Environ. Sci. Techno/. 1995, 29, 2661-2672
Accumulation, Inventory, and Diagenesis of Chlorinated Hydrocarbons in Lake Ontario CHARLES S . W O N G , t GORDON SANDERS,* DANIEL R. ENGSTROM,§ DAVID T. LONG," DEBORAH L. SWACKHAMER,l AND STEVEN J. EISENREICH*jt Gray Freshwater Biological Institute and Department of Civil Engineering, Uniuersity of Minnesota, Navarre, Minnesota 55392, Battelle Geneva Research Centres, Agrochemical Product Development, CH-1227 Geneva, Switzerland, Limnological Research Center, University of Minnesota, Minneapolis, Minnesota 55455, Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824, and Enuironmental and Occupational Health, School of Public Health, Uniuersity of Minnesota, Minneapolis, Minnesota 55455
Five sediment cores were taken in 1990-1991 from the sedimentation basins of Lake Ontario and analyzed for the radionuclide 210Pband hydrophobic organic compounds (HOCs) in order to determine the accumulation, inventory, and diagenesis of these HOCs in the sediments. Two of these sites were sampled earlier in 1981, allowing the study of diagenetic processes affecting these HOCs over a decadelong interval. The shape and details of HOC sediment profiles agreed with the HOC production and usage history, despite evidence of bioturbation in the cores. The 210Pbchronology showed a mixed depth of 2-5 cm, but mixing by deposit-feeding oligochaetes and benthic organisms was insufficient to homogenize the sediment over the time scale of HOC inputs. Recent HOC accumulation rates and inventories showed significant variability among cores, which was removed when corrected for 210Pb-basedsediment focusing. This suggests that particle-reactive compounds like HOCs are mixed and distributed evenly throughout the lake basins and that site-specific differences are due to differing amounts of sediment delivered via focusing of sedimentto depositional basins. Comparison of 1981 and 1990 sediment cores showed expected downcore movement of HOC profiles due to 9 yr of accumulated sediment mass with effectively no loss or gain in mass.
0013-936W95/0929-2661$09.00/0
Q 1995 American Chemical Society
Introduction Hydrophobic organic compounds (HOCs) enter lakes and oceans by atmospheric transport and deposition, direct and indirect discharges, and riverine inputs. Examples of these compounds include polychlorinated biphenyls (PCBs) and chlorinated pesticides (e.g., DDT, mirex) or others generated as byproducts of industrial production (e.g., hexachlorobenzene or HCB). The sorptive properties of HOCs are largely controlled by their hydrophobicity and by the particle organic carbon content, which is generally associated with the clay- or fine-sized particles (I). As a result, HOCs are carried by fine particles, deposited to sediments, and focused into the more quiescent depositional basins. Once delivered to the bottom sediments, HOC burial can be slowed by sediment resuspension (2,3) and bioturbation (4-7). HOCs may also partition into porewater or bind to colloidal organic matter (8-12) and migrate within and from the sediments via diffusion or bioirrigation (13-15). The net effect of these processes is to alter the depositional history of the contaminant as recorded in the sediments and to increase the residence time of the contaminant in the ecosystem. HOCs may also be subject to biotic or abiotic transformation in sediments, which can further alter the sediment input history. The accumulation and behavior of HOCs in dated sediment cores in the Great Lakes have been reported (12, 16-28). These studies have found that the depositional history of HOCs is often preserved in the sediment bed, as local and nonlocal redistribution processes are not sufficiently intense over the time scale of HOC inputs to alter the profiles sigmficantly (24, 25). For many banned compounds (e.g., PCBs, DDT, mirex, HCB), there is a subsurface peak in the sediment contaminant profile corresponding to the period of peak production andlor usage, followed by a slow decline to the sediment-water interface. However, much of this work was done in the early 1980s, and there have been few data available on current loadings of HOCs to sediments. This information is needed to determine the recovery of aquatic ecosystems impacted by HOC deposition, the effectivenessof chemical bans, and the data needed to model chemical behavior. The diagenesis of HOCs within sediments over time is important to evaluate the extent that redistribution and transformation processes have altered the recent historical record. In addition, previous estimates of the total sediment burden of HOCs ( 2 3 , which are necessary to construct a mass balance, have not reported on sufficientcores to form a reliable estimate. In this paper, we examine the accumulation and diagenesis of selected HOCs in five Lake Ontario sediment cores taken in 1990 and 1991 from the three major sedimentation basins of the lake. Two of these cores are taken at the same location as a previous study (24),allowing * Author to whom correspondence should be addressed; e-mail address:
[email protected]. + Gray Freshwater Biological Institute and Department of Civil Engineering. Battelle Geneva Research Centres. § Limnological Research Center. II Department of Geological Sciences. School of Public Health.
VOL. 29, NO. 10, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
2661
m x 0.32 mm i.d. or a 60 m x 0.25 mm i.d. DB-5 glass chromatographic column (both columns are 0.25 pm fdm thickness, J&W Scientific). GC operating conditions are given in Achman etal. (29) and Hornbuckle etal. (30).The Maxima820D software system (Waters)was used to acquire and quantify data. Compound abundances were determined using the internal standard method, and congeners were quantifiedusingastandad calibration mixofAroclors 1232, 1248, and 1262 mixed in the ratios 251618, respectively (27,31,32). Sediment concentrations were calculated FIGURE 1. Map of Lake Ontario showing location of sediment core bydividingtheHOCmass hythedryweight ofthe extracted sites. sediment (ng/g dry wt), determined gravimetrically after dryingaliquotsofsedimentinanovenat50 “C. TotalPCBs the elucidation of decadal trends and diagenetic processes (hereafter referred to as ZPCBs) are reported as the sum of affecting these HOCs. Sediment focusing factors are individualcongeners (n= 80-901, and totalDDT (hereafter introduced as a tool for estimating the total HOC burden referred to as XDDT) is reported as the sum of the parent within sediments and for elucidating the input pathways compound (0.p’-DDT and p,p’-DDT) and its major meof HOCs to sediments. tabolites (o,p’-DDD, p,p’-DDD, and p,p’-DDE). The radionuclide 2LoPbwas measured in all cores to Materials and Methods determine sedimentationrates, mixed depths, and focusing Sediment cores were taken from five sites in Lake Ontario factors. Sediments were analyzed for 210Phusing a modi(Figure 1). In 1990, punch cores (i.d. 6.7 cm) and box cores fication of the method described by Eakins and Morrison (40 x 15 x 15cm) were taken usingthe submersibleJohnson (33). Dried sediment was spiked with a known quantity Sea Link I1 of the R/V Seward Johnson (Harbor Branch of 20sPo,digested in concentrated HCI, and heated to 550 Oceanographic Institute, Ft. Pierce, FL). In 1991, a box “C. Po isotopes were collected and plated onto silver corer (70 x 30 x 40 cm) was deployed from the surface planchettes, and the sample 210Poactivity was counted using the EPA RIVLuke Guardian. This sediment boxcore by a-specuometry. I3’Cs was measured in core F-31 using was subdivided into several subcores (i.d. 6.8 cm, 30-40 nondestructive y-spectroscopy (34) to check the dating cm length) that were used for the study of organic scenario suggested by zlaPbchronology. compounds, radionuclide tracers, metals, and organic Recoveriesof HOCs spiked into matrix blanks consisting carbon. Care was taken not to disturb the sediment surface ofpre-l93Osedimentwere89+31%forXPCBs, 1 0 4 i 18% inalicases bymaintainingabout 10-20cmofclearbottom for ZDDT, 101 k 4% for mirex, and 64 21% for HCB. water above the core. Cores used for organic compound Matrix concentrations of the HOCs of interest in these analysis were sectioned into 1-cm increments with a No correction for spike recovery sediments were negligible. hydraulic extruder aboard ship within 2 h of sample was performed, as spike experiments showed such corcollection,and the sections were placed into solvent-rinsed rections were unnecessary. Average surrogate recoveries glass jars with aluminum foil-lined lids and stored refrigerwere 94 i20% 90 i 17%,and 98 i24% for congeners 14, ated at 4 “C in the dark until analysis. Sediment in contact 65, and 166,respectively. Procedural blanks consisting only withthesides ofthecorewastrimmedanddiscardedduring ofthe solvent were processed with every sample set of 5-7 segmentation to minimize smearing effects. sediment samples and were 0.94 nglg dry wt for XPCBs, For HOC analysis, about 20-30 g of wet sediment was 0.008 nglg dry wt for ZDDT and HCB, and 0.003 ng/g dry homogenized and mixedwithca. 100g ofanhydrous sodium wt for mirex. Method detection limits were estimated as sulfate to remove residual water. This mixture was placed three times the standard deviation of matrix blanks (35) into Soxhlet extractors, spikedwithrecovery standards 3.5and were 1.3 ng/g dry wt for XPCBs, 0.044 ng/g dry wt for dichlorobiphenyl (IUPAC No. 141, 2,3,5,6-tetrachlorobiHCB, 0.51 ng/g drywt for ZDDT, and 0.29 ng/g drywt for phenyl (IUPACNO.651, and 2,3,4,4’,5,6-hexachlorobiphenyl mirex No correction for blanks was done, as these levels (IUPAC No. 166) and extracted for 12-18 hwith 100-175 are low compared to analyte concentrations in samples. mLof dichloromethane (DCM),depending on sample size. Analysis of duplicate sediment samples gave a precision of DCM extractswere transferred to hexane and concentrated 11%. to approximately 1 mL using either Kudema-Danish or rotary evaporators (Bucchi). The extract was then fracResults and Discussion tionated on a liquid-solid chromatography column (10 g of 10% by weight water-deactivated alumina and 3.0 g of Sedimentation and Mixing. The 21nPbprofdes for the five 6% by weight water-deactivated silica, i.d. 1.5 cm) into a cores examined (Figure 2) are representative of sediments hexane fraction (75 mL) containing PCBs, DDT and undergoing bioturhation, sediment accumulation, and metabolites, mirex, and HCB and into a 9 1 (xv) h e m e radionuclide decay. All profiles exhibit a region of constant diethyl ether fraction (75 mL) containing the remaining activity in the surface sediments and an exponential HOCs of interest. Elemental sulfur was removed with decrease below, which can be modeled by a least-squares activated Cu powder. Each fraction was reduced to ca. 500 fit to a rapid steady-state mixing model with a constant pL and spiked with internal standards 2,4,6-trichlorobisedimentation rate (536,371. New material added to the phenyl (IUPACNo. 30) and 2,2’,3,4,4’,5,6,6’-hexachlorobi- sediment surface is assumed to be mixed homogeneously phenyl (IUPAC No. 204) prior to instrumental analysis. and instantaneously to depth S (cm). This mixed zone Sample extracts were analyzed with a Hewlett-Packard movesupwardat the rateofsedimentation W(gcm-zyr-l), 5890 gas chromatograph equipped with a “Ni electron and material leaving the mixed zone is preserved as no capture detector using splitless injection onto either a 30 additional mixing occurs. For a constant Z’oPbflux (cf. ref
+
2662. ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29. NO. 10.1995
0.1
LOO1 -40A
L091-19 1 IO
-
LO9 0 F3 1
L090-E30
1
0.1 0
L090-G32
10
0.1
1
10
1
2
3
4
0
1
2
3
4
1
FIGURE 2. Excess *loPb profiles (open circles, pCi g-l) and lnCs profiles (filled circles, pCi g-l) vs cumulative dry mass of Lake Ontario sediment cores. noPb axis is on top of figures; lnCs axis is on bottom of core F-31 figure. TABLE 1
Sedimentation, Mixing, and Focusing Data for Lake Ontario Core9 LO911 9
1091-40A
L090-E30
LOWF31
1090-G32
43'32.3' N 76'54.3W
43'35.0' N 76'48.0'W
43'39.2' N 76'42.0'W
0.0683
0.0310
0.0815
0.34
0.086
0.23
Location 43O22.2' N 79'21.2' W
43O35.4' N 78'00.7' W
Sedimentation Rates (w) zroPb g ern+ yr-l crn yr-1
0.0302 0.0086 0.18
0.0639 0.0296c 0.43
'37Cs
g c m 2 yr-l
0.0643 0.18
crn yr-'
Mixed Depth (S) P'OPbI g crn-2 crn
0.39 2.3
Y
13.2
4.2
1.07
1.71
0.27 1.8
0.96 4.7
1.28 2.7
0.80 3.5
16.2
9.8
1.72
2.20
Intrinsic Resolution r (S/wl P'OPb] 6.7
zloPb Focusing Factors 2.04
Focusing factors (FF) are derived from the ratio of unsupported 210Pb inventory/210Pb standing crop expected from atmospheric deposition, the latter of which is estimated at 15.5 pCi cmP (52, 53) bSedimentation rate below 7 cm. CSedimentationrate below 13 cm. a
51, the 210Pbactivity as a function of depth A(x) is
A(x) = A,
for x
A(x) =A, exp[-y(x - S/W]
IS
for x
(1) >
S
(2)
where A, (pCi g-l) is the constant activity in the mixed zone and 1 is the decay constant for 210Pb(0.0311yr-l). The effects of compaction are removed if Wis expressed as an accumulation rate (g cm-2 yr-l) and S is expressed as cumulative dry mass (gcm-2). Applyingthese models yields mass sedimentation rates ranging from 0.030 g ern+ yr-l for core 19 in the center of the relatively shallow Niagara Basin to 0.0815 g cm-2 yr-' for core G-32 in the deep hole of the Rochester Basin (Table 1). Given the porosities 4 of the cores (0.9-0.95) and a mean solids density of 2.45 g ~ m - the ~ ,linear sedimentation rate averaged over the top 5 cm of each core ranges from 0.18 to 0.43 cm yr-l (Table 1). The mixed depth S (Table 1) ranges from 0.27 to 1.28 g cm2, or about 1.8-4.7 cm. The sedimentation rates observed in our Lake Ontario cores are in general agreement with those reported in previous studies (24,25,38).Kemp and Harper (38)found sedimentation rates to be highest in the eastern portion of
the lake, reflecting the general counterclockwise circulation pattern in the lake and the trapping of particles in the eastern basin (39, 40). The 210Pb-derivedaccumulation rates at sites E-30 and G-32 in 1990 (0.0682 and 0.0815 g cm-2 y-l, respectively) also compare reasonablywellwith those from cores taken at these locations 9 yr before in 1981 (0.0443 and 0.0767 g cm-2 yr-l, respectively) (24). The rates in this study are somewhat higher than those of the earlier work, likely due to small-scale spatial heterogeneity in sediment deposition. The 210Pbprofiles at sites 19 and 40A showed two regions of exponential decrease. This is interpreted as two sedimentation rates: a slower rate of 0.008 g cm-2 yr-I at site 19 and 0.0296 g cm-2 yr-' at site 40A prior to 1930 or so, and a higher one of 0.0302 g cm-2 yr-I at site 19 and 0.0639 g cm-2 yr-l at site 40A thereafter (Table 1). Kemp and co-workers (41)suggested that sedimentation rates in the Kingston Basin of Lake Ontario have increased since the 1930s, based on the differences between the Ambrosia pollen horizon (1850)(42)and the Castanea horizon (1930) ( 4 4 , with some evidence of an increase in their Niagara Basin core. However, Robbins et al. (43) did not find changes in the 210Pb-derivedsedimentation rate over the VOL. 29, NO. 10, 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY 12883
last 100 yr. They attributed differences in the zlaPb sedimentation rates and pollen-derived rates observed in their Niagara Basin core to loss of sediments laid down at that time from major storm events. Robbins et al. (43) found breaks in the Z1oPbprofiles and deviations from the predicted activity at dated intervals in which there were major storms. We do not observe such deviations in our cores, so we suggest that the abrupt changes observed at sites 19 and 40A are a result of lower sedimentation rates in the western basin at that time. Schelske etal. (44)have suggested that increased productivity in Lake Ontario has resulted in dilution of recent sediment by organic matter and CaC03,thereby changing surficial sedimentation rates. Incorporation of this aspect did not significantly change HOC input histories. Dates obtained by fitting the profiles without taking the effects of mixing into account were inconsistent with the HOC deposition history as observed in the sediments (see next section). The 137Csprofile at F-31 (Figure 2) shows an increase in activity from 2.5 pCi g-l in the suficial sediments to a peak of 4.5 pCi g-l at 1.8 g cm-2 depth or 5-6 cm. The sedimentation rate of 0.0643 g cm-2 yr-’ based on 13’Cs (assuming peak activity at 1963) is twice the rate observed fromthe 210Pbprofile, 0.031 g cm-2yr-1. Usingtheconstant rate of supply (CRS) (45) model for the 210Pbdata yielded sedimentation rates similar to the constant flux-constant sedimentation model for this core. Eisenreich et al. (24) noted that sediment mixing extended the activity of 137Cs 1-3 cm deeper into the 1981 Lake Ontario cores than it would otherwise have been; however, sedimentation rates calculated from both 137Csand 210Pbwere in reasonable agreement. One possible explanation for this discrepancy is that the 210Pbprofile may have been truncated in this core, leading to an overestimation of the supported 210Pb levels. The mixing depths observed in the five Lake Ontario cores suggest that surface bioturbation is intense enough to mix particle-bound tracers on the time scale of decades at these sites, but not in shorter time periods, in agreement with laboratory microcosm studies ( 4 , 6, 46-48). The intrinsic resolution [t = S (cm)/W (cm yr-l) or t = S (g cm-Z)/W (g cm-2 yr-l), y] (7, 241 is the length of time over which a change in the input rate of a tracer will not be observed in the sediment record, assuming that mixing is a long-term steady-state process. This does not take into effect nonlocal integration offered by mixing within the lake itself before the HOCs reach the sediment, which will further degrade the signal. Changes in tracer deposition that are more rapid than t thus do not reside in the mixing zone long enough before change to be completely homogenized. The intrinsic time resolution for the five Lake Ontario cores based on 210Pbranges from 4.2 yr in core 40A to 16 yr in core F-31 (Table 11, with the values at E-30 and G-32 (6.7 and 9.8 y respectively) in reasonable agreement with those calculated at these sites in 1981 (11.3 and 13.5 yr, respectively) (24). Thus, the depositional history of chemicals whose input (or decay) varies over a time period greater than this (i.e., Pb (49))should not be preserved in the sediments, because changes in their input rates are slow enough for bioturbation in the mixing zone to homogenize the profiles. However, it is possible that the decrease in loadings of PCBs, DDT, and mirex since their respective bans may be sufficiently slow to be thoroughly mixed in the bioturbation zone (see next section). Eisen2664
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 10, 1995
reich et al. (24) concluded that mixing rates were not sufficient to destroy the historical profile for HOCs whose input half-lives (ca. 5-10 yr) were significantly less than the half-life for 210Pbdecay. Surface sediments in the Great Lakes are in contact with oxygenated water and often undergo surface mixing by a variety of benthic organisms, such as amphipods (e.g., Diporeia sp.) and oligochaete worms (e.g., Tubifex tubifex) (47,501,as well as by fish and near-bottom currents. The abilityof oligochaetes, a major player in lacustrine sediment bioturbation, to homogenize sediment in the mixed zone can be assessed by comparing the total dry sediment displacement rate to the sedimentation rate (7,24). Robbins (7 defined the oligochaete reworking efficiency E as the ratio of the rate of feeding by the organisms present (g of dry sediment cm-2 yr-l) to the sedimentation rate W (g cm-z yr-l). The first term is divided into the number of organisms present per unit area N (cm-9 and their individual feeding rate r (g yr-9, so that E = NT/W. If E 1 10, then the tracer profile is homogeneous in the zone occupied by the organisms (7).A reasonable estimate for r is about 1 g yr-l (47, 48). While the number of worms was not quantified in this study, the 1981 worm densities at sites E-30 and G-32 (24) provide an estimate of the oligochaete abundance at the same sites in 1990, since the radiochemical data between the two studies at these sites are similar. The earlier study reports worm densities of 0.1450 and 0.3200 individuals cm-2 at E-30 and G-32, respectively, resulting in reworking efficiencies E at these sites of 2.1 and 3.9, respectively. As a result, these cores are not mixed homogeneously by the benthic infauna over the decades-long time scale of the tracer used (210Pb), Therefore, profiles of transient tracers, such as HOCs, may retain much of their chronological structure. The integrated 210Pbinventories of the sediment cores (Table 1) provide a measure of the amount of sediment focusing at a particular site. Focusing is the process by which fine sediment particles and associated HOCs are redistributed laterally moving from shallower nearshore regions to deeper, more quiescent zones by currents and episodic storm events that resuspend sediments (43, 51). Sediment focusing factors (FF) are estimated at each site fromthe ratio of the measured unsupported 210Pbinventory to the value expected from atmospheric deposition, 15.5 pCi cm-2 in mid-North America (52,53). Focusing factors are lower in sediment cores from shallower regions of the lake (site 19, depth 100 m, FF = 1.07)than those from deeper regions (e.g., site E-30, depth 250 m, FF = 2.04). The focusingfactors from the E-30 and G-32 cores of this study (2.04and 2.20, respectively) are comparable to those of the earlier 1981 study (241,if our estimate for the 210Pbstanding crop is used rather than that of the earlier study (21.9pCi cm-21. By using our estimate for the standing crop, the 1981 focusing factors are 1.69 for E-30 and 2.19 for G-32. Organic Carbon. HOCs sorb to the organic-richfraction of silts and clays (I), and HOC retention in sediments depends on the organic matter preservation (54). The organic carbon measured at the sites ranges from 4-5% by weight at the surface to about 1-2% at depth and is typical of sediments from Lake Ontario (24, 41, 55). Surface accumulation rates for organic carbon range from 1.4 mg cm-2 yr-l at site 19 to 3.1 mg cmM2y-l at site G32, and areal inventories from ca. 1940 (the HOC horizon) are as follows: site 19, 69 mg cm-2; site 40A, 98 mg cm-2; site
A
i o zo so
*o
$0
20
so
40
i o zo
30
40
1000
1000 1080 1070
1 080
1070
2
x
1060 lOS0 1040 1030 1020 1010 1000
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2
LO91-19 10 go so
40
L090-40A i o zo ao
40
L090-E30 10 20 so
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IO
L090-40A 15
L090-E30
10
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16
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20
80
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1010 1900
L090-F31
L090-G32
L090-F31
U.S. PCB SALES
D
1900 I080 1070 1060
x
.'l~C LO91 -1 9 0.5
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1970 1060 1050 1040 1030
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PRODUCTION (1' 0
2
s
i
z
s
1060 $ 1050
1040 1020 1910 1000
1
L090-G32
1070
$ 1960 1030
/'
io50
1.0
L090-40A
1.6
0.5
1.0
1020 1010 1 000
L090-E30
1.5
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LO91-19
100 ZOO 300 400
1
1
2
L090-E30 a
100
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2
1060 1040 1030 1020
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1.090-F31
LO9O-40A a
1 000
1020 1000
2
1000
L090-G32
MIREX, SALES ( 1 0 9)
L090-F31
L090-G32 CHLOROBENZENE PROD CTION (10
9)
FIGURE 3. Focus-normalizedaccumulation rates (ng cm-2 yr-l) of (A) XPCB, (B) ZDDT, (C) mirex, and (D)HCB as a function of date in Lake Ontario sediment cores. Sales and production data are from Eisenreich era/. (24.
E-30, 102 mg cm-2; site F-91, 172 mg cm-? site G-32, 118 mg cme2. The organic carbon in the cores is sufficiently abundant such that HOCs are efficiently incorporated into the bottom sediments (i.e.,low benthic recyclingofHOCs). HOC Profiles and Accumulation Rates. The following discussion of HOCs emphasizes four different chemicals or classes of chemicals prominent in the Lake Ontario ecosystem: 2PCBs, XDDT, mirex, and HCB. AU profiles (Figure 3A-D) are characterized by an exponential rise in concentration at depth to a subsurface peak and a decrease in concentration to the surface. The profiles also show a
relatively constant concentration near the surface for all four HOCs. Unlike the previous study (241, in which the concentration peaks for these compounds were observed at the base of the mixing zones, these peaks are below the mixed zone and preserved in the buried sediment. The time-dependent HOC fluxes recorded in the sediment profiles (Figure 3A-D) compare well to the sales and production of these chemicals over time in accord with previous studies in Lake Ontario (24,25). PCBs were first produced in the 1930s, were mass-produced by the early 1950s,peaked in usage in the mid- to late-l960s, and were VOL. 29, NO. 10, 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
2865
o
loo
concentration (ng/gdw) zoo 300 400
500
cf:cs 600
crs
crs
nomix mix
- 1987- 1987- 1985
-
1982- 1980- 1980
- 1975- 1971- 1973
-
1968- 1961- 1964
- 1 9 ~ 1 19481954 - 1952- 1929- 1939
-
1941- 188% 1917
- 1886 1830- 1873
- 183%
1796-1 1817
10
FIGURE 4. ZPCB and ZDDT concentration depth profiles at site 19. Plotted on the right vertical axis are the zroPbdates based on the constant f1ux:constant sedimentationrate model (ckcsl, the constant rate of supply (CRS) variable sedimentation rate model without mixing, and the CRS model with a mixing term included. The nomixing CRS dates are inconsistent with the known depositional history of PCBs and DDT in the lake Ontario system.
banned in the United States and Canada in the early 1970s. DDT was first produced during World War 11, reached peak production and use in North America in 1958-1960, and was banned in the United States and Canada in 1972, although it continues to be used in other parts of the world. Mirex input to Lake Ontario is due to discharges from chemical manufacturing companies on the Oswego (eastern basin) and Niagara Rivers (western basin) (56-58). Mirex sales (57) peaked in the mid- to late-1960s and dropped off in the mid-1970s.HCB is mostly derived as a byproduct of general chlorobenzene production, which has a production history similar to PCBs (59). Sediment HOC profiles may mimic input rates to awater body if attachment to particles and accumulation in sediments is a major loss process from the water column. The average residence time of an HOC in a water column with respect to particle settling is dependent on the amount of suspended organic particulate matter in the water column and the HOCs particle-water distribution coefficient (60). Given the hydrophobicity of the chemicals of interest in this paper and typical values of suspended particulate matter in Lake Ontario, the residence times of particlebound HOCs in Lake Ontario waters is less than 1 yr and short relative to the water residence time of 8 yr, so these contaminants reach the bottom sediments within the time resolution in the sediment core (20, 61). The 210Pbprofiles from sites 19 and 40A indicate a lower constant sedimentation rate in the past. These profiles can also be interpreted with the CRS model as a continuously increasing sedimentation rate (45). However, the HOC profiles do not match the input history when they are compared to CRS model dates (Figure 4). In core 19, the ZPCB peak occurs in 1954 according to the CRS model, while the ZDDT peak occurs in 1938, 22 yr prior to their peak usage. In contrast, the peaks for ZPCB and XDDT occur at 1964 and 1956, respectively, using the steady-state, constant-sedimentation model. These dates more closely correspond to the historical usage rate as discussed previously. However, if a mixing term is incorporated into the CRS model (621,the dates are more in line (but still older) with the dates given using the steady-state, constantsedimentation model (Figure 4). The situation is similar 2666
ENViRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 10, 1995
for the core at 40A, where the ZPCB and CDDT peaks are at 1964 and 1946, respectively, using the no-mixing CRS model, and at 1971 and 1960,respectively,usingthe steadystate, constant-sedimentation model. At other sites, the 210Pbprofiles decrease exponentially below the mixed zone, so mean sedimentation rates derived from either model will be similar. Eisenreich et al. (24) and Oliver et al. (25) have demonstrated that the depositional history of these HOCs is preserved in numerous Lake Ontario sediment cores. This observation shows the utility of using wellknown HOC inputs to verify radionuclide dating. The HOC profiles at site F-31 are consistent with the 13'Cs-derived sedimentation rate, but not the *lOPb-derived rate. ZPCB, ZDDT, mirex, and HCB peaks based on the former occur in this core at 1968, 1953, 1968, and 1968, respectively. The peak dates based on the *loPb-derived sedimentation rates are 1957, 1914, 1957, and 1957, respectively, which is inconsistent with the observed usage pattern and other historical data. This is possibly due to truncation in the 210Pbprofile as discussed in the previous section. ZPCB accumulation rates increase dramatically in the early 1950s, in agreement with other cores taken in the Great Lakes (12, 22-27, 631,peak in 1965-1970 at about 40 ng cm-2yr-l, and decrease to recent rates of about 8-30 ng cm-2yr-l. ZDDT accumulation rates reach a maximum of about 6-17 ng cm-2 yr-l in 1955-1965 (about a decade earlier than ZPCB),with recent values of 1-4 ng cm-2y r l . Mirex accumulation reaches a maximum of 0.8-3 ng cm-2 yr-' in 1965-1970 and decreases to 0.2-0.9 ng cm-2 yr-' in recent times. HCB accumulation rates reach amaximum of 1-4 ng cm-2 yr-' in 1965-1970 with recent values of 0.4-1.7 ng cm-2 y-'. In all cases, HOC accumulation rates today are 15-30% of the peak accumulation rates, and have stayed relatively constant over the last decade or so (Figure 3A-D). Bioturbation of surface sediments may be partially responsible for the constant HOC levels observed. Upward mixing by oligochaetes bring HOCs to the surface sediments from below, putting an upper bound on the observed accumulation rate. But surface mixing is not intense enough to completely homogenize HOC profiles. The reduced levels observed in the surface sediments is largely the response of the system to bans of PCBs, DDT, and mirex and their decreased inputs to the lake. Thus, the lake has clearlyresponded from the previouslyhigh loadings of these compounds. Also, much of the existing HOC burden is now unavailable to the water column, as peak concentrations observed in 1981 (24) were at the base of the mixed zone (and hence available to the water column), have left the zone, and are sequestered in the buried sediment. The variability in recent accumulation rates for ZPCB, ZDDT, mirex, and HCB (Figure5, white bars) is significantly reduced when the accumulation rates are normalized by the respective zloPbfocusing factors of the cores (Figure 5, black bars). Focus-corrected accumulation rates are calculated as focus-corrected accumulation rate = C,W/FF = accumulation rate/FF (3) where C, is the surficial sediment HOC concentration (ng/g dry wt). This correction assumes that 210Pbis a good tracer for HOC particle dynamics in the sediments and water
4
30
3 20 2 10 1
0
LOOl -10
LOOl -40A L000-E30
L000-F31
L000-032
0
LOO(-10
1001-40A
L000-E30
L000-C31
LOOO-032
LOOl-10
LOOl-4OA LOOO-E30 L000-F31
L000-032
2.0
1.3 1.o
0,s
LOO1 -19
LOOl -40A LOOO-E30 L000-FJ1
0.0 LOOO-032
0uncorrected accumulation corrected accumulation FIGURE 5. Recent accumulation rates (ng cm-* yr-') for EPCB, EDDT, mirex, and HCB in Lake Ontario cores. Open bars are the observed accumulation rates; filled bars are the "OPb focus-normalized rates. The lines denote the average focus-normalized accumulation rates for each HOC in the five cores,
column, which is likely since 210Pband HOCs are both concentrated in fine, organic-rich particles. (The focuscorrected surficial accumulation rate for organic carbon itself is 1.47 f 0.099 mg cm-* yr-I.1 FF reflect the amount of lateral movement of sediment to the site in the basin. The fact that the disparate accumulation rates can be normalized by the FFs reducing variability supports our conclusionthat the major factor responsible for differences in accumulation rates within the lake is fine sediment transport and accumulation. Average recent focus-corrected accumulation rates in the cores are 8.77 f 2.2 ng cm-2 yr-' for mCB, 1.40 f 0.17 ng cm-2 yr-' for ZDDT, 0.281 & 0.08 ng cm-2yr-' for mirex, and 0.586 f 0.12 ng ern+ yr-' for HCB and are similar to the 1981 focus-corrected accumulation rates reported at E-30 and G-32 (241,which correspond to the approximate start of the level surface concentrationobserved in our cores. The only exception is the 1990 HCB profile at G-32, which has concentration levels about five times less than the corresponding 1981 core. In comparison, atmospheric deposition rates of ZPCB and XDDT were estimated (64)as ca. 1 and 0.1 ng ern+ y r l , respectively, and were reported by Rapaport and co-workers (65, 66) to be 1-4 and 0.4-1 ng cm-2 yr-' in peat cores north of Lake Ontario. These data are consistent with the hypothesis that only a small fraction (ca. 10%)ofBCB and ZDDT inputs to Lake Ontario are due to atmospheric deposition and that the majority historically has entered via the Niagara River and in-lake cycling of HOCs (via resuspension and sediment transport processes) and is distributed lakewide. The focus-corrected recent accumulation rates translate to lakewide inputs of 1660 kg yr-' for ZPCB, 261 kg yr-' for ZDDT, 51 kg yr-l for mirex, and 111 kg yr-l for HCB. These values are comparable to loadings estimated from Niagara River measurements (e.g., -2000 kg yr-' for ZPCB) (67, 68).
HOC Inventories. Insight into the delivery of HOCs into the Lake Ontario system can be obtained by considering vertically integrated sediment inventories. These inventories represent the total sediment abundance of HOC per unit area, and are calculated as the product of the HOC concentrations, bulk density, and fractional dry weight for each depth increment and summed over all depth increments. Like the accumulation rates, the inventories (Figure 6, white bars) vary sigdicantly among the cores: XPCB, 6001500 ng cm-? XDDT, 240-440 ng cmT2;mirex, 22-54 ng cm-? HCB, 50-100 ng cm-*. However, this variability is largely removed when the inventories are normalized (Figure 6, black bars) by the 210Pb-FFs.The average focuscorrected inventory is 687 & 93 ng cm-2 for B C B , 185 f 26 ng cm-2 for XDDT, 20 f 3 ng cm-2 for mirex, and 44.5 & 8 ng cm-2 for HCB, with the coefficient of variation dropping from 20-30% for the uncorrected inventories to about 15%for the corrected inventories. The differences in the amount of HOC at a particular site in Lake Ontario, therefore, is primarily due to differences in the amount of accumulating sediment. This implies that there is either one source of the HOCs to the entire lake or that the water column dynamics are sufficient to distribute particles and particle-associated HOCs throughout the lake basins and to mask local and point inputs. Both of these processes hold in Lake Ontario. The primary source of the HOCs in this study to Lake Ontario is the Niagara River (20,56,6973). Pickett and Bermick (39)observed that Lake Ontario's counterclockwise circulation pattern carried a plume of mirex from the Niagara River to the eastern basins of the lake. Jaffe and Hites (20, 61) observed that a suite of fluorinated organic compounds originating from a single dump site along the Niagara River was found ubiquitously in the sediments of Lake Ontario. Radiodating showed that VOL. 29. NO. 10,1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
2667
2000
1600
t
800
1 PCB
avg = 687 ng crn -2 %CV = 14.0
-
n
600 J
avg
%CV
-
= 185
ng
15.1
ern-?
n
400 300
1000
1 DDT 3
200 600
1
100
0
LOOI-IO
lakewide total 80
-
L091-40A L000-E30
0 LOOO-F31 LOOO-032
40
100
30
7s
20
60
10
26 LO01-40A L O 0 0 4 3 0 L000-FSI
LO00432
160 126
LOOI - 1 0
LOOI-4OA LOPO-E30 L000-F31
lakewide total = 3 5 , 0 0 0 kg
130,000 kg
60
0
LOOl-10
L000-032
0
LOOI - 1 9
LOOI -40A L000-E30 LOOO-FSI
LOBO-032
lakewide total = 8,500 kg
lakewide total = 3,800 kg
0uncorrected
inventory corrected inventory
FIGURE 6. HOC inventories(ng cnr2) for XPCB. XDOT, mirex, and HCB in Lake Ontario cores. Open bars are the obserued inventories: filled bars are the noPb focus-normalized rates. The lines denote the average focus-normalized inventories for each HOC in the five cores.
peak accumulations of these compounds occurred at roughly the same times (mid-1960s) in all cores examined (20, 64, implying that lake currents dispersed the fine, organic-rich particles to which the HOCs are bound throughout the lake within a relatively short time period (-months), although not at equivalent concentrations at all sites. These observations suggest that HOCs entering Lake Ontario from the Niagara River are thoroughly mixed within the lake and are deposited to the sediments in proportion to the amount of sediment focusing. Focus correction can be used as a tool to elucidate HOC input processes to an aquatic system. If sediment-focus normalized inventories do not agree, then one or more of the assumptions behind the focusing hypothesis are not true, and we can deduce that the HOCs of interest enter the system by other means. Simcik (74)found that 210Pbfocusnormalized PAH inventorieswere similar in Lake Michigan sediments in the north and south basins, implying that PAH inputs derive from a single distributed source (atmospheric deposition) and intra-lake sediment redistribution. However, Golden (27) observed that 210Pbfocusnormalized PCB and DDT inventories were higher in the northern basin of Lake Michigan than they were in the southern basin. This observation was attributed to inputs from PCB contaminated Green Bay and from south-tonorth transport of sediments that was not rapid enough to mask regional inputs. Jeremiason(63, 75) found that focusnormalized P C B sediment inventoriesin the western basin of Lake Superior were much higher than those in the open waters of the lake and attributedthisobservation to localized inputs from the urban area of Duluth-Superior. Focusnormalized P C B sediment inventories in the open waters of Lake Superior were reduced to a common value of -5 ng cm-2,suggesting that HOCs behave in a similar manner in the open waters of the lake as they do in Lake Ontario. 2668 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 10,1995
TABLE 2
Obsened and Focus4onaalized Inventories (ng cm-*) of HOCs at Sites E30 and 632 in 1981 (24) and 1990 (This Study)a XPCBb
PCW
XDDT
mirex
HCB
581 344
268 159
33 19.5
93 55
654 321
429 221
41 21
67 32
711 324
466 213
58 26.5
340 155
791 360
334 173
54 28
100 45
L081-E30
observed norm a Iized L090-E30
observed normalized
1119 549
LO81-G32
observed normalized L090-G32
observed normalized
1557 708
a Focusing factors from the 1981 cores, derived using the current estimate of zroPbstanding crop (15.5 pCi cm-*), are 1.69 for L081-E30 and 2.19 for L081-G32. The same congeners analyzed in both studies are included in the 25 congener listing for direct comparison, b 8 5 congeners. 25 congeners.
The focus-corrected inventories are also useful in determining recent accumulations of HOCs in sediments. Table 2 shows the uncorrected and focus-corrected inventories for ZPCB at sites E-30and G-32 in both 1981 (24) and 1990 (this study). The inventory for the earlier study is normalized using the same estimate for the 210Pbstanding crop (15.5 pCi cm-2) used here rather than the value reported in Eisenreich etal. (24)to allow direct comparison. The same 25 congeners analyzed in both studies are summed here; our recent analysis of ca. 85 congeners shows that early studies that analyzed fewer congeners underestimate the total PCB abundance by60-100% (75).There is no statisticaldifference in the focus-correctedinventories
between the two sampling periods at either site, suggesting that recent deposition over the last decade is low compared to previous deposition. This observation is in agreement with the low accumulation rates noted at the top of the 1990 cores. A maximum of about 12% of the total PCB present in the sediments had accumulated in the period between 1981 and 1990, neglecting upward mixing by oligochaetes. Focus-corrected mirex inventories between the two sampling times are also similar at both sites. ZDDT inventories show more variability. At E-30,the focuscorrected XDDT inventory is 158 ng cm-2 in 1981;in 1990, it is 221 ng cm-*. At G-32, the 1981 inventory is 213 ng cm-2, while in 1990 it is 160 ng cm-*. However, these discrepancies fall within the scatter in the data, as the 1981 values are within the 15%coefficient of variation observed for the average focus-corrected 1990 CDDT inventories. On the other hand, focus-correctedHCB inventories at G-32 are much higher in the 1981 core (155 ng cm-9 than in the 1990 core (45 ng cm-2),due to the much higher concentrations found in the former. The concentrations and inventory in the 1990 G-32 core are more in line with the values reported by Oliver etal. (25). Eisenreich etal. (24)suggested that the 1981G-32 core may be subject to a localized source of HCBs available to it only and not available to that site in 1990,perhaps due to spatialvariability. It is also possible that the higher concentrations in the earlier measurement are incorrect. If sediment focusingaccounts for the variabilitybetween sediment HOC inventory at different sites in a water body, then focus-normalized inventories represent an average areal sediment burden for that region and can be used to estimate the total amount of HOC present in the sediments using relatively few sediment cores. For Lake Ontario, the focus-corrected HOC inventories translate into a total sediment burden of 130 000 f 18 000 kg of XPCBs, 35 000 & 5000 kg of ZDDT, 3800 f 570 kg of mirex, and 8500 kg & 1500 kg of HCB. With the exception of HCB, these estimates are larger than those of Oliver and co-workers (23,who calculated a total of 50 000 kg of PCBs, 30 000 kg of DDT, 2000 kg of mirex, and 10 000 kg ofHCB (comparable to our estimate). These lower estimates were based on normalizing the core inventory by the ratio of upper core concentrations to surficialsediment concentrations for each basin and summing the totals for each basin together. Our total mirex burden is larger than the estimate of 2200 kg by Comba et al. (767, based on a mass balance study (76) of the Lake Ontario and St. Lawrence River systems; the sediment burden in this study was derived from a single core taken in relatively shallow water (70 m) 3 km off the mouth of the Niagara River (13, a location that may be too turbulent to integrate HOC burdens. In addition, the 13’Cs-derived sedimentation rate of the core may be overestimated (25) due to a point source of 137Csalong the Niagara River (77). The focus-dependent inventory is dependent on the 210Pbfocusing factors and is therefore dependent on the expected atmospheric loading of *lOPb. The surficial inventories (Table 3) of XPCB, CDDT, mirex, and HCB in the top cm of the sediment cores and in the 210Pb-derivedmixed zone (1.8-4.7 cm) are useful in estimating the amount of sediment HOC available to the water column. Particulate-bound HOCs at or near the sediment-water interface may be resuspended into the water column (3, 58),and particles bearing HOCs in the mixed zone may be brought to the surface by bioturbation,
TABLE 3
Observed and Focus-Normalized Inventories (ng em-*) of HOCs in Upper 1.0 em and in *l0Pb-Based Mixing Depth in Lake Ontario Sediment Cores LO91-19 0-1 cm mixed depth (2.3 cm) focus-0-1 cm focus-mixed depth LO91-40A 0-1 cm mixed depth (1.8 cm) focus-0-1 cm focus-mixed depth L090-E30 0-1 cm mixed depth (4.7 cm) focus-0-1 cm focus-mixed depth L090-F31 0-1 cm mixed depth (2.7 cm) focus-0-1 c m focus-mixed depth L090-G32 0-1 cm mixed depth (3.5 cm) focus-0-1 cm focus-mixed depth
ZPCB
ZDDT
mirex
HCB
41.38 129.5 38.67 120.98
7.85 25.34 7.33 23.69
1.40 3.60 1.28 3.36
2.69 7.22 2.52 6.75
24.40 52.48 14.27 30.69
3.31 7.63 1.94 4.46
0.70 1.57 0.41 0.92
1.30 3.19 0.76 1.87
49.24 323.9 24.14 158.7
7.76 57.47 3.80 28.17
2.34 16.24 1.14 7.96
3.20 24.50 1.57 12.00
88.66 647.39 51.54 376.39
11.58 75.90 6.73 44.13
1.77 10.12 1.03 5.88
6.55 42.19 3.83 24.67
37.60 175.21 17.09 79.64
7.42 27.85 3.37 12.66
1.25 6.51 0.57 2.96
2.44 11-67 1.11 5.30
where the HOCs may desorb and enter the water column (78). This amount ranges from relatively low in the cores in the western and central basins (e.g., 50-130 ng cm-2 XPCB in the mixed depth in cores 19 and 40A) to high in the cores of the eastern basin (up to 650 ng cm-2 XPCB in the mixed zone of E-30). Variability in the surficial inventories is large, both in the top centimeter and in the mixed zone. Focus correction does not remove this variation because processes like post-depositional mobility can change the amount of HOC present at a particular depth in the core over time. Based on the inventories in the mixed depth, the amount of sediment HOC potentially available to the water column is in the following ranges: 5800-71 000 kg of XPCB, 850-5400 kg of CDDT, 180-1110 kg of mirex, and 600-8000 kg of HCB. The actual depth of sediment exchanging particles with the water column may be much smaller than the 210Pbmixed depth (Le., on the order of 1 cm or less). Using the inventories in the 0-1-cm depth increment as arough estimate of this depth results in 27009800 kg of XPCB, 370-1300 kg of XDDT, 80-270 kg of mirex, and 240-1230 kg of HCB available for sediment-water exchange. Sediment Diagenesis of PCBs. The 1981 and 1990 sediment cores at E-30 and G-32 provide a unique opportunity to study the effects of HOC diagenesis over a decade-long interval and allow us to gain insight into the advection, diffusion, transformation, and partitioning behavior of these compounds. Over the 9 yr between core collection, the HOC profiles should have been moved downcore as fresh material is continuously added. Advection of HOCs in sediments has not been observed to the best of our knowledge as there have been no studies to date taken at the same site over time. From the concentration profiles vs cumulative dry mass of XPCB in the 1981 and 1990 cores (Figure 7A), it is VOL. 29, NO. 10, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
2009
concentrotion (ng g-')
concentrotion (ng g-'1
0
200
400
600
800
0
j000
n
400
600
800
1000
1980
N
'E
200
1990
0
1
1970
pl
v
a
E 2
1960
2
,
1950
Y
3
0
TJ
0 01
1940 1930
.I 4 c
-J
1920
5 5
1910
6
1900
0 1990 PCBs
85 congeners
1981 PCBs
25 congeners
v 1990 PCBs 25 congeners v
i
FIGURE 7. PCB concentration profiles vs cumulative dry mess (A) and date (6)at Lake Ontario site 6-32in 1981 (24) and 1990 (this study). Filled triangles denote 1981 total PCBs (25 congeners), open triangles denote 1990 total PCBs summing up the same 25 congeners analyzed in 1981, and open circles denote 1990 ZPCBs (85 congeners).
apparent that the 1981 study, which analyzed only 25 congeners, underestimated the amount of total PCBs present by 60-100%. Also apparent is the burial of the ZPCB peak as new sediment is added in the 9-yr period between samplings,with the 1981 peak at 4-5 cm and the 1990 peak at 5-6 cm. The distance between the two peaks is 0.67 g cm-2,which is equivalent to the sediment deposited over that time period (average sedimentation rate of the 1981 and 1990 G-32 cores is 0.075 g cm-2 yr-l, resulting in 0.67 g cm-2 sediment over 9 yr or about 0.4 cm). The 4-5cm sample in the 1981core was lost in the analyticalworkup (24),so the peak in this core may be skewed downward by as much as 1 cm. The sedimentation rates in the two E-30 cores are more disparate, with the measured difference of 0.84 g cme2(3 cm) compared to the average sedimentation rate (0.0563 g cm-* yr-l) resulting in a total of 0.5 g cmF2 deposited. This overall behavior demonstrates the longterm burial of HOCs in the sediments. Once deposited to the sediments, HOCs may be redistributed within the sediments via diffusion of dissolved and colloid-bound species in the porewater. Over time, diffusion will broaden the HOC peaks and lower the maximum concentration at the peak. We can compare the profiles from the 1981 and 1990 cores to determine if there is any evidence of diffusion over this time period. The profiles of PCBs at G-32 (Figure 7B) does appear to have broadened over the 9-yr interval, represented by 0.4 gcm-2 or 1 cm of sediment. The amount of broadening expected depends on the HOC effective diffusivity Ded:
where D, is the aqueous diffusivity (cm2 s-l), 4 is the sediment porosity, & is a measure of the tortuosity in the sediment (791,Kd is the solid-solution partition coefficient (cm3g-l), and @b is the sediment bull
