. The 

uncontaminated or least-contaminated end member is defined by the low-As and low-Pb 

samples. This component with 206Pb/204Pb = 18.48 and Pb = 70 ppm is represented by the 

means of the baseline soil sample, SC-00065, and the lowest-Pb sample, SC-00017. 

17
208Pb/204Pb 207Pb/204Pb


15.70 

15.65 

15.60 

15.55 

15.50 

15.45 

15.40 

17.0 17.5 18.0 18.5 19.0 

3 

3 

M 

N 

Single-run 

Uncertainty 

r2 = 0.863 

206Pb/204Pb 

39.0 

38.5 

38.0 

37.5 

37.0 

36.5 

17.0 17.5 18.0 18.5 19.0 

3 33 

M 

N

Single-run 

Uncertainty 

r2 = 0.815 

206Pb/204Pb 

Nominal Range Actual Range 

As < 50 ppm (<11 - 48 ppm) 

As = 50-250 ppm (165-242 ppm) 

As = 250-500 ppm (409 ppm) 

As > 500 ppm (830-1492 ppm) 

Smelter materials 

PAX 

Acme Pb-Arsenate 

3 

M 

N 

Figure 2. 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 

208Pb/204Pb diagrams for selected soil samples from the Vasquez 

Blvd.-I70 study area, coded by As concentrations. Data are from 

Table 2 and represent the mean values for individual samples. Also 

shown are the typical uncertainty (95% C.I.) for and individual mass 

spectrometer run and best-fit lines through all of the data. 

18 
As ppm


1600 

1400 

1200 

1000 

800 

600 

400 

200 

0 

J J 

J 

J 

JJ 

J 

J 

J 

J 

J 

J 

J 

J 

J 

JJ 

J 

J 

J 

J 

J 

J 

J 

J 

J 

J 

J 

J 

J J J J JJJJ JJJ JJJJJJJ

JJJJ J 

J

J JJJJ JJ

JJJJJ

J JJJ

JJ JJJJJJJJJJJJJJ

J

JJJJJJJJ

JJJJ J

J JJJJ

JJJJJJJJ

J

J JJ

J 

JJ 

JJJJJ 

JJ J

JJ 

JJ 

JJJ J

J 

J 

JJ 

J 

J 

J J

J 

J

J 

J JJ JJ

J JJ 

J 

J 

J 

J 

J

J 

J 

J 

J

J 

J 

J

J 

J 

J 

JJ J

J J

J

J 

J

J 

J 

J 

J 

J 

J 

JJ 

JJ

J 

J 

J 

J

J J

J 

J 

JJ 

J

J 

J 

JJ 

J 

J 

JJJ 

J 

Stoichiometric Pb arsenate 

Acme Pb arsenate

Pax

Smelter material 

0 400 800 1200 1600 2000 

Pb ppm 

206Pb/204Pb > 18.5 

206Pb/204Pb = 18.4-18.5 

206Pb/204Pb = 18.3-18.4 

206Pb/204Pb = 18.2-18.3 

206Pb/204Pb = 18.1-18.2 

206Pb/204Pb = 18.0-18.1 

206Pb/204Pb < 18.0 

Figure 3. As vs. Pb concentrations for selected samples from the VBI70 

study area. Samples are coded according to their 206Pb/204Pb values. 

Also shown by the small filled circles are data from samples not analyzed for Pb isotopic compositions? Vectors for PAX, smelter soils and 

materials, Acme lead arsenate, and stoichiometric Pb arsenate 

(PbAsO4), which are all off the scale of the diagram, are also shown. 

19 
Table 3. Summary of identified Pb sources in the VBI70 study area. 

Sample Type Pb ppm As ppm 

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 

Baseline 

Mean1 res soils. 7 0 8 18.476 15.604 38.732 

High-As contaminants 

3-15639-B Acme Pb-Arsenate 128000 273000 18.071 15.568 38.009 

3-15658-B PAX 2 43 00 120000 17.206 15.513 36.886 

3-15643-B Smelter-soil 4520 2 24 00 18.190 15.570 38.101 

3-15645-B Smelter-soil 5660 2 72 00 18.122 15.557 38.026 

3-15646-B Smelter-material 2160 7630 18.121 15.554 38.035 

Mean smelter materials & soils 4113 1 90 77 18.144 15.560 38.054 

Low-As contaminants2 

3-10319-B res. soil 1469 4 8 18.837 15.641 38.534 

3-12152-B res. soil 1730 <11 17.770 15.532 37.470 

1

 Mean of soils 3-15644-B and 3-15654-B 


2

 The residential soils with highest lead contents are used to approximate the contaminants. The true 


contaminant should have higher Pb concentrations and slightly different lead isotopic values.


20 
The data for the PAX sample represent one possible contaminant with 206Pb/204Pb = 

17.21 and Pb = 24300 ppm. A second contaminant is represented by the lead arsenate 

sample with 206Pb/204Pb = 18.07 and Pb = 128000 ppm. The smelter soils with 206Pb/204Pb 

≈ 18.12-18.19 and Pb ≈ 2160-5600 ppm represent a third contaminant source. Each of these 

three sources represents a potential As as well as Pb contaminant. 

Two additional contaminants are approximated by residential soils 3-10319-B and 3-

12152-B. These sources appear to represent Pb contamination, but not As. Because these 

soils are likely mixtures of contaminant and baseline material, the actual Pb concentrations in the 

true contaminants are probably higher than in the soils and the Pb isotopic compositions more 

extreme (206Pb/204Pb ≥ 18.84 and ≤17.77 for 3-10319-B and 3-12152-B, respectively). 

Another high-Pb sample with 206Pb/204Pb = 18.6 may reflect yet another Pb contaminant or a 

mixture of Pb contaminants. 

Two-Component Mixing 

Calculated bulk-mixing curves between average baseline and each of the fivecontaminant sources are shown for reference in Figure 4A. The bulk mixing curves are 

calculated from the following two-component bulk-mixing equations (Faure, 1977): 

(1) CM = CC * (X) + CB * (1-X) 

(2) RM = [RC * CC *(X) + RB * CB * (1-X)] / CM 

where CC, and CB are the concentrations of an element, in this case lead, in the contaminant (C) 

and baseline (B); CM is the concentration of the element in the mixture (a contaminated soil 

sample), and X is the weight fraction of contaminant source A in the mixture (where 0 ≤ X ≤ 1). 

RA, RB, and RM are the isotopic ratios (in this case 206Pb/204Pb) in the contaminant source, 

baseline, and mixture, respectively. The percent contamination is 100*X. The weight fraction of 

an element in a soil, XC, derived from the contaminant is calculated from X in the following 

manner (Church and others, 2002): 

(3) XC = X*CC/CM 

By combining equations (1) and (2) and eliminating X, the following expression is 

obtained (Faure, 1977): 

(4) RM = [CC *CB * (RC – RB)]/ [CM *(CC – CB)] + (CC * RA – CB * RB) / (CC – CB) 

21
19.0 

18.5 

N 

XM A 

152B 

319B 

206Pb/204Pb 206Pb/204Pb


18.0 

17.5 

17.0 

10 100 1000 10000 100000 1000000 

Pb ppm 19.0 

18.5 

18.0 

17.5 

17.0 

0.000 0.003 0.006 0.009 0.012 0.015 

3 3 

M 

N3 

B 

152B 

319B 

1/Pb ppm 

Nominal Range Actual Range 

As < 50 ppm (<11 - 48 ppm) 

As = 50-250 ppm (165-242 ppm) 

As = 250-500 ppm (409 ppm) 

As > 500 ppm (830-1492 ppm) 

Smelter materials 

PAX 

Acme Pb-Arsenate 

Av. Baseline 

High-As high-Pb source 

Low-As high-Pb source 

3 

M 

N 

Figure 4. (A) 206Pb/204Pb vs. Pb concentration and (B) 206Pb/204Pb vs. 1/Pb concentration for 

selected soil samples from the VBI70 study area. Data are from Table 2 and represent the grand 

means of data from individual samples (bold type in Table 2). An average baseline value (cross, 

206Pb/204Pb = 18.48, Pb = 70 ppm) is estimated from data for samples 3-15654-B and 3-15644-B. 

Bulk-mixing curves (lines in Figure 4B) between average baseline and PAX, smelter soil, Acme lead 

arsenate, soil 3-10319-B (319B), and soil 3-12152-B (152B) are shown for reference. The broken 

line in Figure 4B represents a hypothetical mixing line between average baseline and a Pb + As 

source with 206Pb/204Pb 17.8. 

22 
This equation is of the general form: 

RM = a / CM + b 

which is the form for a hyperbolic curve when RM is plotted as a function of CM (a and b are 

constants dependent upon the values of CC, CB, RC, and RB). The mixing hyperbole is 

converted to a straight line if RM is plotted as a function of (1/CM) as is shown in Figure 4B. 

Four points regarding Figure 4 are worthy of mention. (1) The bulk mixing curves for 

Acme lead arsenate and average smelter material are nearly superimposed upon one another. 

This means that the Pb data are not able to resolve contamination from these two sources. (2) 

Most of the low-As samples plot above the mixing curves for lead arsenate and smelter 

material. This observation coupled with the fact that these samples show a wide range in lead 

contents indicates that Pb contamination from a source or sources with radiogenic Pb 

(

206Pb/204Pb > 18.2) is common among the low-As soils. One might reasonably conclude that 

this contamination is also present among the higher-As soils as well, but may be masked by 

other contaminants. (3) The data for many of the high-As residential samples plot between the 

PAX and lead arsenate - smelter material mixing curves. This suggests that the Pb in these 

samples was derived from at least three sources: baseline, PAX and at least one other 

contaminant source. Alternatively, the results may indicate the presence of another contaminant 

with 206Pb/204Pb ≈ 17.8, similar to soil 3-12152-B, but with high As as well as Pb (broken red 

line in Figure 4B). (4) Finally, data for two of the high-As soils (3-13989-B and 3-12524-B) as 

well as three of the intermediate-As soils (3-15648-B, 3-15642-B, and 3-12151-B) plot on or 

near the mixing lines (Figure 4B) between baseline and smelter material or lead arsenate. This 

suggests that lead arsenate or smelter material may represent the principal contaminant in these 

samples. However, if multiple contaminants are involved, then the position of these data 

relative to the lead arsenate and smelter material mixing curves may be fortuitous. 

In a situation involving two-component-mixing, the weight fraction of contaminant (X) can 

be semi-independently calculated from both equations 1 and 2 (equations 1 and 2 are not 

totally independent because the concentration data are used in both equations). The weight 

fraction of contaminant Pb, XPb, in a sample can also be estimated using the Pb isotopic 

compositions alone (Church and others, 1997; 2002) as follows: 

(5) XPb = (RM-RB)/(RC-RB) 

23
Table 4. Two-component mixing parameters for selected soil samples from the VBI70 study area. 

Components Sample Pb ppm As ppm 206Pb/204Pb XC Pb1 XPb 2 X Pb (1)3 X Pb (2)4 

Acme Pb arsenate 3-13989-B 302 904 18.150 0.769 0.805 0.0018 0.0018 and average baseline 3-12524-B 794 1135 18.181 0.912 0.728 0.0057 0.0057 

3-15648-B 115 165 18.410 0.392 0.163 0.0004 0.0004 3-15642-B 422 173 18.242 0.835 0.578 0.0028 0.0028 3-12151-B 540 221 18.174 0.871 0.746 0.0037 0.0037 

Average smelter material 3-13989-B 302 904 18.150 0.782 0.982 0.0574 0.0571 and average baseline 3-12524-B 794 1135 18.181 0.928 0.889 0.1791 0.1792 

3-15648-B 115 165 18.410 0.398 0.199 0.0111 0.0112 3-15642-B 422 173 18.242 0.849 0.705 0.0871 0.0873 3-12151-B 540 221 18.174 0.885 0.910 0.1163 0.1162 

1Weight fraction of contaminant Pb in the soil calculated from Pb concentrations using equation (3).
2Weight fraction of contaminant Pb in the soil calculated from Pb isotopic compositions using equation (5).
3Weight fraction of contaminant in the soil calculated from Pb or As concentrations using equation (1).
4Weight fraction of contaminant in the soil calculated from equation (2).
5Weight fraction of contaminant in the soil calculated from equation (6).


24 
The weight fraction of contaminant, X, can then be calculated from X

in the contaminant and soil in the following manner: 

Pb and the Pb concentrations 

(6) X = XPb * CM/CC 

Equations (3) and (5) provide two independent methods of estimating the relative 

amount of contaminant Pb within a soil, one based solely on the Pb concentrations (3), and one 

based exclusively on the Pb isotopic compositions (5). Equation (6) also provides another 

semi-independent method of estimating the relative amount of contaminant in a soil. The As 

concentration data can also be used in equation (1) to provide an independent check of the 

calculations. 

Two-component mixing calculations for the five soils mentioned above, that plot near 

the baseline-Pb arsenate or smelter material mixing curves, are presented in Table 4. Two 

sets of calculations have been made for mixing between baseline and lead arsenate and 

between baseline and average smelter material. It should be emphasized that these 

calculations represent possible solutions only. Independent verification from geochemical 

analyses and speciation studies are also required in order to determine whether these possible 

solutions are in fact reasonable. Calculations in Table 4 are highlighted in bold type when 

agreement among the three calculations was within a factor of 2 (arbitrary). 

Data for the two high-As samples are consistent contamination of baseline soil with 0.2-

0.6% acme lead arsenate. One of these samples, 3-13989-B is also consistent with 

contamination of baseline soil with 5-6% smelter material. Calculations for the intermediate-As 

sample 3-15648-B are also consistent with a small amount of contamination from either Pb 

arsenate or smelter material. Internally consistent results were not obtained for the other 

samples. 

Three-Component Mixing 

As mentioned above four of the six high-As samples (samples 3-15650-B and 3-

15651-B are duplicate splits of the same soil) and one of the intermediate-As soils plot 

between the three high-As mixing curves in Figure 4. Although these samples plot close to 

the mixing curve for baseline and soil 3-12152-B, this soil can not represent the only 

contaminant in these samples because soil 3-12152-B has low As. Barring the existence of an 

unidentified high-As contaminant with 206Pb/204Pb ≈ 17.8, the Pb data for these samples must 

reflect contamination from more than one source. 

If two contaminant sources are involved then a three-component mixing model may be 

employed where 

25
(7) CM = CC1 * (X) + CC2 * (Y) + CB * (1-X-Y) 

(8) RM = [RC1 * CC1 *(X) + RC2 * CC2 *(Y) + RB * CB * (1-X-Y)] / CM 

where X and Y are the weight fractions of contaminants C1 and C2, respectively. Equations 7 

and 8 may be combined to solve for both X and Y. If more than two contaminant sources are 

involved, then additional data are required in order to obtain unique solutions. 

Three-component mixing calculations for all six high-As and all five intermediate-As soils 

are summarized in Table 5. All calculations are made assuming the soils are mixtures of 

baseline material and two contaminants. The requirements for selecting pairs of contaminants 

are (1) that one of the contaminants must have nonradiogenic Pb (either PAX or soil 3-12152-

B; this requirement is strictly valid only for those soils having 206Pb/204Pb < 18.08) and (2) that 

at least one of the contaminants must have high As (either PAX, lead arsenate, or smelter 

material). Using these criteria, 5 different situations are allowed (Table 5). Weight fractions of 

the two contaminants were calculated using the Pb concentration and Pb isotopic data in 

equations 4 and 5. Arsenic contents were calculated from these proportions and arsenic 

contents of the contaminants using equation 4. The calculated arsenic contents are compared to 

the measured As in the soils in Table 5. Calculations highlighted in bold are for calculated As 

contents that are within a factor of 2 of the measured values. Once again, the calculations only 

indicate possible solutions which must be used in conjunction with other evidence. 

Consistent results were not obtained. Data for some samples (e.g. 3-13569-B and 3-

13989-B) have multiple solutions whereas data for other samples (3-15650-B and 3-00748-

B) yield no satisfactory solutions. If taken at face value, the data in Table 5 suggest that the As 

and Pb contamination in each residential property is derived from a unique set of circumstances. 

The calculations in Tables 4 and 5 are complicated by several factors. First, the 

proportions of lead and arsenic currently present in the soils may be different from those in the 

potential sources as a result of preferential dissolution of specific chemical phases from the soils 

(J. Drexler, oral communication, 2001). Second, the smelter soils present today may have 

different chemical and isotopic signatures from soils present in the past. This is particularly true 

of the smelter in question, which switched from arsenic to cadmium refining during the late 

1920’s. It should also be noted that two other smelters were at one time present in this general 

area, but samples from these smelters are not available. Finally, the samples of PAX and lead 

arsenate represent only two of several similar compounds that might have been commercially 

available during the middle 20th century. The proportions of lead and arsenic as well as the 

lead isotopic signatures of this other material, or of the PAX and Acme lead arsenate for that 

26
Table 5. Three-component mixing calculations for selected resedential soils 

from the VBI70 study area.1 

Sample Pb ppm 206Pb/204P b % Cont. C1 

2 % Cont. C2 3 Calc. As ppm4 

Meas. As ppm 

Contaminants: Pax and Acme Pb arsenate 

Pb ppm 206Pb/204Pb % PAX % Acme Calc. As ppm Meas. As ppm 

3-15641-B 1490 17.910 1.27 0.87 3908 8 3 6 

3-15650-B 1560 17.801 2.13 0.76 4645 1490 

3-15657-B 1140 17.898 1.07 0.63 3023 1490 

3-13569-B 4 0 9 17.930 0.41 0.19 1010 1492 

3-13989-B 3 0 2 18.150 0.02 0.18 5 1 7 9 0 4 

3-12524-B 7 9 4 18.181 -0.28 0.62 1361 1135 

3-15648-B 1 1 5 18.410 -0.05 0.04 7 0 1 6 5 

3-15642-B 4 2 2 18.242 -0.21 0.31 6 1 7 1 7 3 

3-12820-B 6 0 7 17.936 0.52 0.32 1511 2 4 2 

3-12151-B 5 4 0 18.174 -0.13 0.39 9 2 2 2 2 1 

3-00748-B 6 3 3 18.316 -0.60 0.55 7 9 8 4 0 9 

Contaminants: Pax and average smelter material 

Pb ppm 206Pb/204Pb % PAX % smelter Calc. As ppm Meas. As ppm 

3-15641-B 1490 17.910 1.60 25.51 6797 8 3 6 

3-15650-B 1560 17.801 2.42 22.33 7174 1490 

3-15657-B 1140 17.898 1.31 18.60 5130 1490 

3-13569-B 4 0 9 17.930 0.48 5.51 1634 1492 

3-13989-B 3 0 2 18.150 0.09 5.21 1107 9 0 4 

3-12524-B 7 9 4 18.181 -0.05 18.18 3420 1135 

3-15648-B 1 1 5 18.410 -0.03 1.31 2 1 8 1 6 5 

3-15642-B 4 2 2 18.242 -0.09 9.24 1663 1 7 3 

3-12820-B 6 0 7 17.936 0.65 9.41 2578 2 4 2 

3-12151-B 5 4 0 18.174 0.02 11.51 2226 2 2 1 

3-00748-B 6 3 3 18.316 -0.39 16.27 2641 4 0 9 

Contaminants: soil 3-12152-B and Acme lead arsenate 

Pb ppm 206Pb/204Pb % 152-B % Acme Calc. As ppm Meas. As ppm 

3-15641-B 1490 17.910 48.83 0.48 1310 8 3 6 

3-15650-B 1560 17.801 81.87 0.10 2 8 9 1490 

3-15657-B 1140 17.898 41.07 0.30 8 3 7 1490 

3-13569-B 4 0 9 17.930 15.66 0.06 1 7 7 1492 

3-13989-B 3 0 2 18.150 0.81 0.17 4 7 4 9 0 4 

3-12524-B 7 9 4 18.181 -10.78 0.71 1935 1135 

3-15648-B 1 1 5 18.410 -1.94 0.06 1 7 3 1 6 5 

3-15642-B 4 2 2 18.242 -8.00 0.38 1042 1 7 3 

3-12820-B 6 0 7 17.936 20.08 0.16 4 4 3 2 4 2 

3-12151-B 5 4 0 18.174 -4.99 0.43 1188 2 2 1 

3-00748-B 6 3 3 18.316 -23.12 0.74 2028 4 0 9 

27 
Table 5. Cont'd. 

Sample Pb ppm 206Pb/204P b % Cont. C1 

2 % Cont. C2 3 Calc. As ppm4 

Meas. As ppm 

Contaminants: Soil 12152-B and average smelter material 

Pb ppm 206Pb/204Pb % 152-B % smelter Calc. As ppm Meas. As ppm 

3-15641-B 1490 17.910 55.05 12.52 2396 8 3 6 

3-15650-B 1560 17.801 83.21 2.69 5 2 3 1490 

3-15657-B 1140 17.898 45.03 7.98 1530 1490 

3-13569-B 4 0 9 17.930 16.47 1.62 3 1 8 1492 

3-13989-B 3 0 2 18.150 3.04 4.49 8 6 4 9 0 4 

3-12524-B 7 9 4 18.181 -1.56 18.55 3545 1135 

3-15648-B 1 1 5 18.410 -1.15 1.59 3 1 0 1 6 5 

3-15642-B 4 2 2 18.242 -3.05 9.96 1907 1 7 3 

3-12820-B 6 0 7 17.936 22.16 4.18 8 0 6 2 4 2 

3-12151-B 5 4 0 18.174 0.66 11.36 2173 2 2 1 

3-00748-B 6 3 3 18.316 -13.45 19.45 3716 4 0 9 

Contaminants: PAX and soil 3-10319-B 

Pb ppm 206Pb/204Pb % PAX % 319-B Calc. As ppm Meas. As ppm 

3-15641-B 1490 17.910 3.45 41.75 4165 8 3 6 

3-15650-B 1560 17.801 4.04 36.54 4870 1490 

3-15657-B 1140 17.898 2.66 30.44 3210 1490 

3-13569-B 4 0 9 17.930 0.88 9.02 1066 1492 

3-13989-B 3 0 2 18.150 0.47 8.52 5 7 0 9 0 4 

3-12524-B 7 9 4 18.181 1.27 29.75 1544 1135 

3-15648-B 1 1 5 18.410 0.06 2.15 8 3 1 6 5 

3-15642-B 4 2 2 18.242 0.58 15.12 7 1 0 1 7 3 

3-12820-B 6 0 7 17.936 1.33 15.40 1606 2 4 2 

3-12151-B 5 4 0 18.174 0.85 18.84 1038 2 2 1 

3-00748-B 6 3 3 18.316 0.79 26.63 9 6 2 4 0 9 

1

Calculations in bold represent agreement to within a factor of 2 for calculated vs. measured As contents. 

and no significant significant negative contribution from one of the contaminant sources. 

2

 X (*100) calculated from simultaneous solutions of equations 7 and 8. 

3

 Y (*100) calculated from simultaneous solutions of equations 7 and 8. 

4

 As concentration calculated from equation 7 and X and Y values calculated for Pb. 

28 
matter, may have been different from those observed in the existing samples depending 

upon the sources of lead used in the manufacture of the products. 

Focal versus Adjacent Samples 

Samples of residential soils adjacent to high- and intermediate-As soils have been 

analyzed (Table 1) in order to determine whether significant migration of Pb or As from the 

high-As properties has occurred. All of the adjacent samples analyzed in this study have <50 

ppm As (Table 1) which suggests that As migration to surrounding properties has been 

minimal. 

Pb isotopic compositions of the soil samples, this time coded by sample type, are 

shown in Figure 5. Low-As focal samples, which show large variations in Pb contents (Table 

1), span almost the entire range of isotopic values found within the residential soils. The single 

low-As adjacent sample analyzed has the highest 208Pb/204Pb and among the highest 

206Pb/204Pb. Data for the two high-As adjacent samples and the single intermediate-As 

adjacent samples are well within the range observed for other low-As (focal) samples, although 

the 206Pb/204Pb values for the high-As adjacent samples are slightly less radiogenic than those 

in the low-As and intermediate-As adjacent samples. 

Lead contents of the high-As adjacent samples are significantly higher than baseline 

values (150-260 ppm versus ≈70 ppm; Tables 1 and 3), but are well within the range of other 

low-As samples (Figure 6). However, the 206Pb/204Pb values in these two samples appear to 

be slightly lower than in other low-As samples with similar Pb contents. The data are therefore 

consistent with the interpretation that some migration of non-radiogenic Pb has occurred from 

the high-As properties to the surrounding properties. 

Samples from a Single Property 

Five samples, 3-12152-B, 3-12151-B, 3-12040-B, 3-12039-B, and 3-12035-B 

(Table 2), were collected from a single residential property in order to look at mixing 

relationships within a geographically restricted area. The samples show a wide range in Pb 

contents, 256-1730 ppm Pb. Four of the five samples have As < 50 ppm, but sample 3-

12151-B has 221 ppm As. Sample 3-12152-B represents the low-As contaminant source 

with nonradiogenic Pb (Table 3). 

Lead isotopic data for the five sediments are highly correlated with r2 values of 0.947 

and 0.961 for the 206Pb/204 Pb vs. 207Pb/204Pb and 206Pb/204 Pb vs. 208Pb/204Pb diagrams, 

respectively (Figure 7). The high degree of correlation suggests that the data are best 

29
15.7 

15.6 

208Pb/204Pb 207Pb/204Pb


N 

M 

319B 

152B 15.5 

15.4 

17.0 17.5 18.0 18.5 19.0 

206Pb/204Pb 

39.0 

38.5 

38.0 

37.5 

37.0 

36.5 

17.0 17.5 18.0 18.5 19.0 

N 

M 

319B 

152B 

206Pb/204Pb 

High-As focal 

High-As adjacent 

Intermediate-As focal 

Intermediate-As adjacent 

Low-As focal 

Low-As adjacent 

Average baseline 

M PAX 

N Acme lead arsenate 

3 Smelter materials 

Figure 5. 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb 

diagrams for focal vs. adjacent samples from the VBI70 study area. 

Samples are coded by As concentrations. Focal samples are shown by 

filled symbols and adjacent samples by open symbols. 

30

19.0


206Pb/204Pb
18.5 

18.0 

17.5 

17.0 

0.000 0.005 0.010 0.015 0.020 0.025 0.030 

N 

M 

319B 

152B 

1/Pb ppm 

High-As focal 

High-As adjacent 

Intermediate-As focal 

Intermediate-As adjacent 

Low-As focal 

Low-As adjacent 

Average baseline 

M PAX 

N Acme lead arsenate 

3 Smelter materials 

Figure 6. 206Pb/204Pb vs. 1/Pb concentrations for focal vs. 

adjacent samples from VBI70 study area. Samples are coded 

by As concentrations. 

31

explained in terms of mixing between two components. However, the best-fit line on the 

206Pb/204 Pb vs. 208Pb/204Pb diagram does not pass through the average baseline value. 

The 206Pb/204 Pb values are shown as a function of Pb contents (1/Pb) in Figure 8. The 

data are moderately well correlated with r2 = 0.884, but there is obvious scatter about the bestfit line, and the best-fit line passes nowhere near the average baseline value. The results again 

suggest that there are at least two Pb components in these soils. One of the components has 

isotopic characteristics similar to soils 3-12152-B (206Pb/204 Pb ≈ 17.8, Pb ≥ 1730 ppm). The 

other has 206Pb/204 Pb ≥ 18.45 but Pb ≤ 256 ppm. 

The high lead component is clearly a contaminant, but the nature of the other end 

member is somewhat problematic. The trend may suggest that baseline isotopic ratios in this 

soil are much more radiogenic than in other low-As soils (the baseline values lie on the upper 

right extension of the mixing line in Figure 8). 

Another way in which low-Pb end-member could be explained is that this end member 

is a mixture between baseline Pb and a second contaminant with radiogenic Pb. If the two 

contaminants (i.e. those represented by soils 3-12152-B and 3-10319-B) were randomly 

distributed within the soils at this property, then a linear trend in Figure 8 would not be 

expected. The data would simply fall anywhere within an envelope defined by average 

baseline and soils 3-12152-B and 3-10319-B. The fact that a scattered linear trend does exist 

may suggest that variable amounts of contamination from the source represented by soil 3-

12152-B were superimposed onto a soil that was more or less uniformly contaminated by the 

radiogenic -Pb contaminant. 

The presence of two contaminants is consistent with the results from multiple aciddigestions of sample 3-12152-B (Tables 2 and A2-4). There is more isotopic variation among 

the replicate digestions than is easily accounted for if only a single contaminant is involved. The 

isotopic variation may suggest that one of the contaminants is not being totally dissolved by 

the acid-digestion procedure or that one of the contaminants is heterogeneously distributed 

within the soil at a scale comparable to the sample sizes. 

The elevated As content found in sample 3-12151-B is not accounted for by either of 

the scenarios outlined above. This sample is intermediate in terms of both Pb contents and 

isotopic compositions, but has As contents almost an order of magnitude higher than in any of 

the other soils from this property. These observations suggest that yet another contaminant is 

present in this soil. Although the nature of this contaminant can not be determined, it must have 

very high As/Pb such that addition of this material does not significantly affect relationships 

between Pb concentrations and Pb isotopic compositions in Figure 8. 

The results from soils from this property and the fact that there is such a large range in Pb 

contents among low-As soils in general (Figure 4) clearly indicate that the Pb and As 

32
15.7 

15.6 

E E E

E 

E 

E E 

E E

E 

E 

E E E 

E 

E B 

EE E

E 

EE 

B 

B B 

B 

J 

3 

33 

M 

N 

r2 = 0.947 

319B 

208Pb/204Pb 207Pb/204Pb

15.6 

15.5 

15.5 

17.0 17.5 18.0 18.5 19.0 

39.0 

38.5 

38.0 

37.5 

37.0 

36.5 

17.0 17.5 18.0 18.5 19.0 

206Pb/204Pb 

E 

E E

E

E 

EE 

E 

E

E 

E 

E E

E 

E 

E EE B 

EE E 

E 

B 

B B 

B 

J 333 

M 

N

r2 = 0.961 

319B

206Pb/204Pb 

Nominal Range Actual Range 

As < 50 ppm (<11 - 26 ppm) 

As = 50-250 ppm (221 ppm) 

High-Pb soil 3-10319-B 

Smelter materials 

PAX 

Acme Pb-Arsenate 

Av. Baseline 

B 

3 

M 

N 

Figure 7. 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb vs. 208Pb/204Pb diagrams 

for samples from a single residential property. Samples from the property are 

coded by As concentrations. Other samples from the VBI70 study area are shown 

for reference by open circles. Best-fit lines through the five points yield good linear 

arrays with r2 = 0.947 and 0.961 for 206Pb/204Pb vs. 207Pb/204Pb and 206Pb/204Pb 

vs. 208Pb/204Pb diagrams, respectively. 

33 
19.0

206Pb/204Pb18.5 

18.0 

17.5 

17.0 

0.000 0.005 0.010 0.015 

333 

M 

N 

B 

r2 = 0.884 

319B 

1/Pb ppm 

Nominal Range Actual Range 

As < 50 ppm (<11 - 26 ppm) 

As = 50-250 ppm (221 ppm) 

High-Pb soil 3-10319-B 

Smelter materials 

PAX 

Acme Pb-Arsenate 

Av. Baseline 

B 

3 

M 

N 

Figure 8. 206Pb/204Pb vs. 1/Pb concentrations for samples from 

a single residential property. Samples from the property are 

coded by As concentrations. The five points determine a moderately well correlated linear array with r2 = 0.884. The broken line 

represents mixing between average baseline soil and soil 3-

10319-B (319B). 

34

contamination in the soils are not entirely related to the same sources. However, the absence or 

at least paucity of high-As, low-Pb soils within the sample set (Figure 1), the nonradiogenic 

character of the Pb in many of the high-As samples, and the presence of significant amounts of 

Pb arsenate in many of the soils (Laboratory for Environmental and Geochemical Studies, 

2001) suggest that there are high-Pb+As sources responsible for much of the As 

contamination in these soils. Identification of the exact number and nature of the contaminants 

within these soils must await detailed studies of individual properties. 

Conclusions 

(1) At least six potential sources of lead have been found in selected soil samples from 

the Vasquez Blvd.-I70 study area based on the relationships between the isotopic 

compositions and Pb concentrations in the soils: (1) an average “baseline” soil with 206Pb/204Pb 

≈ 18.48 and Pb ≈ 70 ppm, (2) smelter soil and waste with 206Pb/204Pb = 18.12-18.19 and Pb 

= 2100-5600 ppm, (3) PAX, a commercial herbicide, with 206Pb/204Pb = 17.2 and Pb = 24300 

ppm, (4) Acme brand lead arsenate pesticide with 206Pb/204Pb = 18.07 and Pb = 128000 

ppm, (5) an unidentified contaminant with 206Pb/204Pb ≥ 18.8 inferred from residential soil 3-

10319-B, and (6) an unidentified contaminant with 206Pb/204Pb ≤ 17.8 inferred from residential 

soil 3-12152-B. Sources 2,3, and 4 are Pb and As contaminants whereas sources 5 and 6 

appear to be Pb contaminants only. 

(2) The isotopic compositions of the smelter materials and the Acme lead arsenate are 

similar enough that it is not possible to distinguish between these two sources as potential Pb 

contaminants. 

(3) The Pb isotopic data for two of the high-As soils (3-13989-B and 3-12524-B) are 

consistent with the interpretation that they represent contamination of average baseline soil with 

small amounts of either Acme lead arsenate or smelter material. Confirmation of this 

interpretation must await detailed geochemical analyses and speciation studies of these soils. 

(4) The other high-As soils (3-15641, 3-15650-B, 3-15657-B, and 3-13569-B) and 

one intermediate-As soil (3-12820) have 206Pb/204Pb values lower than those found in either 

the smelter materials or Acme lead arsenate. These samples must have within them a 

component of PAX (or a similarly nonradiogenic Pb-rich material). However, the Pb data for 

these samples cannot be explained by simple two-component mixing involving contamination 

of baseline soil with PAX. These data may reflect the presence of more than one Pb 

35Ä
contaminant or the presence of an as yet unidentified Pb and As contaminant with 206Pb/204Pb ≈ 

17.8. 

(5) The Pb and As data for the intermediate- and high-As soils are reasonably 

consistent with three-component mixing models involving baseline soil and two contaminants 

(Table 5). However, different contaminants are required for each of the soils and unique 

solutions were not obtained for most of the soils. The presence of smelter material or Acme 

lead arsenate in these soils are neither eliminated nor required by the calculations. 

(6) Low-As soils show a wide range in Pb concentrations (39-1730 ppm) and isotopic 

compositions (206Pb/204Pb = 17.8-18.8). These results clearly indicate that Pb and As 

contamination is at least partially decoupled within these soils. Furthermore, if such large and 

variable amounts of Pb-only contamination are found in the low-As samples, one might 

logically conclude that this type of Pb-only contamination is also present in intermediate- and 

high-As soils. If true, then the two-component mixing calculations involving average baseline 

Pb would not necessarily be valid. 

(7) Analyses of soils adjacent to high-As soils are consistent with some migration of Pb 

from the high-As, high-Pb soils to the surrounding properties. However, given the wide 

variation in Pb contents and isotopic compositions found among low As focal soils, the 

evidence for Pb migration should be regarded as tentative. 

(8) Analyses of five soils from a single residential property confirm the complex nature 

of the contamination in these soils. The Pb concentration and isotopic data require either the 

presence of two Pb contaminants or a single contaminant and unusual “baseline” with an 

elevated Pb concentration and/or 206Pb/204Pb. In addition a third contaminant with high As and 

low Pb is required to account for the As content in one of the five soils from this property. 

Suggestions for Future Work 

Based upon the preceding discussion, it is quite evident that Pb and As contamination in 

these soils is a complex issue. In some instances Pb and As contamination appear unrelated 

whereas in other instances there is good evidence for a single principal Pb and As contaminant. 

Furthermore, at least 5 potential Pb contaminants and three potential As contaminants have 

been identified. Although the presence of PAX in four of the 6 high-As samples is indicated 

by the data, unique solutions could not be obtained regarding the involvement of other 

potential Pb and As sources. 

Detailed studies of soils from individual properties, such as that represented by Figures 

7 and 8 should be performed. This procedure is based on the assumption that individual 

36
properties would represent less complex systems than the study area as a whole. That is, 

only a restricted number of contaminants would be present at any one property. Five to six 

samples would generally be required and the samples would have to show a significant range 

in Pb (and ideally As) concentrations. Ideally, Pb and As contents should show a good 

correlation among the soils from a single property. The Pb data for each individual property 

would be plotted as in Figures 4, 6, and 8 and the data evaluated as outlined below. 

(1) The data define no linear trend on a plot of 206Pb/204Pb vs. 1/Pb ppm. This would 

mean that multiple Pb contaminants are present and are not homogeneously distributed within 

the soils. In this situation it may not be possible to use Pb isotopic data to identify the 

source(s) of As or Pb contamination. 

(2) The data define a linear trend, but there is no correlation with As contents in the 

samples. This is the situation observed for the samples in Figure 8 and would indicate that Pb 

and As are derived from separate sources. The Pb isotopic data for these samples would 

therefore not be a valid tool for evaluating the source of the As contamination. However, the 

intersection of the mixing trend with the Y-axis would define the 206Pb/204Pb of one of the Pb 

contaminants. 

(3) The data define a linear trend with at least moderately good correlation with As 

contents. A single source for Pb and As contamination would be suggested. The y-axis 

intercept of the mixing trend defines the 206Pb/204Pb of the Pb and As contaminant in this soil. 

Answers to Specific Questions Posed by USEPA 

Question 1. Do the isotopic ratios for lead in residential site soils depend on the level of either 

arsenic or lead present in the sample? If so, what are the characteristic ratios for high-lead or 

arsenic samples compared to low-lead or arsenic samples? 

Answer. Soils with high Pb contents show isotopic values that span the entire range of those 

observed within the soils with 206Pb/204Pb ≈ 17.8-18.8. Low-Pb soils show a much more 

restricted range with 206Pb/204Pb ≈ 18.3-18.5. 

Four of six high-As samples have 206Pb/204Pb < 18.0 whereas two other high-As 

samples have 206Pb/204Pb ≈ 18.1-18.2. All high-As samples analyzed are also high in Pb. 

37
With two exceptions (both from the same residential property), low As soils are 

characterized by 206Pb/204Pb > 18.3. 

Question 2. Do the isotopic ratios observed in residential site soils resemble one or more 

of the potential source materials (Globe soils, PAX, lead arsenate pesticide)? If so, how 

does this depend on lead or arsenic level in the residential soil? 

Answer. Four of six high-As samples have lower 206Pb/204Pb than found in any of the 

identified contaminants except PAX. Two high-As samples have 206Pb/204Pb similar to 

smelter material and Acme lead arsenate. Low-As and low-Pb soils have Pb isotopic 

values similar to baseline values. 

Question 3. Does this approach allow you to identify one or more of the potential source 

materials as being more likely for the observed contamination in site soils than the other 

potential source materials? 

Answer. Four of six high-As samples must have some PAX or isotopically similar material 

within them. Specific sources could not be identified for the other two high-As samples. 

The presence of smelter material or Acme lead arsenate is neither required nor eliminated 

by the Pb isotopic data in any of the soils. 

References cited: 

Cantanzaro, E.J., Murphy, T.J., Shields, W.R., and Garner, E.L., 1968, Absolute isotopic 

abundance ratios of Common, Equal Atom, and Radiogenic lead isotopic standards: 

Journal of research of the national bureau of standards, v. 72A, p. 261-267. 

Church, S.E., Kimball, B.A., Fey, D.L., Ferderer, D.A., Yager, T.J., and Vaughn, R.B., 1997, 

Source, transport, and partitioning of metals between water, colloids, and bed sediments 

of the Animas River, Colorado: U.S. Geological Survey Open-File Report 97-151, 135pp. 

Fey, D.L., Unruh, D.M., and Church, S.E., 1999, Chemical data and lead isotopic 

compositions in stream-sediment samples from the Boulder River Watershed, Jefferson 

County, Montana: U. S. Geological Survey Open File Report 99-575, 147 pp. 

Faure, G., 1977, Principles of Isotope Geology, John Wiley & Sons, New York, 464pp. 

Laboratory for Environmental and Geological Studies, 2001, Extended laboratory report, 

VBI70 project, Denver, Colorado: Department of Geological Sciences, University of 

Colorado Administrative Report. 

38
Ludwig, K.R., 1980, Calculation of uncertainties of U-Pb isotope data: Earth and Planetary 

Science Letters, v. 46, p. 212-220. 

Ludwig, K.R., 1994, Isoplot: a plotting and regression program for radiogenic-isotope data: 

U.S. Geological Survey Open-File Report 91-445, version 2.75, October, 1994, 45pp. 

Murphy, C., Briggs, P., Adrian, B., Wilson, S., Hageman, P., and , P., 1997, Chain of 

Custody—Recommendations for acceptance and analysis of evidentiary geochemical 

samples: U.S. Geological Survey Circular 1138, 26 pp. 

Tatsumoto, M. and Unruh, D.M., 1976, KREEP basalt age: Grain by grain U-Th-Pb 

systematics study of the quartz monzodiorite clast 15405,88: Proceedings of the 7th Lunar 

Science Conference, p. 2107-2129. 

Todt, W., Cliff, R.A., Hanser, A., and Hofmann, 1993, Re-calibration of NBS lead standards 

using a 202Pb-205Pb double spike: Abstracts presented to the 7th meeting of the European 

Union of Geosciences, p. 396. 

Unruh, D.M., Fey, D.L., and Church, S.E., 2000, Chemical data and lead isotopic compositions 

of geochemical baseline samples from streambed sediments and smelter slag, lead isotopic 

compositions in fluvial tailings, and dendrochronology results from the Boulder River 

Watershed, Jefferson County, Montana: U. S. Geological Survey Open-File Report 00-

0038, 75pp. 

U. S. Environmental Protection Agency, 1992, EPA SW 846 Method 3050B, Revision 2, 

December, 1996. 

U. S. Environmental Protection Agency, 1999, Project plan for the Vasquez Boulevard & I 70 

site, Denver, Colorado, pilot scale soil characterization study (Sept. 9, 1999). 

Vandecasteele, C. and Block, C.B., 1993, Modern Methods for Trace Element Determination: 

Wiley and Sons Ltd., U.K., 330 pp. 

39
Appendix 1. Standard analytical procedures for lead isotopic analyses (nitric acidhydrogen peroxide digestion) of metal-rich samples. 

A. Acid digestion 

1. Samples are digested in accordance with Section 7.2 of method 3050B, EPA SW 846, 

revision 2 (12/96), with minor modifications to accommodate smaller sample sizes. 

Although the Pb isotopic analyses would require sample sizes of only a few milligrams 

for most samples, sample sizes of approximately 50 mg to 200 mg are digested in 

order to ensure that they are representative of the bulk sample. 

2. 	2 to 3 ml of 1:1 nitric acid (HNO3) are added to the sample in a PFA-teflon, screw-cap 

container. The sample is heated to near boiling for 15 minutes in the closed container. 

The sample is allowed to cool, and 1ml of concentrated HNO3 is added. If brown fumes 

are observed, another 1 ml of HNO3 is added and the procedure repeated until fuming 

is no longer observed. The solution is then heated to near boiling for approximately 2 

hours in the closed container. 

3. 	The sample is allowed to cool and 1 ml of water and 1 ml of 30% hydrogen peroxide 

(H2O2) are added to the sample. Additional H2O2 is added in 0.1-ml increments until 

either strong effervescence subsides or until the general sample appearance remains 

unchanged. 

4. 	The sample is heated to just below boiling for approximately two hours in the closed 

container. 

5. 	The sample is allowed to cool and is diluted to 15 ml with deionized water. The sample 

is centrifuged and the supernatant transferred to an HDPE bottle. 

6. A lead blank of 0.3 ng (10-9 g) was obtained for this procedure. 

B. Ion Exchange Chromatography 

1. 	An aliquot of the solution, corresponding to approximately 0.5 ug of Pb based on 

preliminary estimates of the Pb concentration provided by the submitter, is 

transferred to a teflon beaker and evaporated to dryness 

2. 	1-2 ml of 6M hydrochloric acid (HCl) is added to the sample and again evaporated 

to dryness. Then 0.5-1 ml of 1.0-1.2 N hydrobromic acid (HBr) is added to the 

sample and is warmed gently for 5-10 minutes. 

3. 	The sample is allowed to cool, centrifuged, and the supernatant is loaded onto an 

anion-exchange column (0.8-1.0 ml resin volume) using Dowex AG1-X8 anion 

exchange resin. 

4. The column is washed with 1.0-1.2N HBr and water. 

40Ä
5. The Pb is eluted from the column with 6-8N HCl. 

6. The sample is evaporated to dryness and ≈0.25 ml of 1.2N HBr is added. 

7. 	The sample is loaded onto a second anion exchange column (AG1-X8 resin) with a 

resin volume of 0.1-0.2 ml. 

8. The column is washed with 1.0-1.2N HBr and water. 

9. Lead is eluted from the column with dilute (≤1M) Nitric Acid. 

10. 	2-3 drops of dilute (0.25-0.5%) phosphoric acid are added to the sample and it is 

evaporated to dryness. 

11. Samples are then ready to be loaded onto filaments for mass spectrometry. 

12. A Pb blank of 0.09 ng was obtained for this procedure. 

C. Mass spectrometry 

Data for samples and standards run during this study were acquired using the following 

procedure: 

1. 	Aliquots of the samples were loaded onto rhenium filaments with colloidal silica gel 

and loaded into the mass spectrometer. Sample filaments were heated to 2.3-2.4 

amp. 

a. 	During the initial stages of the study approximately 1/3 of the sample was loaded 

onto the filament. However, this produced an ion-beam intensity of >10-10 Amp 

for 208Pb, the maximum measurable signal. Consequently, approximately 1/2-

1/3 of this amount was loaded for subsequent analyses. 

2. The temperature of the filament was adjusted to 1250°C ± 5% (1187-1313°C). 

a. 	The temperature was determined by the data-acquisition software from a 

pyrometer supplied with the mass spectrometer. These temperatures may be 

≈ 120°C lower than the true filament temperatures as determined by other 

pyrometers. 

3. The beam was centered and focussed using the most intense peak, 208Pb. In most 

instances the initial current was less than 1250°C so that samples were run at the 

lower end of the temperature window. In many instances, a 208Pb intensity of >10-10 

amp, the maximum signal allowed, was obtained. The filament current was then 

decreased until appropriate beam intensity was achieved. 

4. 	Isotopic data for all samples and standards are obtained using “L1, H1, H2, and H3” 

collectors for 204Pb, 206Pb, 207Pb, and 208Pb, respectively. Data are acquired in “static” 

mode using on line software designed by the manufacturer. Each run consisted of 4 

41Ä
blocks of 12 data cycles. All peaks in each data cycle are measured simultaneously 

with 5 to 8 one-second integrations per cycle. Baselines at the half-mass positions 

are measured at the beginning and end of each block. The software provided by 

the manufacturer (Sector 54-relase 3.974) calculates a running average and statistical 

data. 

42
Appendix 2. Quality Assurance and Quality Control (QA and QC) 

Reagent Preparations and Analytical Blanks 

Nitric, Hydrochloric and Hydrobromic acids are purified by sub-boiling distillation within 

the analyst’s laboratory from commercially available reagent grade acids. Dilute acids are 

prepared by titration of the distilled concentrated acid followed by dilution with the appropriate 

calculated amount of deionized water for the desired final concentration. 

Deionized water is prepared from filtered tap water by passing it through two mixedbed resin filters, an activated charcoal filter, and a “Milli-Q plus” commercial deionizing system. 

Measured Pb blanks for the purified acids and water are typically in the range of 0.001-0.01 

ppb (10-9 g/ g). 

Phosphoric acid is diluted from commercial analytical grade acid with deionized water. 

Hydrogen peroxide is commercial reagent grade material. Ion-exchange resin is commercially 

available analytical grade anion exchange resin 

Reagents are considered acceptable if the overall chemical procedure blank is less than 

the amount required to alter the lead isotopic composition (206Pb/204Pb ) of the sample by 

more than the maximum allowable 1 sigma-mean (0.05%). If the total blank exceeds 

specifications, then work is halted and each of the component reagents is checked to 

determine the source of Pb in the blank. Samples processed with the unacceptable blank are 

discarded and the analyses repeated when the problem has been corrected. Data obtained 

from samples run under conditions of an unacceptable blank are deemed unusable. 

The maximum allowable blank is calculated based on the Pb isotopic differences 

between the samples and the blank. The maximum blank is calculated to be that amount which 

when added to the sample will change the isotopic composition of the sample by no more 

than the target 1 sigma-mean precision. For example, the target precision for 206Pb/204Pb is 

0.05%. If the sample 206Pb/204Pb were 17.0, then the maximum blank contribution would have 

no more than a 0.05% effect on the 206Pb/204Pb of the sample, or an effect of 0.0085 absolute. 

With a blank 206Pb/204Pb of 19.0, the maximum Pb mass allowable in the blank for 0.5 ug of 

sample lead can be calculated in the following manner: 

(0.5*17.0+X*19.0)/(0.5+X) = 17.0085 

Where x is the maximum blank in ug. 

43
Solving for x = 0.0021ug (2.1 ng) or 0.4% of the sample Pb. If the 206Pb/204Pb of the 

sample is closer to that of the blank, then the maximum allowable blank increases, for 

example to 4.5ng for a sample 206Pb/204Pb = 18.0. 

Because samples were aliquotted after acid-hydrogen peroxide digestion, it is 

necessary to evaluate the Pb blank in both the digestion procedure and the total chemical 

procedure in order to ensure that the Pb blank does not exceed specifications at any stage of 

the overall procedure. A Pb blank of 0.3 ng (10-9g) was obtained for the acid and hydrogen 

peroxide digestion procedure (procedure “A” in Appendix 1). The minimum amount of Pb 

present in any of the acid digestions (sample 3-15644-B) was approximately 9 ug. Thus the 

blank contribution was less than 0.004%. 

A total-chemistry blank was processed in the same manner as the samples. A 1-ml 

aliquot from the digestion procedure was processed through ion-exchange chemistry 

(procedure “B” in Appendix 1). Therefore, this blank is a measure of both the Pb contributions 

from the acid digestion to a sample aliquot as well as the subsequent chemical procedures 

applied to the aliquot. A total-chemistry blank of 0.09 ng was obtained. The average amount 

of sample lead processed through chemistry was 0.5 ug so that the blank contribution was less 

than 0.02%. Consequently both blanks were well within specifications and no blank correction 

was applied to the isotopic data. 

Mass Spectrometry Calibrations. 

Pb isotopic data are acquired using a VG-Sector 54, 7-collector, thermal ionization mass 

spectrometer. Up to twenty samples may be loaded into the mass spectrometer at a time. 

The positions of the individual samples on the turret are recorded in the mass spectrometer 

logbook and the software supplied by the manufacturer keeps track of the turret. A turret of 

samples will normally consist of at least two primary standards (NIST SRM-981) which are 

used to monitor mass fractionation induced by the mass spectrometry, a second primary 

standard (NIST SRM 982), and seventeen unknown samples. Data are collected using “static” 

mode (each isotope is measured simultaneously on a separate collector). At least one SRM 

981 standard is run at the beginning, and the other at the end of the analyses of the unknowns. 

At the beginning of each group of analyses or daily when a set of analyses requires 

more than one day to complete, the collector gains are calibrated using a computerized subroutine supplied by the manufacturer of the Mass Spectrometer. 

Should the gain calibrations not be of acceptable precision based on print out from the 

calibration sub-routine, the following steps are taken: (1) the calibration is repeated. (2) If the 

second calibration also fails, the electronics to the mass spectrometer are shut off, and the 

software rebooted. The electronics are then turned back on and the calibration repeated. (3) If 

44
the third calibration also fails, step (2) is repeated. (4) If the 4th calibration fails, then all work is 

stopped until a qualified electronics technician is able to examine and correct the problem. 

Mass Spectrometry Standards 

Analyses of NIST standard SRM 981 (Cantanzaro and others, 1968; Todt and others, 

1993) are used to monitor mass fractionation during mass spectrometry. At least two 

standards are loaded into the mass spectrometer for each batch of samples (at least 10% of 

the runs are primary standards). Analyses of NIST standard SRM 982 (Cantanzaro and 

others, 1968; Todt and others, 1993) are also used to check the mass fractionation corrections 

(at least 1 in 20 or 5%). 

Data are acquired at filament temperatures of approximately 1150-1350C, as 

determined by the pyrometer supplied with the mass spectrometer (Unruh and others, 2000). 

Data are considered acceptable if the standard error (one sigma mean based on the total 

number of cycles) is better than ± 0.05% for 206Pb/204Pb, and better than ±0.02% for 

207Pb/206Pb and 208Pb/206Pb as determined by the on-line data reduction program provided by 

the manufacturer of the mass spectrometer. 

The principal source of analytical uncertainty in mass spectrometric measurements of Pb 

isotopes is mass fractionation induced during mass spectrometry. Lighter-mass isotopes are 

more efficiently evaporated from the filament and transported down the flight tube to the 

detector. Consequently, the measured isotopic ratio of a heavier mass to a lighter mass (e.g. 

206Pb/204Pb) will generally be slightly lower than the true value. Standards with certified isotopic 

compositions are run so that mass fractionation effects may be determined. Corrections to the 

isotopic ratios of the sample data are made based on the amount of mass fractionation 

determined from the analyses of the standards. 

Results for twenty-three analyses of NIST standard SRM 981 are summarized in Table 

A2-1. Apparent mass fractionation factors are calculated for three isotope ratios, 206Pb/204Pb, 

207Pb/206Pb, and 208Pb/206Pb using a linear mass fractionation law as follows (e.g. Ludwig, 

1980): 

Fa = [(Rt / Ra)-1] *100 / ∆M 

Where Fa is the fractionation factor in percent per AMU for a given isotopic ratio, Ra is 

the measured ratio, Rt

 is the true ratio, and ∆M is the nominal mass difference between the two 

isotopes ( 2 for 206Pb/204Pb and 208Pb/206Pb, and 1 for 207Pb/206Pb). The calculated fractionation 

45
Table A2-1. Lead isotopic analyses of NIST standard SRM 981 

Sample Temp 206Pb/204Pb1 Certified Values3 16.9370 

% S.E. 0.0630 

207Pb/206Pb1 

0.914640 

% S.E. 0.0360 

208Pb/206Pb1 

2.16810 

% S.E. 

0.0370 

F 206Pb/204Pb2 F 207Pb/206Pb2 

Redetermined Values4 16.9322 0.0047 0.914561 0.0044 2.16662 0.0060 SRM 981-1 1123 16.8981 0.0018 0.913672 0.0006 2.16241 0.0009 0.101 ± 0.001 0.097 ± 0.001 SRM 981-2 1118 16.8994 0.0015 0.913717 0.0021 2.16243 0.0010 0.097 ± 0.001 0.092 ± 0.002 SRM 981-3 1218 16.9147 0.0064 0.913891 0.0020 2.16390 0.0039 0.052 ± 0.003 0.073 ± 0.006 SRM 981-4 1192 16.9118 0.0022 0.914069 0.0008 2.16418 0.0012 0.060 ± 0.001 0.054 ± 0.001 SRM 981-5 1192 16.9051 0.0031 0.913902 0.0009 2.16341 0.0023 0.080 ± 0.002 0.072 ± 0.001 SRM 981-6 1190 16.9093 0.0046 0.913942 0.0014 2.16400 0.0033 0.068 ± 0.002 0.068 ± 0.001 SRM 981-7 1190 16.8991 0.0028 0.913753 0.0011 2.16281 0.0023 0.098 ± 0.001 0.088 ± 0.001 SRM 981-8 1188 16.8988 0.0024 0.913724 0.0007 2.16280 0.0014 0.099 ± 0.001 0.092 ± 0.001 SRM 981-9 1163 16.9009 0.0037 0.913732 0.0014 2.16282 0.0031 0.093 ± 0.002 0.091 ± 0.001 SRM 981-9 1206 16.9167 0.0044 0.914124 0.0013 2.16495 0.0032 0.046 ± 0.002 0.048 ± 0.001 SRM 981-10 1151 16.9007 0.0025 0.913750 0.0008 2.16277 0.0012 0.093 ± 0.001 0.089 ± 0.001 SRM 981-10 1210 16.9165 0.0022 0.914118 0.0007 2.16474 0.0013 0.046 ± 0.001 0.048 ± 0.001 SRM 981-11 1179 16.9038 0.0021 0.913827 0.0011 2.16320 0.0021 0.084 ± 0.001 0.080 ± 0.001 SRM 981-11 1174 16.9045 0.0035 0.913817 0.0008 2.16316 0.0007 0.082 ± 0.002 0.081 ± 0.001 SRM 981-12 1186 16.9049 0.0035 0.913808 0.0017 2.16314 0.0033 0.081 ± 0.002 0.082 ± 0.002 SRM 981-12 1230 16.9265 0.0045 0.914329 0.0020 2.16580 0.0039 0.017 ± 0.002 0.025 ± 0.002 SRM 981-13 1113 16.8910 0.0042 0.913649 0.0010 2.16190 0.0013 0.122 ± 0.002 0.100 ± 0.001 SRM 981-13 1185 16.9040 0.0032 0.913857 0.0009 2.16322 0.0016 0.083 ± 0.002 0.077 ± 0.001 SRM 981-14 1195 16.8919 0.0042 0.913506 0.0017 2.16148 0.0030 0.119 ± 0.002 0.115 ± 0.002 SRM 981-15 1136 16.8854 0.0033 0.913242 0.0011 2.16407 0.0017 0.139 ± 0.002 0.144 ± 0.001 SRM 981-16 1315 16.9230 0.0089 0.914055 0.0022 2.16517 0.0063 0.027 ± 0.004 0.055 ± 0.002 SRM 981-17 1234 16.9185 0.0133 0.913869 0.0036 2.16381 0.0058 0.040 ± 0.007 0.076 ± 0.004 SRM 981-20 1238 16.9102 0.0064 0.914049 0.0020 2.16435 0.0046 0.065 ± 0.003 0.056 ± 0.002 SRM 981-21 1198 16.9031 0.0028 0.913921 0.0013 2.16343 0.0028 0.086 ± 0.001 0.070 ± 0.001 SRM 981-22 1216 16.9034 0.0039 0.913867 0.0017 2.16326 0.0032 0.085 ± 0.002 0.076 ± 0.002 SRM 981-23 1212 16.9013 0.0067 0.913771 0.0018 2.16294 0.0034 0.091 ± 0.003 0.086 ± 0.002 SRM 981-24 1190 16.9015 0.0023 0.913786 0.0005 2.16302 0.0006 0.091 ± 0.001 0.085 ± 0.001 SRM 981-25 1197 16.8954 0.0024 0.913513 0.0010 2.16186 0.0011 0.109 ± 0.001 0.115 ± 0.001 MEAN, ALL5 N=28 16.9048 0.0570 0.913830 0.0230 2.16326 0.0470 0.081 ± 0.029 0.080 ± 0.023 

MEAN, T<11405 N=4 16.8935 0.0460 0.913570 0.0250 2.16180 0.0540 0.115 ± 0.023 0.108 ± 0.025 

MEAN, T>12205 N=4 16.9200 0.0570 0.914080 0.0280 2.16480 0.0570 0.036 ± 0.029 0.053 ± 0.028 MEAN, T = 1140-12205 N=20 16.9043 0.0440 0.913833 0.0210 2.16325 0.0420 0.083 ± 0.022 0.080 ± 0.021 

Correction factors6 0.166 ± 0.044 0.080 ± 0.021 

MEAN T = 1140-12207 N=20 16.9043 0.0440 0.913833 0.0210 2.16325 0.0420 0.097 ± 0.022 0.088 ± 0.021 

Runs for samples received 04/01-05/01 981-24-VB137 1190 16.9015 0.0023 0.913786 0.0005 2.16302 0.0006 0.091 ± 0.001 0.085 ± 0.001 981-25-VB139 1197 16.8954 0.0024 0.913513 0.0010 2.16186 0.0011 0.109 ± 0.001 0.115 ± 0.001 981-26-VB140 1168 16.9039 0.0036 0.913899 0.0014 2.16207 0.0141 0.084 ± 0.002 0.072 ± 0.001 981-26-VB140 1143 16.9109 0.0038 0.914072 0.0009 2.16425 0.0007 0.063 ± 0.002 0.053 ± 0.001 981-27-VB158 1145 16.9019 0.0069 0.913814 0.0024 2.16321 0.0049 0.090 ± 0.003 0.082 ± 0.002 

46 
Table A2-1, Cont'd. 

Sample Temp 206Pb/204Pb1 % S.E. 207Pb/206Pb1 % S.E. 208Pb/206Pb1 % S.E. F 206Pb/204Pb2 F 207Pb/206Pb2 981-28-VB160 1166 16.9034 0.0022 0.913757 0.0008 2.16302 0.0014 0.085 ± 0.001 0.088 ± 0.001 981-29-VB178 1091 16.9045 0.0019 0.913759 0.0006 2.16244 0.0009 0.082 ± 0.001 0.088 ± 0.001 981-29-VB178 1166 16.9106 0.0031 0.914159 0.0016 2.16354 0.0030 0.064 ± 0.002 0.044 ± 0.002 981-30-VB180 1157 16.9071 0.0039 0.913788 0.0017 2.16306 0.0031 0.074 ± 0.002 0.085 ± 0.002 981-31-VB190 1165 16.9114 0.0029 0.913994 0.0006 2.16374 0.0005 0.061 ± 0.001 0.062 ± 0.001 MEAN, ALL5 N = 10 16.9051 0.0300 0.913850 0.0200 2.16302 0.0340 0.080 ± 0.015 0.078 ± 0.020 

MEAN, T = 1140-12205 N = 9 16.9051 0.0310 0.913860 0.0210 2.16309 0.0350 0.080 ± 0.016 0.077 ± 0.021 

Correction factors6 0.160 ± 0.032 0.077 ± 0.021 

MEAN T = 1140-12207 N = 9 16.9051 0.0310 0.913860 0.0210 2.16309 0.094 ± 0.015 0.085 ± 0.020 

1 Uncertainties are 1 standard error (1 S.E.≈ 1 sigma mean) in percent. 2 Calculated fractionation factors in percent per AMU for each ratio, uncertainties are at 1 S.E. in percent. 

Calculations are relative to the redermined values of Todt and others, 1993, unless otherwise noted. 

3 Certified values from Cantanzaro and others, 1968. 4 Redetermined values from Todt and others, 1993. 5 Uncertainties correspond to 1 sigma. Preferred values shown in bold. 6 mass fractionation correction factors applied to sample isotopic ratios. 7 Calculated relative to the values of Cantanzaro and others, 1968. Uncertainties are 1 sigma. 

47 
factors also account for small possible differences in collector efficiencies and are therefore 

calculated separately for each isotope ratio. 

The amount of mass fractionation for a given run is a function of several variables including 

filament thickness; relative amounts of Pb, phosphoric acid, and silica gel; the particle size of the 

silica gel; and the filament temperature during data acquisition. Although the calculated 

fractionation factors do appear to vary with temperature (Table A2-1), the relationships are not 

particularly well correlated (r2 ≤ 0.6). Consequently, no attempt was made to “fine tune” the 

calculations as a function of running temperature. Instead, a temperature interval of 

approximately 1140°C-1220°C was selected to represent acceptable running conditions. 

This temperature interval shows reasonably consistent calculated fractionation factors, and 

encompasses the bulk of the standard runs (mean values in bold type in Table A2-1). Data 

were accepted for only those samples that were also run within this temperature interval. 

Mean calculated fractionation factors are shown relative to both the original certified 

values for NIST standard SRM-981 (Cantanzaro and others, 1968) and to the more precise 

redetermined values of Todt and others (1993). The calculated fractionation factors relative to 

the redetermined standard values are more internally consistent than those relative to the 

original values. Consequently, the factors calculated relative to the redetermined values were 

used to correct the sample data. 

As a test of the correction procedures, twelve samples of NIST standard SRM-982 

were run. Fractionation factors for each isotope ratio for each SRM 982 standard run have been 

calculated in the manner outlined above. Fractionation factors for the means of the twelve 

analyses have been calculated relative to both the original certified values (Cantanzaro and 

others, 1968) and the redetermined values (Todt and others, 1993). The results relative to the 

original certified values are more internally consistent (bold line in Table A2-2). Agreement 

between the standards is within 0.011% per AMU for 207Pb/206Pb, 0.016% per AMU for 

208Pb/206Pb, and 0.033% per AMU for 206Pb/204Pb. 

Data Reduction 

Raw Data are corrected for mass fractionation based on the analyses of the SRM 981 

standards (Table A2-1) using the following equation (Ludwig, 1980): 

Rc = Ra * ( 1 + ∆M * Fa/100) 

48
Table A2-2. Lead isotopic analyses of NIST standard SRM 982 

Sample Temp 206Pb/204Pb1 % S.E. 207Pb/206Pb1 % S.E. 

Certified Values3 36.7390 0.0990 0.467070 0.0430 

208Pb/206Pb1 

1.00016 

% S.E. 

0.0360 

F 206Pb/204Pb2 F 206Pb/20

Redetermined Values4 36.7537 0.0027 0.467003 0.0011 1.00016 **4 SRM 982-1 1144 36.6569 0.0021 0.466533 0.0006 0.99792 0.0011 0.112 ± 0.001 0.115 ± 0

SRM 982-2 1207 36.6500 0.0041 0.466571 0.0017 0.99793 0.0023 0.121 ± 0.002 0.107 ± 0

SRM 982-3 1194 36.6676 0.0020 0.466648 0.0006 0.99842 0.0014 0.097 ± 0.001 0.090 ± 0

SRM 982-4 1166 36.6427 0.0039 0.466530 0.0016 0.99787 0.0036 0.131 ± 0.002 0.116 ± 0

SRM 982-5 1195 36.6564 0.0050 0.466643 0.0019 0.99833 0.0042 0.113 ± 0.003 0.092 ± 0

SRM 982-6 1206 36.6668 0.0048 0.466686 0.0018 0.99857 0.0040 0.098 ± 0.002 0.082 ± 0

SRM 982-7 1192 36.6498 0.0054 0.466658 0.0012 0.99831 0.0020 0.122 ± 0.003 0.088 ± 0

SRM 982-8 1215 36.6302 0.0106 0.466778 0.0015 0.99855 0.0024 0.149 ± 0.005 0.063 ± 0

SRM 982-9 1222 36.6699 0.0031 0.466712 0.0013 0.99863 0.0026 0.094 ± 0.002 0.077 ± 0

SRM 982-10 1193 36.6460 0.0030 0.466624 0.0011 0.99812 0.0021 0.127 ± 0.002 0.096 ± 0

SRM 982-11 1197 36.6637 0.0028 0.466734 0.0006 0.99863 0.0007 0.103 ± 0.001 0.072 ± 0

SRM 982-12 1197 36.6457 0.0027 0.466628 0.0011 0.99818 0.0026 0.127 ± 0.001 0.095 ± 0

SRM 982-13 1165 36.6471 0.0032 0.466542 0.0009 0.99789 0.0021 0.125 ± 0.002 0.113 ± 0

SRM 982-14 1170 36.6381 0.0028 0.466487 0.0009 0.99759 0.0016 0.138 ± 0.001 0.125 ± 0

SRM 982-15 1166 36.6660 0.0023 0.466680 0.0010 0.99845 0.0020 0.100 ± 0.001 0.084 ± 0

SRM 982-16 1150 36.6687 0.0035 0.466653 0.0013 0.99831 0.0027 0.096 ± 0.002 0.089 ± 0

SRM 982-17 1186 36.6663 0.0031 0.466617 0.0012 0.99824 0.0024 0.099 ± 0.002 0.097 ± 0

Mean5 36.6549 0.0330 0.466631 0.0170 0.99823 0.0320 0.115 ± 0.017 0.094 ± 0

Mean6 36.6549 0.0330 0.466631 0.0170 0.99823 0.0320 0.135 ± 0.017 0.080 ± 0

1 Uncertainties are 1 standard error (1 S.E. ≈ 1 sigma mean) in percent.2 Calculated fractionation factors in percent per AMU for each ratio, uncertainties are at 1 S.E. in percent. 

Calculations are relative to the certified values of Cantanzaro and others, 1968, unless otherwise noted.

3 Certified values from Cantanzaro and others, 1968.4 Redetermined values from Todt and others, 1993. Calculations based on an assumed 208Pb/206Pb = 1.00016.5 Preferred values shown in bold. Uncertainties correspond to 1 sigma.6 Calculated relative to the values of Todt and others, 1993. Uncertainties are 1 sigma.

49 
Where Rc is the corrected ratio, Ra is the measured ratio, Fa

as defined above, and ∆M is the nominal mass difference between the numerator and 

is the calculated fractionation factor 

denominator isotopes. Both the Fa values and the correction factors (= ∆M * Fa) for each 

isotope ratio are shown in bold type in Table A2-1. Also shown are the 1-sigma uncertainties 

for both sets of values. 

Precision for the mass fractionation-corrected data for individual mass spectrometry runs 

listed in Table 2 is calculated at the 95% confidence interval (C.I.) using the equations given by 

Ludwig (1980; 1994) with the following parameters: 3.2 times the measured standard error 

(based on 4 blocks of data in each run) and the 2.05 times the 1-sigma uncertainty from the 

average of the SRM-981 standard runs (Table A2-1; based on 20 runs). Weighted means 

and uncertainties for data from individual samples are then calculated from the data for individual 

runs also at the 95% C.I. 

Replicate Analyses 

Three types of replicate samples were analyzed. (1) Blind duplicates were submitted 

by USEPA. (2) Replicate acid-hydrogen peroxide digestions of the same sample were 

performed in order to evaluate sample homogeneity as well as the reproducibility of the 

digestion procedure (digestion replicates). These are designated by different suffix letters in 

Tables 1 and A2-4 (e.g. 642B-A, 642B-B, 642B-C). (3) Replicate loads of the same 

sample from a single chemical procedure (mass spectrometry replicates) were run in order to 

evaluate the reproducibility of the mass spectrometric procedures. These replicates are 

designated by separate entries under a single sample number in Tables 2 and A2-3. 

Two sets of blind duplicates were submitted by USEPA. These were the baseline 

soil, samples 3-15653-B and 3-15654-B (EPA number SC-000650), and high-As-focal 

samples 3-15650-B and 3-15651-B (EPA number SC-00046). Results for the duplicates of 

the baseline soil are in reasonably good agreement and in fact the variation between the two 

samples is less than that found among duplicate acid digestions of sample 3-15654-B (Table 

2). Results for the duplicate analyses of the high-As-focal sample are in agreement within 

analytical uncertainty (206Pb/204Pb = 17.801 ± 0.009 and 17.789 ± 0.009; Table 2). 

Samples from most individual acid-digestions were run at least twice under acceptable 

temperature and beam-intensity conditions outlined above (Table 2). Data were corrected for 

mass fractionation and a weighted mean and uncertainty were calculated also as outlined 

above. 

50
Table A2-3. Lead isotopic compositions from replicate runs of 

Sample 3-15642-B, Vasquez Blvd.-I70 Project 

Sample/Lab No.1 Temp 206Pb/204Pb2 207Pb/206Pb2 208Pb/206Pb2 

3-15642-B 

642B-A 1078 18.1958 ± 0.0020% 0.85370 ± 0.0006% 2.08992 ± 0.0005% 

642B-A 1188 18.2064 ± 0.0031% 0.85404 ± 0.0014% 2.09149 ± 0.0024% 

642B-A 1167 18.2260 ± 0.0064% 0.85436 ± 0.0023% 2.09344 ± 0.0053% 

642B-A 1194 18.2150 ± 0.0093% 0.85402 ± 0.0025% 2.09192 ± 0.0045% 

642B-A 1273 18.1949 ± 0.0045% 0.85359 ± 0.0010% 2.08987 ± 0.0012% 

642B-A 1213 18.2196 ± 0.0063% 0.85400 ± 0.0025% 2.09185 ± 0.0054% 

642B-A 1191 18.2284 ± 0.0127% 0.85371 ± 0.0028% 2.09137 ± 0.0044% 

Mean-All3 18.212 ± 0.075% 0.85392 ± 0.031% 2.0914 ± 0.059% 

Mean-T=1140-12203 18.219 ± 0.049% 0.85403 ± 0.027% 2.0920 ± 0.040% 

1

 Data are for separate loads of a single processed aliquot of sample 3-15642-B 

2

 Raw data uncorrected for mass fractionation. Corrected data are shown in Table 1. 

Uncertainties for individual runs are 1 standard error in % (1 S.E. ≈ 1 sigma mean) 

3

Uncertainties for means are 1 sigma in percent 

51 
Table A2-4. Lead isotopic data for replicate acid-digestions of selected soils samples from the Vasquez Blvd.-I70 Project. 

Sample/Lab No1 Pb ppm2 Sample wt (g) 206Pb/204Pb3 207Pb/206Pb3 208Pb/206Pb3 207Pb/204Pb3 208Pb/204Pb3 

3-15642-B 422 Mean 642B-A 0.126 18.249 ± 0.007 0.85471 ± 0.00021 2.0953 ± 0.0008 15.598 ± 0.009 38.237 ± 0.029 Mean642B-B 0.072 18.219 ± 0.031 0.85570 ± 0.00065 2.0962 ± 0.0036 15.590 ± 0.039 38.191 ± 0.126 Mean642B-C 0.092 18.243 ± 0.009 0.85511 ± 0.00021 2.0960 ± 0.0010 15.600 ± 0.012 38.237 ± 0.038 Mean 642B A-C 18.242 ± 0.029 0.8551 ± 0.0009 2.0956 ± 0.0006 15.599 ± 0.007 38.235 ± 0.023 3-15644-B 39 Mean 644B-A 0.233 18.452 ± 0.012 0.84521 ± 0.00025 2.0963 ± 0.0013 15.595 ± 0.014 38.681 ± 0.046 644B-B 0.255 18.497 ± 0.017 0.84213 ± 0.00036 2.0947 ± 0.0018 15.577 ± 0.021 38.746 ± 0.068 644B-C 0.191 18.610 ± 0.017 0.84048 ± 0.00037 2.1133 ± 0.0019 15.641 ± 0.021 39.328 ± 0.069 Mean 644B A-C 18.52 ± 0.20 0.8426 ± 0.0061 2.101 ± 0.023 15.604 ± 0.080 38.92 ± 0.90 3-15654-B 101 Mean654B-A 0.254 18.869 ± 0.010 0.82794 ± 0.00021 2.0506 ± 0.0010 15.623 ± 0.012 38.693 ± 0.039 654B-B 0.177 18.459 ± 0.017 0.84513 ± 0.00037 2.0887 ± 0.0018 15.600 ± 0.021 38.556 ± 0.068 654B-C 0.164 18.454 ± 0.017 0.84637 ± 0.00037 2.0898 ± 0.0018 15.619 ± 0.021 38.564 ± 0.068 654B-D 0.078 18.496 ± 0.017 0.84416 ± 0.00036 2.0887 ± 0.0018 15.614 ± 0.021 38.633 ± 0.068 654B-E 0.272 18.385 ± 0.017 0.84836 ± 0.00037 2.0894 ± 0.0018 15.597 ± 0.021 38.414 ± 0.068 654B-F 0.219 18.460 ± 0.017 0.84481 ± 0.00036 2.0886 ± 0.0018 15.595 ± 0.021 38.554 ± 0.068 Mean 654B A-F 18.52 ± 0.18 0.8428 ± 0.0079 2.083 ± 0.016 15.610 ± 0.012 38.571 ± 0.097 Mean 654B B-F 18.451 ± 0.050 0.8458 ± 0.0020 2.0890 ± 0.0008 15.605 ± 0.009 38.544 ± 0.096 3-15655-B 151 Mean 655B-A 0.260 18.297 ± 0.011 0.85249 ± 0.00026 2.0894 ± 0.0013 15.598 ± 0.014 38.231 ± 0.046 655B-B 0.152 18.312 ± 0.017 0.85129 ± 0.00037 2.0865 ± 0.0018 15.589 ± 0.021 38.209 ± 0.067 Mean 655B A-B 18.303 ± 0.073 0.8519 ± 0.0073 2.0880 ± 0.0173 15.595 ± 0.011 38.224 ± 0.037 3-15656-B 175 Mean 656B-A 0.208 18.480 ± 0.010 0.84476 ± 0.0002 2.0711 ± 0.0018 15.611 ± 0.012 38.274 ± 0.038 656B-B 0.204 17.985 ± 0.016 0.86567 ± 0.00037 2.0975 ± 0.0018 15.569 ± 0.021 37.722 ± 0.067 656B-C 0.180 18.572 ± 0.017 0.84076 ± 0.00037 2.0722 ± 0.0018 15.614 ± 0.021 38.483 ± 0.068 656B-D 0.524 18.402 ± 0.017 0.84755 ± 0.00037 2.0726 ± 0.0018 15.597 ± 0.021 38.140 ± 0.067 656B-E 0.601 18.426 ± 0.017 0.84663 ± 0.00037 2.0705 ± 0.0018 15.600 ± 0.021 38.151 ± 0.067 Mean 656B A-E 18.37 ± 0.28 0.849 ± 0.011 2.077 ± 0.014 15.599 ± 0.021 38.16 ± 0.34 Mean 656B A, C-E 18.47 ± 0.12 0.8449 ± 0.004225 2.0714 ± 0.001 15.607 ± 0.008 38.26 ± 0.25 3-15657-B 1140 Mean 657A 0.133 17.900 ± 0.011 0.86944 ± 0.00026 2.1125 ± 0.0013 15.563 ± 0.014 37.815 ± 0.045 657B-B 0.199 17.894 ± 0.016 0.87011 ± 0.00037 2.1135 ± 0.0018 15.570 ± 0.021 37.821 ± 0.067 Mean 657B A-B 17.898 ± 0.009 0.8698 ± 0.0040 2.1128 ± 0.0010 15.565 ± 0.011 37.817 ± 0.037 3-12152-B 1730 Mean 152A 1.004 17.614 ± 0.008 0.8810 ± 0.0003 2.1198 ± 0.0011 15.517 ± 0.012 37.336 ± 0.035 Mean 152B 0.269 17.899 ± 0.008 0.8683 ± 0.0003 2.0997 ± 0.0010 15.542 ± 0.012 37.582 ± 0.035 Mean 152C 0.593 17.786 ± 0.008 0.8736 ± 0.0003 2.1084 ± 0.0011 15.537 ± 0.012 37.498 ± 0.035 Mean 152A-C 17.77 ± 0.36 0.874 ± 0.016 2.109 ± 0.025 15.532 ± 0.030 37.47 ± 0.31 1 Separate acid-digestions of the same sample are listed as "-A" through "-F". Only the mean values are shown when 2 or more 

mass spectrometer runs of the same subdample were obtained. Data are corrected for mass fractionation based on analyses of NIST standard SRM-981 (Table A2-1). 

2 Pb concentrations provided by USEPA from acid digestions of splits of the samples. 3 Uncertainties are absolute and are at the 95% confidnece level (Ludwig, 1980; 1994). 

52 
Seven aliquots of one sample, 642B-A (3-15642-B), were run at a variety of 

temperatures as shown in Table A2-3. Mean values of all three isotope ratios were calculated 

for all data and only for those data run between approximately 1140°C and 1220°C as was 

done for the SRM 981 standard (Table A2-1). The calculated analytical uncertainties for the 

mean values are similar to those obtained for to those obtained for the mass fractionation 

correction factors calculated from analyses of the SRM 981 standard (Table A2-1). 

Samples 3-15642-B (422 ppm Pb), 3-15644-B (39 ppm), 3-15654-B (110 ppm) 

and 3-12152-B (1730 ppm) were acid-digested in triplicate in order to evaluate the overall 

procedure and sample homogeneity. In addition, duplicate acid-digestions were performed on 

samples 3-15655-B (151 ppm), 3-15656-B (175 ppm), and 3-15657-B (1140 ppm). 

Significant isotopic variability was observed among samples 3-15654-B and 3-15656B, so 

three additional acid digestions of these two samples were performed (Table A2-4). 

Two of the samples with moderate to high Pb content (3-15642-B, and 3-15657-B) 

show only minor variation in isotopic ratios among the data from replicate acid digestions (Table 

A2-4). In contrast, there is considerable isotopic variation observed among replicate aciddigestions of sample 3-12152-B. 

Significant isotopic variation is observed among most of the samples with <200-ppm 

Pb. Data from one acid digestion of sample 3-15654-B show anomalously radiogenic Pb 

isotopic values whereas data for one from sample 3-15656-B show anomalously 

nonradiogenic values. Data for sample 3-15644-B also show rather large variation (0.85% in 

206Pb/204Pb). These variations are not related to mass fractionation corrections, but instead 

represent real variations induced either by the acid digestion procedures or as a result of 

sample heterogeneity. 

Some isotopic heterogeneity within urban soils is perhaps to be expected. Soils at an 

individual site may be composed of a variety of natural and man made materials. The isotopic 

compositions of soils low in Pb are subject to influence by even small amounts of “exotic” 

material such as metal, solder, fertilizer, gasoline, etc. Such material may be randomly 

distributed at a given site and even within a given sample at that site. For example, inclusion of 

a single 0.0001 gram fragment of lead solder in a 1 gram sample of soil with 100 ppm Pb 

would effectively double the lead concentration in that 1-gram sample. The Pb isotopic 

composition would also be altered to the approximate average of that in the soil and that in the 

solder. 

Samples higher in lead are generally much less sensitive to the effects of exotic 

materials. For example, the same 0.0001 gram fragment of solder in a 1 gram sample of soil 

3-15657-B (1140 ppm) would alter the apparent Pb concentration by less than 10 percent 

53
and would have only about one-fifth the effect on the lead isotopic composition as in the former 

case. Consequently, the variation in isotopic compositions observed among the replicate 

acid-digestions of sample 3-12152-B is somewhat more difficult to explain. The results may 

indicate that the high-Pb phase in this sample is very heterogeneously distributed within the 

soil. Alternatively the results may indicate that there is more than one contaminant in the soil and 

that they are either not uniformly distributed within the soil or that the acid-digestion procedure is 

not completely dissolving at least one of the contaminants. 

The inclusion of small amounts of exotic material appears to be responsible for the 

atypical isotopic compositions of samples 654B-A (3-15654-B) and 656B-B (3-15656-B). 

The data for these two fractions is included in Pb isotopic variation diagrams (Figure 1), but are 

excluded from the mean calculations for the 206Pb/204Pb vs. Pb concentration diagram in Figure 

2. 

Method Determination Limit (Practical Quantitation Limit) 

The method determination limit (MDL) is defined as the lowest analyte concentration that 

can be determined with a specified relative standard deviation (Vandecasteele and Block, 

1993). For concentration measurements, the method determination limit is generally 3 to 5 

times the detection limit (Vandecasteele and Block, 1993). EPA often refers to the method 

determination limit as the practical quantitation limit. In the case of isotopic measurements, 

where absolute concentrations are not measured, the definition is modified as the lowest ionbeam intensity at which a specified relative standard deviation is obtained. The chemical 

procedures employed prior to mass spectrometry are designed to produce a purified Pb 

fraction and thus to eliminate or at least minimize matrix effects. Consequently, the method 

determination limit is equivalent to the instrument determination limit. The method 

determination limit is defined as that intensity for 204Pb (the least abundant isotope) below 

which the one-sigma-mean uncertainty in 206Pb/204Pb exceeds ±0.05%. 

The method determination limit was determined as follows: 

1. Seven aliquots of Pb standard solution (SRM-981) were loaded into the mass 

spectrometer and run using the normal software routine, but were repeatedly run at a variety of 

beam intensities. 

2. Mean and standard errors (1 sigma mean) were calculated for the data from the 

seven runs at discreet beam intensity intervals. 

3. The data were then plotted on a graph of relative standard deviation as a function of 

204Pb beam-intensity (Figure A2-1). Best-fit curves were calculated for each of the seven 

samples and the intensity at which the relative uncertainty equals 0.05% were calculated. The 

54Ä
method determination limit was then calculated from the mean of the seven samples as 3.7 ±


0.2 x 10-14 amps (Table A2-5, Figure A2-1). Beam intensities for normal running conditions


are also shown in Figure A2-1 and are approximately one to two orders of magnitude larger


than the MDL (2.5 x 10-13A to 2.5 x 10-12 A).


55
56

Table A2-5. 

determination limit (practical quantitation limit).

Standard Run coeff. a1 coeff. b1 r

2 2 Minimum

Intensity3

SRM981-38-00 -25.595 -0.86087 0.997 3.81 x 10-14

SRM981-39-00 -30.038 -0.87641 0.995 3.98 x 10-14

SRM981-40-00 -29.891 -0.87029 0.992 3.79 x 10-14

SRM981-41-00 -29.364 -0.85108 0.988 3.50 x 10-14

SRM981-42-00 -28.378 -0.81856 0.983 3.41 x 10-14

SRM981-43-00 -29.213 -0.84817 0.996 3.77 x 10-14

SRM981-44-00 -26.309 -0.75428 0.939 3.77 x 10-14

MEAN = MDL 3.7 ± 0.2 x 10-14

1

Equations follow a power law in the form Y = ea * Xb, where y is relative uncertainty and

x is 204Pb beam intensity in amps.

2

 r2 is the regression correlation coefficient.

3

 The minimum intensity in amps for each standard is calibrated for a 1 S.E. uncertainty

of 0.05%.

Summary of results for determination of the method
1

2

3

4

5

6

7

1 S.E. (%) in 206Pb/204Pb


0.40 

0.35 

0.30 

0.25 

0.20 

0.15 

0.10 

0.05 

0.00 

1.0-15 1.0-14 1.0-13 1.0-12 1.0-11 

204Pb Beam Intensity (A) 

B B

B B

B 

B 

B 

B 

J J

J J 

J 

J 

J 

J 

H H H 

H 

H 

H 

H 

H 

F F F 

F 

F 

F 

F 

F 

M M M M 

M 

M 

M 

M 

N N 

N N 

N 

N 

N 

N 

Normal Range of 

data acquisition 

Minimum Beam 

3.7 * 10-14A 

B 

J 

H 

F 

M 

N 

Run 

Figure A2-1. Percent Standard Error in 206Pb/204Pb vs. 204Pb 

beam intensity for seven analyses of NIST standard SRM 981. 

Seven standards were run at a variety of beam intensities. Best-fit 

curves (unweighted) through data for each standard are show. The 

method determination limit (MDL) is defined as that 204Pb beam 

intensity below which the 1S.E. for 206Pb/204Pb exceeds 0.05%. 

57