Environ. Sci. Technol. 1996, 30, 178-187
Dissolved Trace Metals in Lakes Superior, Erie, and Ontario J E R O M E O . N R I A G U , * ,† G R E G L A W S O N , ‡ HENRY K. T. WONG,‡ AND VEN CHEAM‡ Department of Environmental & Industrial Health, School of Public Health, The University of Michigan, Ann Arbor, Michigan 48109, and National Water Research Institute, P.O. Box 5050, Burlington, Ontario L7R 4A8, Canada
In spite of continuing large inputs from anthropogenic sources, average concentrations of many trace metals in the Great Lakes remain quite low: 2.8-4.5 ng L-1 for Cd, 3.2-11 ng L-1 for Pb, and 87-277 ng L-1 for Zn. These metals are rapidly scavenged by the seston (suspended particulates) and have a rapid turnover rate in the water column. Factors that affect the distribution of dissolved trace metals include water depth, seston abundance, and biological processes. Higher concentrations are generally found in nearshore stations and especially near the urban centers and polluted river mouths. In summer, there is a marked depletion of Zn (and other bioactive metals) in the epilimnion of the offshore waters. The patchiness in the concentrations of the metals is attributed to spatial differences in the biological processes. With the exception of Cr, there is no systematic increase in concentration down the drainage basin from Lake Superior to Lake Ontario. Most of the Pb, Cd, and Zn loadings into the lakes are retained in the basin, but there is a significant export of dissolved Cu, Ni, and Cr via the St. Lawrence River.
Introduction The Great Lakes receive large inputs of toxic metals from sources within and outside their watershed (1, 2). Of the 10 states that release the largest quantities of toxic substances into the U.S. environment, five are located in the Great Lakes basin, and atmospheric pollutants released in three more of these states can reach the lakes after just 1 day of travel time (3). With the growing perception that the metals and persistent organic compounds are a threat to the entire Great Lakes ecosystem, the principle of “virtual elimination” of persistent toxic substances from the Lakes was adopted in the Canada-U.S. Great Lakes Water Quality Agreement (4). The governments of Canada and the United States as well as the eight Great Lakes states and the province of Ontario are moving toward this goal, using the instruments of regulation, public education, and pollution prevention strategies (4, 5). In spite of the heightened * Corresponding author fax: 313-764-9424; e-mail address:
[email protected]. † The University of Michigan. ‡ National Water Research Institute.
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concern and pollution prevention programs, very little is currently known about the distribution, chemical behavior, and effects of trace metals in the Great Lakes. The improvements in the collection, handling, and analysis of samples developed by marine chemists have yet to be widely applied to a study of the Lakes. As a consequence, most of the published data are severely compromised by sample contamination and hence are useless for indentifying the fundamental processes and relationships involved in the cycling of the trace metals in the Great Lakes (6, 7). This paper presents a comprehensive set of data on trace metals in the Great Lakes that have been obtained using the ultraclean laboratory methods. The focus of the study has been on processes that control the vertical profiles and the spatial distribution of dissolved trace metals in Lake Superior (one of the Upper Great Lakes) as well as in Lakes Erie and Ontario (the Lower Great Lakes). The paper also provides the current baseline levels for the trace metals, which can later be used in time trend analysis to ascertain the response of the lakes to the virtual elimination program.
Methodology Materials. High-density linear polyethylene (HDLPE) bottles used in collecting the samples were decontaminated by the nine-step procedure described by Nriagu et al. (7). The sequence of key steps in the procedure include degreasing with Versa Clean (Fisher Scientific) soap, washing with acetone, soaking in concentrated HCl, soaking in concentrated HNO3, soaking in warm (40-50 °C) 2 M HNO3, soaking in 0.5% HNO3, and then rinsing thoroughly with Milli-Q water (Millipore Corp., Bedford, MA). Final soaking in acid and rinsing with Milli-Q water was done in a class 100 clean laboratory. Each bottle was filled with Seastar (Sidney, British Columbia) 0.2% HNO3 and individually sealed in acid-washed inner and outer polyethylene bags. Six of the bottles were sealed with a larger polyethylene bag and then stored in a plastic cooler box for transportation to the field. All the filtering and sampling equipment that come into contact with the sample were decontaminated in the same manner as the sample bottles. After cleaning, each unit was triple bagged and then stored in a cooler box. The polycarbonate filter membranes (Nuclepore, 0.45 µM pore size and 95 mm in diameter) were leached with 20% Seastar NHO3 for a least 1 week before use and then left soaking in Milli-Q water until used in the field. Subboiled, doubly distilled nitric and hydrochloric acids (from Seastar) were used to acidify the samples and prepare the standard solutions. The “distilled” water used in the study was obtained using the following demineralization processes: (a) the in-house deionized water (by reverse osmosis) was used as the feed for (b) a quartz still (Corning AG-3 system) to derive the doubly distilled water, which was then used as the feed for (c) the Milli-Q water system. The final Milli-Q water routinely gave the following blank values: Cd < 1.0, Cu < 0.4, Ni < 0.2, Pb < 0.2, and Zn < 0.05 ng L-1. Sample Collection and Analysis. Samples were obtained from Lake Superior from August 29 to September 7, 1991; from Lake Erie during August 16-20, 1993; and from Lake Ontario between May 31 and June 4, 1993. The Lake
0013-936X/96/0930-0178$12.00/0
1995 American Chemical Society
show thermal stratification, samples were obtained at the following depths: mid-epilimnion, thermocline, midhypolimnion, and about 1.0 m from the sediment-water interface. One would expect the vertical metal profiles to be related to thermal stratification. At selected stations with stratification however, samples were obtained every 10 m down the water column. Where there was no thermal stratification, subsurface samples were obtained at the midcolumn and 1.0 m off the bottom. Upon retrieval, the samples were immediately passed through a Teflon in-line filter connected directly to the Go-Flo bottles or to the reservoir into which the surface samples were emptied. Details of the filtering system used have already been described (7). The filtered samples were acidified with 2 mL of Seaster HNO3, which lowered the pH to about 1.0-2.0. Bottles filled with the samples were put back into their individual polyethylene bags and then packed and sealed in a cooler box, which was stored in a cold room. At each station, an electronic hydrolab (bathothermograph) was used to obtain continuous profiles of temperature, transmissivity, and conductivity. The pH of each sample was also measured on board the cruise ship using a combination electrode.
FIGURE 1. Lakes Superior, Erie, and Ontario showing the sample stations.
Ontario stations were resampled during October 4-8, 1993. The sampling stations used in the study (Figure 1) are those that have been monitored for years by Environment Canada. Data on the basic chemistry of the water are thus available for each location. The sampling cruises were made using the CS Limnos of Environment Canada, Burlington, Ontario. Surface samples were collected from a rubber raft rowed at least 200 m upwind of the mother ship (CS Limnos). Gloved hands were used to open, fill, and close the sample bottles under water. Subsurface samples were obtained by means of 5-L Go-Flo bottles (General Oceanics, Miami, Florida) attached to a Kevlar line and trigggered with a Teflon-coated messenger (7). For most of the stations that
Samples were analyzed for lead using a custom-built laser-excited atomic fluorescence spectrometer (8). Since the detection limit for lead on the instrument was < 0.1 ng L-1, all the samples were analyzed without preconcentration. All the other metals were analyzed using a graphite furnace atomic absorption spectrometer (Varian Spectra AA-400) equipped with a Zeeman background corrector. Multiple injections and a standard addition method were used where the metal concentrations were too low to be detected by single sample injection into the graphite tube (9).
Results Field or procedural blanks were used to estimate the amount of each metal introduced to the samples from the reagents and storage containers and during the handling and analysis of the samples. The blanks were aliquots of Milli-Q water that were filtered, stored, processed, and analyzed in exactly the same way as the samples. The blanks were prepared usually in triplicate at every other station. Average concentrations of the trace metals in field blanks during the various cruises are shown in Table 1. The blank values were subtracted from measured concentrations during each cruise. With the exception of Pb, the field blanks were generally 1000 ng L-1) are found in nearhore stations (nos. 43 and 18). Since high concentrations minimize the effects of sample contamination, it is not surprising that the current results for Lake Erie are in reasonable agreement with previously reported mean levels: 680 ng L-1 by Coale and Flegal (10), 700 ng L-1 by Rossmann and Barres (11), and 550 ng L-1 by Lum and Leslie (12). Spatial differences in dissolved Cu concentrations are more marked in Lake Ontario especially during the May/June cruise (Table 3). The highest concentrations are found in nearshore waters off Buffalo, NY (station 21), Rochester, NY (station 87), and Kingston, Ontario (station
TABLE 5
Dissolved Trace Metal Concentartions in Lake Superior, Aug 29-Sep 7, 1991 station LS-2 LS-12 LS-25
LS-102
LS-127
LS-171 LS-201
LS-189 LS-80
LS-43 LS-22
depth (m)
Fe (ng/L)
Mn (ng/L)
Zn (ng/L)
Cu (ng/L)
Cr (ng/L)
Pb (ng/L)
temp (°C)
transm (%)
conduct (µS)
pH
12 23 70 12 80 120 10 45 90 180 10 20 65 110 150 12 18 32 80 160 240 12 80 130 14 26 70 140 174 6 15 80 2 10 20 50 140 200 251 21 80 160 10 21 32 96 150
1,524 806 989 609 488 345 548 459 358 399 188 333 229 272 773 581 709 215 317 278 489 598 295 369 854 596 481 362 641 643 449 303 98 64 82 123 162 117 175 377 119 264 52 67 48 123 36
327 152 52 243 57 33 62 14 23 29 44 62 19 9 43 89 95 19 54 26 42 202 16 28 15 17 8 18 11 117 52 25 93 83 41 28 25 15 26 105 19 26 84 45 15 5 12
867 446 456 336 323 269 208 239 276 318 144 212 289 265 374 196 157 245 264 254 335 262 293 328 169 268 244 255 234 192 184 303 176 182 183 259 258 263 281 558 278 284 145 179 203 347 229
771 789 741 774 791 791 692 629 711 732 706 768 755 748 832 775 695 673 729 776 782 752 743 769 739 769 749 763 787 655 743 769 809 789 796 828 725 678 789 813 749 807 776 713 789 834 749
60 56 39 62 51 25 38 38 34 32 80 26 28 30 40 97 71 54 63 66 94 68 79 66 53 34 43 29 49 63 70 86 49 65 61 55 43 43 45 77 61 65 64 79 74 88 82
25.36 17.12 4.77 9.04 3.14 2.66 3.66 2.63 1.62 2.72 1.04 3.32 0.31 0.87 3.52 2.75 2.33 1.84 0.57 1.19 1.25 3.07 2.14 1.12 2.02 4.08 1.88 1.16
21 18 3.8 15 3.9 3.9 16 5.3 3.9 3.8 14 14 6.8 3.9 3.8 14 6.4 4.6 3.9 3.9 3.6 14 3.7 3.6 16 4.2 3.8 3.7 3.7 19 6.5 3.8 15 14 8.5 4.9 3.8 3.6 3.5 8.5 3.8 3.7 14 9.3 4.5 3.7 3.6
90 91 91 90 92 92 88 89 92 92 87 87 82 92 91 86 86 82 91 92 92 88 91 88 82 85 89 92 89 90 90 92 89 88 85 93 94 94 93 86 93 94 88 84 91 93 93
98 97 101 96 104 103 96 100 103 103 97 98 100 102 103 100 101 101 102 103 103 95 103 103 99 103 103 103 103 95 100 102 96 97 99 102 103 102 102 97 103 100 97 98 103 102 103
7.59 7.78 7.33 7.5 7.71 7.13 7.72 7.69 7.41 7.35 7.71 7.86 7.95 7.49 7.47 7.86 7.88 7.67 7.53 7.53 7.46 7.83 7.4 7.43 7.98 7.64 7.49 7.49 7.46 7.86 7.84 7.55 7.75 7.87 7.98 7.99 7.67 7.54 7.47 7.78 7.56 7.54 7.34 7.33 7.28 7.35 7.33
84). The lowest concentrations are found in the offshore waters at stations 33 (
