Environ. Sci. Technol. 1994, 28, 1691-1697
Transport of Detroit River Pollutants from Lake Erie by Episodic Resuspension Events Michael J. Howdeshell and Ronald A. Hltes’
School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana The transport of contaminated sediment out of Lake Erie is an important process controlling the fate of potentially toxic organic compounds in the lower Great Lakes. This transport was investigated by electron capture gas chromatographic mass spectrometry analysis of 2,4-di-tertpentylphenol(24DP) and 2-chloro-4,6-di-tert-pentylpheno1 (24DP6Cl)derivatized with pentafluorobenzylbromide (PFBBr) to give PFB ether derivatives. 24DP and 24DP6C1 are unusual sediment-bound pollutants originating from a single point source on the Trenton Channel of the Detroit River. The concentrations of these compounds were determined in suspended solids collected between 1980 and 1993 in the Niagara River. Loads were found to be proportional to the third power of 24-h averagedwind velocities. We suggestthat the phenol loads may be controlled by episodic resuspension of near-shore sediments in Lake Erie’s eastern basin. The model suggests that about 2 % of Detroit River sediment-bound pollutants is transported to Lake Ontario annually. Introduction The Laurentian Great Lakes are an important natural resource. They cover 250 000 km2and contain about 95 % of the surface water in the United States. About 10% of Americans get their drinking water from the lakes, and about 40 % of the United States’ industry is located in the area (1). Despite their vastness, the Great Lakes are sensitive to the effects of anthropogenic pollutants. Sources of pollution include runoff of farm chemicals, discharge of city sewage,dumping of industrial waste, and deposition from the atmosphere. In order to understand man’s impact on the Great Lakes, one must be able to quantitatively predict the transport and fate of anthropogenic pollutants throughout the system. Of the five Great Lakes, Lake Erie has had the most significant environmental problems. Although Lake Erie accounts for only 2 % of the volume and 10% of the surface areaof the Great Lakes, its basin is populated by 12 million people, almost 40 % of the total living in the Great Lakes basin (2). This combination of limited water supply and high anthropogenic activity overwhelmed the lake, and during the late 1960s,the lake suffered substantial damage as a result of oxygen depletion. Although the general water quality of Lake Erie has improved in recent years (because of reduced nutrient loadings and a subsequent increase in the dissolved oxygen level), inputs of toxic organic compounds have not. In fact, the influx of these persistent pollutants is now one of the most serious problems confronting the lake. Lake Erie is the shallowest of the Great Lakes; its average depth is only 19 m. The lake is oriented southwest to northeast, and it is approximately 390 km long and 80 km wide near the midpoint of its axis. It is divided into three
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0 1994 American Chemical Society
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basins, conveniently called the western, central, and eastern basins. The western basin is shallow and flat, with an average depth of only 7.3 m. The central basin has a mean depth of 18 m and accounts for almost twothirds of the lake’s surface area. Like the western basin, the central basin is a broad flat basin of almost uniform depth. The eastern basin has an average depth of 24 m; it is a bowl-shaped depression with a maximum depth of about 64 m. It is separated from the central basin by the Pennsylvania Ridge, which runs southward from Long Point. Lake Erie receives the vast majority of its organic contaminants from the Detroit River, which flows into the western basin at an average rate of 5800 m3/s, accounting for almost 90% of the water input to the lake. In addition to water, the Detroit River carries 1.4 X lo6 t of sediment into the lake each year (3). Industrial and municipal activities in the area have led to high concentrations of persistent environmental pollutants in the Detroit River. Many of these pollutants are hydrophobic and are sorbed to particulate matter. This particulate matter is carried by currents into calm areas of the lake where it settles and accumulates in bottom sediment. Over time, the bottom sediments can become resuspended, continuing the transport process, or they can be buried, which removes both the sediments and any sorbed pollutants. Thus, the sediment is often a sink for hydrophobic pollutants. Those pollutants that are not buried are resuspended and transported from Lake Erie into Lake Ontario via the Niagara River. Our laboratory has previously identified a series of unusual alkylphenols in the sediments of the Trenton Channel, a heavily polluted part of the southwest Detroit River (4-6). The most abundant of these compounds were 2,4-di-tert-pentylphenol(24DP) and 2-chloro-4,6-di-tertpentylphenol(24DP6Cl); see Figure 1for structures. The concentrations of 24DP were 430 000 ng/g of dry sediment near its source and 1 2 ng/g at the mouth of the Detroit River (6). These compounds were produced by the acidcatalyzed reaction of 2-methylbutene with phenol at a chemical manufacturing plant on the Trenton Channel. This plant was the only major source of 24DP in the Great Lakes system until 1991, when it ceased production. Because these phenols have a point source and because they are hydrophobic (log KO, = 4.61 and 5.01, respectively), and thus sorbed to sediment, they are useful tracers for studying the transport and fate of similar pollutants in Lake Erie. Carter and Hites measured 24DP in dated sediment cores throughout Lake Erie and established average fluxes for each core (7). They found the highest fluxes (1-2 ng cm-2yr-l) within 20-30 km of the Detroit River mouth in the western basin of Lake Erie; fluxes in the western basin decreased as the distance from the Detroit River increased. Fluxes in the central and eastern basins were about 0.010.09 ng cm-2 yr-l. Using a simple three-compartment model, the 24DP distribution throughout the lake was Environ. Sci. Technol., Vol. 28, No. 9, 1994
1691
(M-CH,C,F,)'
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the bottom to avoid sampling bed-load. The samples are separated into aqueous and particulate phases by a Westfalia continuous-flow centrifuge, which is operated for 24 h. The particulate phase is freeze-dried and analyzed by CCIW for a variety of trace metals and pesticides (8). The unused portions of these samples were then analyzed in our laboratory for 24DP and 24DP6C1. The CCIW also provided suspended solid concentrations and river flow rates. Reagents. The alkylphenols and deuterated standards were previously synthesized and purified in our laboratory (5). A pentafluorobenzyl bromide (PFBBr) solution was made by adding 0.1 g of PFBBr (Lancaster; Windham, NH) to 1.9 mL of anhydrous acetone. This stock solution was stored at 4 OC in a desiccator. Fresh reagent was prepared biweekly. The KzCO3 solution was prepared by dissolving 10 g of anhydrous KzC03 in distilled water and diluting it to 100 mL. Sample Preparation and Extraction. A known amount of suspended solids was placed in a glass thimble of a Soxhlet extractor and spiked with a known amount of 2,4-di-tert-butylphenol-dzo(24DB-d). The sample was then extracted for 24 h with methanol and for another 24 h with dichloromethane. The extracts were combined, rotary evaporated, exchanged into hexane, and reduced to a final volume of about 5 mL. The combined extracts were chromatographed on columns containing 20 g of 1% water deactivated silica gel with 2 g of activated copper placed at the bottom of each column to remove elemental sulfur. Each column was capped with 1 g of anhydrous NazS04 to remove water. Samples were charged to the columns and eluted with 50 mL of hexane, 50 mL of 10% dichloromethane in hexane, and 50 mL of dichloromethane. The dichloromethane fraction contained the alkylphenols. It was spiked with 2 mL of acetone and 250 pL of the KzC03 solution, rotary evaporated to 5 mL, and transferred to a centrifuge tube. The volume was reduced to 0.8 mL under agentle nitrogen stream. The flask from the rotary evaporator was rinsed four times with 2-mL aliquots of acetone, and the rinsings were combined in the same centrifuge tube to give a final volume of about 9 mL. Derivatization and Cleanup. Derivatization followed the procedure of Lee et al. to react phenols with PFBBr to give PFB ether derivatives (9). A total of 100 pL of the K&O3 solution and 100 pL of PFBBr reagent solution were added to theextract in the centrifuge tube, stoppered tightly, and mixed on a vortex mixer. The tube and its contents were then heated for 1 h a t 60 "C. After completion of the reaction, the solution was evaporated to 0.5 mL under a gentle nitrogen stream. A 3-mL sample of hexane was added, and the evaporation was repeated to a final volume of 0.5 mL. The derivatized phenols were separated from the excess reagents on a chromatographic column made from a Pasteur pipet (23 cm long X 0.5 cm i.d.). The pipet was plugged with precleaned glass wool, filled with 5 cm of 5% water deactivated silica gel in a hexane slurry, and capped with 0.5 cm of anhydrous NanSO4. The concentrated derivatized extract was transferred to the column, and the centrifuge tube was rinsed twice with 1mL of hexane, which was added to the column. The hexane fraction from the minicolumn was discarded, and the derivatized phenols were eluted with 8 mL of 25% toluene in hexane.
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Flgure 1. Mass spectra of (top) 24DB-d, (middle)24DP, and (bottom) 24DP6CI.
used to determine 24DP accumulation rates in each basin (7). A total of 16 kg of 24DP/yr was accumulating in the western basin, 4.3 kg/yr in the central basin, and 1.1kg/yr in the eastern basin. From these accumulation rates, it was calculated that a total of 22 kg of 24DP/yr entered Lake Erie from the Detroit River. Of this total, 6.0 kg/yr moved from the western basin into the central, 1.7 kg/yr moved from the central basin into the eastern, and 0.6 kg/yr was lost to the Niagara River. In each case, about 30% of the sediment entering a basin was transported to the next basin, and about 70% remained in the basin. The above results were based on a steady-state mass balance model of Lake Erie. We suspected, however, that the transport of contaminated sediment was episodic, driven by storm events. Thus, the goal of this study was to determine the dependence of sediment transport from Lake Erie into Lake Ontario as a function of storms. To do this, we measured 24DP and 24DP6C1concentrations in suspended solids from the Niagara River, converted these concentrations into load rates, and related these loads to wind velocities. We have addressed several questions: Are hydrophobic pollutants from the Detroit River making their way to the Niagara River? Are winds associated with storm events driving this transport? How is pollutant transport related to wind velocities? How does the load rate compare to the interlake transport value derived from our previous mass balance study? Experimental Section
Sampling. Suspended solid samples were collected by the Canada Centre for Inland Waters (CCIW) once a week at Niagara-on-the-Lake (the mouth of the Niagara River). The sampling intake is at a depth such that it will not interfere with river navigation, but it is far enough from 1692
Environ. Sci. Technol., Vol. 28, No. 9, 1994
Quantitation. The alkylphenolswere quantitated with electron capture gas chromatographic mass spectrometry using a Hewlett Packard 5985 instrument equipped with a 30 m X 250 pm DB-5 fused-silica capillary column, which had a 0.25-pm film thickness (J & W Scientific; Folsom, CA). The ion source was held at 100 "C and the transfer line at 280 "C. Helium was used as the carrier gas at a flow of 40 cm/s, and methane was the reagent gas at an ion source pressure of 0.4 Torr. Samples of 1 pL were introduced by on-column injection. The column temperature was increased from 40 to 100 "C at 30 "Clmin, then at 5 "C/min to a final temperature of 280 "C, where it was held for 5 min. Response factors for 24DP and 24DP6C1 relative to 24DB-d were determined in a series of calibration experiments. Standards were analyzed with each set of five samples to confirm the response factors. The most abundant ions of the analytes were used for quantitation, and the second most abundant ions were used for confirmation. The quantitation and confirmation ions ( r n l z ) are as follows: 24DB-d, 225, 226; 24DP, 233, 163; 24DP6C1, 267, 269. The mass spectra of 24DB-d, 24DP, and 24DP6C1 are shown in Figure 1. Concentrations were converted to load rates using
L = TFC,
than the loads for 1980-1991 (see Table 1). This may be due to the halt of 24DP production in 1991, or it may be due to the explosion of the Zebra mussel (Dreissena polymorpha) population, which occurred at about that time. These mussels are efficient filter feeders, and they may have substantially reduced the particle load with which these phenols are associated. Theory of Sediment Transport. The amount of sediment that can be resuspended per unit area ( Ein g/cm2) is given by (10)
E = a0r2
In this equation, a0 depends on the compaction (water content) of the sediment; however, for our purposes, we will assume that it is constant. In eq 2, 7 is the shear stress, which is the force per unit area (in dyn/cm2)exerted between the water and the sediment at the sedimentwater interface. This stress is produced by wave action and currents. Often this equation includes a "critical" shear stress for the onset of sediment movement (10). However, preliminary analysis of our data indicates that this critical shear stress is very small; thus, it has been ignored in the following discussion. The shear stress exerted on the bottom sediments is given by
(1) 7
where L is the load rate in (g/day), T is the total of suspendedsolids (indryg/L),Fis thewater flow (inL/day), and C, is the concentration of the analyte (in g of analyte/g of dry sediment). The derivatization of the phenols to PFB ethers gave limits of detection of about 0.001 g/day. This is about 2 orders of magnitude better than detection limits for the underivatized phenols, which were previously analyzed by electron impact mass spectrometry (4-7). Quality Control. All solvents were of glass-distilled grade, and all glassware was acid washed and heated at 450 "C overnight. The copper, sodium sulfate, and silica gel were cleaned for 24 h by Soxhlet extraction using dichloromethane and subsequently heated at 160 "C overnight. Procedural blanks were run with each batch of five samples; they were taken through all phases of extraction, isolation, and analysis. All blanks were less than the detection limits of the individual phenols. Recoveries of the derivatized alkylphenols were 81-84 $6 as determined by standard additions. Precision was assessed by triplicate analysis of selected samples; it was f 17% (rsd).
Results and Discussion Phenol Loads. Table 1summarizesthe sampling dates, the phenol concentrations, the water flow rates, the suspended solid concentrations,the phenol load rates, and the wind velocities (Ull see next section). Clearly, the phenols are carried by suspended solids from Lake Erie into Lake Ontario. The Pearson correlation coefficient ( r ) for 24DP loads with the sample date is -0.24, which is not significant at the 95% confidence level; thus, there has been no significant change in this contaminant load over the 14 yr of this study. The range for 24DP loads is
