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Assessment of sediment and porewater after one year of subaqueous capping of contaminated sediments in Hamilton Harbour, Canada
@)
Pergamon
Wal. Sci. Tech. Vol. 37, No. 6-7. pp. 323-329,1998. @ 1998 IA WQ. Published by Elsevier Science Ud Printed m Great Britam.
PH: S0273-1223(98)00214-5
0273-1223/98 $19'00 + 0-00
ASSESSMENT OF SEDIMENT AND POREWATER AFTER ONE YEAR OF SUBAQUEOUS CAPPING OF CONTAMINATED SEDIMENTS IN HAMILTON HARBOUR, CANADA Jose M. Azcue*, Alex 1. Zeman, Alena Mudroch, Fernando Rosa and Tim Patterson National Water Research Institute. P.O. Box 5050. Burlington. Ontario. Canada L7R 4A6
ABSTRACf In this manuscript, we present data from a demonstration in situ capping site (100m x 100m) in Hamilton Harbour. Lake Ontario. Canada. A layer of clean medium to coarse sand Wtth the average thickness of 35 cm was placed at the site in the summer of 1995. Concentration of Zn. Cr, and Cd in the original sediments reached values over 6000. 300 and 15 Ilg!g. respectively. The predicted consolidation of the uppermost one meter of sediment was about 14 em. which was in good agreement with values obtained from comparisons of moisture content values of pre-eapping and post-capping cores. A thin layer of fresh moderately contaminated sediments has started to develop on the top of the cap. In general. the concentrations of elements were greater in porewater than in the overlying water. e.g.• the concentration of Fe and soluble reactive phosphorus were 1000 times. and those of Mn 100 times greater. There was a significant reduction in the vertical fluxes of all the trace elements after the capping of the contaminated sediments. © 1998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Canada; contaminated sediments; in situ; porewater; remediation; subaqueous capping. INTRODUCTION Contaminated sediments have been identified as a one of the major environmental concerns both in industrialised and developing countries. In recent years several treatment techniques have been used to deal with severely contaminated sediments. No particular method is ideal and problem free for all circumstances. SUbaqueous capping is one of the most recent alternatives used to isolate contaminated sediments from the overlying water and biota. In essence, capping consists in the placement of a layer of clean sand evenly spread over the contaminated sediments. The first capping projects were carried out in the United States (Bokuniewicz et al. 1978) and thereafter several countries, such as Japan (Kikegawa, 1983), Norway (Skei, 1992; Instanes, 1994), and Canada have engaged in capping projects. Presently, capping is considered as one • Present address: Laboratorio Nacional de Engenharia Civil. Departamento de Hidraulica Av. Brasil 101. 1799 Lisboa - Portugal. 323
324
J. M. AZCUE et al.
of several sediment remediation measures in Areas of Concern on the Great Lakes identified by the International Joint Commission. Capping can also be a viable alternative to disposal of contaminated sediments that must be dredged and placed in another location. Among the advantages of the capping technique are its significantly lower costs relative to other forms of handling contaminated sediments; its suitability for organic and inorganic contaminants; and a low potential for environmental adverse effects, which is normally intrinsic to in situ remediation techniques. In the summer of 1995 Environment Canada carried out a pilot-size demonstration project of capping contaminated sediments in Hamilton Harbour, Lake Ontario. The sediments from the selected site (tOO m x 100m) exceeded the Ontario Ministry of Environmental and Energy (OMEE) sediment quality guidelines at the severe effect level for Zn, Cu, Pb, Cr, Ni, Cd, As and Hg. For example, the concentrations of Zn and Cr reached values of 6000 and 300 ~g1g, respectively. The in situ capping is a relatively new technique and therefore, there is a definitive need for field information, particularly on sediment and porewater geochemistry. The objectives of this study were to assess the long term mobility of trace elements through the cap material and the physical stability of the cap. Sediment porewater is the medium for migration of different elements within the sediments and plays an important role in the linking of bottom sediments and overlying lake water. Preliminary results of monitoring the migration of different elements and compounds at the demonstration site after one full year of cap placement are discussed in this paper. MATERIALS AND METHODS Study area The capping demonstration project was carried out in Hamilton Harbour, Lake Ontario. The harbour has been identified as one of the Great Lakes Areas of Concern by the U.S.-Canada International Joint Commission. The sediments at the selected site exceeds the OMEE sediment quality guidelines at the severe effects levels for trace element and organic contaminants. Bottom sediments at the site consist of about 30 cm of very soft (undrained shear strength su<12.5 KPa) black silty clay underlain by very soft greyish brown silt and clay. Physical properties of the sediments and the extent of their contamination were discussed elsewhere (Rukavina and Versteeg, 1996; Versteeg et aI., 1996). A detailed side-scan sonar survey of the Hamilton Harbour showed that the surficial sediment of the selected capping site had a limited disturbance by human activities, such as anchor scours, dredging and dumping. The average water depth is IS m and maximum slope of the site is about 1.9'. Several factors, such as bottom currents, ship traffic, and wave• induced shear stress were considered in the selection of the site (Zeman, 1994). Results of the pre-capping geotechnical and sedimentological testing were discussed by Zeman et al. (1995). Cat!t!in~
technique
Precise placement of capping material with minimum sediment disturbance and mixing is one of the critical factors for capping projects. The cap layer consisted of clean medium coarse sand, with an average grain size diameter of 0.5 mm. The spreader barge used for the transport and application of sand could hold up to about 400 tonnes of sand. A custom-designed hopper and tremie tube system distributed the sand. A 3-m wide hopper fixed on the end of the barge distributed the sand through a series of twenty 130-mm diameter and approximately 12-m long tremie tubes. A generator driven pump supplied harbour water through the tremie tubes to prevent plugging or bridging of the sand. The system was calibrated to deliver approximately 0.75 tonnes of sand per minute. The combined three layers of applied sand produced a total cap thickness of approximately 35 cm. Horizontal and vertical positioning of the barge was controlled using Differential Global Positioning System (DGPS), with an accuracy of ±a. I m. It took approximately 20 working days to complete the capping of the I ha site. Before and after capping operations, surveys of the cap site for sediment texture were undertaken using a ROXANNTM acoustic sea-bed classification system (Rukavina and Cadden, 1997). Before cap placement, divers installed two cables on the harbour bottom inside the cap site, to be used as the diver guides. Eight settlement gauges were installed to measure both short and long term
Subaqueous capping of contaminated sediments
32S
consolidation. FolIowing completion of the cap, divers also made underwater observations of cap coverage and thickness. Samplin~
and analytical methods
Sediment cores were colIected using a modified (Kajak-Brinkhurst) c6rer with a 6.6 cm plastic liner (Mudroch and Azcue, 1995). In addition several cores were colIected by vibracorers, which penetrated the sand cap. After retrieval, the cores were kept upright, sealed and stored at 4'C until analysis. AlI cores were X-rayed for sediment structure before being subsampled into 2-cm sections using an hydraulic extruder. Water content (oven drying at IIO'C) and the concentration of organic matter (loss on ignition at 450'C) were determined in all sediment samples. Particle density (g1cm 3) was determined with a Multivolume Pycnometer 1305. Particle size distribution was determined by the sieve and sedigraph method (Duncan and Lahaie, 1979). Undrained shear strength was determined by the fall-cone test (Hansbo, 1957). The quantitative determination of major and trace elements in sediments was carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Dialysis samplers or "peepers" (Hesslein, 1976), were used to collect in situ sediment porewater at I cm depth intervals. Each peeper contained 100 compartments which were filled with oxygen-free deionized, double distilled water. The open side of the compartments was covered with a 0.45 )lm cellulose membrane (Gelman Scientific, Inc.). The peepers were assembled following standard procedures recommended by Rosa and Azcue (1993) and kept in oxygen free double distilled water until they were vertically deployed by divers. Upon retrieval the peepers were quickly rinsed with lake water to dislodge adhering sediment particles. The porewater from each compartment was immediately removed using disposable syringes. The samples were stored in polystyrene vials and acidified with Ultrapure Seastar concentrated HN0 3 to pH<2 and stored at 4'C prior to analysis. Concentrations of trace elements in pore water samples were measured by direct injection of acidified pore water solution in a graphite furnace atomic absorption spectrophotometer equipped with background corrector. The calibration standards consisted of mixed solutions of high purity elements in 2% HN0 3. Reagent blanks and certified reference materials of the National Institute of Standards and Technology were analysed simultaneously with all samples. RESULTS AND DISCUSSION Temporal and spatial variations of the concentration of dissolved oxygen is common in Hamilton Harbour. The concentration of dissolved oxygen rapidly decreases from saturation in early spring to approximately 7 mgll in the epilimnion and to 1-2 mgll in the hypolimnion during summer. Water quality monitoring carried out during the cap placement indicated turbidity plumes around the demonstration site. The data obtained with an Acoustic Doppler Current Profiler and water sampling at the site during cap placement showed that the suspended material was almost entirely composed of particles associated with the capping material. After one year of placement the cap thickness ranged between 27 and 47 em, with an average thickness of 35 em. Sedjments The sediment composition varied considerably with depth. However, some general trends were observed. In the surficial 10 cm, silt and clay content was found to be approximately equal. Silt content in the cores ranged between 23 and 60%, while clay content ranged between 31 and 71 %. GeneralIy sand content was less than 20% within each core, although discontinuous sand pockets were found at different depths in several sediment cores. The average content of organic matter decreased from 21 % at the sediment surface to 9% at 40 cm depth. The sediments from the study site were of very soft consistency with average undrained shear strength (su) below 5 KPa. AlI cores had Su values below I KPa above 30 cm depth. More geotechnical and sedimentological information can be found in Zeman et al. (1995). A comparison of water content profiles in sediment cores collected before and after capping indicated approximately 14 cm consolidation of the uppermost one meter of the sediment. This observation is in good agreement with predictions obtained from consolidation tests (Zeman and Patterson, 1995).
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Subaqueous capping ofcontaminated sediments
327
The concentrations of major and trace elements in the original sediment (before capping) are given in Table I. Major ele~ents, i.e.: F~,. Si, AI, and Ca represent the geochemical character of the sediments. One year after the cappmg, no slgmflcant changes were observed in the total concentrations of these elements in the sediment profile, with the exception of Ca. The concentration of CaO decreased from 9.2% at the sediment surface to 3.6% at 30 cm sediment depth. Similar trend in CaO concentrations has been observed in Lake Ontario fine-grained sediments (Mudroch unpublished data). The concentration of the major elements fluctuates around a general uniform value in the sediment profile suggesting that they have not been significantly affected by anthropogenic inputs. The greatest concentrations of trace elements were found between 6 and II cm sediment depth, with subsequent decrease of concentrations towards the sediment• water interface. Considering a sedimentation rate of about 3 mm/y, the greatest inputs of the trace elements occurred between 1957 and 1977. However, the concentration of some trace elements remained elevated in the surficial sediments, for example up to 2500 Jlglg of Zn. Two main observations can be made from the analyses of sediment cores collected one year after the cap placement. First, a significant consolidation of the capped sediments resulted in changes of the location and thickness of the layer with the greatest concentrations of the trace elements (i.e., thickness approximately 4 to 14 cm in the original sediments, reduced to about 4 to 5 cm in the capped sediment). This finding is in general agreement with the decrease in the water content values referred to in the preceding text. Second, a layer (l to 3 cm thick) of contaminated sediments has started to develop on the top of the capping material. Mercury in the capping material is <5 nglg. However, in some cores mercury concentrations were up to 560 nglg in the surficial 1 cm. The average concentration of Fe in the sand material was approximately 1.5%. However, concentrations up to 9.7% were observed in the surface 1 cm of all collected sediment cores. It appears that these elevated concentrations originated from fine-grained sediments recently deposited on the sediment cap. Porewater The concentration profiles of major and trace elements in porewater reflect the physico-chemical conditions during the sampling period. The porewater data presented in this manuscript correspond to samples collected at two stations (capped and uncapped sediments). In general, the concentrations of the determined elements in the porewater were considerably greater than those in the overlying lake water. The vertical profile of oxygen-sensitive elements, such as Fe, Mn, and S04' indicated anoxic conditions below the surface 4 cm of capping material. The concentrations of Fe and soluble reactive phosphorus were 1000 times, and those of Mn 100 times greater than those in the overlying water (Table 2). The profiles of total dissolved concentrations of trace elements in the porewater of uncapped sediment are characterised by maxima below the sediment-water interface and decreasing below the zone of reduced sediments. The concentration maxima were always located within the top 3 cm of the sediment, which corresponded to the oxic layer. The concentration profile of Fe in the porewater indicated remobilization of soluble Fe in the anoxic sediments with subsequent precipitation in the oxic sediment-water interface. Table 2. Average concentrations (mg/l) of different elements in porewater collected in June 1996 Fe Lake water (+2em) Interface (-2cm) Cap (-IOcm) Cap (-2Ocm) Sediment (-30 em) Sediment -60em
Uncapped sediments Mn S04 SRP
0.05 18.5
0.03 5.8
88 33
0.002 1.7
24.8 17.2
4.3 4.4
25 5
1.5 1.8
Fe 0.05 1.8 15.9 35 29.6 17.7
Capped sediments Mn SRP S04 0.03 2.9 6.2 4.4 4.8 4.9
65 23 8.3 15 1.5 1.4
0.002 0.4 0.2 0.7 1.3 1.3
The vertical diffusion fluxes of trace elements from the bottom sediments was calculated in order to evaluate the efficiency of the capping material in preventing upward flux. The dissolved gradient across the sediment-water interface and Fick's first law in one dimension (F -0Ds(dCldZ)Z=Q were used to estimate
=
328
J. M. AZCUE et al.
the vertical fluxes (Lerman, 1988). During the summer time, when the oxygen levels in the hypolimnion were very low, the porewater concentrations showed bi-directional gradients. In the anoxic zone of the sediments the trace elements were transported deeper into the sediments and above the region of high concentrations they were also migrating across the sediment-water interface into the overlying water column. The calculated upward diffusion fluxes for some of the trace elements are presented in Table 3. The calculated fluxes ranged from 796 ~glcm2.y for Si02 in the uncapped sediments to II ~glcm2.y for soluble reactive phosphorous in the capped sediments. Significant reduction was observed in the vertical fluxes of all the trace elements after the capping of the contaminated sediments. However, these estimates represent the physico-chemical conditions during the sampling period. Considerable differences in the diffusion fluxes due to seasonal conditions have been reported in other environments (Azcue et al., 1994). The calculated fluxes show that under reducing conditions redox-sensitive elements are mobile and the upwardly diffusing elements are trapped in the oxic zone near capping-water interface. The seasonal changes in the benthic fluxes due to periods of oxic and anoxic conditions in the investigated site are under investigation. Table 3. Average calculated upward dissolved fluxes (~glcm2.y) for different elements Fe Mn Zn
Uncapped sediments 418 174 224 605
Sicn
796
cr SRP
61
Capped sediments 68 87 30 434 II 473
CONCLUSIONS The preliminary results presented here are part of a large multidisciplinary project carried out in Hamilton Harbour, Lake Ontario, Canada. The consolidation of the uppermost one meter of sediment of about 14 cm has altered the location of the layer of greatest concentrations of trace elements in the sediments. The resuspension of surrounding contaminated sediments or deposition of new suspended particles can be considered critical factors after the placement of the capping layer. In general, the concentrations of different elements in sediment porewater were considerably greater than those in the overlying lake water. The oxygen-sensitive elements, such as Fe and Mn, are characterised by remobilization in the anoxic sediments and precipitation in the oxic interface. Significant reductions in the fluxes (up to 80%) were observed after capping of the contaminated sediments. ACKNOWLEDGEMENTS The capping demonstration project in Hamilton Harbour is sponsored by the Great Lakes Cleanup Fund of Environment Canada. The contractor in charge of the cap placement was Greenwood Environmental Inc. of Niagara Falls, Ontario. Field monitoring was mostly carried out by the Technical Operations Section of the National Water Research Institute. We also thank Mr. J. Rajkumar for the analytical support conducting sediment and porewater analyses. REFERENCES Azcue, J. M, Nriagu, J. M. and Schiff, S. (1994). Role of sediment pore water in the cycling of arsenic in a mine polluted lake. Environ. Intern., 20, 517-527. Bokuniewicz, H. J., Gebert. J., Gordon, R. B., Higgins, J. L.• Kaminsky. P., Pilbeam. C. C., Reed. M. and Tunle, C. (1978). Field Study of the Mechanics of the Placement of Dredged Material at Open-water Disposal Sites. Report 0-78-7. US Army Engineer Waterways Experiment Station, Vicksburg. Miss. Duncan, G. A. and LaHaie. G. G. (1979). Size analysis procedures used in the sedimentology laboratory. Hydraulics Division Manual, NWRI. CCIW.
Subaqueous capping of contaminated sediments
329
Hansbo, S. (1957). A new approach to the determination of the shear strength of clay by the fall-cone test. Proc. Royal Swedish Geotech. Insr., No 14, 46 pp. Hesslein, R. (1976). An in situ sampler for close interval pore water studies. Limnol. Oceanogr.• 21, 912-914. Instanes, D. (1994). Pollution control of a Norwegian fjord by use of geotextiles. 5th Int. Conf. Geotextiles, Geomembranes and related products. Singapore. 5-9 September, 1053-1056. Kikegawa, K. (1983). Sand overlying for bottom sediment improvement by sand spreader. Proc. 7th US/Japan Experts Meeting. US Army Engineer Waterways Experiment Station, Vicksburg, MS, pp. 79-103. Lerman. A. (1988). Geochemical Processes· Water and Sediment Environments. Robert E. Kriger Publishing Company. Florida. Mudroch. A. and Azcue, J. M. (1995). Manual ofAquatic Sediment Sampling. Lewis Publisher, 238 pp. Rosa, F. and Azcue. J. M. (1993). Peeper Methodology - detailed procedure from Field Experience. National Water Research Institute Contribution No 93-33. Rukavina. N. A. and Versteeg, J. K. (1996). Surficial sediments of Hamilton Harbour: physical properties and basin morphology. Waf. Qual. Res. J. Can .• 31, 529-551. Rukavina. N. A. and Caddell. S. (1997). Application of an acoustic sea-bed classification system to environmental research and remediation in Canada. Conf. Remote Sensing for Marine Coastal Envir., Orlando, 1-317 Skei. J. M. (1992). A review of assessment and remediation strategies for hot spot sediments. Hydrobiologia, 2351236,629-638. Versteeg, J. K., Morris, W. A. and Rukavina, N. A. (1996). Mapping contaminated sediments in Hamilton Harbour. Geoscience Canada, 22(4). 145-151. Zeman. A. J. (1994). Subaqueous capping ofvery soft contaminated sediments. Can. Geotech. J., 31. 57()"577. Zeman. A. J. and Patterson, T. S. (1995). Evaluation of primary and secondary consolidation of Hamilton Harbour and Lake Ontario sediments due to in situ capping. National water Research Institute Contribution 95-02. Zeman, A. J.• Patterson, T. S., Mudroch. A., Rosa, F., Reynoldson. T. B. and Day, K. E. (1995). Results of baseline geotechnical. chemical and biological tests for a proposed in situ sediment capping site in Hamilton Harbour. National Water Research Institute Contribution 95-03.
Pergamon
Wal. Sci. Tech. Vol. 37, No. 6-7. pp. 323-329,1998. @ 1998 IA WQ. Published by Elsevier Science Ud Printed m Great Britam.
PH: S0273-1223(98)00214-5
0273-1223/98 $19'00 + 0-00
ASSESSMENT OF SEDIMENT AND POREWATER AFTER ONE YEAR OF SUBAQUEOUS CAPPING OF CONTAMINATED SEDIMENTS IN HAMILTON HARBOUR, CANADA Jose M. Azcue*, Alex 1. Zeman, Alena Mudroch, Fernando Rosa and Tim Patterson National Water Research Institute. P.O. Box 5050. Burlington. Ontario. Canada L7R 4A6
ABSTRACf In this manuscript, we present data from a demonstration in situ capping site (100m x 100m) in Hamilton Harbour. Lake Ontario. Canada. A layer of clean medium to coarse sand Wtth the average thickness of 35 cm was placed at the site in the summer of 1995. Concentration of Zn. Cr, and Cd in the original sediments reached values over 6000. 300 and 15 Ilg!g. respectively. The predicted consolidation of the uppermost one meter of sediment was about 14 em. which was in good agreement with values obtained from comparisons of moisture content values of pre-eapping and post-capping cores. A thin layer of fresh moderately contaminated sediments has started to develop on the top of the cap. In general. the concentrations of elements were greater in porewater than in the overlying water. e.g.• the concentration of Fe and soluble reactive phosphorus were 1000 times. and those of Mn 100 times greater. There was a significant reduction in the vertical fluxes of all the trace elements after the capping of the contaminated sediments. © 1998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Canada; contaminated sediments; in situ; porewater; remediation; subaqueous capping. INTRODUCTION Contaminated sediments have been identified as a one of the major environmental concerns both in industrialised and developing countries. In recent years several treatment techniques have been used to deal with severely contaminated sediments. No particular method is ideal and problem free for all circumstances. SUbaqueous capping is one of the most recent alternatives used to isolate contaminated sediments from the overlying water and biota. In essence, capping consists in the placement of a layer of clean sand evenly spread over the contaminated sediments. The first capping projects were carried out in the United States (Bokuniewicz et al. 1978) and thereafter several countries, such as Japan (Kikegawa, 1983), Norway (Skei, 1992; Instanes, 1994), and Canada have engaged in capping projects. Presently, capping is considered as one • Present address: Laboratorio Nacional de Engenharia Civil. Departamento de Hidraulica Av. Brasil 101. 1799 Lisboa - Portugal. 323
324
J. M. AZCUE et al.
of several sediment remediation measures in Areas of Concern on the Great Lakes identified by the International Joint Commission. Capping can also be a viable alternative to disposal of contaminated sediments that must be dredged and placed in another location. Among the advantages of the capping technique are its significantly lower costs relative to other forms of handling contaminated sediments; its suitability for organic and inorganic contaminants; and a low potential for environmental adverse effects, which is normally intrinsic to in situ remediation techniques. In the summer of 1995 Environment Canada carried out a pilot-size demonstration project of capping contaminated sediments in Hamilton Harbour, Lake Ontario. The sediments from the selected site (tOO m x 100m) exceeded the Ontario Ministry of Environmental and Energy (OMEE) sediment quality guidelines at the severe effect level for Zn, Cu, Pb, Cr, Ni, Cd, As and Hg. For example, the concentrations of Zn and Cr reached values of 6000 and 300 ~g1g, respectively. The in situ capping is a relatively new technique and therefore, there is a definitive need for field information, particularly on sediment and porewater geochemistry. The objectives of this study were to assess the long term mobility of trace elements through the cap material and the physical stability of the cap. Sediment porewater is the medium for migration of different elements within the sediments and plays an important role in the linking of bottom sediments and overlying lake water. Preliminary results of monitoring the migration of different elements and compounds at the demonstration site after one full year of cap placement are discussed in this paper. MATERIALS AND METHODS Study area The capping demonstration project was carried out in Hamilton Harbour, Lake Ontario. The harbour has been identified as one of the Great Lakes Areas of Concern by the U.S.-Canada International Joint Commission. The sediments at the selected site exceeds the OMEE sediment quality guidelines at the severe effects levels for trace element and organic contaminants. Bottom sediments at the site consist of about 30 cm of very soft (undrained shear strength su<12.5 KPa) black silty clay underlain by very soft greyish brown silt and clay. Physical properties of the sediments and the extent of their contamination were discussed elsewhere (Rukavina and Versteeg, 1996; Versteeg et aI., 1996). A detailed side-scan sonar survey of the Hamilton Harbour showed that the surficial sediment of the selected capping site had a limited disturbance by human activities, such as anchor scours, dredging and dumping. The average water depth is IS m and maximum slope of the site is about 1.9'. Several factors, such as bottom currents, ship traffic, and wave• induced shear stress were considered in the selection of the site (Zeman, 1994). Results of the pre-capping geotechnical and sedimentological testing were discussed by Zeman et al. (1995). Cat!t!in~
technique
Precise placement of capping material with minimum sediment disturbance and mixing is one of the critical factors for capping projects. The cap layer consisted of clean medium coarse sand, with an average grain size diameter of 0.5 mm. The spreader barge used for the transport and application of sand could hold up to about 400 tonnes of sand. A custom-designed hopper and tremie tube system distributed the sand. A 3-m wide hopper fixed on the end of the barge distributed the sand through a series of twenty 130-mm diameter and approximately 12-m long tremie tubes. A generator driven pump supplied harbour water through the tremie tubes to prevent plugging or bridging of the sand. The system was calibrated to deliver approximately 0.75 tonnes of sand per minute. The combined three layers of applied sand produced a total cap thickness of approximately 35 cm. Horizontal and vertical positioning of the barge was controlled using Differential Global Positioning System (DGPS), with an accuracy of ±a. I m. It took approximately 20 working days to complete the capping of the I ha site. Before and after capping operations, surveys of the cap site for sediment texture were undertaken using a ROXANNTM acoustic sea-bed classification system (Rukavina and Cadden, 1997). Before cap placement, divers installed two cables on the harbour bottom inside the cap site, to be used as the diver guides. Eight settlement gauges were installed to measure both short and long term
Subaqueous capping of contaminated sediments
32S
consolidation. FolIowing completion of the cap, divers also made underwater observations of cap coverage and thickness. Samplin~
and analytical methods
Sediment cores were colIected using a modified (Kajak-Brinkhurst) c6rer with a 6.6 cm plastic liner (Mudroch and Azcue, 1995). In addition several cores were colIected by vibracorers, which penetrated the sand cap. After retrieval, the cores were kept upright, sealed and stored at 4'C until analysis. AlI cores were X-rayed for sediment structure before being subsampled into 2-cm sections using an hydraulic extruder. Water content (oven drying at IIO'C) and the concentration of organic matter (loss on ignition at 450'C) were determined in all sediment samples. Particle density (g1cm 3) was determined with a Multivolume Pycnometer 1305. Particle size distribution was determined by the sieve and sedigraph method (Duncan and Lahaie, 1979). Undrained shear strength was determined by the fall-cone test (Hansbo, 1957). The quantitative determination of major and trace elements in sediments was carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Dialysis samplers or "peepers" (Hesslein, 1976), were used to collect in situ sediment porewater at I cm depth intervals. Each peeper contained 100 compartments which were filled with oxygen-free deionized, double distilled water. The open side of the compartments was covered with a 0.45 )lm cellulose membrane (Gelman Scientific, Inc.). The peepers were assembled following standard procedures recommended by Rosa and Azcue (1993) and kept in oxygen free double distilled water until they were vertically deployed by divers. Upon retrieval the peepers were quickly rinsed with lake water to dislodge adhering sediment particles. The porewater from each compartment was immediately removed using disposable syringes. The samples were stored in polystyrene vials and acidified with Ultrapure Seastar concentrated HN0 3 to pH<2 and stored at 4'C prior to analysis. Concentrations of trace elements in pore water samples were measured by direct injection of acidified pore water solution in a graphite furnace atomic absorption spectrophotometer equipped with background corrector. The calibration standards consisted of mixed solutions of high purity elements in 2% HN0 3. Reagent blanks and certified reference materials of the National Institute of Standards and Technology were analysed simultaneously with all samples. RESULTS AND DISCUSSION Temporal and spatial variations of the concentration of dissolved oxygen is common in Hamilton Harbour. The concentration of dissolved oxygen rapidly decreases from saturation in early spring to approximately 7 mgll in the epilimnion and to 1-2 mgll in the hypolimnion during summer. Water quality monitoring carried out during the cap placement indicated turbidity plumes around the demonstration site. The data obtained with an Acoustic Doppler Current Profiler and water sampling at the site during cap placement showed that the suspended material was almost entirely composed of particles associated with the capping material. After one year of placement the cap thickness ranged between 27 and 47 em, with an average thickness of 35 em. Sedjments The sediment composition varied considerably with depth. However, some general trends were observed. In the surficial 10 cm, silt and clay content was found to be approximately equal. Silt content in the cores ranged between 23 and 60%, while clay content ranged between 31 and 71 %. GeneralIy sand content was less than 20% within each core, although discontinuous sand pockets were found at different depths in several sediment cores. The average content of organic matter decreased from 21 % at the sediment surface to 9% at 40 cm depth. The sediments from the study site were of very soft consistency with average undrained shear strength (su) below 5 KPa. AlI cores had Su values below I KPa above 30 cm depth. More geotechnical and sedimentological information can be found in Zeman et al. (1995). A comparison of water content profiles in sediment cores collected before and after capping indicated approximately 14 cm consolidation of the uppermost one meter of the sediment. This observation is in good agreement with predictions obtained from consolidation tests (Zeman and Patterson, 1995).
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Zn
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Pb
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0.43 0.54 0.46 0.49 0.52 0.63 0.80 0.91 0.96 0.96 0.79 0.62 0.65 0.56 0.53 0.51 0.49 0.52 0.53 0.56 0.53 0.56 0.61 0.58 0.54 0.49 0.49 0.44 0.39
163 163 167 179 196 232 295 320 328 306 249 213 211 159 146 129 129 110 95 86 78 68 65 62 60 55 53 50 49
61 63 63 66 70 77 85 87 87 85 80 77 78 75 73 68 70 68 64 62 58 54 54 53 49 47 45 41 42
HI H2 HI H6 135 138 155 157 157 147 136 131 136 126 127 H9 123 118 119 116 109 103 105 105 93 84 81 72 67
2476 2446 2508 2791 3272 4023 5183 5496 5477 5040 3983 3338 3201 2516 2356 1960 1922 1656 1582 1527 1436 1333 1261 1121 956 818 669 593 571
6,1 6,2 6,6 7,0 8,2 9,4 12,9 14,0 14,7 14,6 13,5 13,0 14,4 12,2 12,0 11,2 11,5 10,7 10,7 9,8 8,7 7,7 7,2 6,1 5,3 4,4 3,4 2,8 2,5
158 161 164 181 198 205 200 180 157 147 116 89 86 72 79 61 65 54 56 36 38 35 33 38 27 21 20 25 13
291 290 296 324 359 422 531 574 590 563 478 420 409 332 327 271 267 232 226 214 204 188 175 161 144 127 112 102 99
13 19 17 20 21 27 29 32 33 32 28 30 28 26 29 25 26 28 23 24 21 19 20 18 17 12 15 II 13
41,4 41,1 40,7 40,6 40,6 40,9 41,1 41,5 41,7 42,0 43,1 44,4 44,3 45,7 45,4 46,4 46,5 47,4 47,4 47,4 48,4 49,2 48,7 48,1 48,9 49,S 49,S 49,2 50,S
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9,2 9,4 9,5 9,5 9,0 8,2 6,3 4,9 4,5 4,6 5,0 5,4 5,2 5,3 5,5 5,1 4,6 4,5 4,5 4,9 4,8 4,5 4,2 4,1 4,1 4,1 4,2 4,4 3,8
1,0 1,0 1,1 1,1 1,2 1,4 1,5 1,4 1,5 1,5 1,3 1,2 1,1 1,1 1,0 0,8 0,8 0,8 0,6 0,6 0,6 0,5 0,5 0,5 0,5 0,4 0,4 0,4 0,3
21,3 21,3 21,4 21,3 21,2 20,5 19,2 18,4 18,2 18,0 17,3 16,6 16,6 16,0 16,2 16,0 15,6 15,6 15,5 15,6 15,5 15,1 15,6 15,6 15,6 15,4 15,7 15,8 15,1
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Subaqueous capping ofcontaminated sediments
327
The concentrations of major and trace elements in the original sediment (before capping) are given in Table I. Major ele~ents, i.e.: F~,. Si, AI, and Ca represent the geochemical character of the sediments. One year after the cappmg, no slgmflcant changes were observed in the total concentrations of these elements in the sediment profile, with the exception of Ca. The concentration of CaO decreased from 9.2% at the sediment surface to 3.6% at 30 cm sediment depth. Similar trend in CaO concentrations has been observed in Lake Ontario fine-grained sediments (Mudroch unpublished data). The concentration of the major elements fluctuates around a general uniform value in the sediment profile suggesting that they have not been significantly affected by anthropogenic inputs. The greatest concentrations of trace elements were found between 6 and II cm sediment depth, with subsequent decrease of concentrations towards the sediment• water interface. Considering a sedimentation rate of about 3 mm/y, the greatest inputs of the trace elements occurred between 1957 and 1977. However, the concentration of some trace elements remained elevated in the surficial sediments, for example up to 2500 Jlglg of Zn. Two main observations can be made from the analyses of sediment cores collected one year after the cap placement. First, a significant consolidation of the capped sediments resulted in changes of the location and thickness of the layer with the greatest concentrations of the trace elements (i.e., thickness approximately 4 to 14 cm in the original sediments, reduced to about 4 to 5 cm in the capped sediment). This finding is in general agreement with the decrease in the water content values referred to in the preceding text. Second, a layer (l to 3 cm thick) of contaminated sediments has started to develop on the top of the capping material. Mercury in the capping material is <5 nglg. However, in some cores mercury concentrations were up to 560 nglg in the surficial 1 cm. The average concentration of Fe in the sand material was approximately 1.5%. However, concentrations up to 9.7% were observed in the surface 1 cm of all collected sediment cores. It appears that these elevated concentrations originated from fine-grained sediments recently deposited on the sediment cap. Porewater The concentration profiles of major and trace elements in porewater reflect the physico-chemical conditions during the sampling period. The porewater data presented in this manuscript correspond to samples collected at two stations (capped and uncapped sediments). In general, the concentrations of the determined elements in the porewater were considerably greater than those in the overlying lake water. The vertical profile of oxygen-sensitive elements, such as Fe, Mn, and S04' indicated anoxic conditions below the surface 4 cm of capping material. The concentrations of Fe and soluble reactive phosphorus were 1000 times, and those of Mn 100 times greater than those in the overlying water (Table 2). The profiles of total dissolved concentrations of trace elements in the porewater of uncapped sediment are characterised by maxima below the sediment-water interface and decreasing below the zone of reduced sediments. The concentration maxima were always located within the top 3 cm of the sediment, which corresponded to the oxic layer. The concentration profile of Fe in the porewater indicated remobilization of soluble Fe in the anoxic sediments with subsequent precipitation in the oxic sediment-water interface. Table 2. Average concentrations (mg/l) of different elements in porewater collected in June 1996 Fe Lake water (+2em) Interface (-2cm) Cap (-IOcm) Cap (-2Ocm) Sediment (-30 em) Sediment -60em
Uncapped sediments Mn S04 SRP
0.05 18.5
0.03 5.8
88 33
0.002 1.7
24.8 17.2
4.3 4.4
25 5
1.5 1.8
Fe 0.05 1.8 15.9 35 29.6 17.7
Capped sediments Mn SRP S04 0.03 2.9 6.2 4.4 4.8 4.9
65 23 8.3 15 1.5 1.4
0.002 0.4 0.2 0.7 1.3 1.3
The vertical diffusion fluxes of trace elements from the bottom sediments was calculated in order to evaluate the efficiency of the capping material in preventing upward flux. The dissolved gradient across the sediment-water interface and Fick's first law in one dimension (F -0Ds(dCldZ)Z=Q were used to estimate
=
328
J. M. AZCUE et al.
the vertical fluxes (Lerman, 1988). During the summer time, when the oxygen levels in the hypolimnion were very low, the porewater concentrations showed bi-directional gradients. In the anoxic zone of the sediments the trace elements were transported deeper into the sediments and above the region of high concentrations they were also migrating across the sediment-water interface into the overlying water column. The calculated upward diffusion fluxes for some of the trace elements are presented in Table 3. The calculated fluxes ranged from 796 ~glcm2.y for Si02 in the uncapped sediments to II ~glcm2.y for soluble reactive phosphorous in the capped sediments. Significant reduction was observed in the vertical fluxes of all the trace elements after the capping of the contaminated sediments. However, these estimates represent the physico-chemical conditions during the sampling period. Considerable differences in the diffusion fluxes due to seasonal conditions have been reported in other environments (Azcue et al., 1994). The calculated fluxes show that under reducing conditions redox-sensitive elements are mobile and the upwardly diffusing elements are trapped in the oxic zone near capping-water interface. The seasonal changes in the benthic fluxes due to periods of oxic and anoxic conditions in the investigated site are under investigation. Table 3. Average calculated upward dissolved fluxes (~glcm2.y) for different elements Fe Mn Zn
Uncapped sediments 418 174 224 605
Sicn
796
cr SRP
61
Capped sediments 68 87 30 434 II 473
CONCLUSIONS The preliminary results presented here are part of a large multidisciplinary project carried out in Hamilton Harbour, Lake Ontario, Canada. The consolidation of the uppermost one meter of sediment of about 14 cm has altered the location of the layer of greatest concentrations of trace elements in the sediments. The resuspension of surrounding contaminated sediments or deposition of new suspended particles can be considered critical factors after the placement of the capping layer. In general, the concentrations of different elements in sediment porewater were considerably greater than those in the overlying lake water. The oxygen-sensitive elements, such as Fe and Mn, are characterised by remobilization in the anoxic sediments and precipitation in the oxic interface. Significant reductions in the fluxes (up to 80%) were observed after capping of the contaminated sediments. ACKNOWLEDGEMENTS The capping demonstration project in Hamilton Harbour is sponsored by the Great Lakes Cleanup Fund of Environment Canada. The contractor in charge of the cap placement was Greenwood Environmental Inc. of Niagara Falls, Ontario. Field monitoring was mostly carried out by the Technical Operations Section of the National Water Research Institute. We also thank Mr. J. Rajkumar for the analytical support conducting sediment and porewater analyses. REFERENCES Azcue, J. M, Nriagu, J. M. and Schiff, S. (1994). Role of sediment pore water in the cycling of arsenic in a mine polluted lake. Environ. Intern., 20, 517-527. Bokuniewicz, H. J., Gebert. J., Gordon, R. B., Higgins, J. L.• Kaminsky. P., Pilbeam. C. C., Reed. M. and Tunle, C. (1978). Field Study of the Mechanics of the Placement of Dredged Material at Open-water Disposal Sites. Report 0-78-7. US Army Engineer Waterways Experiment Station, Vicksburg. Miss. Duncan, G. A. and LaHaie. G. G. (1979). Size analysis procedures used in the sedimentology laboratory. Hydraulics Division Manual, NWRI. CCIW.
Subaqueous capping of contaminated sediments
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Hansbo, S. (1957). A new approach to the determination of the shear strength of clay by the fall-cone test. Proc. Royal Swedish Geotech. Insr., No 14, 46 pp. Hesslein, R. (1976). An in situ sampler for close interval pore water studies. Limnol. Oceanogr.• 21, 912-914. Instanes, D. (1994). Pollution control of a Norwegian fjord by use of geotextiles. 5th Int. Conf. Geotextiles, Geomembranes and related products. Singapore. 5-9 September, 1053-1056. Kikegawa, K. (1983). Sand overlying for bottom sediment improvement by sand spreader. Proc. 7th US/Japan Experts Meeting. US Army Engineer Waterways Experiment Station, Vicksburg, MS, pp. 79-103. Lerman. A. (1988). Geochemical Processes· Water and Sediment Environments. Robert E. Kriger Publishing Company. Florida. Mudroch. A. and Azcue, J. M. (1995). Manual ofAquatic Sediment Sampling. Lewis Publisher, 238 pp. Rosa, F. and Azcue. J. M. (1993). Peeper Methodology - detailed procedure from Field Experience. National Water Research Institute Contribution No 93-33. Rukavina. N. A. and Versteeg, J. K. (1996). Surficial sediments of Hamilton Harbour: physical properties and basin morphology. Waf. Qual. Res. J. Can .• 31, 529-551. Rukavina. N. A. and Caddell. S. (1997). Application of an acoustic sea-bed classification system to environmental research and remediation in Canada. Conf. Remote Sensing for Marine Coastal Envir., Orlando, 1-317 Skei. J. M. (1992). A review of assessment and remediation strategies for hot spot sediments. Hydrobiologia, 2351236,629-638. Versteeg, J. K., Morris, W. A. and Rukavina, N. A. (1996). Mapping contaminated sediments in Hamilton Harbour. Geoscience Canada, 22(4). 145-151. Zeman. A. J. (1994). Subaqueous capping ofvery soft contaminated sediments. Can. Geotech. J., 31. 57()"577. Zeman. A. J. and Patterson, T. S. (1995). Evaluation of primary and secondary consolidation of Hamilton Harbour and Lake Ontario sediments due to in situ capping. National water Research Institute Contribution 95-02. Zeman, A. J.• Patterson, T. S., Mudroch. A., Rosa, F., Reynoldson. T. B. and Day, K. E. (1995). Results of baseline geotechnical. chemical and biological tests for a proposed in situ sediment capping site in Hamilton Harbour. National Water Research Institute Contribution 95-03.