U.S. Department of the Interior

U.S. Geological Survey

USGS Scientific Investigations Report 2005–5112

Lead Isotopic Compositions of Soil and Near-Surface Till 

Profiles from a Watershed Containing Arsenic-Enriched 

Groundwater in Coastal Maine
Cover.  Soil and till sites were selected for detailed chemical and lead isotope analysis from an area of coastal Maine known to have groundwater with anomalously high arsenic contents, in order to determine the source of the lead and, by inference, possible sources of arsenic.
Lead Isotopic Compositions of Soil 

and Near-Surface Till Profiles from a 

Watershed Containing Arsenic-Enriched 

Groundwater in Coastal Maine

By Robert Ayuso, Nora Foley, Gregory Wandless, Jeremy Dillingham, and Anna 

Colvin

Scientific Investigations Report 2005-5112

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

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Suggested citation:

Ayuso, Robert; Foley, Nora; Wandless, Gregory; Dillingham, Jeremy; Colvin, Anna; 2005, Lead Isotopic Compositions 

of Soil and Near-surface Till Profiles from a Watershed Containing Arsenic-enriched Groundwater in Coastal Maine: 

Reston, Virginia.

Library of Congress Cataloging-in-Publication Data

Ayuso, Robert.

Title of book/ Author List

p. cm. –(U.S. Geological Survey Science Investigations Report 2005-5112)

Includes bibliographic references.

1. Subject heading. 2. Subject heading. 3. Subject heading. I. Main entries. 

II.Main entries. III. Main entries.

TN24.M9 M55 2002

553’.09786’6—dc21

2001051109
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

The Penobscot Formation.............................................................................................................................3

Bedrock in the Northport area ...........................................................................................................3

Previous work........................................................................................................................................3

Near-surface soil profiles in the Northport area......................................................................................3

Analytical Techniques...................................................................................................................................4

Results .............................................................................................................................................................5

Lead isotope data..................................................................................................................................5

Major and trace element contents: Penobscot formation...........................................................11

Lead and arsenic contents: Soils and near-surface till profiles .................................................12

Discussion.....................................................................................................................................................13

Conclusions...................................................................................................................................................16

Acknowledgments.......................................................................................................................................16

References....................................................................................................................................................16

Figures

1 Topographic map of study area. .........................................................................................................2

2 Plot showing acid-leach and residue fractions of soils and tills. ...............................................11

3 Summary plot showing all leach and residue data for soil and till profiles. .............................12

4 Plot showing isotopic compositions of soil and till profiles, and average isotopic 

composition of Penobscot Formation......................................................................................13

5a Average compositions and ranges of total carbon, total sulfur, pH, lead, and arsenic for 

bulk fractions of the soil and till profiles, shown as a function of depth...........................14

5b Average compositions and ranges of lead and arsenic for leach fractions of soil and till 

profiles shown as a function of depth.....................................................................................14

6 Plot showing isotopic compositions of soil and till profiles, field of iron-hydroxides,

average composition of galena and the field of most commonly used pesticides

in New England. ..........................................................................................................................16

Tables

1 Range of lead isotope compositions and contents of lead and arsenic in soil profiles............5

2 Average and range in major- and trace-element compositions of Penobscot Formation. ......6

3 Average and range of trace-element compositions of leach fractions and total carbon, 

total sulfur, and pH of bulk fractions of soil profiles...................................................................7
Abstract

Lead isotope compositions of soils and near-surface tills 

from an area of coastal Maine known to have groundwater 

with anomalously high arsenic contents were measured in 

order to determine the source of the lead and, by inference, 

possible sources of arsenic. Five soil and till sites were 

selected for detailed chemical and isotopic analysis. To construct profiles of the soil and till horizons, five samples were 

collected at 10-cm intervals from the surface to the base of 

each horizon. Total lead and arsenic concentrations and lead 

isotopic compositions were measured for 48 leaches and bulk 

residues. The soils and tills are underlain by sulfidic schists 

of the Penobscot Formation. Several generations of minerals containing arsenic and lead exist in the regional bedrock, 

including rock-forming silicates (feldspar and micas), sulfide 

minerals formed during diagenesis (for example, arsenic-rich 

pyrite), and sulfide and oxide minerals that formed as a result 

of Silurian metamorphic and igneous events (for example, 

arsenopyrite, galena, iron-oxides, and arsenic-sulfides). A 

young group of secondary minerals (for example, iron-hydroxides, arsenic-hydroxides, lead-sulfate, and arsenic-jarosite) 

formed from recent weathering and pedogenic processes.

Acid-leach compositions of the soils and tills represent 

lead that is adsorbed to mineral surfaces or held in soluble 

minerals (iron- and manganese-hydroxides, carbonate, and 

some micaceous minerals), whereas residue compositions 

may reflect bedrock. Labile lead (acid-leach fractions) 

from the soils and tills shows a moderate range in values of 

206Pb/207Pb = 1.1870 to 1.2069, 208Pb/207Pb = 2.4519 to 2.4876, 

and 206Pb/204Pb = 18.583 to 18.836; values vary as a function of depth. Residues, which range in 206Pb/207Pb = 1.2056 

to 1.2328, 208Pb/207Pb = 2.4661 to 2.5000, and 206Pb/204Pb = 

18.831 to 19.303, are distinctly more radiogenic than the leach 

fractions. The profiles display an isotopic stratigraphy with no 

sharp isotopic discontinuities. Generally, the lowest 208Pb/207Pb 

ratios in the leach fractions are in the uppermost horizons (the 

surface soil layer) compared to horizons deeper in the profiles 

(50 cm). The highest 206Pb/207Pb ratios are in the surface soil 

layers. Average lead contents of the leach fractions decrease 

with depth (from 252 ppb in the surface soil layer to 17 ppb in 

the bottom interval). Average arsenic contents show no clear 

trend with depth (from 985 to 716 ppb). The profiles show 

that the average values of Ctotal (surface soil layer 6.17 wt percent, bottom of pit 1.73 wt percent) and pH vary as a function 

of depth (surface layer 4.55, bottom of pit 5.11). The values of 

Stotal show no clear trend (surface soil layer 0.03, bottom of pit 

0.02). 

The range in lead isotope values in the soil and till profiles is generally consistent with multiple sources that include 

both natural and anthropogenic lead. The most likely geologic 

sources of lead (and, by inference, of arsenic) contributing to 

the surface soil layer and to most tills in the watershed are (1) 

the breakdown of arsenic-bearing sulfides in the Penobscot 

Formation during glaciation and (2) weathering. The lead 

isotope compositions of the profiles overlap values of U.S. 

aerosols (from burning of fossil fuels), and also values of 

pesticides used historically and extensively in New England. 

The leach fractions at the top of the profiles (horizons where 

organic matter is abundant) closely overlap lead isotope values 

for recent aerosols (1997-1999). A multicomponent mixing 

scheme that included lead from young aerosols (1997-1999) 

derived from combustion of fossil fuels, lead from the ironhydroxides (reflecting an assortment of natural and anthropogenic sources), and lead from the local parent rock (Penobscot 

Formation) could account for lead isotope variations in the soil 

profiles. Our results show that, although a genetic bond likely 

exists between the soil and till profiles and the local country 

rocks, lead isotope variations and lead contents cannot be conclusively related to the arsenic contents. Solubility differences 

for lead and arsenic in the surface soil layers, together with the 

effects of elemental redistribution during weathering and soil 

formation may best explain decoupling of lead and arsenic.

Introduction

The predominant source of lead and arsenic in soils and 

near-surface tills is often thought to be local rocks, although 

substantial input from non-local rocks and other metal sources 

Lead Isotopic Compositions of Soil and Near-Surface Till 

Profiles from a Watershed Containing Arsenic-Enriched 

Groundwater in Coastal Maine

By Robert Ayuso, Nora Foley, Gregory Wandless, Jeremy Dillingham, and Anna Colvin
 Lead Isotopic Compositions of Soil and Near-Surface Till Profiles in Coastal Maine

Figure 1. Topographic map of study area showing locations of 34 sites selected for soil and till profiles and boreholes for water and core 

samples (C, Cummings site; B, Baker site). Large squares identify locations of five soil and till profile sites used for the lead isotope 

study; smaller squares are locations of other soil and till profiles used in the larger geochemical study (R. Ayuso, unpublished data, 

2004). Dashed line encloses an area containing groundwater with high measured arsenic contents (As > 0.1 mg/L) (Lipfert and Reeve, 

2004). Solid line encloses area of the watershed.

0 500

meters

contour interval 10 feet 

N

RA-ME-02-S09

RA-ME-02-S26

JJ-ME-02-S03

JJ-ME-02-S01

RA-ME-02-S02

Solid line delineates 

area down-gradient from

Mount Percival 

Dashed line delineates cluster of high As 

in groundwater (As > 0.1 mg/L) 

Cummings (C) core site 

Baker (B) core site 

X

X

N

0 200 Km 

AREA 

 OF STUDY 

Maine

44o22'30"

69o57'30"
generated as a result of human-induced activities may also be 

present. Anthropogenic metal sources are related to industrial and urban activities (for example, burning of fossil fuels, 

mining and metal alloy industries, smelting, manufacturing, 

wood preservation, glass and ceramics industries, sewage, 

and so on), as well as agricultural activities (for example, 

pesticide and herbicide use). In areas containing groundwater 

with anomalously high arsenic concentrations, a critical step 

toward establishing the ultimate source of arsenic depends on 

properly evaluating the relative contributions from natural and 

anthropogenic sources.

Several areas in New England have groundwater wells 

supplied by bedrock aquifers that are anomalously rich in 

arsenic (Marvinney and others, 1994; Peters and others, 1999; 

Ayotte and others, 2003). This preliminary study is focused 

on a small watershed in the Northport area in coastal Maine 

(fig. 1). This watershed is the focus of extensive geological 

studies to determine the processes leading to arsenic mobilization and groundwater enrichment (Ayuso and Foley, 2002; 

Foley and others, 2004a). Detailed sampling of rocks, stream 

sediments, and areas containing abundant sulfide minerals 

have been used to establish the geochemical characteristics 

of the natural background (Ayuso and Foley, 2004). Samples 

were obtained from drill core to characterize the bedrock, as 

well as soils and near-surface tills (summary in this study; 

R. Ayuso, unpublished data, 2004) and to obtain information 

relating to groundwater chemistry (Lipfert and Reeve, 2004). 

The objectives of this study are to examine the distributions of the isotopic compositions in the watershed, to 

determine whether the soil and near-surface till profiles in the 

Northport area (fig. 1) have distinct lead isotope signatures 

as a function of depth, and, most importantly, to test whether 

there is compelling genetic evidence establishing the source of 

lead and arsenic in soils and tills. Petrogenetic links between 

lead and arsenic in the soil and till profiles and in the Lower 

Ordovician Penobscot Formation (bedrock underlying the till 

in the area) or between the soils and tills and anthropogenic 

sources will help to identify the source of metals and metalloids in the near-surface environment. Assessing the influence 

of lead (and related elements) from outside sources depends 

on establishing analytically distinguishable lead isotope differences between the natural background materials (for example, 

bedrock, sulfide mines, and mineral occurrences) and other 

sources resulting from human activities (for example, gasoline 

and pesticide use). This report summarizes the geochemical features of leach fractions of soils and tills from different 

depth intervals at five sites as well as the average major- and 

trace-element compositions of the Penobscot Formation 

obtained from outcrop and drill core.

The Penobscot Formation 

Bedrock in the Northport area

Rusty-weathering iron-sulfide-rich rocks of the Lower 

Ordovician Penobscot Formation constitute the regional bedrock in the Northport area (Stewart and Tucker, 1998). Pelitic 

beds (with sandy siltstone layers) are abundant in the upper 

part of the formation. Limestone and calcareous sandstone 

are rare. Diorite dikes and muscovite granite pods and dikes 

are found locally. Regional metamorphism of the Penobscot 

Formation produced coarse-grained andalusite in the pelitic 

beds (Stewart and Tucker, 1998). Ridges and higher elevations in the Northport area have outcrops of the Penobscot 

Formation covered by a layer of glacial sediments (till) that 

had been scavenged from the underlying rocks by the Late 

Wisconsian Laurentide ice sheet (Hunter and Smith, 2001). In 

the coastal lowlands of eastern Maine, glaciomarine sediments 

were deposited as ice withdrew at about 14,000 and 12,200 

yr B.P. (Dorion and others, 2001). The till deposits, which 

predominate in the watershed, characteristically lack bedding 

and consist of a jumbled assortment of clay, silt, sand, cobbles, 

and boulders.

Previous work

A major conclusion of previous investigations by Ayuso 

and Foley (2002) and Foley and others (2004a) is that several 

generations of metal sulfides and secondary minerals containing arsenic and lead exist in the Penobscot Formation. In 

addition to the oldest generation of sulfide minerals (including arsenic-rich pyrite and pyrrhotite), which likely formed 

by diagenesis, there is a distinct sulfide and oxide mineral 

population (including galena, pyrite, arsenopyrite, iron-oxide 

minerals, and arsenic-sulfides) that formed as a result of Silurian metamorphic and igneous events affecting the Penobscot 

Formation (Foley and others, 2004a, b). Moreover, secondary oxide and sulfate minerals (for example, iron-hydroxides, 

anglesite, arsenic-hydroxides, and arsenic-jarosite) have also 

formed from recent weathering of the till and from pedogenic 

processes. We have suggested that low-temperature oxidation 

and decarbonation reactions involving the young group of 

arsenic-bearing minerals in contact with local groundwater are 

likely to account for the bulk of the arsenic contributed to the 

regional groundwater (Foley and others, 2004a).

Near-surface soil profiles in the 

Northport area

The major protolith of the soils in this portion of Waldo 

County is glacial till, but marine and lacustrine sediments can 

also be locally important. The predominant soil type in the 

watershed belongs to the Peru-Marlow-Brayton association, 

which is found along the coast and consists of deep, nearly 

level to steep soils that range from well drained to poorly 

drained; the soils developed in predominantly moderately 

coarse-textured compact glacial till (Hedstrom and Popp, 

1981). Another soil type in the watershed—one that is somewhat less extensively developed—belongs to the deep, nearly 

The Penobscot Formation 
 Lead Isotopic Compositions of Soil and Near-Surface Till Profiles in Coastal Maine

level to steep soils of the Boothbay-Swanville-Lyman association (Hedstrom and Popp, 1981), generally found landward 

from the predominant Peru-Marlow-Brayton soil association. 

The Boothbay-Swanville-Lyman soil association differs from 

the Peru-Marlow-Brayton association mainly in that some of 

the soils are moderately well drained to poorly drained and 

that they formed in marine and lacustrine sediments. The 

Boothbay-Swanville-Lyman soil association also includes 

excessively drained soils in moderately coarse textured glacial 

till (Hedstrom and Popp, 1981). Thus, the dominant soils 

in the watershed developed on glacial till and were derived 

mostly from the gneiss and schist local bedrock, perhaps 

with contributions from nearby granitic rocks. Glaciofluvial 

sediments, organic material, and alluvium may also constitute 

significant components in the soils. 

The profiles typically contained unstratified clay to boulder-sized constituents (generally up to about 10 cm and infrequently up to 20 cm). The soil, till, and other near-surface 

materials generally make up a relatively thin (< 1.5 m thick) 

layer that overlies bedrock. A predominantly local provenance 

for the glacial till in the watershed appears likely, on the basis 

of the types and amounts of pebbles and cobbles in the profiles. Organic-rich and irregularly bedded layers (decomposing forest litter: roots, leaves, twigs, and other organic debris) 

dominate the top of the profiles, extending generally to about 

8 to 12 cm. Intensely weathered, saprolitized, and disaggregated, rusty-sulfidic schistose bedrock was encountered at 

many sites at the bottom of the profiles, starting at about 60 

cm. The large pebbles and boulders in the intermediate depth 

intervals were predominantly rusty-sulfidic schist, in addition 

to minor granite, mafic rocks, and quartzite.

Abrupt color changes were occasionally found in the 

depth profiles, and color variations in the dominant soil 

association in the watershed generally are characterized by a 

surface layer of dark brown soils with fine sandy loam at the 

top (~10-15 cm thick), overlying a layer of gray to orangebrown fine sandy loam (~4-7 cm). Deeper in the profiles, 

the subsoils ranged from dark orange-brown silty to sandy 

loam to gray and green-brown pebbly to sandy loam (~25-35 

cm). The profiles were usually dug shallower than about 1 

m and ended below the subsoils in a dense layer (substratum) 

of disaggregated schist and gneiss fragments or intensely 

weathered bedrock. Locations dug at higher elevations (Mt. 

Percival area) (fig. 1) were characterized by abundant outcrop 

and consisted of soils and tills shallowly underlain by bedrock. 

In the predominant soils, permeability above the substratum 

and bedrock is moderate. In the spring, the water table is commonly perched above the substratum and bedrock (Hedstrom 

and Popp, 1981).

The profiles of the Northport area contain various silicate, carbonate, oxide, and rare sulfide minerals identified by 

petrography, X-ray diffraction, and electron microprobe analysis. The tills also have abundant organic constituents at the top 

of the profiles. The soils display several horizons that characteristically reflect the general incorporation of organic material 

and the effects of weathering. Leaching of the topmost layer, 

breakdown of organic material, and removal of soluble ions 

enhance the acidity of the soils and result in the dissolution 

of oxides, reduction of iron, and formation of soluble metalorganic complexes (Hedstrom and Popp, 1981). The metal 

complexes migrate lower in the profile, and many precipitate 

as a result of chemical and biological reactions as hydroxides 

of iron, aluminum, and manganese. The silicate mineralogy 

is dominated by quartz, feldspar, and mica, including sericite 

and chlorite, all of which are distributed throughout the depth 

profiles. Our studies suggest that iron-hydroxides are the 

predominant secondary minerals in the profiles and underlying 

bedrock (Ayuso and Foley, 2002; Foley and others, 2004a, b; 

N. Foley, unpublished data, 2005). The tills contain goethite, 

lepidocrocite, and a variety of iron-manganese-hydroxide minerals. In general, the iron and manganese-iron oxyhydroxide 

minerals occur throughout the till profiles, although they are 

most abundant in the upper depth intervals, particularly below 

the organic layer. Rare sulfide minerals in some near-surface 

tills include pyrrhotite, greigite, sphalerite, galena, djurlite, 

and arsenopyrite. Carbonate (calcite, manganoan siderite) and 

iron-bearing phosphate minerals also were identified in some 

of the depth intervals. 

Analytical Techniques

Profiles were obtained at five sites and represent a subset 

of 34 sites (fig. 1) that have been investigated for a geochemical study in progress. Pits approximately 30 to 40 cm in diameter were hand dug in the midsummer to expose profiles from 

the ground (free of forest litter) to at least 60 cm. Five or six 

samples (about 150-500 g) were obtained approximately 10 

cm apart in order to select representatives of the morphological variations in the soils and tills. For example, the samples 

from the tops of the profiles are from the uppermost surface 

layers of the soils, as characterized by color (for example, 

most are dark brown), particle size, and morphology (silt or 

sandy loam). Most soils in the area exhibited contrasting color 

and compositional features distinguishing the soils, subsoils, 

and lower horizons. The surface layer was clearly evident in 

the watershed (mostly representative of the A1 horizon, and in 

some cases the Ap soil horizon) (Hedstrom and Popp, 1981). 

This topmost layer, which is at or near the surface, consists 

of humified organic matter and minerals and may include a 

plowed surface horizon that was part of the underlying mineral 

horizon (B). Often, bedrock was encountered at depths of less 

than 60 cm. The samples were air dried and prepared using 50 

to 100 mg of the <0.2-mm-sized fraction following established 

techniques for lead isotope analysis at the U.S. Geological 

Survey, Reston (Ayuso and others, 2004b). 

Analysis of the secondary minerals in soils and tills provides a means to establish the composition of labile lead and 

a way to distinguish labile lead from lead inherited from the 

parent rocks and the original rock-forming minerals (silicates, 

oxides, sulfides). Although hot and prolonged leaching of min-
erals using acid mixtures (HCl and HNO3

) is a standard dissolution procedure for secondary minerals (Tessier and others, 

1979), such leaching also attacks the original rock-forming 

minerals, especially sulfide constituents. The resulting acid 

solution mixes lead from secondary minerals and from original 

minerals. Thus, the standard leach method was unsuitable for 

our study. In order to attack only the secondary minerals, we 

devised a cool and mild acid leach (1.5N HCl+3N HNO3). 

This solution was passed through lead columns by means of 

established elution protocols (Ayuso and others, 2004b). The 

acid leach aliquots represent anthropogenic and labile lead 

(Erel and others, 1997) trapped in the iron-hydroxide, carbonate, or organic materials or lead from other secondary minerals 

and organic material likely available for rock/fluid reactions 

(such as clay minerals). These minerals are the constituents 

that may have been important metal point sources to groundwater. They can also contain lead and arsenic (and other metals) derived from outside of the watershed.

Mixed solutions of HF-HNO3 were used for final 

dissolution of the residual fractions. All isotopic compositions 

were measured by using a Finnigan-MAT 262 thermal 

ionization mass spectrometer (TIMS). Mass fractionation 

was monitored by frequent analysis of the NIST-SRM 981 

standard. Lead blanks during the course of the study were 

less than 50 nanograms and thus are not significant compared 

to the lead abundances in the samples. A mass fractionation 

correction per amu of about 0.1 percent was applied to 

all samples. Maximum analytical uncertainties for ± 2 

standard error of the mean are as follows: 206Pb/204Pb < 0.002, 206Pb/207Pb < 0.00003, and for 208Pb/207Pb < 0.00004). Table 1 

summarizes the lead isotope data. 

Major- and trace-element compositions and mineralogy 

of the Penobscot Formation and material from the near-surface 

till depth intervals in the watershed were obtained by various analytical techniques including XRD, XRF, ICP-MS, and 

INAA. Table 2 summarizes the major- and trace-element data 

for samples from outcrops and rocks obtained from drill core. 

Table 3 is a summary of the trace-element data for the leach 

aliquots obtained on the soils and tills used in the lead isotope 

study.

Results

Lead isotope data

A summary of lead isotope compositions of a total of 

48 leach and residues from five soil and subsoil (mostly till 

and, in some cases, marine and lacustrine sediments) profiles 

is given in table 1 and figures 2 and 3. Fields showing the 

isotopic compositions of galena from sulfide mines and bedrock (Penobscot Formation) (fig. 4), lead and arsenic contents 

(fig. 5), and the isotopic compositions of secondary mineral 

alteration products (predominantly iron-hydroxides) are also 

shown for comparison (fig. 6) (Ayuso and Foley, 2002, 2004; 

Foley and others, 2004a, b). The minerals and rocks represent 

lead that would be available to the groundwater directly from 

the sulfide-rich Penobscot Formation bedrock and from galena 

associated with sulfide-mineralized mines and occurrences. 

Table 1. Range of lead isotope compositions and contents of lead and arsenic in the soil profiles, Northport area, coastal Maine

Depth Profile

Leach

Average (range) content (in ppb) 206Pb/204

Pb 206Pb/207Pb 208Pb/207Pb

Pb As

0-10 cm horizon 296 (86.5-606) 1221 (70.5-3960) 18.601-18.751 1.19046-1.20111 2.45188-2.46186

10-20 cm horizon 42.3 (20.4-29.8) 638 (17.6-2030) 18.603-18.774 1.19351-1.20252 2.46314-2.47236

20-30 cm horizon 26.4 (11.9-47.4) 874 (47.1-2150) 18.583-18.836 1.18703-1.20355 2.47327-2.48761

30-40 cm horizon 29.1 (9.6-65.9) 843 (34.1-1860) 18.731-18.822 1.19147-1.20685 2.46895-2.47630

40-50 cm horizon 16.9 (12.2-19.7) 716 (48.9-1780) 18.733-18.810 1.19128-1.23280 2.46994-2.49296

 Depth Profile

Residue (bulk)

Average (range) content (in ppm) 206Pb/204

Pb 206Pb/207Pb 208Pb/207Pb

Pb As

0-10 cm horizon 35.9 (23.7-59.6) 176 (37.9-517) 18.831-18.976 1.20560-1.21375 2.46654-2.48104

10-20 cm horizon 18.7 (15.0-22.1) 172 (13.0-606) 18.925-19.303 1.21182-1.23280 2.47863-2.49296

20-30 cm horizon 21.8 (16.6-24.3) 221 (62.0-657) 18.893-19.267 1.21042-1.23151 2.48156-2.50001

30-40 cm horizon 21.9 (20.5-23.2) 203 (55.0-616) 18.937-19.041 1.21163-1.21884 2.48024-2.49473

40-50 cm horizon 18.9 (8.9-28.0) 225 (59.0-640) 18.905-19.026 1.20992-1.21774 2.48008-2.49028

Analytical Techniques 
   Lead Isotopic Compositions of Soil and Near-Surface Till Profiles in Coastal Maine

Outcrops (in wt percent) Core samples (in wt percent)

Average (n=9) Minimum Maximum Average (n=19) Minimum Maximum

SiO2 61.25 46.94 75.75 61.53 45.55 83.91

Al2O3 19.90 13.47 28.69 18.49 7.16 27.57

Fe2O3 5.55 1.08 8.26 6.95 2.63 9.73

MnO 0.06 0.01 0.12 0.07 0.03 0.27

MgO 1.37 0.06 2.31 2.17 0.66 6.92

CaO 0.73 0.02 3.78 0.89 0.1 6.98

Na2O 1.97 0.43 4.59 1.32 0.34 3.82

K2O 4.28 0.99 7.36 3.37 0.3 6.93

TiO2 0.90 0.03 1.63 0.93 0.48 1.42

P2O5 0.09 0.06 0.16 0.11 0.04 0.20

LOI 3.53 0.64 5.82 3.57 1.34 5.81

As1 28.3 7.7 65.0 82.6 0.7 730

Pb 28.8 9.7 52.7 17.2 7.5 42

Ba 850 52 1620 696 106 1330

Rb 166 56 288 177 41 320

Sr 141 26 294 144 67 449

Y 25 2 43 40 26 60

Cs 10 1 16 28 5 98

La 29.45 4.35 44.22 62 26 138

Ce 59.75 9.78 90.27 132.68 58.69 312.43

Pr 6.40 1.03 9.90 14.19 5.84 33.58

Nd 24.75 3.42 38.48 50.89 20.71 121.31

Sm 5.03 0.66 8.21 9.64 4.05 22.38

Eu 1.40 0.14 2.54 2.19 1.03 5.25

Gd 4.64 0.54 7.82 2.19 1.03 5.25

Tb 0.79 0.09 1.31 1.21 0.74 2.26

Dy 4.52 0.48 7.25 6.65 4.33 11.13

Ho 0.95 0.09 1.49 1.30 0.83 1.90

Er 3.02 0.26 4.61 3.60 2.25 5.16

Tm 0.47 0.04 0.70 0.53 0.32 0.80

Yb 2.94 0.27 4.38 3.39 2.05 5.04

Lu 0.46 0.04 0.68 0.50 0.29 0.72

Zr 153.1 30.3 279.2 206.7 95.7 276.5

Hf 4.7 1.3 8.3 5.32 2.53 7.23

Nb 17.3 5.6 24.9 17.1 8.1 25.5

Ta 1.4 0.1 2.3 1.3 0.3 2.6

Th 10.4 0.6 18.6 15.2 6.3 21.35

U 3.9 0.9 6.1 4.8 1.7 11.3

Be 3.4 2.0 5.0 nd nd nd

Sb 0.4 0.2 0.6 0.5 0.1 1.3

Table 2. Average and range in major- and trace-element compositions of the Penobscot Formation, Northport area, Maine

[nd, not determined]
1

 Arsenic content in sulfide-rich band; maximum content away from sulfide-rich bands is 120 ppm. 

2

 Gold and mercury content in ppb.

Table 3. Average and range of trace-element compositions of the leach fractions and Ctotal, S, and pH of the bulk fractions of the soil 

profiles

Outcrops (in wt percent) Core samples (in wt percent)

Average (n=9 Minimum Maximum Average (n=19) Minimum Maximum

Hg 9.3 5.0 14.3 nd nd nd

Bi 0.3 0.1 0.8 0.4 0.1 1.1

Se 0.6 0.1 1.2 0.7 0.2 3

Tl 1.5 0.6 2.9 1.3 0.2 2.7

Outcrops (in ppm) Core samples (in ppm)

Average (n=9) Minimum Maximum Average (n=19) Minimum Maximum

Sc 18.3 0.7 32.2 18.9 7.6 32.7

V 178.8 15.9 516.6 115.0 30.2 212.8

Ni 39.9 26.7 63.8 576.1 24.7 4220

Co 8.8 2.0 31.0 24.9 8.0 54.0

Cr 98.8 27.0 179.0 2404 83.0 20800

Ga 26.9 17.4 40.4 25 8.2 40.1

Ge 2.3 1.2 3.1 1.9 0.9 3.1

Cu 29.9 17.7 55.4 30.8 10.6 70.9

Zn 105.5 66.9 127.9 119.8 36.0 198.2

Mo 5.4 2.9 12.3 5.8 2.6 14.3

Sn 3.6 2.3 4.8 8.7 1.9 67.1

W 1.6 0.7 3.5 4.5 0.7 20.7

Au2 2.7 0.8 10.0 0.6 0.2 1.4

Ag 2.5 0.2 8.8 nd nd nd

Hg2 9.3 5.0 14.3 nd nd nd

Leach 0-10 cm (in ppb) 10-20 cm (in ppb) 20-30 cm (in ppb)

Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum

As 985 40.3 3960 639 17.6 2030 702 14.2 2150

Pb 252 76.0 606 42.3 20.4 112 25.9 11.9 47.4

Ba 779 229 1970 2504 255 7430 2737 512 7700

Rb 174 28.1 676 229 25.0 499 200 52.8 343

Sr 949 58.7 4150 733 55 3750 643 107 3140

Y 120 36.4 295 91.2 18.7 511 126 35.1 613

Cs 3.05 0.16 26.70 4.80 0.34 29.00 8.20 0.90 59.90

La 72.0 36.9 123.0 49.6 15.7 129.0 59.1 28.3 134.5

Ce 120.9 76.1 211.0 108.8 31.1 348.0 118.2 44.2 438.5

Pr 22.1 8.8 41.2 16.9 4.1 72.8 23.6 7.9 85.6

Nd 96.0 35.6 205.0 78.5 16.9 429.0 116.2 32.8 515.5

Table 2. Average and range in major- and trace-element compositions of the Penobscot Formation, Northport area, Maine—Continued

{nd, not determined}

[bdl, below detection limit]

Results   
   Lead Isotopic Compositions of Soil and Near-Surface Till Profiles in Coastal Maine

Leach 0-10 cm (in ppb) 10-20 cm (in ppb) 20-30 cm (in ppb)

Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum

Sm 25.1 7.7 62.7 20.9 4.0 140.0 31.5 7.8 158.5

Eu 6.2 2.2 15.8 5.5 1.5 35.3 7.9 2.5 39.2

Gd 22.8 6.9 58.8 19.2 3.7 134.0 29.2 7.1 154.0

Tb 4.5 1.3 11.3 3.6 0.7 25.6 5.4 1.3 28.9

Dy 23.9 7.5 61.1 20.0 4.2 137.0 28.9 7.3 152.0

Ho 4.5 1.2 11.4 3.9 0.8 25.0 5.5 1.5 28.7

Er 11.7 3.7 29.0 10.6 2.3 60.6 14.8 4.1 70.4

Tm 1.6 0.5 4.1 1.4 0.3 6.2 1.9 0.6 7.5

Yb 9.6 3.0 25.5 9.1 1.9 34.8 12.3 3.5 50.6

Lu 1.5 0.4 3.8 1.3 0.3 4.9 1.7 0.5 7.5

Zr 98.7 20.8 268 124 23.9 879 158 34.9 1060

Hf 3.0 0.8 7.9 3.7 1.0 24.7 4.7 1.1 31.9

Nb 15.6 5.1 40.8 7.3 2.9 17.5 6.3 2.9 17.4

Ta 2.8 0.7 15.1 0.4 0.1 0.9 0.3 0.1 1.1

Th 58.4 18.6 115.0 51.6 11.4 181.0 50.0 13.3 167.5

U 7.1 2.7 28.8 16.8 4.1 114.0 17.2 7.2 46.1

Be 12.0 4.6 27.2 17.4 3.4 39.7 22.7 2.9 49.9

Sb 10.3 3.3 20.8 7.0 1.5 15.5 5.3 1.5 16.0

Hg bdl bdl bdl 1.2 1.2 1.2 1.1 1.1 1.1

Bi bdl bdl bdl bdl bdl bdl 0.8 0.1 1.3

Se 42.2 29.9 68.9 65.4 30.9 166.0 29.0 11.0 59.8

Tl 1.6 0.2 6.4 2.2 0.4 5.2 2.0 0.3 4.4

Cd 18.9 7.4 28.2 17.6 6.9 34.3 16.4 5.1 32.8

Sc bdl bdl bdl bdl bdl bdl 138.5 138.5 138.5

V 307.9 150.0 826.0 294.4 87.1 631.0 270.2 102.0 751.0

Ni 261.3 117.0 564.0 235.8 137.0 497.0 63.6 13.0 162.5

Co 463.4 28.7 2230.0 632.1 53.8 2370.0 1031.0 67.5 3500.0

Cr 130.9 53.0 285.0 167.3 59.1 428.0 151.2 82.0 284.0

Ga 18.0 5.7 63.8 12.4 4.0 35.0 10.4 4.1 24.2

Ge 1.4 0.5 2.2 1.7 0.7 4.3 1.2 0.5 3.8

Cu 24.5 5.8 153 17.1 5.23 89.7 24.2 3.49 69.9

Zn 934 233 1780 832 67 2350 865 68.2 1950

Mo 10.0 1.23 34.6 6.4 1.06 33.7 4.16 1.01 16.9

Sn 4.88 1.08 18.4 3.36 1.17 10.7 2.66 2.66 2.66

W 10.4 1.5 63.4 11.1 1.19 79 12.38 1.225 77.3

Br 2143 662 4140 2586 542 5020 2464 446 4430

I 2215 581 3920 4864 665 11800 5971 525 17900

Table 3. Average and range of trace-element compositions of the leach fractions and Ctotal, S, and pH of the bulk fractions of the soil 

profiles [bdl, below detection limit]­—Continued
Bulk 0-10 cm (in wt percent) 10-20cm (in wt percent) 20-30 cm (in wt percent)

Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum

Ctotal 6.17 2.85 7.25(16.45) 4.18 1.78 8.15 3.13 0.47 6.85

S 0.03 0.03 0.02

pH 4.55 3.09 5.88 4.69 3.88 5.96 4.86 4.28 5.83

Leach

30-40 cm (in ppb) 40-50 cm (in ppb)

Average Minimum Maximum Average Minimum Maximum

As 678 16.0 1860 577 17.8 1780

Pb 27.7 9.6 65.9 19.0 12.2 27.5

Ba 2401 532 5530 2115 297 5210

Rb 200 45.9 361 256 46.2 410

Sr 717 82 3380 1225 92 4220

Y 132 25.5 575 174 31.7 733

Cs 8.66 1.16 59.25 6.65 1.21 26.30

La 70.3 26.7 148.5 83.0 31.2 150.0

Ce 111.1 37.8 299.0 174.8 67.6 540.0

Pr 25.1 7.0 69.9 33.0 10.8 86.6

Nd 113.8 28.9 412.0 148.7 36.1 437.0

Sm 30.4 6.4 146.5 38.2 8.0 126.0

Eu 7.6 1.9 36.2 8.9 2.0 29.1

Gd 28.4 5.9 143.0 36.9 7.6 131.0

Tb 5.5 1.1 29.1 6.4 1.3 22.7

Dy 30.1 6.2 161.5 35.1 7.3 127.0

Ho 5.8 1.0 30.7 6.8 1.1 25.1

Er 16.2 2.8 84.0 18.9 3.2 69.7

Tm 2.2 0.4 11.3 2.6 0.5 9.3

Yb 15.1 2.5 76.9 16.7 3.1 59.8

Lu 2.2 0.3 11.7 2.5 0.4 9.5

Zr 166 33.1 1150 251 34.2 1090

Hf 4.9 1.1 33.1 7.7 1.3 32.0

Nb 6.3 2.7 14.0 7.2 3.1 11.4

Ta 0.3 0.1 0.9 0.4 0.2 0.8

Th 42.6 13.3 83.4 49.2 19.4 168.0

U 21.5 7.2 44.4 25.1 10.0 62.3

Be 23.1 3.4 40.8 20.5 2.4 38.4

Sb 5.4 1.5 19.7 6.3 1.6 26.0

Hg bdl bdl bdl bdl bdl bdl

Bi 0.8 0.0 1.4 0.9 0.9 1.0

Se 24.0 11.9 43.6 17.0 6.3 37.5

Table 3. Average and range of trace-element compositions of the leach fractions and Ctotal, S, and pH of the bulk fractions of the soil 

profiles [bdl, below detection limit]­—Continued

Results   
10 Lead Isotopic Compositions of Soil and Near-Surface Till Profiles in Coastal Maine

Labile lead obtained from the leach fractions shows a 

moderate range in values of 208Pb/207Pb = 2.4519 to 2.4876 

and 206Pb/204Pb = 18.583 to 18.836 (figs. 2, 3) and 206Pb/207Pb = 

1.1870 to 1.2069 (fig. 4). In every site, the leach fractions are 

lowest for 208Pb/207Pb in the uppermost soils (surface 

layers within 10 cm from the surface), in contrast to the values 

obtained deeper in the profiles (up to about 50 cm from the 

surface). Considering all of the sites in the Northport watershed, as a group the leach fractions of the surface layers of 

the soils have the least radiogenic values, and in general, the 

profiles display an isotopic stratigraphy (fig. 3). Ratios of 

206Pb/207Pb are also higher in the uppermost soil horizons 

(table 1). Moreover, samples from approximately equivalent 

horizons and representing the different sites in the watershed 

have a narrow range in isotope compositions (figs. 2, 3). For 

example, the uppermost soil surface layers have a limited 

range in values of 206Pb/204Pb = 18.619-18.751 (~ 0.7 percent), 

except for the middle depth interval (representative of 20-30 

cm), where the values of 206Pb/204Pb range from 18.583 to 

18.836 (~ 1.3 percent). In this portion of the profiles 

(20-30 cm), soil and glacial processes controlling the dissolution and re-precipitation of the secondary minerals did 

not homogenize the labile lead isotope compositions (see 

similar results obtained on temperate forest near-surface tills 

by Emmanuel and Erel, 2002). Compositions of the residues 

range in 206Pb/207Pb = 1.2056 to 1.2328, 208Pb/207Pb = 2.4661 

to 2.5000, and 206Pb/204Pb = 18.831 to 19.303 and are distinctly more radiogenic than the leach fractions (table 1, fig. 

2). Notably, the leach and the residue fractions are not aligned 

along a tight systematic trend but scatter about a broad band. 

The residue fractions have isotope compositions that overlap 

the Penobscot Formation (fig. 4). Sulfide minerals (galena) 

from massive sulfide deposits in coastal Maine and from 

other regional mineral occurrences are distinct from the ironhydroxides in the Penobscot Formation, being much lower 

in 208Pb/207Pb and 206Pb/207Pb (fig. 6). The mineral deposits 

do not appear to have been involved as direct sources of the 

lead in the till from the Northport area, although the relatively 

unradiogenic nature of compositions of the leach fractions 

(fig. 6) may be attributed to lead redistribution emanating from 

mineral deposits and occurrences in the area (Foley and others, 

2004b).

Leach

30-40 cm (in ppb) 40-50 cm (in ppb)

Average Minimum Maximum Average Minimum Maximum

Tl 2.2 0.2 5.3 2.0 0.3 8.0

Cd 16.3 3.0 32.5 13.5 3.7 28.5

Sc 118.8 117.5 120.0 132.0 109.0 172.0

V 256.2 77.0 830.5 243.9 108.0 590.0

Ni 79.5 6.5 329.5 325.5 129.0 1460.0

Co 1337.8 66.4 3230.0 1354.9 74.7 4830.0

Cr 134.7 68.9 240.5 88.2 46.5 154.0

Ga 13.0 4.5 30.2 11.4 6.3 20.0

Ge 1.1 0.0 1.8 1.6 0.8 2.4

Cu 16.1 0.08 56.7 13.2 3.44 31.7

Zn 734 34.7 1900 644 169 1670

Mo 4.17 1.36 11.4 4.75 1.07 12.4

Sn 2.98 2.98 2.98 1.54 0.932 3.48

W 9.3 1.23 61.8 5.4 1.2 45

Br 2478 311 5220 1876 219 4080

I 6472 826 17900 3186 383 5930

Bulk

30-40 cm (in wt percent) 40-50 cm (in wt percent)

Average Minimum Maximum Average Minimum Maximum

Ctotal 2.44 0.39 5.95 1.73 0.94 4.9

S 0.02 0.02

pH 4.95 4.27 6.00 5.12 4.40 6.27

Table 3. Average and range of trace-element compositions of the leach fractions and Ctotal, S, and pH of the bulk fractions of the soil 

profiles [bdl, below detection limit]­—Continued
Figure 2. 208Pb/207Pb vs. 206Pb/204Pb plot showing the acid-leach (solid symbols) and the residue (open symbols) fractions of the 

soils and tills at successive depth intervals. (A) 0-10 cm. (B) 10-20 cm. (C) 20-30 cm. (D) 30-40 cm. (E) 40-50 cm. Note that data for nearsurface till profiles at 20 to 30 cm show more scatter than other depth intervals. Analytical error is less than symbol size

18.5 18.7 18.9 19.1 19.3

2.44

2.46

2.48

2.50

Pb/ Pb 206 204 

Pb/ Pb 208 207

leaches

residues

0-10 cm 

A.

18.5 18.7 18.9 19.1 19.3

2.44

2.46

2.48

2.50

Pb/ Pb 206 204 

Pb/ Pb 208 207

leaches

residues

10-20 cm 

B.

18.5 18.7 18.9 19.1 19.3

2.44

2.46

2.48

2.50

Pb/ Pb 206 204 

Pb/ Pb 208 207

leaches

residues

20-30 cm 

C.

18.5 18.7 18.9 19.1 19.3

2.44

2.46

2.48

2.50

Pb/ Pb 206 204 

Pb/ Pb 208 207

leaches

residues

30-40 cm 

D.

18.5 18.7 18.9 19.1 19.3

2.44

2.46

2.48

2.50

Pb/ Pb 206 204 

Pb/ Pb 208 207

leaches

residues

40-50 cm 

E.

Major and trace element contents: Penobscot 

Formation 

The major- and trace-element variations of the sulfidic 

Penobscot Formation are consistent with its overall diversity, 

which ranges from pelite dominated to sandy silts containing wide variations in contents of sulfide mineral and which 

includes andesitic to dioritic dikes and felsic granitic pods 

and dikes (table 2). Calcareous rocks are not abundant in the 

Penobscot Formation but are locally distributed. A wide range 

in the contents of Fe2O3, Al2O3, MgO, and CaO, for example, 

can be attributed to the various relative proportions of iron-rich 

sulfides, pelitic rocks, and calcareous rocks. Outcrop and drill 

core samples of the Penobscot Formation have equivalent contents and ranges of the major and trace elements in general but 

can exhibit significant contrast in their arsenic content (table 

2). Drill core whole-rock samples are somewhat depleted in 

Results 11
12  Lead Isotopic Compositions of Soil and Near-Surface Till Profiles in Coastal Maine

18.5 18.7 18.9 19.1 19.3

2.44

2.46

2.48

2.50

Pb/ Pb 206 204 

Pb/ Pb 208 207

0-10 cm 

10-20 cm 

30-40 cm 

40-50 cm 

Leaches

Residues

Residues

0-20 cm 

30-50 cm 

Figure 3. 208Pb/207Pb vs. 206Pb/204Pb summary plot showing all leach and residue data for the soil and till profiles. Generalized trends 

of leach fractions for each depth from the five sites are plotted from the surface (0-10 cm) to the bottom of the profile (40-50 cm). (The 

20- to 30-cm interval is omitted because the data show significant scatter.) The leach fractions, which correspond to labile lead, are 

represented by lines of decreasing slope as a function of depth. The residue fractions, which represent silicate lead, are grouped 

into shallow (0-20 cm) and deep (30-50 cm) sets. The data show that the residues deeper than 30 cm are more homogeneous than the 

residues higher in the profile.

lead (average ~17 ppm) and enriched in arsenic (average = 83 

ppm; up to 730 ppm in a sulfide mineral rich layer), compared 

to the outcrop samples (average lead ~29 ppm; average arsenic 

~28 ppm) (table 2). 

Lead and arsenic contents: Soils and nearsurface till profiles

Baseline concentrations of arsenic in U.S. near-surface 

till are ~7.4 mg kg-1 (Shacklette and others, 1971). Locally, 

arsenic concentrations can vary widely because of the content 

of sulfide minerals (for example, sulfide-rich shales, sulfiderich hydrothermal veins, and so on) and redox conditions. 

Pyrite oxidation in black-shale parent rock results in acid-sulfate soils, for example, that can be enriched in arsenic (Smedley and Kinniburgh, 2002). In the Northport area, the average 

contents of the bulk fractions of the soils and tills used in this 

study show a wide range of lead and arsenic contents (fig. 

5, table 1) and closely parallel the variations found for data 

representing all of the sites in the Northport area (R. Ayuso, 

unpublished data, 2004). In the bulk fractions of the profiles, 

the content of lead is highest in the surface layer (average ~34 

ppm). The content of arsenic shows no systematic trend with 

depth in the soil profiles (~ 144 ppm at the top; ~225 ppm at 

the bottom). High contents of arsenic in all the depth intervals 

of sample RA-ME-02-S26 account for the wide range and 

overall enrichment of arsenic in the average compositions of 

the sites used for the isotope study. As a group, the sites used 

for the isotope study (tables 1, 3), compared to all of the sites 

in the geochemical study of the Northport watershed, appear to 

contain about twice as much arsenic (average arsenic 62 ppm 

at 0-10 cm, 73 ppm at 40-50 cm) (R. Ayuso, unpublished data, 

2004). Variations in the contents of lead and arsenic in the 

leach fractions of the depth intervals (fig. 5) also closely parallel the variations found when the data for all of the sites in the 

Northport area are considered. Again, sample RA-ME-02-S26 

accounts for the highest contents of lead in the leach fractions (tables 1, 3). Lead contents decrease with depth (from an 

average of 252 ppb in the surface soil to an average of 19 ppb 

in the bottom horizon). Average arsenic contents, however, 

show no clear trend with depth (an average of 985 ppb in the 

uppermost soil surface layer and 577 ppb in the bottom horizon). The leach fractions are also most variable for lead and 

arsenic in the soil surface layer (lead ~ 76 ppb to ~606 ppb; 

arsenic ~ 40 ppb to ~3900 ppb). Other elemental contents, 

which are the subject of a detailed study in preparation, vary 

as a function of depth: for example, Cd, Zn, and V contents 

decrease from the top to the bottom of the profiles, and Zr, Hf, 

U, and Co increase (table 3). The average contents of Ctotal 
Figure 4. 206Pb/207Pb vs. 208Pb/207Pb plot showing the isotopic compositions of the soil and till profiles and the average isotopic 

composition of the Penobscot Formation (representing bedrock in the area and the likely parent rock of the till). Compositions of U.S. 

aerosols evolved from 206Pb/207Pb ~1.15 in the mid-1960s (dominated by Mississippi Valley type lead from United States and lead 

imports from Mexico (Mississippi Valley type lead data from Doe and Delevaux, 1972) to 206Pb/207Pb ~1.23 in the 1970s and early 1980s 

(as a result of predominant use of Mississippi Valley-type lead) and then decreased to 206Pb/207Pb ~1.20 in the early 1990s (Rosman 

and others, 1994; Erel and others, 1997; Bollhöfer and Rosman, 2001). Also shown for reference: the range of U.S. aerosols for 1971-1989, 

eastern U.S. aerosols 1997-1999 (Rosman and others, 1994; Erel and others, 1997; Bollhöfer and Rosman, 2000; 2001), U.S. atmospheric 

lead in the 1970s (Chow and others, 1975), and lead ore from Mexico and Peru (Chow and others, 1975). See text for additional 

explanation and references.

2.42 2.44 2.46 2.48 2.50 2.52

1.15

1.20

1.25

208 207 Pb/ Pb 

Pb/ Pb 206 207

U.S. aerosols

1971-1989

Leaches

Residues

Peruvian, Mexican 

ores

*

U.S. atmospheric Pb 

1970s

Eastern U.S. 

aerosols 1994-1999 

0-10 cm horizon 

0-10 cm horizon 

30-40 cm horizon 

40-50 cm horizon 

Residue

Leach

10-20 cm horizon

10-20 cm horizon

30-40 cm horizon 

20-30 cm horizon

20-30 cm horizon

40-50 cm horizon 

(labile Pb) 

Penobscot Fm. 

average (silicate Pb) 

(surface soil 6.17 wt percent, bottom horizon 1.73 wt percent) 

and pH (surface soil 4.55, bottom horizon 5.11) show consistent differences as a function of depth (table 3). The values for 

S, however, show no clear trend along the profiles (surface soil 

0.03 percent, bottom horizon 0.02 percent) (table 3). 

Discussion

Lead isotope ratios are useful for investigating metal 

mobility because biological, physical, and chemical processes 

in the near-surface environment do not disturb the isotopic 

signatures. Natural and anthropogenic point sources frequently 

can be distinguished. The isotope signatures can be used to 

evaluate the contributions of lead from geological background 

materials (rocks, minerals, near-surface tills, and so on) and 

lead originating from human activities. Sources providing lead 

may also be the same as those providing other metals. 

In temperate forest soils similar to those found throughout most of New England, the affinity for lead is greatest in 

the iron-hydroxides, followed by organic matter and by silicate 

minerals (Emmanuel and Erel, 2002). Arsenic, like lead, has 

a strong preference for residing in the iron-hydroxides under 

surface conditions (Korte and Fernando, 1991). The abundance of newly formed secondary minerals that can accept 

metals and metalloids that are potentially harmful to human 

health results in a very short residence in water (for example, 

lead, Jaffe and Hites, 1986; arsenic, Smedley and Kinniburgh, 

2002). Moreover, lead residence in a temperate forest floor is 

also thought to be short, probably less than 80 yr (Miller and 

Friedland, 1994; Erel and others, 1997). 

The dominant source of anthropogenic lead in the near 

surface environment is generally from industrial emissions 

involving combustion of alkyl-lead gasoline additives (Chow 

and others, 1975). The most important domestic sources of 

lead ore used for industrial applications in the United States 

(for example, gasoline additives) were Mississippi Valley 

type deposits. As the supply of lead shifted from domestic to 

foreign sources, values of 206Pb/207Pb in aerosols and gasoline 

became more variable (fig. 4). By the 1970s, the lead isotopic 

composition of U.S. alkyl lead (fig. 4) nearly matched that 

of atmospheric lead (206Pb/207Pb ~1.23 and 208Pb/207Pb ~2.46) 

(Chow and others, 1975; Bollhöfer and Rosman, 2000). From 

Discussion 13
14  Lead Isotopic Compositions of Soil and Near-Surface Till Profiles in Coastal Maine

+

+

+

+

0 200 400 600

As (ppm) 

0 20 40 60

Pb (ppm) 

40-50 cm 

30-40 cm 

20-30 cm 

10-20 cm 

0-10 cm 

4 5 6

pH

20

Total C (%) 

0 10 0.04

S (%) 

0.08

+

0

Figure 5a. Average compositions (red ovals) and ranges (solid lines) of total carbon, total sulfur, pH, lead, and arsenic for bulk fractions 

of the soil and till profiles are shown as a function of depth. For arsenic, ranges (dashed lines) and averages (red x) are also shown for 

depth profile, omitting two sites in the watershed that contain much higher values of arsenic. 

Figure 5b. Average compositions (red ovals) and ranges (solid lines) of lead and arsenic for leach fractions of the soil and till profiles are 

shown as a function of depth. For arsenic, ranges (lines) and averages (red x) are also shown for depth profile, omitting two sites in the 

watershed that contain much higher values of arsenic. 

0 200 400 600

Pb (ppb)

40-50 cm

30-40 cm

20-30 cm

10-20 cm

0-10 cm

+

0 2000 4000

As (ppb)

+

+

+

+
1982 to 1984, the lead isotopic compositions of industrial 

emissions in the eastern United States had values of 206Pb/207Pb 

equal to 1.213 (Sturges and Barrie, 1987). By 1997 to 1999 

(fig. 4), aerosols in the eastern United States were less radiogenic (Bollhöfer and Rosman, 2001) than those measured in 

alkyl lead in the 1970s (Chow and others, 1975). 

Emissions from fossil fuel combustion are extensively 

distributed through long-range atmospheric transport (Nriagu 

and Pacyna, 1988), resulting in heavily polluted soils enriched 

in lead and, notably, in near-surface materials that are also 

associated with enrichments in arsenic, zinc, and cadmium 

(Steinnes, and others, 1989). In addition to the effects of 

fossil fuel combustion, the natural geochemical cycle of lead 

also has been greatly disturbed by emissions from industrial 

plants and ore smelters (Graney and others, 1995; Dunlap 

and others, 1999; Ayuso and others, 2001; Tarzia and others, 

2002), the effects of urbanization (Hopper and others, 1991; 

Erel and others, 1997; Hansmann and Koppel, 2000; Teutsch 

and others, 2001), and other human activities that emit metals 

(arsenic, antimony, cadmium, selenium, mercury, and so on) 

into the atmosphere and surface environment (Steinnes and 

others, 1992). Other studies have shown that water reactions 

with metal-contaminated soils and runoff into ponds, lakes, 

and streams can enhance the effects of anthropogenic activities 

(Erel and Patterson, 1994; Kober and others., 1999; DeVivo 

and others, 2001). Atmospheric precipitation, however, is 

thought to contribute very little arsenic to surface and groundwater bodies unless there is a significant and local source of 

contamination (Smedley and Kinniburgh, 2002). 

The most important natural source of lead (and, by 

inference, of arsenic) to the surface soil layer and to most 

tills in the watershed is likely the breakdown of sulfide-bearing parent rocks during glaciation and subsequent weathering 

of the oxidation zone overlying the groundwater (Ayuso and 

Foley, 2002, 2004; Foley and others, 2004a). The lead isotope 

compositions of the profiles overlap the iron-hydroxides from 

the local bedrock, the average Penobscot Formation, and the 

established fields of U.S. aerosols and pesticides that had been 

extensively used in New England (Ayuso and others, 2004a) 

(figs. 4, 6). Decomposition of the sulfide minerals as a result 

of fluid migration would have leached and oxidized bedrock 

and soils, producing iron-hydroxides and other secondary 

minerals (carbonates, sulfates, sulfides, oxides). Arsenic- and 

lead-bearing iron-sulfides, such as pyrite, would have been 

converted to limonite, goethite, and other secondary iron 

oxyhydroxide minerals. Generally, the bulk of the lead (and, 

by inference, of arsenic) in the original sulfide minerals may 

have been captured by the secondary minerals. Subsequent 

reduction of such iron-hydroxide minerals is thought to release 

metals (for example, arsenic) into the groundwater (Matisoff 

and others, 1982; Stuben and others, 2003).

Although the soil and till profiles show some lead 

isotopic variations from top to bottom (leach fractions are 

lowest in 208Pb/207Pb in the surface soil layer, 10 cm from the 

surface), there is no obvious discontinuity that can be used 

to distinguish a boundary for the possible effects of near-surface contaminants (such as industrial lead from atmospheric 

deposition) and lead derived from weathered parent rock. The 

trend from top to bottom of the profiles shows that the leadisotope compositions of U.S. aerosols overlap the values of the 

leach fractions of the surface soil layers (fig. 4). Lead isotope 

contributions from the youngest aerosols (1997-1999) appear 

to decrease or be absent in a trend from the top, where organic 

matter is most abundant, toward the leach compositions of 

the deeper till fractions. The field of the iron-hydroxides also 

overlaps the compositions of the leach fractions at the top of 

the profiles (fig. 6). Deeper leach fractions in the profiles (> 10 

cm) are shifted from the field of aerosols and gasoline (1997-

1999) and the field of the iron-hydroxides toward somewhat 

higher values of 208Pb/207Pb and the field of the arsenical 

pesticides (figs. 4, 6). The lead isotope field for the 1997-1999 

aerosols is extensive, and is almost as large as the field for all 

of the U.S. aerosols and gasoline produced since the 1960s 

(for example, 206Pb/207Pb as low as ~1.15 according to Rosman 

and others, 1994). 

Considered as a group, results for labile lead in the soil 

and till profiles suggest the involvement and mixing of at 

least three components: (1) lead from eastern U.S. aerosols 

(1997-1999), perhaps preferentially concentrated at the top of 

the profiles, (2) lead from the iron-hydroxides (representing a 

rough average of various natural and anthropogenic sources), 

and (3) lead from the parent rock (Penobscot Formation) (figs. 

4, 5). Values of 208Pb/207Pb higher than those of the ironhydroxides and pesticides in the deeper profiles may indicate 

a more prominent role for foreign lead sources (Peruvian and 

Mexican lead ores) or perhaps a natural source that has not 

been identified (figs. 4, 6). However, no evidence has been 

documented for such foreign lead ores in the region; moreover, 

local bedrock with the appropriate high values of 208Pb/207Pb 

is unknown. The arsenical pesticides do not have a distinctive 

lead isotope range (Ayuso and others, 2004a) that would allow 

for an exact evaluation of their contribution to the labile lead, 

separate from that of the iron-hydroxides (fig. 6).

This study was carried out as a test of whether the lead 

isotopes in the soils and tills can be linked to the likely parental rocks (Penobscot Formation) and to investigate whether 

additional constraints on relative metal contributions from 

natural and anthropogenic sources are possible on the basis 

of these data. Although our data show that a general link 

between the soil and till profiles and the local country rocks 

is feasible, a conclusive interpretation of results that directly 

relate the lead contents and lead isotope variations to the 

arsenic contents must remain equivocal. At this time, it seems 

that the contrasting solubilities of lead (low solubility at high 

soil pH) and arsenic (complexing and solubility contrasts 

with pH) in the near-surface layers, and the various effects of 

mechanical disturbance (tillage, and digging) near the surface 

redistributed lead deeper into the soil, subsoil, and till (Peryea, 

1998, 1999) and, more importantly, effectively decoupled lead 

from arsenic. 

Discussion 15
16 Lead Isotopic Compositions of Soil and Near-Surface Till Profiles in Coastal Maine

Conclusions

Lead isotope compositions of near-surface soil and till 

profiles were measured in an area known to have groundwater with anomalously high arsenic contents to determine the 

source of the lead and, by inference, to provide constraints on 

possible sources of arsenic. Labile lead from the soils and tills 

shows a moderate range in isotope values, distinctly less radiogenic than the residue fractions. Leach fractions are lowest 

in values of 208Pb/207Pb in the soil surface layers. Acid-leach 

compositions represent lead that is loosely bound in minerals 

(iron- and manganese-hydroxides, carbonate, and micaceous 

minerals) and likely approximate the composition of lead 

available to the regional groundwater. The profiles show that 

average lead (34 ppm) and arsenic (144 ppm) contents are generally highest in the surface soil layer. Contents of major and 

trace elements vary widely in the till parent rock, primarily 

variably sulfidic schists of the Penobscot Formation. Samples 

of drill core extracted from the Penobscot Formation show 

wide ranges in their arsenic content. Overall, core samples 

are depleted in lead (average 17 ppm) and enriched in arsenic 

(average 83 ppm), compared to outcrop samples (average lead 

29 ppm; average arsenic 28 ppm). Our lead isotope and other 

geochemical data show that a connection exists between the 

soil and till profiles and the local Penobscot Formation, but a 

common source for lead and arsenic cannot be conclusively 

identified at this time.

Acknowledgments

We thank William C. Cannon and Bruce R. Doe for constructive comments.

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