History of anthropogenic activities in Hamilton Harbour as determined from the sedimentary record

Environmental Pollution 86 (1994) 341-347 © 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0269-7491/94/$07.00 ELSEVIER

HISTORY OF ANTHROPOGENIC ACTIVITIES IN HAMILTON HARBOUR AS DETERMINED FROM THE SEDIMENTARY RECORD T. Mayer Rivers Research Branch, National Water Research Institute, Burlington, Ontario, Canada, L7R 4A6

&

M. G. Johnson Department of Fisheries and Oceans, Box 5050, Burlington, Ontario, Canada, L7R 4A6 (Received 31 March 1993; accepted 29 November 1993)

Abstract Bottom sediment cores collected from two closely spaced locations in the depositional basin of Hamilton Harbour (Lake Ontario, Canada) were analyzed for organic matter, bulk density, heavy metals and phosphorus concentrations. Combined data on dredging and steel production records in relation to core composition, together with 21°pb dating were used to develop core chronology. Identification and enumeration of chironomid taxa and molluscs in both cores were carried out to interpret the paleoenvironmental conditions in the harbour. Chemical geochronological and paleolimnological profiles of investigated cores indicate perturbation of the natural sedimentation processes by dredging and spoil disposal with definite evidence of an infill of extrinsic littoral sediments. Decreasing metal concentrations in sediments reflect a positive response of sediments to decreased metal loadings to the harbour. The recent sediment accumulation rates, estimated from the 2~°pb profiles of the two cores, are 38 and 97 mg cm -2 y e a r ~, Mass sedimentation rates of the pre-dredging era were higher (189 and 142 mg cm -2 year -1) due to intensive harbour activities, municipal development and intensive crop production in the late 1800s.

mented. Rehabilitation, however, requires better understanding of pollution trends in the past, which benthic sediments can provide. Presently, little information (Nriagu et al., 1983; Painter et aL, 1989; Yang et al., 1993) is available on the past activities in the harbour, as documented by the metal contaminant distribution within a sediment column and biological status of sediments. The aim of this work is to reconstruct the recent history of anthropogenic activities in the harbour, as documented by changes in chemical and paleolimnological profiles in benthic sediments. Combined data on dredging and the steel production record, together with 21°pb dating, are used to establish core chronology.

MATERIALS AND METHODS Gravity cores of bottom sediments were collected from two closely spaced locations (stations 19 and 26) in the depositional basin of Hamilton Harbour (Fig. 1) in 1987. One core from each location was used for taxonomy work. A second core was used for all chemical analysis and bulk density measurements. The cores were extruded and sliced at l-cm intervals to a depth of 20 cm and at 2-cm intervals further down. Wet sediments were used for taxonomy work. Sediments used for bulk density measurement and for chemical analysis were freeze dried and finely ground. Bulk density (gcm -3) of sediment was determined as a ratio of the mass (g) of a dried sample to the wet volume (cm-3) of that sample (McKeague, 1978). The organic content was determined as loss on ignition (LOI), measured gravimetrically after overnight heating of dry sediments to 550°C. Forms of P, non-apatite inorganic P (NAI-P), and apatite-P (AP) were determined by sequential extraction as described by Williams et al. (1976) and Mayer & Williams (1981). Total P (TP) concentrations were determined by the ignition of samples at 550°C and subsequent 16-h 1N

INTRODUCTION Hamilton Harbour, one of the most polluted bodies of water in North America, was identified in 1985 by the International Joint Commission as an Area of Concern. Despite recent reduction in pollutant loads, beneficial uses of the harbour have been impaired as a result of excessive loadings of nutrients and toxic contaminants in the past. The principal sources of nutrients and toxic contaminants have been the sewage treatment plants of the surrounding cities of Hamilton and Burlington and the steel industry located on the south shore. A Remedial Action Plan (RAP) for rehabilitation of the harbour is being developed and imple341

342

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Fig. 1. Hamilton Harbour, showing locations of the sampling sites. HC1 extraction. Organic P was determined by difference between the TP and the sum of the NAI-P and AP concentrations. The metal concentrations in sediments were determined by digestion of samples in hot aqua regia, followed by atomic absorption spectrometry. Taxonomy data were obtained by identification and enumeration of chironomid taxa and molluscs in wet sediments (Johnson, 1993). Chronology of the core was established using the ~l°Pb methodology. Determination of ~l°Pb was carried out by Dr R. J. Flett (Flett Research, Winnipeg), using the technique of Eakins & Morrison (1978), in which the activity of 2~°Pb was measured via the activity of its grand-daughter, 2~°Po. Polonium in sediments was converted to volatile chloride, distilled from the sediments at 500°C and captured in a Pyrex-wool plug. This was refluxed in HNO3 for 3 h, and the solution was decanted and evaporated to dryness. The polonium was dissolved in dilute HC1 and plated on a silver disc for counting of a activity. Selected samples were analyzed for 226Ra by Dr G. J. Brunnskill (Freshwater Institute, Winnipeg) to determine supported 2~°pb activity directly. Mass sedimentation rate (MSR) was calculated from the regression of logo unsupported 2~°pb activity on cumulative sediment mass with depth, below the base of the mixed surface layer. Dredged spoils were identified by visual inspection and from the bulk density data.

The time interval corresponding to dredged spoil layers was estimated from the unsupported 2~°pb activities immediately above and below the spoils. RESULTS AND DISCUSSION Sediment parameters, such as bulk density and LOI are valuable for interpretation of processes in freshwater sedimentary environments. A gradual increase of the bulk density and decrease of LOI through the length of the core would be expected, as a result of compaction and mineralization of organic matter. However, a large fluctuation in bulk densities and LOI (Fig. 2a,b), at both locations, suggests that processes other than just natural sedimentation were responsible for deposition of these sediments. Visual observation (change in texture and colour), water content (unpublished data), bulk density and LOI reveal horizons of dredged spoils between 9 ~'!. cm and 7-24 cm in cores 19 and 26, respectively. Variability of P forms is also greater than expected from the post-depositional mobility and is consistent with the LOI (Fig. 2a,b). The variability was greatest among the NAI-P fraction which accounted for most (81%) of the variation in TP concentration. The use of the naturally occurring radionuclide 2~°Pb to determine sedimentation rates in freshwater sediments has been well established (Matsumoto, 1975;

343

History o f anthropogenic activities in Hamilton Harbour Bulk Deeeity (g/erai) 0,2

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Fig. 2a. Metal, bulk density, LOI and phosphorus profiles in Hamilton Harbour sediments, Station 19 (left panel). The right panel shows the dredging history of the harbour (data from Holmes, 1986). Robbins & Edgington, 1975; Robbins, 1978; Nriagu et aL, 1979; Durham & Joshi, 1980, 1981; Nriagu et aL, 1983; Anderson et aL, 1987). The technique assumes that the concentration of unsupported 2~°pb in the depositing material and the rate of sediment delivery is constant, and that there is no post-depositional redistribution of sediment. Furthermore, it is assumed that 21°Pb, 21°Bi, and 2t°Po are in secular equilibrium and there is no post-depositional mobility of any of these radionuclides, and the 21°pb background is constant through the sediment column. If these assumptions hold, then the distribution of excess :l°Pb (i.e. the total 21°pb less that supported by 226Ra in the sediment) is described by Ax = (P/to) e -x'~s° + A o

where Ax and Ao are the total activity at the compaction-corrected depth x and radium-supported ~l°Pb activity, respectively. P is the flux of unsupported ~l°Pb at the sediment-water interface, to is the MSR in g cm -2 year-~, A is the 2~°Pb decay constant (0.0311 year-I), and So is the sedimentation rate in cm year -I at the sediment-water interface. The plots of ln(A~ - Ao) versus cumulative sediment mass, from which the MSRs were calculated, are presented in Fig. 3. The profiles of both cores show four distinct segments (Fig. 3). At the top (up to 4 cm

in both cores), relatively uniform 21°pb values indicate the zone of mixed sediment. Although physical processes, such as gas evolution or bottom currents, are to some extent responsible for mixing of surface sediments, high density of the oligochaetes (Reynoldson, unpublished data) in the depositional basin of the harbour suggests that bioturbation is the dominant mixing process in the harbour surficial sediments. The effect of mixing, which results from the rapid steady state biological activity of benthic organisms, is to suppress the surface 21°pb activity over that of newly deposited material (Robbins, 1982). Data of Johnson & Matheson (1968) and Reynoldson (unpublished) indicate that oligoehaete worms occur in sufficient densities to completely homogenize the upper layers of benthic sediments. The data also suggest that the bioturbation is not of recent onset, but has been a constant phenomenon in the harbour over many years. The exponential decrease in the ~l°Pb activity used for the estimates of MSRs below the mixed layer is normally interpreted to indicate the radioactive decay of 21°pb if mixing below the depth of the mixed layer is insignificant. The interruption of the exponential decay by fluctuating activities suggests disturbance in sedimentation rates in the recent history of the harbour, and indicates the presence of dredged spoils (dashed lines in Fig. 3). The 21°Pb-derived MSRs of the two

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Fig. 2b. Metal, bulk density, LOI and phosphorus profiles in Hamilton Harbour sediments, Station 26 (left panel). The right panel shows the dredging history of the harbour (data from Holmes, 1986). cores were highly discordant, differing by a factor of about 2-5 for the upper parts of the cores and somewhat less (1-3) for the lower parts of the cores. The MSRs calculated for the upper part of the cores (above dredged spoils) are 38 mg cm -2 year-~ for core 19 and 97 mg cm-2 year l for core 26. The calculated MSRs are upper limits, as any mixing below the mixing zone will increase the apparent sedimentation rate above the true value (Anderson et al., 1987). Applying the above MSRs to the weight of sediments above spoils, the surface of spoils was estimated to be 1935 and 1969 in cores 19 and 26, respectively. Using unsupported 21°pb activities immediately above and below the spoil horizons, estimated time intervals were 5 years in core 19 and 22 years in core 26. The MSRs in lower parts of both cores, corresponding to the pre-dredging era, were 189 and 142 mg cm -2 year in cores 19 and 26, respectively. Nriagu et al. (1983) attributed the high sedimentation rates during the early phases of the harbour development to construction associated with the enlargement of the ship canal and other construction activities connected with industrial and port expansion. Higher export from farmland, because of extensive grain cropping (Johnson & Nicholls, 1989) in the late 1800s, may have also contributed to high sedimentation rates during the early stages of the harbour. The small peak in bulk density close to the

bottom of core 26 occurs sometime just before 1850 and may be related to the activities (Johnson, 1993) associated with building of the Desjardins Canal (1837) and construction of the Great Western Railway into Hamilton (1853). Applying the calculated pre-dredging MSRs to the sediment weight below the spoils, the bottom of the cores were dated at 1895 and 1830 for sites 19 and 26, respectively. Although high variability in the sedimentation rates and 2t°pb flux to surface sediments within the lake was found by Robbins & Edgington (1975) and Edgington & Robbins (1975), the proximity of the two sampling sites and their bathymetric similarities suggest that the sedimentation rates should be similar at both sites. Thus, the substantial differences in sedimentation rates estimated from the 21°pb activities in two cores, especially above the dredged spoils, point out the ambiguity encountered in using the 2~°Pb method for dating Hamilton Harbour sediments. Sedimentary profiles of heavy metals, particularly of Zn and Fe can be used to validate the 2]°pb chronology by relating them to trends in land use and steel production. Sedimentary profiles of Fe and other heavy metals (Fig. 2a,b) show substantially greater stratigraphic variability than could be attributed to their generally low post-depositional mobility. Moreover, metal profiles reveal notable spatial differences between the two sites,

345

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show dredged-spoils horizons. Solid lines are fitted by the least square regressions, from which MSRs were calculated. although the two stations are close to each other. Consistent, however, are the depths of metal maxima (5-6 cm) in both cores, which provide evidence of metal contamination of sediments in the harbour. In core 19, this interval (cumulative weight 1.10 g cm 2) corresponds to a time 1.10/0.038 = 28.9 years or a date of AD 1958. In core 26, this interval (1.22 g cm -2) corresponds to a time of 12.6 years (1.22/0.097) or a date of AD 1974. The 21°pb geochronology, steel production data (STELCO, unpublished data), and RAP loading data (RAP, 1988) suggest that, in core 19, this depth does not coincide with the maximum loadings of metals in 1977, possibly because of underestimation of the sedimentation rates, or because of post-depositional mixing and redistribution of sediments by oligochaetes. Data of Johnson & Matheson (1968), and Reynoldson (unpublished data) show large populations of oligochaetes present in Hamilton Harbour sediments. In core 26, however, the sediment layer with highest metal concentrations is reasonably close to the expected depth. RAP (1988) data indicate that the loadings of Fe have steadily decreased since 1977 and the loadings of other metals are decreasing as well, which is also evident from the metal profiles (Fig. 2a,b). According to Robbins (1982), exponential decrease in sediment contaminant concentrations is observed in sediments where mixing occurs, in response to an abrupt reduc-

tion in inputs of contaminants. The metal concentrations in the surficial sediments are still, however, higher than the Severe Effect Levels of metals, specified in the current Provincial Sediment Quality Guidelines (MOE, 1992). According to these Guidelines these levels of metals are detrimental to the majority of benthic species. An abrupt drop in metal concentrations consistent with sharp changes in bulk density, LOI and P concentrations at 9 cm in core 19 and 7 cm in core 26 is likely to correspond to material accumulated at the end of open water disposal in the main bay. Between 13 and 22 cm, lower Fe and other metal concentrations (Fig. 2a) than those of uncontaminated fine-grained Great Lakes sediments (Kemp et aL, 1978), with a minimum of 2.1% Fe at 19-20 cm in core 19, suggest the presence of sandy sediments of littoral origin, which is consistent with the bulk density, LOI and P data (Fig. 2a). In this interval of the core, the littoral molluscs (Amnicola sp., Helisoma sp., Gyraulus sp., and Physa sp.) were most abundant (Table 1) and the chironomid community was largely composed of littoral forms (Johnson, 1993). The geochemical (Fig. 2a) and paleolimnological data (Table 1; Johnson, 1993) suggest that the 22-44 cm interval of core 19 contains much less entrained spoils. Absence of littoral molluscs (Table 1) and the low number and limited diversity of the littoral chirono-

346

T. Mayer, M. G. Johnson Table 1. Number of molluscs observed in cores 19 and 26

Depth (cm) Core 19 11-12 12-13 13-14 14-15 15-16 16-17 17-18 18-19 19-20 20-22 22-24 Core 26 44-50

Observations

1 Amnicola sp. 25 Amnicola sp., 25 Amnicola sp., 13 Amnicola sp., 10 Amnicola sp., 1 Helisoma 3 Amnicola 3 Amnicola 1 Amnicola 1 Helisoma

sp. sp., sp. sp., sp.

1 Valvata sp.,

4 Physa sp. 4 Physa sp., 1 Physa sp., 1 Gyraulus sp.,

1 Helisoma sp., 2 Helisoma sp.

1 Physa sp.,

1 Helisoma sp.

1 Gyraulus sp.,

1 Armiger sp.

5 Gyraulus sp.

Pisidium sp.,

mids (Johnson, 1993) in core 26, suggest that dredgeate from a later period, possibly impacted by metal discharges, was dumped here. This is also evident from generally higher metal concentrations in this layer of dredged spoils than that of core 19. The differences between the two sites in geochemistry and abundance of littoral species of molluscs (Table 1) in dredged spoils horizons support the 2t°pb results, which indicate that the spoils are of a different age and/or different origin. According to the 21°pb geochronology, dredged spoils (9-44 cm) were deposited at station 19 between 1930 and 1935, whereas at station 26, a thinner layer of dredged spoils (7-24 cm) was deposited later over a longer time (1956-1969). Dredging records (Holmes, 1986) indicate that the largest sediment quantities, dredged recently in the littoral areas and disposed in open water, were between 1956 and 1960 (Fig. 2a,b). Although large quantities of material were dredged between 1972 and 1973, the dredging records indicate no open-water disposal in the harbour at that time. According to these records, substantial dredging took place in the early 1930s. In summary, this study illustrates an approach in which combined data on dredging and steel production were related to core composition, 2t°Pb profiles and paleolimnology and used to interpret the complex sedimentary environment of Hamilton Harbour. The 21°pb profiles reveal the perturbation of continuous sedimentation, resulting from dredging and spoil disposal in the harbour. Geochemical and paleolimnological data show the presence of littoral spoils of different age at different locations in the harbour. Inconsistencies in the 21°pb geochronology of the two cores reveal the ambiguity of this method for dating highly disturbed environments, such as Hamilton Harbour. The results of this study are of major significance, as they provide a first approximation of the past conditions in Hamilton Harbour based upon the integration of geochemical, geochronological and paleolimnological data and historical dredging and steel production records.

Sphaerium sp.

ACKNOWLEDGEMENTS The authors wish to thank Dr R. Flett of Flett Research Ltd. for expert analytical work on the 21°pb analysis. We are grateful to E. Delos Reyes, A. Whitmel, K. Gracey, L. Culp, O. MacNeil, and S. Smith for their capable assistance and Dr D. Jeffries for his review of the manuscript. Dr J. Nriagu provided helpful comments on the earlier drafts of this paper. Thorough review and thoughtful comments of the anonymous reviewer were helpful in preparing the major revision of this manuscript. REFERENCES

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