lead biogeochemistry in the littoral zones of south-central ontario lakes, canada, after the...

LEAD BIOGEOCHEMISTRY IN THE LITTORAL ZONES OFSOUTH-CENTRAL ONTARIO LAKES, CANADA, AFTER THE

ELIMINATION OF GASOLINE LEAD ADDITIVES

P. M. OUTRIDGEEnvironmental and Resource Studies Program, Trent University, Peterborough, Ontario K9J 7B8,

Canada(Present address: Geological Survey of Canada, 601 Booth Street, Ottawa K1A 0E8, Canada,

e-mail: [email protected])

(Received 19 October 1998; accepted 26 March 1999)

Abstract. Stable Pb isotope ratios were used to trace the sources and pathways of Pb betweenthe atmosphere, surficial sediment fractions, the white water-lilyNymphaea odorata, and waterscollected at 26 littoral sites in 23 Ontario lakes in summer 1993, three years after alkyl Pb additiveswere finally eliminated from Canadian gasoline. Based on similarities of isotopic composition, theexchange of Pb between lakewater and sediment ‘carbonate’, and subsequently between ‘carbonate’,‘oxide’ and other sediment fractions, was the most likely water-sediment pathway of Pb movement.pH controlled Pb fractionation within surficial sediments, with the ‘organic’ pool comprising 80–97% of total Pb in most acidic lakes and 15–60% in alkaline lakes. About 28% of the Pb inN. odoratashoots was accumulated directly from water, whereas there was no evidence of root uptake of Pbfrom sediments. The Pb in plant tissues was isotopically homogeneous and dissimilar to the variablecomposition exhibited in ambient waters and sediments. Plant Pb isotopes strongly resembled the his-torical Canadian atmospheric (alkyl Pb) signature. A possible explanation is that, like essential tracemetals, historically-accumulated Pb was highly conserved during the annual growth cycle of thislong-lived, clonal macrophyte, being stored over-winter in underground rhizomes and recycled intospring growth. Given the low rate of ‘new’ Pb uptake, historical alkyl Pb may continue to dominateplant tissues for some time, even though it was not detectable in littoral waters and sediments.

Keywords: atmosphere, bioaccumulation, isotope ratios, lead, macrophytes, sediments

1. Introduction

Lead is unique among the metals of environmental significance by exhibiting nat-ural variations in the proportions of its stable isotopes (atomic masses 204, 206,207 and 208), which are related to the radiogenic ontogenies of the latter threeisotopes (Dickin, 1995). Different rock formations and Pb ore deposits thus con-tain Pb with characteristic though not necessarily unique isotopic ratios (Brown,1962). This fact has been exploited to identify the provenance of Pb pollutionfrom different anthropogenic sources. Sturges and Barrie (1987, 1989) identifiedalkyl Pb from gasoline combustion in the atmosphere over southern Ontario in the1980s, and isotopically distinguished air masses originating from Canadian andAmerican urban/industrial centres as well as from various smelters within Canada.

Water, Air, and Soil Pollution118: 179–201, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

180 P. M. OUTRIDGE

The Canadian/American dichotomy in isotopic signatures was confirmed by Flegalet al. (1989) in Great Lakes waters.

The introduction of isotopically-distinctive anthropogenic Pb into the atmo-sphere has provided the setting for an unintended, large-scale biogeochemical fieldexperiment. Because of the dominance of alkyl Pb from gasoline combustion,atmospheric Pb across southern Ontario in the 1980s was isotopically quite ho-mogeneous (Sturges and Barrie, 1987). Deposition of this homogeneous Pb into aseries of lakes containing sediments of varying Pb isotopic composition would al-low the mixing of leads to be studied in the various biotic and abiotic compartmentsof lake ecosystems (water, exchangeable sediment fractions, and biota), particu-larly in systems where the compartments had not come into isotopic equilibriumbecause of rapid changes in the strength or composition of one or more of thesources.

In the present study, the biogeochemistry of Pb in the littoral zones of fresh-water lakes was investigated by studying the patterns of Pb isotope ratios andconcentrations in the atmosphere, lakewaters, various operationally-defined geo-chemical compartments in littoral sediments, and the above- and below-groundbiomass of a common littoral macrophyte, the white water lilyNymphaea odorataL. Lead may be accumulated byN. odoratafrom lakewater, sediment and possiblyatmospheric sources, depending on its mode of uptake (see Outridge and Noller,1991). The lakes studied in rural south-central Ontario, Canada, in 1993 wereselected to cover a range of physico-chemical types and catchment geologies, aswell as varying distances from prominent industrial point-sources of Pb. While noatmospheric Pb isotope data have been reported from southern Ontario/northernU.S.A. since the phase-out of alkyl Pb in the late 1980s, Pb deposition loadings inOntario declined dramatically after the introduction of ‘Pb-free’ gasoline in 1974and continued declining until alkyl Pb’s complete phase-out in 1990 (Reidet al.,1993). It was expected that, in 1993, significant isotopic differences would existbetween atmospheric inputs, still influenced by anthropogenic sources to some de-gree (Reidet al. 1993), and natural sediment and groundwater inputs derived fromsurrounding catchment rocks. Lithologies underlying the study lakes ranged fromPrecambrian Shield metamorphic terrains to Paleozoic carbonates (GSC, 1996),which will manifest marked Pb isotope differences (Dickin, 1995).

2. Materials and Methods

2.1. FIELD SAMPLING

Twenty-six sampling sites were visited in 23 lakes across south-central Ontario(Figure 1). Sampling took place during July 1993, and occurred in a south-to-northpattern to sampleN. odorataat approximately the same phenological stage acrossall lakes, just after the commencement of flowering. At each site, lakewater in the

LEAD BIOGEOCHEMISTRY AFTER THE ELIMINATION OF GASOLINE LEAD ADDITIVES181

Figure 1.Location map of the study lakes ( ) and Environment Canada’s air sampling sites (N) usedin this study.

littoral zone was measured for pHin situwith a VWR pH meter, and water samplestaken in sterile WhirlpaksTM for subsequent laboratory analyses. One water sample(200 mL) was acidified with 2 mL of trace grade nitric acid for total metal determ-inations. Six replicate surficial sediment samples were taken from within a 10 msection of the littoral zone (<1 m depth) with acid-washed perspex coring tubes(10 cm length, 2.5 cm diameter). The tubes were pushed into the sediment, andwithdrawn with the bottom of the tube sealed by hand. The position of the visibleoxidized/reduced boundary was marked on the tube, the ends of the tubes sealedwith parafilm, and the water and sediment samples frozen on site on dry ice. Com-plete freezing of the cores occurred usually within three hours of sampling. Onlythe oxidized portion (usually to 2–4 cm depth) of each core was used in subsequentanalyses. From within the same section, 1–3 plants ofN. odoratawere removed,and washed with lakewater on site to remove adhering sediment and algae. Theywere covered with waxed paper and dried in plant presses in the field, and later inthe laboratory at 60◦C.

Airborne particulate filters were obtained from Environment Canada atmosphericmonitoring stations located at Burnt Island and Point Petre, Ontario (Figure 1).Eight filters containing samples accumulated over 10–14 day periods spanning1993 (see Table I), and two blanks, were obtained from each station. The filterswere leached with 10 mL 1 N HNO3 + 1.75 N HCl for 90 min at 35◦C and made

182 P. M. OUTRIDGE

up 25 mL with distilled deionized water. This dilute acid treatment was found byGraneyet al. (1995) to remove all acid-soluble Pb from lake sediments, and gavePb isotope and concentration data identical to full strength aqua regia digestion.

2.2. LABORATORY ANALYSES

After thawing, subsamples of lakewater were analysed for conductivity with aCole-Parmer model 1500-32 conductivity meter, and for dissolved organic carbon(DOC) and inorganic carbon (DIC) with a Shimadzu TOC-5000 total organic car-bon analyser calibrated against stock solutions of potassium hydrogen phthalateas per manufacturer’s instructions. Samples for DOC and DIC were first filteredthrough 0.45µm Whatman GF/F glass fibre filter papers, which had been ashedat 400◦C for 24 h and washed with distilled deionized water to remove volatileorganic contaminants.

The upper oxidized components of all six sediment samples from each site werecombined and separated under nitrogen into several subsamples for determinationof various parameters. One subsample (10–20 g) was dried at 60◦C and ashed at500 ◦C overnight for dry-wet weight determination and loss-on-ignition (organicmatter) measurements. A second subsample (1–4 g) was dried for particle sizeanalysis. The dry sediment was lightly ground in a mortar and pestle to break upsilt and clay aggregates, then shaken for 20 min through a series of copper sieves.For the purposes of this study, the particle size fractions were defined as silt + clay(<250µm) and sand + gravel (≥250µm).

A further subsample (3–5 g) of sediment, sealed under nitrogen in a screw-capped vial, was centrifuged at 2500 rpm for 10 min to remove porewater, andsequentially extracted according to the Tessieret al. (1979) scheme as modified byRapinet al. (1986). Much of the literature using this or other extraction schemesreports data on dried, ground sediment samples. However, the present study usedfresh sediments which had been rapidly frozen in the field as a means of measuringPb fractionation under conditions as close toin situ as possible (see Rapinet al.,1986). Four operationally-defined Pb fractions were extracted:

– F1 (exchangeable), shaken in 1 M MgCl2 at pH 7.0 for 30 min;– F2 (carbonate), shaken for 5 h in 1 M sodium acetate buffered at pH 5.0 with

acetic acid;– F3 (oxide-adsorbed), heated and shaken in a water bath at 96◦C for 6 h in

0.04 M NH2OH·HCl in 25% acetic acid; and– F4 (organic), heated and shaken at 70◦C overnight in 30% H2O2 buffered at

pH 2.0 with HNO3, followed by 3.2 mL ammonium acetate in 20% HNO3 atroom temperature for 30 min.

Each stage was extracted with 20 mL of reagent. Extracted solutions were filteredthrough Whatman GF/A paper, and made up to a final volume of 25 mL with 10%


TABLE I

Airborne Pb concentrations and isotope ratios at Burnt Island and Point Petre monitoringstations, Ontario, 1993. (Isotope data from this study, Pb concentrations and air masssource information supplied by R. Hoff, Environment Canada. Sampling periods incor-porate air samples accumulated during previous 10–14 days.∗ = Average isotope ratioswere weighted for Pb concentration on each sampling date)

Site/sampling period Airborne [Pb] Likely main source 206/207 208/207

in 1993 (ng m−3) of air mass

(a) Burnt Island(45◦48′N; 82◦57′W)

16 January 1.44 Local background 1.189 2.443

28 January 1.29 Local background 1.079 2.378

21 February 6.70 Sudbury, Ont. 1.144 2.435

05 March 4.71 Sudbury, Ont 1.133 2.440

03 July 7.33 Sault Ste. Marie, Ont. 1.210 2.448

27 July 1.12 Local background 1.162 2.411

08 August 1.10 Local background 1.157 2.425

09 December 9.42 Saginaw, Mich. 1.219 2.455

Annual avergae 1.179∗ 2.442∗CDN sources 1.163∗ 2.436∗USA sources 1.219∗ 2.455∗

(b) Point Petre(43◦50′N; 77◦09′W)

16 January 10.17 Buffalo, N.Y. 1.198 2.444

21 February 1.59 Local background 1.168 2.425

29 March 11.65 Montreal, Que. 1.161 2.423

10 April 1.41 Local background 1.145 2.406

22 April 1.23 Local background 1.140 2.404

09 June 8.66 Buffalo, N.Y. 1.199 2.442

09 December 11.35 Buffalo, N.Y. 1.213 2.455

21 December 1.37 Local background 1.207 2.434

Annual average 1.189∗ 2.438∗CDN sources 1.163∗ 2.422∗USA sources 1.204∗ 2.448∗

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trace grade nitric acid. Multiple procedural blanks and replicate samples (N = 3) ofa standard reference material (NBS River Sediment) were also carried through theextraction scheme.

DriedN. odorataplants were separated into ‘shoot’ (stems, leaves, flowers) and‘root’ (roots and rhizomes) parts, and ground in a Wiley mill equipped with stain-less steel blades. Multiple shoot and root specimens from each site were combinedinto a single sample of each per site. A standard reference material (NBS PineNeedles) was carried through the grinding phase to check for Pb contamination.Subsamples of powdered plants (0.1–0.2 g dry wt) were refluxed with 2 mL HNO3

at 80◦C, and made up to 25 mL with distilled deionized water. Reagents used inthis study were of commercial analytical trace grade or better.

2.3. LEAD ANALYSIS

2.3.1. Lead ConcentrationsAll of the sediment extracts and macrophyte digests were diluted 1:25 with 1%HNO3, and these together with acidified lakewater samples analysed for Pb con-centrations by flow injection inductively-coupled plasma mass spectrometry (Elan5000 ICP-MS with Model 400 Flow Injection Auto-Sampler; Perkin-Elmer SCIEX,Norwalk, CT, U.S.A.). Measurement was by external calibration with Bi at 10µg L−1 as an internal standard; calibration accuracy was checked against NIST1643c Trace Elements in Water SRM. For sediment extracts, the external calib-ration standards were made up with extraction reagent diluted 1:25. Preliminarytests showed that this procedure, together with Bi as internal standard, negated anyspectral and non-spectral mass interferences that might have affected measurementaccuracy. Results for NBS Pine Needles reference material (10.2± 1.0µg g−1, N= 8) were within error of the certified Pb value (10.8± 0.5µg g−1).

2.3.2. Lead IsotopesLead isotopes (206, 207, 208m/z) were determined on different sample typeswith two different Elan ICP-MS instruments. Accuracy in both cases was checkedagainst NIST 981 Common Pb Isotopic Standard at regular intervals during eachrun. Corrections were made to account for mass bias and drift during measurement.

Lead isotopes on macrophyte digests and sediment extracts were determinedwith an Elan 5000 ICP-MS in solution nebulization mode, using 200 ms peakdwell times and a total measurement time of 3 min. The reference materials usedin this study do not have certified Pb isotope compositions. However, isotope ratioprecisions on Pb fractions in NBS Buffalo River Sediment were 0.2–0.7% RSD (N= 3) except for the F1 fraction (1.3–2.1% RSD) which had a very low Pb content(average207Pb count, 2390 cps). The overall weighted average ratios (206Pb/207Pb= 1.224;208Pb/207Pb = 2.474) in NBS Buffalo River Sediment were within 1% ofthe ratios identified by Flegalet al. (1989) as American industrial Pb in the GreatLakes. Ground samples of NBS Pine Needles gave mean±SD (N = 8) isotope


ratios of 1.179±0.003 (206Pb/207Pb) and 2.449±0.005 (208Pb/207Pb) which werewithin one standard deviation of those in unground samples (1.184±0.005 and2.445±0.006; N = 4).

Lead isotopes in lakewaters and airborne Pb samples were analysed with anElan 5000A ICP-MS, using similar dwell and total measurement times as previ-ously. Unfortunately, half of the water samples were lost before analyses were com-pleted, and so data will be presented from 13 sites. Replicate analyses of a 20µgL−1 commercial Pb standard interspersed with the samples gave ratio precisions of0.15% for206Pb/207Pb and 0.3% for208Pb/207Pb. Duplicate samples of macrophytedigests (N = 13) were analysed with both Elan instruments, and gave very similarresults (Elan 5000:206Pb/207Pb = 1.153±0.014,208Pb/207Pb = 2.427±0.029; Elan5000A:206Pb/207Pb = 1.156±0.011,208Pb/207Pb = 2.423±0.009).

2.4. DATA ANALYSIS AND STATISTICS

Across a group of sites exhibiting a range of isotopic composition in waters and/orsediments, the slopes of simple linear regressions between the isotope ratios in eachenvironmental compartment (water, sediment fractions, plants) identify the overallisotopic similarities or dissimilarities between those compartments. The regressionslope may thus be taken as an average measure across all sites of the proportionof Pb in the compartment from each putative contributing source. A significantcorrelation indicates that the source in question explained a significant amount ofthe variance in isotopic composition of the compartment.

Isotope ratios were arc-sine square-root (x/10) transformed, and concentrationdata log (x+1) transformed to address statistical assumptions of normality and het-eroscedasticity (Sokal and Rohlf, 1981). Regression analysis was performed withSigmaStat (Jandel Scientific, U.S.A.).

3. Results

3.1. ATMOSPHERIC LEAD

Airborne particulate samples gathered at Burnt Island and Point Petre, Ontario,in 1993 showed that while the temporal variability of Pb concentrations and Pbisotopic ratios during the year was great, the overall weighted average isotope ratiosat both sites were similar (Table I). There were distinct differences in the isotopiccomposition of Pb originating from Canadian and US sources, with air masses fromthe U.S.A. having higher ratios than those in Canada.

3.2. LAKEWATER AND SEDIMENT CHEMISTRY

Water chemistry of the lakes reflected the diversity of physico-chemical types andcatchment characteristics included in the study (Table II). pH ranged from 4.6 to

186 P. M. OUTRIDGE

Figure 2. Relationship between lakewater pH and the proportions of Pb in various operation-ally-defined sediment fractions (F1 – ‘exchangeable’; F2 – ‘carbonate-bound’; F3 – ‘oxide-bound’;F4 - ‘organic’. Middle Lake is indicated by an arrow).

8.8, and waterborne Pb concentrations from 1.1 to 130µg L−1. pH was signific-antly positively correlated with dissolved organic carbon (r2 = 0.20, P = 0.025) andwith dissolved inorganic carbon (r2 = 0.66, P<0.001), however, waterborne Pb wasnot significantly correlated with any lakewater parameters.

Sediments at most of the study sites were predominantly sand/gravel, reflectingthe littoral, high wave-action environment (Table III). Sediment Pb at the majorityof sites occurred mainly in the F3 (‘oxide’) and F4 (‘organic’) fractions. In termsof both concentration and as a percentage of total Pb, F4 was positively correlatedwith sediment organic content (loss-on-ignition), while the proportion of F3 wasnegatively correlated with LOI (Table IV). Lakewater pH and DIC were also relatedto the partitioning of Pb between sediment fractions. Acid lakes contained 80–97%of total sediment Pb in the F4 fraction, and proportionately less in F1–F3 thanalkaline lakes (Figure 2). Middle Lake was a conspicuous outlier in the data set(indicated in Figure 2). This lake was also unusual in its water chemistry, beingthe only acidic lake of low DOC and DIC to exhibit a relatively high conductivity(Table II). The explanation for the high conductivity is unknown; anion chemistrydata are not available.

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TABLE II

Lake geographical co-ordinates, sampling sites and water chemistry

Lake Site Latitude Longitude pH Conductivity DIC+ DOC+ Water Pb

(◦N) (◦W) (µS) (mg L−1) (mg L−1) (µg L−1)

Plastic W shore 45:11 78:50 5.4 20 0.56 2.2 25.8

Wren outflow 45:11 78:52 6.3 47 0.60 3.4 73.9

Wren W shore 45:11 78:52 5.9 28 0.47 3.6 8.8

Loch Garry outflow 45:15 74:44 7.4 184 15.0 14.5 59.6

Shiner outflow 45:15 76:53 8.3 190 13.8 3.7 1.1

Dempsey N shore 45:16 76:40 8.7 180 13.4 12.5 46.1

Middle outflow 45:17 74:40 6.0 230 1.67 3.8 40.6

Eleanor N shore 45:21 77:03 8.2 108 7.40 3.2 3.3

Lumber W shore 45:24 78:53 4.6 18 0.57 9.0 34.6

Middle (#2) outflow 45:25 81:01 8.0 165 8.86 6.9 7.1

Toad N shore 45:26 78:56 6.8 47 1.61 4.4 24.2

Cobb’s inflow 45:28 75:15 7.7 350 10.7 22.7 11.3

Cobb’s outflow 45:28 75:15 8.8 430 11.7 23.8 8.4

March Hare W shore 45:34 78:42 – 23 0.96 9.5 58.1

Loxton NW shore 45:57 79:13 6.5 24 0.79 3.2 6.5

Loxton SW shore 45:57 79:13 6.5 16 1.18 2.4 98.6

Traverse inflow 45:58 78:03 6.4 – 0.25 7.0 44.8

Hinsburger S shore 45:58 79:14 7.0 24 0.32 2.5 69.1

+ DIC and DOC = dissolved inorganic and organic carbon, respectively.

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TABLE II

(continued)

Lake Site Latitude Longitude pH Conductivity DIC+ DOC+ Water Pb

(◦N) (◦W) (µS) (mg L−1) (mg L−1) (µg L−1)

Tyne inflow 45:59 79:11 6.3 24 1.31 2.7 130

Radiant outflow 46:00 78:17 6.3 26 1.47 2.9 27.5

Rains N shore 46:06 83:55 6.9 117 4.27 8.2 46.2

Wavy N shore 46:17 81:06 4.8 53 0.32 3.1 22.9

Macfarlane S shore 46:25 80:57 7.9 275 3.63 4.5 20.9

Geneva outflow 46:46 81:33 6.5 27 1.29 4.0 29.6

Bailey outflow 46:52 81:40 6.3 43 0.96 5.4 8.6

Low Water boat ramp 47:10 81:40 5.5 50 1.45 5.3 82.1

Range – – – 4.6–8.8 16–430 0.25–15.0 2.2–23.8 1.1–130

+ DIC and DOC = dissolved inorganic and organic carbon, respectively.

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TABLE III

Lake sediment texture, organic matter and lead fractionation, and Pb concentrations in plant tissues (LOI = loss on ignition. For operational definitionof sediment fractions F1 through F4, see the Methods section)

Lake Site % Silt % Sand LOI Total Pb % F1 % F2 % F3 % F4 Root Pb Shoot Pb

+ Clay + Gravel (% DW) (µg g−1 DW) (µg g−1 DW) (µ g−1 DW)

Bailey outflow 19.2 80.8 61.1 144 0.6 0.7 1.7 97.0 8.8 2.0

Cobb’s inflow 32.9 67.1 17.9 13.5 29.6 2.8 8.2 59.4 11.7 3.6

Cobb’s outflow 30.1 69.9 19.2 10.0 24.6 3.8 28.4 43.2 20.8 2.8

Dempsey N shore 55.4 44.6 2.8 5.2 33.5 2.5 35.9 28.1 12.1 2.4

Eleanor N shore 15.1 84.9 4.5 3.0 53.0 5.2 28.3 13.4 83.7 5.0

Geneva outflow 12.3 87.7 9.0 69.5 2.5 1.8 3.1 92.6 4.0 2.6

Hinsburger S shore 28.6 71.4 3.4 20.6 8.6 0.4 4.8 86.2 115 8.1

Loxton NW shore 10.9 89.1 1.7 9.7 1.3 0.9 5.7 92.1 14.1 3.6

Loxton SW shore 11.3 88.7 1.9 14.9 17.6 0.4 9.1 72.8 32.4 0.7

Loch Garry outflow 19.7 80.3 7.4 7.4 1.1 7.8 59.4 31.7 10.5 13.5

Low Water boat ramp 30.8 69.2 1.9 7.5 0.4 2.3 11.7 85.6 5.8 4.9

Lumber W shore 26.5 73.5 86.2 716 0.9 0.1 2.6 96.4 26.2 7.8

MacFarlane S shore 77.9 22.1 1.3 15.4 9.8 39.6 5.8 44.8 10.5 5.0

March Hare W shore 40.6 59.4 4.3 6.6 3.8 1.0 4.4 90.8 583 3.9

Middle outflow 35.8 64.2 3.5 8.3 20.2 5.2 43.4 31.2 21.0 2.4

Middle #2 outflow 26.8 73.2 6.6 90.4 1.1 13.0 6.2 79.6 76.4 7.1

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TABLE III

(continued)

Lake Site % Silt % Sand LOI Total Pb % F1 % F2 % F3 % F4 Root Pb Shoot Pb

+ Clay + Gravel (% DW) (µg g−1 (µg g−1 (µ g−1

DW) DW) DW)

Plastic W shore 30.3 69.7 41.9 165 2.2 0.3 3.0 94.5 92.8 11.9

Radiant outflow 67.5 32.5 7.6 21.9 1.8 1.4 8.5 88.3 61.4 8.7

Rains N shore 17.3 82.7 79.2 380 1.2 0.2 1.7 96.9 10.7 3.7

Shiner outflow 10.9 89.1 1.4 3.9 23.4 9.0 45.8 21.8 148 3.0

Toad N shore 27.8 72.2 26.4 275 1.3 0.1 3.6 94.9 105 5.7

Traverse inflow 62.4 37.6 1.2 12.8 1.8 0.5 15.4 82.2 194 16.4

Tyne inflow 14.4 85.6 3.2 23.5 3.0 0.5 8.9 87.7 5.3 5.7

Wavy N shore 22.6 77.4 76.9 725 5.8 2.8 3.8 87.6 5.2 6.9

Wren outflow 9.5 90.5 88.3 138 2.6 0.3 3.9 93.2 21.1 6.1

Wren W shore 47.0 53.0 5.0 13.3 6.9 1.0 10.8 81.3 1.6 17.9

Range – 9.5–77.9 22.1–90.5 1.2–88.3 3.0–725 0.4–53.0 0.1–39.6 1.7–59.4 13.4–97.0 1.6–583 0.7–17.9

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TABLE IV

Co-efficients of determination (r2 values) between selected water and sediment parameters+ and sediment Pb fractions (∗ = correlationsignificant at P<0.05;∗∗ = at P<0.01. Negative sign indicates a negative correlation)

Sediment Pb fractions

F1 F2 F3 F4 F1 F2 F3 F4

(µg g−1) (µg g−1) (µg g−1) (µg g−1) (% total) (% total) (% total) (% total)

pH 0.02 0.05 0.01 –0.22∗ 0.36∗∗ 0.25∗ 0.24∗ –0.47∗∗DIC 0.01 0.05 0.01 0.12 0.26∗∗ 0.27∗∗ 0.36∗∗ –0.50∗∗DOC 0.02 0.00 0.00 0.02 0.02 0.02 0.08 0.07

LOI 0.10 0.01 0.04 0.55∗∗ 0.10 –0.13 –0.22∗∗ 0.24∗

+ DIC = dissolved inorganic carbon in mg L−1; DOC = dissolved organic carbon in mg L−1; LOI = sediment loss on ignition (% DW).

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Figure 3. Relationships between206Pb/207Pb in lakewater and sediment fractions. (Dashed diag-onal line represents a slope of 1.0; solid lines represent the slopes of significant linear regressions.Non-significant regression lines are not plotted).

Regression analysis of Pb concentrations in sediment fractions and in lakewa-ters found few significant associations, with positive correlations only betweenF2 and F3 fractions and between F1 and waterborne Pb (Table Va). Analysis ofthe isotope ratios revealed several additional relationships (Table Vb, c; Figure 3).Across the study lakes, all of the sediment fractions were significantly correlatedindicating that in general they shared at least some Pb from the same source orcombination of sources. Lakewater Pb isotope ratios were significantly correlatedonly with those in the F2 fraction, with an average slope of 0.96 (slopes of 0.88and 1.03 for the two ratios). At some sites the lakewater had slightly lower isotoperatios than sediment F2, which led to a constant offset from 1:1 for the regressionline (Figure 3b). The lakewater-sediment F2 regression was particularly influencedby two Cobb’s Lake sites showing the highest Pb isotope values of any lake (seeFigure 3). When these were excluded, there was no significant correlation between


TABLE V

Co-efficients of determination (r2 values) and slopes of linear regressions for Pb concentrations andPb isotope ratios inN. odoratatissues, lakewaters and sediment fractions. (For operational definitionof Pb in sediment fractions F1 through F4, see Methods. Total Pb = sum of concentrations in fractionsF1 through F4. Significant correlations between ratios or concentrations indicated by:∗ = P <0.05,∗∗= P <0.01; only the slopes of significantly correlated regressions are included in the table)

(a) Pb concentrations

r2 co-efficient

F1 F2 F3 F4 Total Plant Plant Lakewater

Pb roots shoot

F1 – 0.0 0.09 0.07 0.07 0.00 0.06 0.19∗F2 – 0.16∗ 0.02 0.07 0.05 0.00 0.02

F3 0.74 – 0.07 0.17∗ 0.01 0.06 0.00

F4 – 0.93∗∗ 0.02 0.04 0.13

Total Pb 0.28 0.74 – 0.02 0.03 0.05

Plant roots – 0.00 0.06

Plant shoots – 0.20∗Lakewater 0.41 0.28 –

Regression slope

(b) 206Pb/207Pb ratio

r2 co-efficient

F1 F2 F3 F4 Plant Plant Lakewater

roots shoots

F1 – 0.36∗∗ 0.37∗∗ 0.31∗∗ 0.02 0.13 0.17

F2 0.52 – 0.87∗∗ 0.51∗∗ 0.11 0.32∗∗ 0.50∗∗F3 0.44 0.79 – 0.62∗∗ 0.10 0.27∗∗ 0.44∗F4 0.39 0.52 0.66 – 0.02 0.13 0.35∗Plant roots – 0.01 0.09

Plant shoots 0.21 0.16 – 0.61∗∗Lakewater 0.88 0.78 0.64 0.29 –

Regression slope

lakewater and F2, however, because there is no known reason to exclude these sitesthey remain part of the data.

Differences between isotopic ratios were apparent. For example, the206Pb/207Pbratio showed correlations between lakewater Pb and the sediment F3 and F4 frac-

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TABLE V

(continued)

(c) 208Pb/207Pb ratio

r2 co-efficient

F1 F2 F3 F4 Plant Plant Lakewater

roots shoots

F1 – 0.21∗ 0.24∗ 0.26∗ 0.26∗∗ 0.09 0.03

F2 0.58 – 0.69∗∗ 0.28∗∗ 0.10 0.21∗ 0.29∗F3 0.49 0.35 – 0.40∗∗ 0.07 0.21∗ 0.04

F4 0.43 0.20 0.54 – 0.00 0.14 0.10

Plant roots 0.26 – 0.07 0.00

Plant shoots 0.29 0.23 – 0.28∗Lakewater 1.03 0.27 –

Regression slope

tions. However, these were not corroborated by208Pb/207Pb and thus the Pb in F3and F4 and in lakewater was not isotopically similar.

3.3. MACROPHYTE TISSUES

Pb concentrations inN. odorataroots were generally higher than in shoots (Table III).Lead concentrations in sediment fractions and in roots and shoots were not signi-ficantly correlated (Table Va). The Pb content of shoots was positively correlatedwith lakewater Pb concentrations, but root Pb concentrations were not. Isotopicanalyses bore out the significant association between shoot Pb and lakewater Pb,although the slope of the regression was only about 0.28 (Table Vb, c; Figure 4),indicating that on average 28% of shoot Pb was identical with lakewater Pb. Plantshoots were also significantly correlated with sediment F2 and F3, but becauseof the correlation between water and the F2 fractions it is possible that a degreeof autocorrelation may account for the significant association between shoots andF2 or F3. Lead isotope ratios in plant roots were not significantly related to Pbeither in waters or sediments. The range of Pb isotope ratios in plant tissues wasalso extremely narrow compared to those in the surrounding waters and sediments(compare Figure 4f with 4a–e). The calculated shoot-root correlation co-efficientwas small and non-significant, because of the narrow range of isotopic valuescompared with the analytical error (3δ of Pine Needle SRM analysis = 0.76% for206Pb/207Pb). However, there was no consistent difference between root and shootisotopes.


Figure 4. Relationships between206Pb/207Pb ratios in plant shoots and sediment fractions (a–d),lakewater (e) and plant roots (f). (Lines represented as in Figure 3).

4. Discussion

4.1. SOURCES OFPbIN LAKEWATERS AND PLANTS

Atmospheric Pb deposition integrated over the year was isotopically homogeneousacross southern Ontario in 1993, as it was in the 1980s (Sturges and Barrie, 1987),

196 P. M. OUTRIDGE

even though there was a 7-fold decline in atmospheric Pb concentrations between1983 and 1992 (Reidet al., 1993). The geographic variation observed in lakewaterPb isotopes was therefore probably due to Pb regeneration from littoral sediments,most likely from the ‘carbonate’ (F2) fraction judging by the significant correla-tion between water and F2 isotopes. Given the lack of historical trend data on Pbisotopes from these lakes, it is not possible to estimate the relative contributionsof natural (geological) and industrial Pb sources to the lakes with great accuracy.However, the isotopic variation in littoral waters and sediments indicates that alkylPb, which dominated Canadian and US atmospheric Pb and Great Lakes watersduring the 1980s (Sturges and Barrie, 1987; Flegalet al., 1989), was of minimalimportance in littoral waters and sediments in 1993, only three years after its com-plete elimination from gasoline in Canada (Reidet al., 1993). The absence of alkylPb in 1993 suggests that the rapid, post-1985 decline in airborne Pb concentra-tions across southern Ontario was matched by a corresponding decline in lake Pbconcentrations. The most likely fate of the historical alkyl Pb was transport to pro-fundal areas of lakes, through sediment focussing of particulates by resuspensionand offshore transport due to wind and wave action (Blais, 1996). Profundal lakesediments in southern Ontario and Quebec surveyed in the early 1990s by Blais(1996) displayed much lower Pb isotopic variability than the littoral sedimentsin the present study, and were dominated (on average 93%) by anthropogenic Pbwhich had a mixed Canadian/U.S.A. industrial signal.

A slight offset in lakewater Pb isotopes relative to sediment F2 was noted at afew sites, and could be explained by inputs from a localized Pb source with consist-ently lower isotope ratios than most sediments. Most of the six sites showing thegreatest deviation from the 1:1 relationship with sediment F2 (see Figure 3b; LochGarry, Middle, Loxton NW shore, Tyne, Rains, and Macfarlane) were generallywithin the upper quartile of lakewater Pb concentrations and exhibited water iso-tope ratios close to the average Canadian industrial signature (206Pb/207Pb≈1.14–1.16, Sturges and Barrie, 1987; Flegalet al., 1989). Middle, Tyne and Macfarlanelakes were located near the smelter town of Sudbury, whose emissions (206Pb/207Pb≈ 1.16–1.17; Sturges and Barrie, 1989) also matched the isotopic composition oftheir waters.

The remarkably homogeneous isotopic composition of Pb in plant shoots androots, compared to the variability seen in ambient waters and sediments, presentsan apparent contradiction. The consistently low slopes of regressions between lake-water and sediment isotope ratios and those in the shoots, and the absence of anycorrelations between environmental Pb and root Pb, indicate that ambient sedi-ments and waters were relatively minor contributors to the total Pb tissue poolin 1993. Table VI illustrates this point in another way. Both sediments and lake-waters contained Pb that had on average higher isotopic values than those inN.odorata. No combination of Pb from these sources could account for the iso-topic composition of the plants. Because of correlations between the F2 and F3fractions, and between lakewaters and F2, it is difficult to unequivocally estimate


Figure 5. Lead isotope ratios in plant shoots and lakewaters, and in Canadian and U.S.A.urban/industrial air, pre-1989 and 1993. (Error bars represent one S.D. around mean. 1993 atmo-sphere data from this study; pre-1989 data for Canadian and U.S.A. sources from Sturges and Barrie(1987, 1989) and Flegal et al. (1989)).

the contributions of Pb to shoots from each of these sources. However, using theslope of the regression between Pb concentrations in waters and shoots as a roughestimate, only 28% of the Pb in shoots was from the ambient environment, a fig-ure corroborated almost exactly by the shoot-water isotopic data. To identify analternative source of the Pb in plants, isotope signatures of known North Americanurban/industrial Pb emissions were compared against the plant shoot data. The res-ulting plot indicates that Pb isotope ratios in plants from most lakes fall close to thepre-1989 Canadian alkyl Pb signature (Figure 5). The 1993 Canadian atmospherecould also have been a contributor of Pb to plants, although the fit is better with theold gasoline additive signal.

Possible explanations for the putative dominance of historical gasoline Pb inrecent plants are speculative until further work is undertaken, however, a hypo-thesis is suggested by considerations of plant physiology.N. odoratais a clonalplant which dies back to a living underground tuber (rhizome) each winter. Likemost clonal plants, it has an almost infinite theoretical lifespan due to the yearlyincremental growth of new clones from ‘parental’ root stock (Cook, 1985). Studiesof other rhizomatous aquatic plants show that essential elements including Fe, Cu,Mn, Zn and Ca are highly conserved during the senescence of parent clones, beingtranslocated to new clones with minor or no losses (Smithet al., 1988; Outridgeand Hutchinson, 1990). There is evidence of significant Pb translocation in somemacrophyte species (Mayeset al., 1977), but not in others (Peteret al., 1979).

198 P. M. OUTRIDGE

TABLE VI

Stable Pb isotope ratios inN. odorata, lakewaters and ex-tracted sediment fractions (Data shown as mean± SD overall 26 study sites)

Sample 206Pb/207Pb 208Pb/207Pb

N. odoratashoots 1.155±0.010 2.427±0.022

N. odorataroots 1.159±0.011 2.423±0.022

Mean (shoots + roots) 1.157±0.011 2.425±0.021

Sediment F1 1.197±0.024 2.447±0.035

Sediment F2 1.200±0.028 2.470±0.028

Sediment F3 1.217±0.033 2.477±0.035

Sediment F4 1.206±0.040 2.459±0.041

Lakewaters 1.180±0.027 2.453±0.027

If Pb was similarly conserved and translocated betweenN. odorataclones, it ispossible that isotopically-distinctive historical Pb could continue to dominate liv-ing plant tissues for some years after the historical source (gasoline additives) hadbeen eliminated, particularly as the uptake of ‘new’ Pb was small relative to thesize of the existing tissue Pb pool. If the hypothesis is correct, plant tissue Pbconcentrations in 1993 must have been considerably higher than in pre-industrialtimes. Eventually, as ‘new’ Pb comes to replace historical Pb in plants, the plantisotopic signatures may change and Pb concentrations decrease. Future studies areplanned to plot this trend.

Although the plant accumulation of ‘new’ Pb was small, it is likely that shoots(i.e. the stems and under-sides of the floating leaves) were the main uptake tissues,and that lakewaters were the penultimate source of shoot Pb. The Pb isotopes inshoots were significantly correlated with sediment F2 and F3 fractions, however,there is obviously no direct connection between shoots and sediments; the onlypossible routes of shoot uptake are either from sediments via the roots or fromwater. The lack of an isotopic association between Pb in roots and sedimentsindicated that roots were not involved in Pb uptake, whereas the F2 (carbonate)fraction was correlated with waterborne Pb, and waterborne Pb was correlatedwith shoot Pb. Thus, the most likely pathway was sediment ‘carbonate’→ water→ shoots. Together with Gelinas and Schmit (1997), the present study providesisotopic evidence to support Denny and collegues’ assertion that regeneration ofPb from sediments followed by shoot uptake was the dominant pathway of Pb ac-cumulation in freshwater macrophytes (Peteret al., 1979; Welsh and Denny, 1980;


Everard and Denny, 1985). These findings suggest that the utilization of somemacrophyte species as proxies of sediment Pb bioavailability in biogeochemicalstudies may be conceptually unsound.

4.2. PbDYNAMICS IN THE ATMOSPHERE, WATERS AND SEDIMENTS

The 1993 atmospheric data again demonstrated a U.S.A.-Canadian dichotomy inisotopic signatures, first reported by Sturges and Barrie (1987, 1989). However,as Figure 5 shows, while there was no marked shift in the Canadian signatureover time, the208Pb/207Pb ratio of US-origin air declined by about 2% since thephase-out of leaded gasoline in the late 1980s. Thus, the recent US atmosphericsignature can now be clearly distinguished from the historical gasoline-dominatedsignal. Although this shift has not been previously reported, the overall data fromthis study are comparable to other recent work in southern Quebec. The weightedmean 1993 data from southern Ontario reported here (206Pb/207Pb: 1.179–1.189;208Pb/207Pb: 2.438–2.442) were almost identical to the annual average206Pb/207Pbratio in deposition (1.188±0.015; Gelinas and Schmit, 1997), and to the isotopiccomposition of Pb in lichens (206Pb/207Pb: 1.182–1.185;208Pb/207Pb: 2.438–2.442;Carignan and Gariepy, 1995), sampled south of Montreal near the US border in1994–95. This agreement increases confidence that the post-1989 shift in the USisotope signature was stable over at least several years and was constant across thenortheastern U.S.A. bordering Ontario and Quebec.

Using thermodynamic evaluation of sediment-water chemistry, Carrollet al.(1998) concluded that carbonate and oxyhydroxide uptake dominated waterbornePb binding in stream sediments. However, based on isotopic data in this study,while carbonate was an important sink (on average 96% of lakewater Pb wasisotopically identical to sediment ‘carbonate’), oxide-bound Pb was not directlyrelated to lakewater Pb. Rather, oxide Pb in sediments was associated with car-bonate Pb, suggesting that partitioning between lakewater and carbonate, and sub-sequently between carbonate and oxide species, was the dominant pathway ofwaterborne Pb entry into sediments. The positive correlation between water DICand the proportion of carbonate-bound Pb supports this hypothesis, indicating asit does that precipitation of Pb carbonate was a rate limiting step. Of the waterand sediment quality parameters measured, pH, DIC and sediment organic contentmost strongly controlled Pb partitioning between the various sediment fractions,with greater association with the F4 ‘organic’ pool in acidic lakes. This finding ac-cords with the general Pb-organic association under acidified conditions observedin field and laboratory experiments (Nelson and Campbell, 1991). In the acidifiedcatchment of Plastic Lake, for example, LaZerte et al. (1989) reported that organicmatter controlled Pb movement in soil profiles.

Because isotope ratios directly indicate associations between the leads in dif-ferent environmental compartments, the exploitation of natural isotopic variationsby studies such as this and Gelinas and Schmit (1997) can provide insights into Pb

200 P. M. OUTRIDGE

biogeochemistry that concentration data alone can not. One qualification, however,is that the Pb composition needs to be characterized as fully as possible. A numberof recent environmental studies have based their conclusions on only one ratio, usu-ally 206Pb/207Pb. However, as the present study demonstrated for US atmosphericPb and for the lakewater-F3 and -F4 correlations, different conclusions may resultif another ratio such as208Pb/207Pb is also considered.

Acknowledgements

The author was supported by a post-doctoral fellowship from the Natural Sciencesand Engineering Research Council of Canada, and a grant from the CanadianWildlife Foundation. Gordon Earle assisted in the field and laboratory. I thankRay Hoff and Len Barrie for samples and Pb data from Environment Canada’sair monitoring stations. The study benefitted in its early stages from discussionswith Ann P. Zimmerman and Lesley A. Warren. Part of the work was carried outwhile the author was resident in the laboratory of R.D. Evans. Ian Jonasson andtwo anonymous reviewers improved the manuscript with their comments.

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