Lead concentration profiles in lead-210 dated Lake Ontario sediment cores

The Science of the Total Environment, 10 (1978) 117-127 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

LEAD CONCENTRATION PROFILES IN LEAD-210 DATED LAKE ONTARIO SEDIMENT CORES

JOHN G. F A R M E R

Department of Forensic Medicine, University of Glasgow, GlasgowG12 8QQ, Scotland (Great Britain) (Received December 10th, 1977; in final form March 1st, 1978)

ABSTRACT

Lead concentrations were measured in sediment cores from four sites distributed among the three major sedimentary basins - - Niagara, Mississauga and Rochester - of Lake Ontario for which sedimentation rates had been previously determined by 2 lopb dating. Around 90~o of the total lead present in fine-grained surface sediments was removed by CH3CO2H/NH2OH • HC1 leaching, concentrations ranging from 137 #g/g in surface sections to a constant background of 12-13 pg/g at unpolluted depths. Lead-210 dating indicated that increases in lead concentrations commenced ca. 1850-1875 with 5, 10, 35 and 50~o of the total "excess" lead inventory in the sediment column being assigned to the pre-1900 period and successive 25-year intervals during the 20th Century, respectively. Anthropogenic inputs of lead from such sources as the combustion of leaded gasoline and coal are responsible for these increases. About 10 /~g/g lead remained in the sediment residue after leaching. The total natural lead flux to the sediments ranged from 0.4-1.3 #g/cmZ/yr while "excess" lead of anthropogenic origin varied from 1.2--6.7 #g/cme/yr and totalled 0.5-1.5 g/m 2 at the four sites. Various potential modes of introduction of anthropogenic lead and of 210pb to the lake are considered in conjunction with the ratio of Pb/2t °Pb fluxes to the sediment. INTRODUCTION

Man's use of fossil fuels as energy sources has contributed significantly to the creation and development of industry and technology in the western world since ca. 1750. Against the material benefits of our Industrial Revolution there can be set certain disadvantages of environmental concern, directly linked to the combustion of fossil fuels. Ranking high in the list of such environmental pollution is the injection of lead into the atmosphere from the burning of coal and the combustion of leaded gasoline in automobiles, the latter source being of growing and relatively greater significance since the 1920's. There is a considerable literature not only on the distribution of lead, derived from the anti-knock compounds added to gasoline, in the air, dust, soil and vegetation

118 adjacent to highways (Chow, 1973; Day et al., 1975; Farmer and Lyon, 1977), but also on the controversy surrounding the possible detrimental effects associated with this dispersion of lead on the general health of the populace (Ter Haar and Aronow, 1974; Schroeder, 1974; Waldron, 1975). Rather less well-documented has been the general growth in environmental lead concentrations in the last 10(O200 years. A few recent studies, however, have illustrated the potential of coastal and lacustrine sediment cores in recording trends in lead inputs to the environment (Crecelius and Piper, 1973; Bruland et al., 1974; Iskandar and Keeney, 1974; Edgington and Robbins, 1976; Goldberg et al., 1977; Miiller et al., 1977). The value of such information is enhanced when a time-scale provided, for example, by 21 opb or pollen dating, can be attached to the sediment column. This paper considers the growth in lead concentrations in recent Lake Ontario sediments at various sampling stations on the lake and discusses the findings in the light of the time-scale suggested by 210pb dating of the sediment cores. MATERIALS AND METHODS

Sediment cores for which lead data are reported here were collected as part of an intensive sampling programme on Lake Ontario - - the most easterly lake in the Laurentian Great Lakes Chain - - during 1972 and 1973, aimed primarily at an evaluation of the distribution and behaviour of transuranic elements in a freshwater I

I

I

VINCENT

44" N INSHORE

430N

I 79*W

I 78*W

I 77*W

I 76*W

Fig. 1. Lake Ontario, showing the positions of stations 72-2, 72-4, 73-13 and 73-14, and the major sedimentary basins (after Thomas et al., 1972).

119 TABLE 1 SEDIMENT CORE SAMPLINGSITESON LAKE ONTARIO

Station

Sampling date

Latitude

Longitude

Water depth (m)

72-2 72-4 73-13 73-14

12/9/72 13/9/72 25/8/73 25/8/73

43 °45'N 43 °24'N 43 °31'N 43 °42'N

77 °42"W 79 °00'36"W 76°58'W 77 °W

90 90 245 105

system (Farmer et al., 1977). Core samples were taken using a 21.3 cm internal diameter gravity corer with sphincter core retainer (Burke, 1968). The cores were sub-sectioned into 1, 2 or 4 cm slices, dried at 110°C and hand-ground using a mortar and pestle. Lead analyses were performed on cores from each of the three main sedimentary basins in Lake Ontario - - Niagara (72-4), Mississauga (72-2) and Rochester (73-13, 73-14) - - as shown in Fig. 1 and listed in Table 1. Thomas et al. (1972) report that these basins are filling with post-glacial muds, composed of soft, fluid, fine-grained silty clays, coarsening to silt size around the periphery of each basin and towards regions of glacial outcrops. Sedimentation rates based on the 210pb dating method have been determined and reported for each of the above sites (Farmer, 1978). The chemical procedure adopted here was based on the three-step leaching and dissolution technique of Presley et al. (1972), designed to gain insight into the associations of lead and other heavy metals with various sedimentary phases. This approach has also been adopted by Bruland et al. (1974) in their study of the history of metal pollution in the Southern California coastal zone. The procedure as employed in this study consisted of: (1) overnight leaching of 1-2 g dried, ground sediment at 60°C with 100 ml 25% CHaCO2H solution, 0.25 M in NH2OH • HC1, which removed 10-15% of the sediment by weight; (2) leaching of residual sediment with hot 30~o H 2 0 2 , which removed a further 10 % of the sediment by weight; (3) dissolution of the resistant residue, typically 75-80% of the original dry weight, with HF/HNO3. The three liquid fractions were processed in a similar fashion - - evaporation to dryness followed by dissolution in 100 ml 1 M HC1 and direct analysis for lead via flame atomic absorption (2, 217 nm; slit, 320/tin) using an IL 151 single beam atomic absorption spectrophotometer equipped for background correction with a hydrogen continuum lamp. Good agreement was found with analytical checks based on standard additions and solvent extraction (dithizone) techniques. Replicate analyses indicated an overall analytical precision of i 5 %. RESULTS

Weak acid leaching, such as the initial acid leaching procedure used here, is believed to remove metals that are loosely bound to mineral surfaces, located in the

120

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20

40

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Lead c o n c e n t r a t i o n

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40

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81

100 120 ~ug/gJ

Lead concentration . ...i.d

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171~

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Lead ¢ c r , ~ n t r o t i o n

1~o (~g/g)

1~o

1:o

tt,

,o

,'o

~'o

0~

Lead concentration

1oo

1;o

1'o

(~ug/g}

Fig. 2. Lead concentration profiles (CHsCOsH/NHsOH • HC1 fraction) for Lake Ontario sediment cores 72-2 (a), 72-4 (b), 73-13 (c) and 73-14 (d).

ion-exchange rites of clay minerals or incorporated in the CaCO3 fraction of the sediment, but not those strongly bound in organic material nor incorporated in clay mineral lattices. More specifically, while 25 % CH3COzH dissolves the iron oxide phases, NHzOH • HCI dissolves the manganese oxide phases of ferromanganese minerals (Agemian and Chau, 1976). Consideration of the amounts of lead removed by the successive treatments of CH3COzH/NH2OH • HCI, 30 % H,O2 and HF/HNO3 showed that typically - - (1) less than 2 #g Pb/g dry sediment was removed by H202, with no significant variations between cores or with depth in the cores; and (2) a fairly constant 10 4- 5 #g Pb/g dry sediment was present in the residues digested with HF/HNO3. In contrast, the concentrations of lead removed by the CH3CO2H/NH2OH • HCI leaching were not only significant relative to those removed by H202 but varied between cores and exhibited variations with section depth in the uppermost sections of the cores. Consequently, it is clear that in this study the information concerning anthropogenic input of pollutant lead to Lake Ontario is contained in the lead levels removed by CH3COzH/NH2OH • HC1. Accordingly, the lead profiles for cores 72-2, 72-4, 73-13 and 73-14 in Fig. 2(a)-(d) are based on the lead concentration for each sediment section as indicated by CH3CO2H/NH2OH • HCI treatment.

121 TABLE 2 "BACKGROUND" AND SURFACE tt CONCENTRATIONS OF LEAD IN LAKE ONTARIO CORES (25~/o C H s C O ~ H /

0.25M NH~OH • HCI FRACTION)

Core Onset depth of "background" Pb eoncn. No. of sections analysed below "background" Pb depth Range of "background" Pb conchs. Mean background Pb conch. Surface Pb concn. Total "excess" Pb in core 0-1 cm: 72-2, 73-13, 73-14; 0-2 era: 72-4.

72-2

72-4

73-13

73-14

(cm)

6

14

14

14

(#g/g) ~g/g) (#g/g) (rag)

8 3-6 4 4- 1 35 13.8

16 9-16 13 4- 1 120 31.2

17 10-18 12 4- 1 133 30.5

19 9-15 12 4- 2 137 43.2

It is evident in all four cases that a constant "background" level is attained at depth in the cores. Not included in Fig. 2(b)-(d) are results of lead analyses of several sections below 26 cm. For cores 72-4, 73-13 and 73-14, five, eleven and twelve core sections respectively, chosen at intervals between 26 cm and the bottom of the cores, ranged from 9-18 #gPb/g. Mean "background" lead levels below the depth indicated for each core in Table 2 were calculated from a weighted averaging of all the sections analysed below that depth and ranged from 4 #gig (72-2) to 12-13 #g/g (72-4, 73-13 and 73-14). Thus, adding the constant lead concentration removed by HF/HNO3, the total "baseline" lead levels for these Lake Ontario sediment cores were 14 #gig (72-2), 23 pg/g (72-4), 22 #g/g (73-13) and 22 #g/g (73-14). Kemp and Dell (1976) have reported a mean lead concentration of 30 #g/g (range 26-32 #g/g) in Lake Ontario "baseline" (i.e. > 120 yr. B.P. as indicated by the Ambrosia horizon) sediment while 23 Fg/g has been reported for Lake Michigan sediment by Edgington and Robbins (1976). Surface lead concentrations in all four cores were significantly enhanced relative to the CHaCO2H/NH2OH • HC1 "background" levels (Fig. 2(a-d), Table 2). The growth in lead concentrations is discussed below in terms of sediment age as indicated by 21 opb dating. Mean sedimentation rates (compaction-corrected), in cm/yr, for cores 72-2(0.02), 72-4(0.11), 73-13(0.08) and 73-14(0.11) had been previously determined (Farmer, 1978). The age of each sediment section was then calculated by dividing the "compacted" section depth (obtained after normalisation of the wet/dry volume ratio of each section t o t h e constant wet/dry volume ratio, corresponding to constant porosity, generally attained at depth in lake sediments) in cm by the compaction-corrected sedimentation rate in cm/yr. This was done for the four cores and the corresponding calendar years appended to the axes in Fig. 2(a)-(d). DISCUSSION Whereas lead concentrations in the H F / H N O 3 fraction are virtually identical in cores 72-2, 72-4, 73-13 and 73-14, "background" and surface concentrations of lead

122 (CH3CO2H/NH2OH • HC1 fraction) are noticeably greater in 72-4, 73-13 and 73-14 (Table 2). Similar variations in the specific activity of 2 l opb had been observed between 72-2 and the other cores. Lead-210 and stable lead transport to the sediments are primarily via scavenging by settling particles. Thus the nature of these particles, especially with respect to grain size and abundance of sites for adsorption, is critical in explaining 21°pb specific activities and lead concentrations in the sediments. In contrast to the coarser-grained 72-2 (37.5 ~o water content, by volume, at 15 cm), taken near the north-eastern periphery of the Mississauga basin with the northern inshore zone, cores 72-4, 73-13 and 73-14 (80-85~o water content, by volume, at 50 cm) are composed of finer-grained particles. It seems probable that preferential scavenging of 2 ~Opb and lead from the lake waters by fine-grained particles, reflecting the difference in adsorption properties due to grain size (de Groot et al., 1976), is responsible for the differences in "background" and in surface lead concentrations between 72-2 and the group, 72-4, 73-13, 73-14 listed in Table 2. The "background" lead values of 4/~g/g (72-2) and 12-13/~g/g (72-4, 73-13, 73-14) probably reflect the input of "acid-leachable" lead to the lake in pre-industrial times, i.e., they represent the component of the total baseline lead level that is derived from natural erosion and break-down of mineral material in the lake drainage basin. The flux of total "natural" lead (i.e., CH3CO2H/NH2OH • HC1 -4- HF/HNO3 fractions) to the sediments at each location is shown in Table 3. The range is from 0.4-1.3 /~g/cm2/yr, very similar to the 0.8-1.0 /~g/cm2/yr reported by Kemp and Thomas (1976) for Lake Ontario. An increase in lead concentrations (to 120-137/~g/g for surface sediment in cores 72-4, 73-13 and 73-14) since the mid-19th Century is clear in Fig. 2(a)-(d) with obvious similarities in the onset and trend of the increase for each core. It is possible to compare the trends in 72-4, 73-13 and 73-14 by integrating the "excess" lead (obtained for each core section by subtracting the appropriate "background" lead concentration and multiplying by the section dry weight) for each core (Table 2) and expressing the inventories for approximate 25-year periods as a percentage of the total excess lead inventory (Table 4). Approximately 5 ~o of the total lead was deposited prior to 1900, mainly since 1850; then about 10, 35 and 50~o in successive quarters of TABLE3 "NATURAL" LEAD FLUXES TO LAKE ONTARIO SEDIMENTS

Core

Mean "background"Pbconcn. (/zg/g) (CHaCO2H/NH2OH • HCi) Mean Pb concn, in residue {jzg/g) (HF/HNOo) Pb conch. (combined) (~g/g) Sedimentationrate (g/cm~/yr) (Farmer, 1978) "Natural" Pb flux (~ug/cm2/yr)

72-2

72-4

73-13

73-14

4± 1

13 + 1

12 -4- 1

12 :k 2

10 ± 5

10 ± 5

10 ~ 5

10 :k 5

14 -4- 5

23 -4- 5

22 ± 5

22 -4- 5

0.0313 (-4- 0.0049) 0.44 -4-0.17

0.0555 (-4- 0.0064) 1.28 -4-0.31

0.0308 (+ 0.0019) 0.68 -4-0.16

0.0472 (-4- 0.0025) 1.04 -4-0.24

123 TABLE 4 INTEGRATED "EXCESS" LEAD INVENTORIES OF LAKE ONTARIO SEDIMENT CORES FOR 25-YEAR TIME PERIODS EXPRESSED AS A PERCENTAGE OF THE TOTAL "EXCESS" LEAD INVENTORY

Core

Depth span (cm)

Time span

°//o of total "'excess" Pb

72-4

0- 5 5- 9 9-11 11-14 0- 3 3- 6 6- 9 9-14 0- 3 3- 7 7-10 10-14

1952-1972 1921-1951 1903-1920 pre-1903 1950-1973 1925-1949 1899-1924 pre-1899 1954-1973 1926-1953 1903-1925 pre-1903

49.3 36.5 9.1 5.1 49.9 33.6 11.6 4.9 48.9 35.1 11.3 4.8

73-13

73-14

TABLE 5 "EXCESS" LEAD AND ~ l ° P b FLUXES AT SURFACE (0 c m

Core

Sedimentation rate "Excess" Pb "Excess" 210pb "Excess" Pb flux "Excess" elopb flux

(g/cm~/yr) ~g/g) (d.p.m./g) ~g/crn~/yr) (d.p.m./cm2/yr)

Flux ratio Pb/~10pb ~g/d.p.m.)

depth) OF LAKE ONTARIO SEDIMENT CORES

72-2

72-4

73-13

73-14

0.0313 38 5.42 1.19 4- 0.22 0.17 4- 0.07 0.05 7.00 4- 2.79

0,0555 121 19.49 6.72 -4- 1.06 1.08 4- 0.18 0.17 6.22 4- 1.41

0.0308 136 22.42 4.19 + 0.53 0.69 4- 0.07 0.06 6.07 4- 0.96

0.0472 139 19.11 6.56 4- 0.79 0.90 4- 0.09 0.08 7.29 4- 1.12

N.B. The error term (4- la) associated with the "excess" Pb flux is based on a combination of the

,-, 4- 10% uncertainty accorded the estimate of "excess" Pb cone. at 0 em, from extrapolation of the lead profiles in Fig. 2, and the confidence limit attached to the sedn. rate (cf. Table 3). The "excess" ~lopb flux was obtained through multiplication of Ao, the surface al0pb activity obtained through setting Z = 0 in the equations (A = e-az+b) describing "excess" zl0pb activity vs. "compacted depth" (Z) (Farmer, 1978), by the sedn. rate. The error term (4- ltr) associated with the "excess" ~lOpbflux is based on a combination of the confidence limits of the "excess" ~10pb activity at 0 cm (as indicated by the regression analysis of A on Z) and of the sedn. rate. The error term ( + lo) associated with the flux ratio Pb/~X°Pb is based on a combination of the errors on the "excess" Pb and "excess" a~0pb flux and has utilized the average of the errors on the "excess" s~0Pb flux. the 20th Century. A l t h o u g h the time intervals for the sections of 72-2 do n o t allow such specifically c o n v e n i e n t analysis, a similar trend is clearly in evidence. A s n o t e d by E d g i n g t o n a n d R o b b i n s (1976) in their study o n Lake Michigan, s u b s t a n t i a l increases since ca. 1920 coincide with the i n t r o d u c t i o n a n d rapid growth in c o n s u m p tion of leaded gasoline. T h e c o m b u s t i o n of coal is generally held to be the chief c o n t r i b u t o r y source to "excess" lead in sediments prior to this date.

124 There are dangers in estimating the total lead pollution inventory of lake sediments by extrapolating from the results on a few cores. Quite clearly, although cores 72-4, 73-13 and 73-14 exhibit similar integrated "excess" lead totals in relation to 72-2, there is a sizeable ( ~ 3070) difference in total "excess" lead between 73-13 and 73-14 (Table 2), two cores from the same (Rochester) basin. Similarly it is unlikely that 72-2, due to its peripheral location, is representative of the Mississauga basin. However, in expressing lead on a per unit area basis, the total "excess" lead in the lake sediments varies from 0.5 g/m 2 (72-2) to 1.5 g/m 2 (73-14). The present flux of "pollutant" lead to the sediments can be obtained through multiplication of the sedimentation rate by the estimated "excess" lead concentration at depth 0 cm in the core (Table 5). This latter value has been calculated here through extrapolation of the lead profiles in Fig. 2 to give an estimated surface lead concentration from which is subtracted the "background" lead concentration (Table 2). The range is from 1.19-6.72/~g/cm2/yr; Kemp and Thomas have reported 7.4-8.1 #g/cm2/ yr near the centres of the three main basins while Edgington and Robbins (1976) have indicated an average 5 #g/cm2/yr in Lake Michigan. Since up to 50 70 of the fine-grained sediment input to Lake Ontario is believed to come from erosion of shoreline bluffs (Kemp and Dell, 1976) of lead concentrations comparable to "baseline" sediment, it is clear that the "excess" lead in the sediment originates through attachment of atmospherically derived lead to sinking particles and by stream and river input of particles from the drainage basin. According to Kemp and Thomas (1976), the atmospheric input of lead is only 1.4 gg/cm2/yr based on the Lake Ontario rainfall data of Shiomi and Kuntz (1973). The difference between this and the total anthropogenic input was attributed to "river inputs and local atmospheric sources" and "atmospheric inputs of dry particulate matter". However, it should be borne in mind that both horizontal translocation of finer-grained sediment material from the sides, after deposition, to the centres of the sedimentary basins and general enhanced deposition of lead-containing sediment in the basins relative to the lake as a whole, could result in regions of "enhanced" and of "depleted" lead flux to the sediments, which would tend to balance out and perhaps yield a~-average flux more closely in agreement with estimates of the atmospheric flux. Although the behaviour and transport of lead and 21 opb deposited directly onto the lake surface from the atmosphere is believed chemically identical, there is likely to be a difference on land between automobile-exhaust-emitted lead and 21°pb deposited from the atmosphere. Lead-210 is derived almost exclusively from radioactive decay of 222Rn which has diffused from the earth's crust into the atmosphere. Due to the high affinity of soils for lead, only a small fraction of the 21 opb deposited overland will be washed into streams and then, via run-off, into the lake. In contrast, much of the lead emitted to the atmosphere is readily transported to streams because of efficient surface drainage along roads where some of the lead is deposited postemission (Edgington and Robbins, 1976). In the case of Lake Ontario, the range of"excess" 2~°pb fluxes to the sediments for cores 72-2, 72-4, 73-13 and 73-14 is 0.17-1.08 d.p.m./cm2/yr (Table 5). While these fluxes probably exhibit some dependence on sediment particle characteristics (e.g.,

125 contrast 72-2 vs. 72-4), they suggest, when compared with direct atmospheric measurements, that 210pb input to the lake is almost exclusively derived from atmospheric deposition and that one need not necessarily require to postulate additional significant contributions of 21 opb from rivers and streams. Peirson et al. (1966) calculated the atmospheric 2 ~0pb flux from rainwater concentrations of 210pb at nearby Ottawa at 0.99 d.p.m./cm2/yr. More generally, Moore and Poet (1976) concluded that the atmospheric 210pb flux may range from 0.53-2.0 d.p.m./cm2/yr for the north temperate zone for which Jaworowski's (1969) 0.44 d.p.m./cm2/yr had previously been considered the best estimate. Edgington and Robbins (1976) suggested that enhanced values of the "excess" pb/21°pb flux ratio in Lake Michigan sediments would be expected in those areas influenced by a significant input of terrigenous material via stream run-off. They found that some in-shore pb/21°pb ratios were up to twice as high as at off-shore stations, where typical values of 4-5/~g/d.p.m. were obtained. In this Lake Ontario study, the "excess" Pb/2~ °Pb flux ratio at the four stations ranged from 6.2-7.3 (Table 5) with a mean of 6.65. Combining the Lake Ontario rainfall lead flux data of Shiomi and Kuntz (1973) with the 2X°pb flux estimates of Jaworowski and Peirson et al., the rainfall Pb/2~ °Pb flux over Lake Ontario would range from 1.4-3.2 suggesting, when compared with the sediment flux ratio, that only 20--50 % ofthe"excess" lead influx to the sediments is derived directly from atmospheric input to Lake Ontario. Such conclusions and the usefulness of pb/2~°pb ratios in distinguishing input sources of lead can only be considered highly speculative, however, especially in the absence of adequate information concerning inshore sediment pb/2~°pb ratios and atmospheric Pb fluxes over the lake at this time. Although about 80 % of the river inflow to Lake Ontario is contributed by the Niagara River, which connects the lake with Lake Erie, and suspended particulate matter in the Niagara averages 110 #g/g lead (Kemp and Thomas, 1976), comparable to the mean concentration in modern Lake Erie sediments, the effect of this inflow with respect to sediment lead inventories throughout the lake is uncertain. While there is a close resemblance of the "excess" Pb/zx °Pb sediment flux ratio (7-8) of a core taken in a high sedimentation rate (> 0.5 cm/yr) area from the Niagara delta at the mouth of the river (73-6, Farmer, unpublished data) to that in the three main sediment basins, both the Pb and 210pb sediment fluxes at 73-6 are enhaficed by a factor of 3--4 relative to 72-4. This dual enhancement and the "normality" of sediment fluxes of 2 ~Opb in the cores of this study suggest that lead contributions from the Niagara River (at least on suspended particulates) to Lake Ontario may be confined to deposition in a localised high sedimentation area at the mouth of the Niagara. While discussing enhanced lead concentrations in the upper sections of Lake Ontario sediment cores in terms of increasing anthropogenic input, it is recognised that lead (and other trace metal) profiles in the lake sediment may be subject to modification by other processes such as sediment diagenesis and upward migration of chemical species in the pore waters. Although these potential influences may operate, one might expect that, under the reducing conditions generally found in

126 Lake Ontario sediments, lead should remain immobile and fixed in the sediment due to formation of the very insoluble lead sulphide. Even under oxidising conditions as may prevail in the thin surface layer of the sediment, where migration of lead might be anticipated, scavenging of lead through adsorption on hydrated oxides of iron and manganese formed at the sediment/water interface and on other clay minerals would tend to counteract lead mobility. Quantification of such effects is not possible here but the general ten-fold increase in surface lead concentrations relative to the baseline at all sites and the temporal concordance of onset of increases in lead concentrations strongly supports the contention (Kemp and Thomas, 1976) that anthropogenic loading of sediments is by far the major factor responsible for such lead profiles at Great Lakes' locations. Acting in the opposite direction to diagenetic/mobilisation effects would be downward transport of lead through post-depositional mechanisms such as bioturbation. Nuclear-fallout studies have indicated the presence of small amounts of fallout nuclides (e.g. Pu, 1a TCs) in some of these cores at sediment depths dated earlier than the "nuclear" era by 2 t opb dating. However, consideration of the likely relative amounts involved (with respect to Pu, ~aTCs and 2~°Pb) suggested that corresponding 2 ~°Pb activities had not been affected to a degree sufficient to alter the sedimentation rates determined by 210pb beyond the expressed confidence limits of about ± 10% (Farmer, 1978). If stable lead levels at these sediment depths were also enhanced, to some extent, through operation of similar post-depositional mechanisms as for Pu and ~aTCs, then the qualitative implication would be that the 25-year inventories of lead presented earlier would be distorted in the ,direction of underestimating the relative amount of lead introduced to the environment in recent years. Clearly, however, from the shape of the lead profiles, any surficial mixing has been insufficient to produce a homogeneous lead concentration over upper sediment depths greater than 1 cm. CONCLUSIONS

The 2 l opb.dated sediments of Lake Ontario preserve a temporal record of the increasing input of lead to the environment as a consequence of man's activities. Lead concentrations in surface sediments from near the centres of the major sedimentary basins range from 120-137 #g/g, some 10 times greater than the average 12-13 #g/g (CHaCO2H/HN2OH • HCI fraction) found at unpolluted depths. The date of onset of increasing lead concentrations in the sediment columns appears to be 1850-1875. About 50 % of the total "excess" lead inventory of the sediments has been laid down post-1950 and 85 % of the total since the date of introduction of leaded gasoline in the 1920's; the pre-1900 period has contributed only 5 %. It is difficult to assess the relative magnitude of atmospheric and stream/river sources of lead input to the lake at the present time although measurements of the Pb/21 °Pb flux ratio in Lake Ontario sediment may be useful. Detailed direct measurements of the atmospheric lead flux to the lake would be highly advantageous.

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