Isotope record of anthropogenic lead
pollution in lake sediments of Florida, USA
Jaime Escobar, Thomas J. Whitmore,
George D. Kamenov & Melanie
A. Riedinger-Whitmore
Journal of Paleolimnology
ISSN 0921-2728
J Paleolimnol
DOI 10.1007/s10933-012-9671-9
1 23
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DOI 10.1007/s10933-012-9671-9
ORIGINAL PAPER
Isotope record of anthropogenic lead pollution in lake
sediments of Florida, USA
Jaime Escobar • Thomas J. Whitmore •
George D. Kamenov • Melanie A. Riedinger-Whitmore
Received: 13 July 2011 / Accepted: 26 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract We examined the anthropogenic lead (Pb)
burden that accumulated in sediment of lakes in the
southeastern USA during the last *150 years. Mining,
smelting, agriculture, and fossil-fuel combustion are
known to have contributed to Pb pollution in lakes of
other regions. Few studies, however, have examined Pb
sequestered in lakes of the southeastern USA, particulary peninsular Florida, which is subject to less continental atmospheric influence than other regions of the
eastern USA. We obtained sediment cores from Little
Lake Jackson and Little Lake Bonnet in Highlands
County, Florida and used Pb isotopes in the records to
identify principal sources of Pb contamination. The
sediment records showed that changes in Pb concentration and isotope ratios correspond temporally with
J. Escobar (&)
Departamento de Ingenierı́a Civil y Ambiental,
Universidad del Norte (Uninorte), km 5 vı́a Puerto
Colombia, Barranquilla, Colombia
e-mail: [email protected]
J. Escobar
Center for Tropical Paleoecology and Archaeology,
Smithsonian Tropical Research Institute (STRI), Balboa,
Panama
gasoline consumption in the USA, as well as with
changes in lead ores used to produce leaded gasoline.
Lead concentrations in the study lakes showed temporal
variations that were similar to those found in peat
records from east-central Florida. Isotope trends were
similar to the mean USA atmospheric Pb deposition
record, and to Pb isotope records from Bermuda and
Atlantic corals. We modeled the isotopic composition of
the anthropogenic Pb in lake sediments and found that
the overall trend is controlled by Pb that was released
during leaded gasoline combustion. There is, however,
additional Pb at each site that comes from sources that
are not fully represented by the natural, background Pb.
Lead isotope ratios and Pb/arsenic (As) ratios provide
evidence that Pb deposition in lakes during the middle
1900s might have been influenced by lead arsenate
applications to golf courses, a source that is often
ignored in Pb isotope studies. Isotope evidence confirms, however, that following cessation of commercial
lead arsenate use in the 1960s, atmospheric alkyl lead
was again the primary influence on Pb in sediments of
the study lakes.
Keywords Lead isotope Alkyl lead Florida Sediment Lead arsenate
T. J. Whitmore G. D. Kamenov
Department of Geological Sciences, University of Florida,
Gainesville, FL 32611, USA
Introduction
M. A. Riedinger-Whitmore
Department of Biological Sciences, University of South
Florida St Petersburg, St. Petersburg, FL 33701, USA
Anthropogenic activities such as mining, smelting and
fossil fuel combustion have produced a global-scale
123
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lead (Pb) fingerprint that is recognized in ice cores,
snow, and sediments from Greenland and Antarctica
(Murozumi et al. 1969; Rosman et al. 1993, 1997;
Bindler et al. 2001). Lead deposition also has been
documented in corals of the Atlantic, Pacific and
Indian Oceans (Shen and Boyle 1987), in peat bogs
(Shotyk et al. 1998; Bindler et al. 2004; Kamenov et al.
2009) and in lacustrine sediments (Renberg et al.
2000; Siver and Wozniak 2001; Rose et al. 2004;
Merilainen et al. 2011). Lead is an ideal pollution
indicator in lacustrine environments because it is
relatively immobile in sediments and has four stable
isotopes (204Pb, 206Pb, 207Pb, 208Pb) that permit the
recognition of various sources. Therefore, Pb concentrations, together with isotope signatures, have been
used widely to reconstruct spatial and temporal
patterns of Pb pollution in lake sediments (Shirahata
et al. 1980; Farmer et al. 1996; Brännvall et al. 2001).
Most atmospheric lead studies to date come from
peat bogs and lakes in Europe (Renberg et al. 1994;
Shotyk et al. 1998; Brännvall et al. 2001; Eades et al.
2002) and in the northern United States (Graney et al.
1995; Heivaert et al. 2000; Simonetti et al. 2000; Siver
and Wozniak 2001), with few studies in the Neotropics
(Cooke et al. 2007, 2008). Florida (USA) lies at subtropical latitudes, possesses approximately 8,000
lakes, and is the second densest lake district in the
continental USA. Recently, Kamenov et al. (2009)
conducted a detailed trace element and Sr–Nd–Pb
isotope study on a 500-year-old peat-core record from
Blue Cypress Marsh in the Upper St. Johns River
Basin of east-central Florida (Fig. 1). Variations in the
lead isotopes in the Blue Cypress Marsh followed the
historical record of different Pb ores used to produce
the gasoline additive (Kamenov et al. 2009). In
another study, Schottler and Engstrom (2006) investigated sediment profiles of heavy metals, 137Cs, PCBs
and pollen to validate use of 210Pb as a chronological
marker in cores from shallow Lake Okeechobee
(Fig. 1). The record showed that the changes in Pb
concentrations in Lake Okeechobee sediments correlated temporally with leaded gasoline consumption in
the USA (Schottler and Engstrom 2006). Lead isotope
data, however, were not available to assess the alkyl
lead contribution in that study. Peninsular Florida is
subject to more atmospheric influence from the Gulf of
Mexico than from the continental USA, so it is unclear
whether the Pb record observed in the marshes of eastcentral Florida is typical of lake records throughout the
123
rest of the state, or whether Pb profiles from Florida
lakes might differ in Pb isotope composition from
those in more northerly lakes of the continental USA.
In this study, we investigated stratigraphic changes
in Pb concentrations and Pb isotope signatures from
two lakes, Little Lake Bonnet and Little Lake Jackson,
located in south-central Florida. Our objective was to
examine the anthropogenic influence on the Pb burden
and the Pb isotope record in the sediments during the
last century in peninsular Florida.
Study site
Little Lake Jackson is located at 27°280 N and 81°280 W
(Fig. 1) in Highlands County, Florida, USA, within
the city of Sebring, which was settled in 1913. The
lake has a surface area of 63 ha, a watershed of 424 ha,
and is connected on the north side to larger Lake
Jackson. Joined US Highways 27 and 98 and State
Highways 25 and 700 are adjacent to the north shore of
Little Lake Jackson, and state Highway 634 is in the
western part of the watershed. Since the early 1900s,
automobile traffic frequented the roadway on the north
shore, which was expanded in the 1960s to a four-lane
highway.
Paleolimnological study of Little Lake Jackson
concluded that the lake became more eutrophic and
alkaline in response to nutrient and ionic loading
(Whitmore et al. 2006). Significant amounts of arsenic
accumulated in sediments because of arsenical herbicide applications to turf lawns on golf courses
immediately adjacent to the lake (Whitmore et al.
2008). Land-use maps and historical records show no
indication of significant agriculture during the past in
the watershed (Whitmore et al. 2008).
Little Lake Bonnet is situated in northern Highlands
County at 27°330 N and 81°280 W (Fig. 1) within the
city limits of Avon Park. The lake has a surface area of
34 ha. Much of the watershed has been used for citrus
agriculture. The Atlantic Coast Line Railroad (now
CSX Transportation Railroad) was built immediately
adjacent to the lake on the southwestern shore in 1912,
and subjected the lake first to low-level coal emissions, then progressively through the 1930s–1950s to
diesel emissions. Little Lake Bonnet is situated
between State Highways 17 and 17A, each about
1.4 km from the lake, and it is approximately 2.8 km
east of joined US Highways 27 and 98 and State
Highways 25 and 700. Florida Power Corporation
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Fig. 1 Map of Florida
showing the location of Blue
Cypress Marsh, Little Lake
Bonnet, and Little Lake
Jackson
(now Progress Energy) constructed a 59,200-kW,
coal-fired power plant 2.1 km northwest of the lake in
1928. This power plant was expanded in 1945, 1952
and 1970, and was converted to diesel operation using
number 6 fuel oil. Pinecrest on Lotela Golf Club owns
the property on the west, north, and east shores of the
lake, and remnant citrus agriculture is present on the
property. Fairways on the north and west shores of
Little Lake Bonnet are apparent on aerial photographs
from 1944, and probably were part of the original golf
course, established in 1926. A canal enters Little Lake
Bonnet from a discharge area on Lake Lotela, and
water exits Little Lake Bonnet to Lake Letta on the
east.
Materials and methods
A sediment core (27°28.0130 , 81°27.8300 ) was
retrieved from Little Lake Jackson in June 2005 at a
water depth of 7.52 m in the deep, central depositional
zone of the lake. In Little Lake Bonnet, a sediment
core was collected in April 2008 in the central portion
of the lake (27°33.6830 , 81°28.5180 ). Both cores were
collected with a 7-cm-diameter, 1.83-m-long polycarbonate piston corer (Fisher et al. 1992) and sectioned
vertically at 5-cm intervals in the field.
Little Lake Bonnet and Little Lake Jackson sediment samples were analyzed for total Pb content at
Waters Agricultural Laboratory in Camilla, Georgia
using EPA method 6020. Samples were digested with
concentrated HCl and HNO3 for 1 h at 95 °C, then
filtered with a WhatmanTM Grade No. 1 (11-lm pore)
filter. Total Pb content of digestates was measured with
a Thermo Scientific ICAP 6000 series ICP Spectrometer. The minimum detection limit was 0.01 mg L-1.
Precision was measured by 3 replicate readings on
10 % of samples, and the average standard deviation
for replicate samples was 0.15 mg L-1. Digestion
efficiency was measured using three National Institute
of Standards and Technology (NIST) Standard Reference Material 2,702 samples within the sample run,
and total Pb content of the NIST samples
(mean = 133.5 mg L-1, range 133.2–133.8 mg L-1)
demonstrated 100.5 % recovery with respect to the
certified Pb content of 132.8 ± 1.1 mg L-1.
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All reagents used for sample preparation for the
isotopic analysis of Pb in Little Lake Jackson and
Little Lake Bonnet sediments were Optima grade.
Sample preparation was performed in a class 1000
clean laboratory, equipped with class 10 laminar flow
hoods, at the Department of Geological Sciences,
University of Florida. About 1 g of sediment was
weighed in acid-cleaned ceramic crucibles and combusted at 550 °C for 2.5 h to remove organic matter
and to determine weight loss on ignition (LOI). No
elemental Pb loss was expected during the combustion, as Pb has a boiling point higher than 550 °C.
About 50 mg of the resultant ash was digested in HFHNO3 mixture overnight in closed Teflon beakers.
The resultant sample solution was evaporated to
dryness and Pb was separated following procedures
described in Kamenov et al. (2009). In short, the
residue was dissolved in 1 N HBr and loaded on
columns packed with Dowex 1X-8 resin to separate Pb
for isotope analyses. Samples were washed 3 times
with 1 ml of 1 N HBr, and the Pb fraction was
collected in 1 ml of 3 N HNO3. Lead isotopes were
determined on a ‘‘Nu-Plasma’’ MC-ICP-MS with Tlnormalization (Kamenov et al. 2009). The Pb isotope
data are relative to the following values of NBS 981:
206
Pb/204Pb = 16.937 (±0.004, 2r), 207Pb/204Pb =
15.490 (±0.003, 2r), and 208Pb/204Pb = 36.695
(±0.009, 2r).
Arsenic (As) content in the Little Lake Bonnet and
Little Lake Jackson cores also was measured at Waters
Agricultural Laboratory, using EPA method 7062
(U.S. EPA 1994a) for digestion of soils to assess As
content by atomic absorption spectrometry. Samples
were digested with concentrated HCl and HNO3 for
1 h at 95 °C, then filtered with a WhatmanTM Grade
No. 1 filter. Total As content of digestates was
measured with a PS Analytical Millennium Excalibur
Analyzer. The minimum detection limit was 0.01 mg
L-1, and digestion efficiency was evaluated using
National Institute of Standards and Technology
(NIST) Standard Reference Material 2702.
Lead-210 dates for cores from Little Lake Jackson
and Little Lake Bonnet were obtained by direct
gamma counting (Schelske et al. 1994) with an
intrinsic germanium detector. Unsupported 210Pb
activity was calculated by subtracting 226Ra activity
from total 210Pb activity. Sediment ages were calculated using the constant rate of supply model (Appleby
and Oldfield 1983).
123
Results
In Little Lake Bonnet, Pb concentrations, accumulation rates and isotope ratios showed distinct stratigraphic changes (Table 1; Fig. 2). Total Pb content
appeared low (15.2–23.4 mg L-1) in samples from the
undated section of the core (65–45 cm). At 40 cm the
total Pb content increased to *28.2 mg L-1, and was
approximately 28.0 mg L-1 in the samples from 1874
to 1920. Lead content then increased to 37.6 mg L-1
in the 1949 sample, and showed a peak of 71.7 mg L-1
in the 1990 sample. Lead concentrations declined to
*59.9 mg L-1 in the 4–0-cm interval (post 2001).
The Pb accumulation rate in Little Lake Bonnet began
to increase in samples above the 1874 level, peaked in
the 1949 sample, then declined by more than half in
the 1964 sample. The Pb accumulation rates from
1974 to the present (13.3–15.8 mg m-2 year-1) were
approximately equal to the accumulation rate in 1920.
Lead isotope ratios varied with changes in Pb
concentration (Fig. 2). In Little Lake Bonnet, Pb
isotope ratios in the undated portion of the core
(75–45 cm) remained fairly constant, with a
206
Pb/204Pb value of *18.97, a 207Pb/204Pb value of
*15.66, and a 208Pb/204Pb value of *38.98
(Table 1). Lead isotope ratios decreased from 1920
to 1964, with a decline in 206Pb/204Pb from *18.74 to
*18.61, in 207Pb/204Pb from *15.63 to *15.62, and
in 208Pb/204Pb from *38.63 to *38.38. Isotope ratios
then increased between 1964 and 1974, with
206
Pb/204Pb values increasing from *18.61 to
*18.78, 207Pb/204Pb increasing from *15.62 to
*15.64, and 208Pb/204Pb from *38.38 to *38.42.
Isotope ratios remained fairly constant thereafter.
Little Lake Jackson Pb concentrations, accumulation rates, and isotope ratios (Fig. 3) showed stratigraphic changes similar to those in Little Lake Bonnet.
Lead concentrations in Little Lake Jackson peaked
between the mid-1970s and early 1980s, then declined
to the present day as they did in Little Lake Bonnet. In
Little Lake Jackson, Pb isotope ratios in the undated
portion of the core (95–75 cm) remained fairly
constant, with a 206Pb/204Pb value of *18.93, a
207
Pb/204Pb value of *15.66, and a 208Pb/204Pb value
of *38.90. Lead isotope ratios showed a gradual
decrease from 1902 to 1965 (Table 2; Fig. 3),
206
Pb/204Pb declining from *18.90 to *18.64,
207
Pb/204Pb declining from 15.66 to 15.63, and
208
Pb/204Pb from 38.82 to 38.45. From 1965 to 1987,
19.002
18.999
18.990
18.933
18.902
18.926
18.742
18.754
18.607
18.682
18.615
18.777
18.756
18.748
206/
204
15.668
15.668
15.670
15.666
15.668
15.659
15.657
15.662
15.639
15.640
15.626
15.632
15.627
15.647
15.644
15.645
207/
204
Lead Isotope ratios
19.000
27.2
15.2
4.8
8.0
4.4
6.6
3.9
3.6
Pb/As
ratio
75–80
0.2
0.9
12.7
11.1
9.7
2.0
3.9
4.4
As accumulation
rate
mg m-2 year-1
18.990
23.37
65–70
6.7
13.4
61.2
88.4
42.1
13.3
15.4
15.8
Pb accumulation
rate
mg m-2 year-1
70–75
22.27
27.45
27.95
60–65
24.50
48.00
15.18
1874
35–40
45.72
37.63
55–60
1920
30–35
133.88
234.86
52.01
21.12
1955
1949
20–25
25–30
81.02
69.07
21.97
1964
15–20
19.32
71.66
59.89
50–55
1974
10–15
21.55
26.37
45–50
1990
5–10
Pb
concentration
mg kg-1
28.15
2001
0–5
Mass sedimentation
rate
mg cm-2 year-1
40–45
Age mid interval
depth
year
Depth
Lake Bonnet
39.010
39.005
39.031
39.033
39.027
38.933
38.862
38.927
38.635
38.652
38.448
38.555
38.387
38.428
38.410
38.406
208/
204
2.0531
2.0539
2.0539
2.0544
2.0551
2.0564
2.0559
2.0568
2.0614
2.0610
2.0662
2.0637
2.0623
2.0465
2.0479
2.0486
208/
206
Table 1 Little Lake Bonnet sediment age, mass sedimentation rate, lead concentration, lead accumulation rate, arsenic accumulation rate, and lead isotope data
0.8246
0.8250
0.8246
0.8246
0.8250
0.8271
0.8283
0.8276
0.8344
0.8339
0.8398
0.8367
0.8395
0.8332
0.8341
0.8345
207/
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Fig. 2 a Variations in lead
concentrations versus depth
and age, b 206Pb/204Pb,
c 207Pb/204Pb, d 208Pb/204Pb
for Little Lake Bonnet
206
Pb/204Pb increased from *18.64 to *18.92,
Pb/204Pb increased from *15.63 to *15.66, and
208
Pb/204Pb increased from *38.46 to *38.58. From
1987 to the present time in Little Lake Jackson
sediments, 206Pb/204Pb decreased from *18.92 to
*18.84, 207Pb/204Pb decreased from *15.66 to
*15.65, and 208Pb/204Pb decreased from *38.58 to
*38.52 (Table 2; Fig. 3).
Arsenic accumulation rates in the Little Lake Bonnet
core, like Pb accumulation rates, were related strongly to
mass sedimentation rates (Table 1). At the base of the
dated profile, ca. 1874, the As accumulation rate was
207
123
approximately 0.2 mg m-2 year-1, and it increased
about 3.5 times by 1920. Arsenic accumulation rates
increased between the ca. 1949 and 1955 samples, with a
maximum value of 12.7 mg m-2 year-1 in 1995.
Arsenic accumulation rates decreased rapidly after
1964, but they remained an order of magnitude higher
after the 1960s than they were prior to 1920.
Arsenic accumulation rates in the Little Lake
Jackson core, like Pb accumulation rates, are also
related strongly to mass sedimentation rates (Table 2).
At the base of the dated profile, at ca. 1922, the As
accumulation rate was approximately 1.8 mg m-2
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Fig. 3 a Variations in lead
concentrations versus depth
and age, b 206Pb/204Pb,
c 207Pb/204Pb, d 208Pb/204Pb
for Little Lake Jackson
year-1, and it increased about 4.0 times by 1940.
Arsenic accumulation rates increased rapidly from ca.
1955 to 1977, with a maximum value of 122.3 mg m-2
year-1 in 1977. Arsenic accumulation rates decreased
after 1977, but remained an order of magnitude higher
than they were prior to 1940.
Discussion
Significant increases in sediment Pb concentrations
during the last *150 years indicate that anthropogenic activities are responsible for contribution of this
heavy metal to Lakes Little Bonnet and Little Jackson.
Several anthropogenic activities, such as fossil-fuel
combustion, mining, and agriculture probably contributed to this Pb increase.
There is localized mining of heavy minerals (rutile,
ilmenite, leucoxene, and zircon) in northeastern Florida. Studies have shown, however, that airborne
particles from this mining are not a major source of
anthropogenic Pb to peat deposits in east-central
Florida (Kamenov et al. 2009), and they would not
have had significant influence on our study lakes.
South Florida has been a major phosphate producer
since 1889 (Florida Institute of Phosphate Research,
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123
1965
1955
1940
1922
1902
50–55
55–60
60–65
65–70
70–75
82.92
283.19
60.88
56.61
43.65
55.55
112.46
11.20
18.66
41.15
77.79
88.38
94.39
97.90
12.23
1972
45–50
99.57
110.79
95–100
1977
40–45
54.96
71.04
89.46
13.76
1987
1982
30–35
35–40
63.98
73.71
15.65
1991
25–30
63.31
56.29
90–95
1994
20–25
59.53
58.65
51.93
85–90
1997
15–20
62.55
68.46
15.65
2000
10–15
52.80
14.41
2003
5–10
80.27
Pb
concentration
mg kg-1
80–85
2005
0–5
Mass sedimentation
rate
mg cm-2 year-1
75–80
Age mid interval
depth
year
Depth
Little Lake Jackson
11.4
23.3
34.0
49.1
106.2
81.2
54.7
78.7
57.2
46.7
33.5
36.7
35.6
42.4
Pb accumulation
rate
mg m-2 year-1
1.8
7.5
17.3
34.7
96.3
122.3
63.0
96.8
59.9
50.0
38.2
43.5
37.1
37.3
As accumulation
rate
mg m-2 year-1
6.4
3.1
2.0
1.4
1.1
0.7
0.9
0.8
1.0
0.9
0.9
0.8
1.0
1.1
Pb/As
ratio
18.943
18.942
18.943
18.922
18.894
18.908
18.813
18.768
18.654
18.647
18.746
18.836
18.921
18.916
18.900
18.880
18.861
18.842
18.845
18.841
206/
204
15.664
15.661
15.663
15.660
15.658
15.657
15.640
15.639
15.632
15.632
15.640
15.648
15.659
15.657
15.658
15.656
15.653
15.650
15.654
15.652
207/
204
38.934
38.927
38.927
38.885
38.844
38.823
38.686
38.571
38.452
38.459
38.580
38.584
38.585
38.590
38.579
38.569
38.545
38.525
38.532
38.526
208/
204
Lead Isotope ratios
2.0553
2.0550
2.0550
2.0550
2.0559
2.0533
2.0564
2.0551
2.0615
2.0625
2.0579
2.0484
2.0392
2.0401
2.0412
2.0428
2.0436
2.0446
2.0447
2.0448
208/
206
Table 2 Little Lake Jackson sediment age, mass sedimentation rate, lead concentration, lead accumulation rate, arsenic accumulation rate, and lead isotope data
0.8268
0.8268
0.8268
0.8276
0.8287
0.8281
0.8313
0.8333
0.8380
0.8383
0.8343
0.8307
0.8276
0.8277
0.8285
0.8292
0.8299
0.8306
0.8307
0.8307
207/
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http://www1.fipr.state.fl.us/PhosphatePrimer). Phosphate fertilizers are common soil amendments applied
to golf courses, such as those present in the watersheds
of both study lakes, and these applications exerted
influence on water quality in Little Lake Jackson
(Whitmore et al. 2006). Florida phosphates have high
radiogenic Pb isotopic values (Kamenov et al. 2009),
with 206Pb/204Pb values as high as 34.34 (Table 3;
Fig. 4c). The observed Pb isotope ratios in sediments
of Little Lake Bonnet and Little Lake Jackson, however, have Pb ratios distinctly different from those of
Florida phosphates, which indicates that phosphate
fertilizers did not contribute substantially to the Pb in
these lakes.
Another potential Pb source to Little Lake Bonnet
might have been coal combustion from electric power
plants. Florida Power Corporation began electric
power production near Little Lake Bonnet in 1928
using coal combustion, although the plant was
converted in subsequent decades to being powered
with number 6 fuel oil, which typically contains
\3 ppm Pb (Irwin et al. 1997). Lead fallout from this
power plant might have contributed, in part, to the
early total Pb accumulation in sediments of Little Lake
Bonnet. Detailed information about the timing of the
conversion from coal to fuel–oil combustion could not
be obtained for this Florida Power Corporation plant,
nor do we have information about the Pb isotope
signature of coal used at that time.
Table 3 Lead isotope values for different sources
206/204
207/204
208/204
208/206
207/206
Sahara
18.76
15.69
Lead arsenate
18.395
15.585
38.353
Acme arsenate
of Pb
17.07
15.468
36.876
160 phosphate
18.935
15.651
280 phosphate
19.599
15.698
38.51
2.0339
0.8266
38.952
1.9875
333 phosphate
22.454
0.801
15.838
38.779
1.7271
447 phosphate
0.7053
34.34
16.297
38.753
1.1285
0.4746
Coquina
18.822
15.646
38.504
2.0457
0.8313
Ocala limestone
19.292
15.646
38.737
2.008
0.8111
Tampa 1994
18.88
15.551
38.13
2.0196
0.8237
Tampa 1997
19.01
15.736
38.697
2.0356
0.8278
Tampa 1998
19.39
15.751
38.905
2.0064
0.8123
Data for Saharan dust from Abouchami et al. (1999), lead arsenates
from Ayuso et al. (2004), phosphates, coquina and Ocala limestone
from Kamenov et al. (2009), and Tampa aerosol particles from
Bollhöfer and Rosman (2001)
We consider, however, whether modern coal-fired
power plants exert influence on Pb isotope ratios in the
vicinity of the study lakes. Bollhöfer and Rosman
(2001) reported Pb isotope data for three aerosol
samples collected between 1994 and 1998 in the
Tampa area, approximately 110 km southwest of our
study lakes. They observed that airborne Pb isotope
values were influenced largely by Pb emitted from the
coal power plant of Tampa Electric Company, one of
the ten largest Pb releasers in the United States. The
reported ratios for the Tampa aerosols (Table 3) differ
greatly from Pb isotope values observed in the
sediments of Little Lake Bonnet and Little Lake
Jackson (Fig. 4b). In addition, close examination of Pb
isotope data shows that Tampa aerosols are distinct
from the isotope values in the lake sediments (Fig. 4b;
Table 3), indicating that even mixing relationships
cannot account for significant pollution from the
Tampa power plant. Therefore, Pb emission from coal
combustion in recent decades does not appear to have
influenced the Pb isotope signal in sediments from
these lakes. Similarly, Kamenov et al. (2009) concluded that coal-fired power plant emissions were not
important Pb contributors to Blue Cypress Marsh in
east-central Florida.
There are two potential agricultural sources of Pb
deposition in our study lakes, and they are use of
lead arsenate as a pesticide in citrus agriculture and
for weed control on turf lawns. Commercial application of lead arsenate to citrus agriculture began in
Florida by 1893 (Miller et al. 1932). Lead arsenate
was applied on citrus until 1927, when a moratorium
was imposed on the use of the pesticide (Harding
1945). During an outbreak of the Mediterranean fruit
fly, the moratorium was lifted between 1929 and
1933 and lead arsenate once again was applied to all
citrus crops. Lead arsenate application to orange and
tangerine agriculture was not legal after 1933
(Harding 1945), but application to grapefruit trees
continued until 1988 (U.S. EPA 1988). Historical
records show no evidence of past citrus agriculture
in the watershed of Little Lake Jackson (Whitmore
et al. 2008). Aerial photographs show that citrus
agriculture was active in the watershed of Little
Lake Bonnet from 1953 to the present, but aerial
photographs from 1944 show no established citrus
agriculture, so we conclude that lead arsenate would
not have been applied to citrus in the Little Lake
Bonnet watershed (Fig. 5).
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LLB
LLJ
Saharan dust
Limestone
LLB
LLJ
Tampa aerosols
A
B
LLB
LLB
LLJ
Florida phosphate
LLJ
Lead arsenates
C
D
Fig. 4 Comparison between 206Pb/204Pb and 207Pb/204Pb for
Little Lake Bonnet (solid circles), Little Lake Jackson (open
circles) and a Saharan dust (solid squares), and Florida
limestones (solid triangles), b Tampa aerosols (solid squares),
c Florida phosphate (solid diamonds), and d lead arsenates (plus
sign). Saharan dust data from Abouchami et al. (1999), Florida
limestone and phosphate data from Kamenov et al. (2009), lead
arsenates data from Ayuso et al. (2004), Tampa aerosols data
from Bollhöfer and Rosman (2001)
Lead arsenate was used for weed control on golf
courses in the first half of the 1900s. A 1933 publication
by the US Department of Agriculture, for example,
recommended application of 1,525 kg ha-1 of
lead arsenate during the establishment of golf-course
lawns, and seasonal maintenance applications of
218–653 kg ha-1 (USDA 1933), but other sources
recommended maintenance applications as high as
1,089 kg ha-1 every 3 years (Brown 1958). The use
of lead arsenate decreased substantially by the 1960s
because of toxicity issues (Murphy and Aucott 1998).
Sebring Municipal Golf Course was established beside
Little Lake Jackson in 1926, and 1944 aerial photographs of Little Lake Bonnet show fairways of Pinecrest
on Lotela Golf Course on the north and west shores, and
these probably were constructed in the 1920s. Both
study lakes, therefore, are likely to have had lead
arsenate applications to golf courses in their watersheds.
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J Paleolimnol
Fig. 5 a 1944 aerial photograph of Little Lake Bonnet and
watershed. b 1953 aerial photograph of lake and watershed. By
1953, citrus had been planted through the golf fairways on the
northwest side of the lake, adjacent to fairways on the northeast
side, and to the west and south sides of the lake. A channel had
been cut through the golf fairways on the northeast side, and it
drained into Little Lake Bonnet
We are not aware of published Pb isotope ratios of
lead arsenates that might have been applied on golf
courses in the early 1900s in Florida. We sought
information about historic lead arsenate products in
published literature, as well as from a Florida State
citrus extension agent (Stephen Futch, University of
Florida/Institute of Food and Agricultural Services)
who queried the library at the Citrus Research and
Education Center in Lake Alfred, but no documented
information was available about early Pb-based pesticides in Florida. Consequently, we turn to Pb isotope
values documented for other agricultural lead-arsenate
pesticides that were used in the USA during that time
period.
Ayuso et al. (2004) published lead isotope data for
pesticides commonly used on apple orchards in New
England, USA (Table 3). The two most common lead
arsenates show isotope values of 206Pb/204Pb = 18.395
and 17.070, 207Pb/204Pb = 15.586 and 15.469, and
208
Pb/204Pb = 38.353 and 38.876 (Ayuso et al. 2004).
At the peak of probable lead arsenate application, i.e.
1930s–1960s, in the watersheds of Little Lake Bonnet
and Little Lake Jackson, 206Pb/204Pb ratios show a slight
decrease, as might be expected if lead arsenates with Pb
isotopic composition similar to those reported by Ayuso
et al. (2004) were used on golf courses. The 206Pb/204Pb
ratio for sediments in Little Lake Bonnet, for example,
declines from *18.75 to 18.61–18.68 during
1949–1964, then resumes values of about 18.75 to the
top of the core. Consequently, we suspect that lead
arsenates contributed to a portion of the Pb burden in the
sediment profiles during the 1920s–1960s.
Maximum As accumulation rates in Little Lake
Bonnet occur during the 1940s to the 1960s, similar to
the pattern shown by Pb accumulation rates (Table 1).
Both Pb and As accumulation rates are influenced
greatly by mass sedimentation rates, which increase
during this period. Increased sedimentation probably
resulted from shoreline disturbance because this time
period corresponds to the establishment of citrus
groves in an area that traverses the golf-course
fairways on the western and northwestern shores of
Little Lake Bonnet (Fig. 5). Citrus groves also were
planted immediately adjacent to the fairways on the
northeastern shore. A canal was constructed at this
time through the fairways on the northeastern shore,
and it carried runoff into Little Lake Bonnet. Soil
disturbance from the planting of citrus groves probably mobilized lead arsenate in the soils adjoining the
golf-course fairways, particularly because lime
amendments applied to citrus groves increase the
mobility of Pb and As in soils as a result of carbonate,
bicarbonate, and potassium loading (Davenport and
Peryea 1991; Murphy and Aucott 1998; Florida DEP
2002).
The Pb/As ratio in Little Lake Bonnet (Table 1)
showed that the amount of As increased relative to the
amount of Pb over time. At the base of the Little Lake
Bonnet core, the Pb/As ratio was about 15–27, the
ratio declined to 8 by the 1940s, then to 4.4–4.8 in the
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1950s–1960s. The ca. 1974 sample in Little Lake
Bonnet showed a Pb/As ratio of 6.6, but the ca. 1990
and 2001 samples values were 3.9 and 3.6, respectively. The Little Lake Jackson sediment core showed
a similar pattern of decline in Pb/As values over time.
In the samples from Little Lake Jackson that represented 1922–1965, the Pb/As ratio declined sequentially from 6.4 to 1.4, then Pb/As values remained at
approximately 1.0 in the ten samples that represent
1972–2005 (Table 2).
Lead arsenate used in turf applications typically
was in the form of acid lead arsenate with the formula
PbHAsO4, so a 1:1 ratio might be expected between Pb
and As if lead arsenate were the primary source of
these metals, and if both metals were transported
equally from soils to the lake. The observed correspondence in accumulation rates of Pb and As during
the 1900s, increases in Pb/As ratios, as well as the
decline in 206Pb/204Pb ratio in both lakes suggests that
some lead arsenate might have influenced deposition
of these metals. Although trends in Pb/As decline were
similar in both lakes, ratios were higher, in general, for
samples from Little Lake Jackson than for Little Lake
Bonnet. Greater As deposition might be expected in
Little Lake Jackson because the lake had three golf
courses developed progressively in the watershed, as
well as a stream that transported golf-course runoff to
the lake (Whitmore et al. 2008). Lead arsenate use
declined by 75 % by the 1960s (Murphy and Aucott
1998), so greater As content in sediments of Little
Lake Jackson and Little Lake Bonnet after the 1960s
probably would have resulted from As application to
golf-course soils in the form of monosodium methylarsonate (MSMA) (Whitmore et al. 2008). MSMA
application after the 1960s would not have contributed
to Pb deposition. Isotope ratios support the conclusion
that alkyl lead was the primary contributor of Pb to the
lakes in recent decades.
Widespread leaded gasoline use began in the mid1920s, and rapid population growth in Florida began in
the 1930s. By the middle 1950s to 1969, a major
interstate highway system (e.g. Highways I-75 and
I-95) was constructed in Florida (www.us-highways.
com). Careful observation of the lead record shows
that the changes in Pb concentrations and isotopic
ratios in Little Lake Bonnet and Little Lake Jackson
sediments correlate temporally with gasoline consumption in the USA as well as with changes in the
lead ore used to produce leaded gasoline. The isotope
123
values of leaded gasoline varied significantly
throughout the last century (Hurst et al. 1996).
Lead isotope ratios in Little Lake Jackson show a
gradual decrease from 1902 to 1965 (Table 2; Fig. 3).
This gradual decrease also is seen in Little Lake
Bonnet from 1920 to 1964 (Table 1; Fig. 2). The
decline reflects the use of relatively non-radiogenic Pb
from Idaho ore deposits in leaded gasoline until the
1960s (Hurst et al. 1996). Before 1967 the average
206
Pb/207Pb value was calculated to be around 1.153
(Erel and Patterson 1994). An increase in isotope
ratios from the mid 1960s to 1974 in Little Lake
Bonnet and from the 1960s to 1987 in Little Lake
Jackson (Fig. 3) reflects the use of more radiogenic
lead from Mississippi Valley Type. Mississippi lead
ore 206Pb/207Pb values range between 1.28 and 1.33
(Heyl et al. 1974). Little Lake Jackson sediments show
a decrease in isotope ratios from 1987 to the present
(Fig. 3), whereas isotope ratios in the sediments of
Little Lake Bonnet remain fairly constant after 1974
(Fig. 2). Although there is a decline in isotope values
in the top sediments, values continue to be more
radiogenic than bottom sediments. This indicates that
lake sediments might be receiving inputs of anthropogenic Pb that were deposited on watershed soils
during the past, but are still being transported to the
lake. Alternatively, values might not have returned to
natural background values for the Florida peninsula
because modern deposition continues to be influenced
by anthropogenic activities (Kamenov et al. 2009).
Following mixing equations from Bacardit et al.
(2012), we modeled the isotopic composition of the
anthropogenic Pb added to the lake sediments (Fig. 6).
Lead isotope values from the bottom of the cores were
assumed to be representative of natural Pb (Fig. 6,
dashed lines). We used Pb accumulation rates shown
in Tables 1 and 2 in the calculations and assumed
4,000 g km-2 year-1 for Little Lake Bonnet, and
5,000 g km-2 year-1 for Jackson in the pre-210Pbdated intervals. Note that we do not calculate the
anthropogenic Pb component for the pre-210Pb-dated
parts of the cores because we do not know the exact
accumulation rates. The calculated Pb isotope values
for the anthropogenic component were similar overall
to the trends from the bulk sediment analyses. The
calculated anthropogenic Pb isotope values in the Blue
Cypress Marsh peat core also showed Pb values
similar to the bulk analyses (Kamenov et al. 2009).
This is not surprising, given that anthropogenic Pb
Author's personal copy
J Paleolimnol
Fig. 6 Bulk sediment 206Pb/207Pb (solid lines) versus modeled
206
Pb/207Pb (dotted lines) of the anthropogenic Pb added to
a Little Lake Jackson, b Little Lake Bonnet, and c Blue Cypress
Marsh. Lead isotope values from the bottom parts of the cores
were assumed to be representative of the natural Pb (dashed
lines). Anthropogenic Pb isotopic composition is calculated
after equations in Bacardit et al. (2012). Lead accumulation
rates shown in Tables 1 and 2 were used in the calculations. For
the pre-210Pb-dated intervals we assumed 4,000 g km-2 year-1
for Lake Bonnet and 5,000 g km-2 year-1 for Little Lake
Jackson accumulation rates
dominates the record in the upper, 210Pb-dated part of
the cores (Tables 1, 2). Before the 1920s, the Pb
isotope record for Lake Jackson and Blue Cypress
Marsh was close to the assumed natural Pb values
from the bottom parts of the cores (Fig. 6). Only the
ca. 1874 datum for Little Lake Bonnet was farther
away from the natural value. This is most likely a
calculation artifact that is a consequence of the rapid
drop in Pb accumulation rate in Little Lake Bonnet at
that time (Table 1). The anthropogenic Pb fraction in
the ca. 1874 interval was much smaller compared to
the intervals above. In order to account for the isotope
change, the model requires Pb with a very distinct
isotope composition, assuming the simple two-component mix between natural Pb and anthropogenic Pb
modeled by the equations of Bacardit et al. (2012).
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LLB
LLJ
mean US lead
Bermuda corals
Atlantic corals
A
LLB
LLJ
BCM
Lake Annie
Lake Okeechobee
B
Fig. 7 a Variations in 206Pb/207Pb versus dated sediments. Little
Lake Bonnet, LLB (circles), Little Lake Jackson, LLJ (squares),
mean USA atmospheric lead deposition record (diamonds),
Bermuda corals (triangles), Atlantic corals (circle in filled
square). USA atmospheric lead data from Desefant et al. (2006),
Atlantic corals data from Reuer et al. (2003), and Bermuda corals
data from Shen and Boyle (1987). b Variations in lead
concentrations in Florida sediments versus 210Pb age. Little Lake
Bonnet, LLB (circles), Little Lake Jackson, LLJ (squares), Blue
Cypress Marsh, BCM (diamonds), Lake Annie (open circles),
Lake Okeechobee, three sediment cores (triangles). Blue Cypress
Marsh data from Kamenov et al. (2009), Lake Annie and
Okeechobee data from Schottler and Engstrom (2006)
Regardless of the overall similarity in the trends,
the calculated anthropogenic Pb component isotope
values are not identical in the two lakes and the Blue
Cypress Marsh core, suggesting that the simple, two-
123
end-member mixing model cannot fully account for
the observed isotope trends. This indicates that
although the overall trend is controlled by a dominant
common source at the three sites, which is Pb released
during leaded gasoline combustion, there are additional discrete Pb sources at each site that are not fully
represented by the presumed natural Pb background
from the deeper parts of the cores. Similar findings
were noted in several lakes in the Central Pyrenees
(Bacardit et al. 2012). The modeled modern anthropogenic Pb component did not show the same
composition at the Pyrenees lakes and was interpreted
to be a result of slightly different Pb sources for the
different lakes. The fact that the calculated anthropogenic Pb component from the mixing model is slightly
different in each core shows that the Florida lakes
experienced the addition of alkyl lead, as well as Pb
with isotopic composition distinct from the assumed
natural Pb from the bottom parts of the cores. Given
the complex development history of the two lakes
during the last century, it might be expected that the
bottom portions of the cores are not really representative of the ‘‘natural Pb’’ mixed with the anthropogenic Pb released during leaded gasoline combustion.
During the last 100 years or so, a number of local
factors, such as possible use of lead arsenates,
highway construction near the lakes, and extensive
application of lime and fertilizer soil amendments
(Whitmore et al. 2006) have likely contributed new
sediments and/or dust with distinct Pb isotopes
compared to the pre-210Pb-dated bottom parts of the
cores from the two lakes. Therefore, our modeling
shows that the 210Pb-dated parts of the cores contain
leaded-gasoline Pb and an additional discrete Pb
component intrinsic to each site, which cannot be
simply represented by the bottom parts of the cores.
The observed Pb isotope changes in sediments from
the two lakes show chronological patterns very similar
to the east-central Florida peat record reported by
Kamenov et al. (2009), as well as to the mean USA
atmospheric lead deposition record (Desefant et al.
2006), to lead isotope records from Bermuda (Shen
and Boyle 1987) and Atlantic corals (Reuer et al.
2003), and to the Pb record obtained from Lake
Okeechobee (Schottler and Engstrom 2006) (Fig. 7).
This suggests that regional patterns of atmospherically
derived Pb deposition are comparable throughout the
peninsula, despite the relatively reduced continental
atmospheric influence on this region.
Author's personal copy
J Paleolimnol
Conclusions
Lead concentrations and isotope ratios show significant stratigraphic changes in sediment cores from
Little Lake Bonnet and Little Lake Jackson. The
observed increase in Pb concentrations in the lake
sediments during the last *100 years indicates
anthropogenic contribution of this heavy metal to the
lake. The increase in the Pb concentration is also
accompanied by changes in the Pb isotope values,
which allowed us to determine which anthropogenic
activities played a role in the increase in Pb accumulation during the last century. The sediment records in
Little Lake Bonnet and Little Lake Jackson show that
the changes in Pb deposition and isotope ratios
correlate temporally with gasoline consumption in
the USA as well as with changes in the ore used to
produce leaded gasoline. The observed Pb depositional changes and isotopic trends show patterns
similar to other Florida records, as well as to the
mean USA atmospheric Pb deposition record, and to
Pb isotope records from Bermuda and Atlantic corals.
Regional synchrony of Pb depositional records and Pb
isotope trends support the use of Pb concentrations and
isotope ratios as a chronological marker in Florida
paleolimnological studies. Several other Pb pollutants
can influence the sedimentary record in Florida lakes,
but their occurrence is largely within the time span of
210
Pb dating, and their relative contribution appears
small with respect to the atmospheric contribution
from alkyl lead sources.
Acknowledgments Funding for this project was provided in
part by a U.S. Environmental Protection Agency STAR grant to
the University of South Florida—St. Petersburg. We thank Keith
Dominey and Jessica Moss at Waters Agricultural Laboratory in
Camilla, Georgia for analytical assistance with total Pb content.
Natalia Hoyos assisted with figures.
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