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Synthetic Musk Fragrances in Lake Michigan 

Environ. Sci. Technol. 2004, 38, 367-372 1jan04

AARON M . PECK AND KERI C. HORNBUCKLE *

Department of Civil and Environmental Engineering, The University of Iowa, Iowa City, Iowa 52242

* Corresponding author e-mail: keri-hornbuckle@uiowa.edu; phone: (319)384-0789; fax: (319)335-5660.

Synthetic musk fragrances are added to a wide variety personal care and household products and are present in treated wastewater effluent. Here we report for the time ambient air and water measurements of six polycyclic musks (AHTN, HHCB, ATII, ADBI, AHMI, and DPMI) and two nitro musks (musk xylene and musk ketone) in North America. The compounds were measured in the and water of Lake Michigan and in the air of urban Milwaukee, WI. All of the compounds except DPMI were detected. HHCB and AHTN were found in the highest concentrations in all samples. Airborne concentrations HHCB and AHTN average 4.6 and 2.9 ng/m3, respectively, in Milwaukee and 1.1 and 0.49 ng/m3 over the lake. The average water concentration of HHCB and AHTN in Lake Michigan was 4.7 and 1.0 ng/L, respectively. A lakewide annual mass budget shows that wastewater treatment plant discharge is the major source (3470 kg/ of the synthetic musks while atmospheric deposition contributes less than 1%. Volatilization and outflow through the Straits of Mackinac are major loss mechanisms (2085 and 516 kg/yr for volatilization and outflow, respectively). Concentrations of HHCB are about one-half the predicted steady-state water concentrations in Lake Michigan.

Introduction

Synthetic musk fragrances (Figure 1) are a group of chemicals used extensively in detergents, perfumes, shampoos, and other personal care products. Nitro musks, mainly musk xylene and musk ketone, have been used for many decades. In 1996, about 2000 t of musk xylene and musk ketone was produced worldwide (1). In recent years, polycyclic musks have become the most important commercial synthetic musks due in part to concern about the environmental distribution and toxicological effects of the nitro musks and subsequent reduction in use of the nitro musks. In 1996 the worldwide production of polycyclic musks, 95% of which were HHCB and AHTN, was 5600 t (2). U.S. sales estimates for these compounds are unavailable although musk ketone, musk xylene, HHCB, AHTN, ATII, AHMI, ADBI, and DPMI are listed on the Toxic Substances Control Act (TSCA) Inventory (3). Additionally, HHCB is a high production volume (HPV) chemical as defined by the U.S. EPA (more than 1 million lb is produced in or imported into the United States each year) (4).

Although created to replace the more expensive and rare natural musks, polycyclic and nitro musks are not structurally or chemically similar to their natural counterparts. The physical-chemical properties (Table 1) of synthetic musks have more in common with hydrophobic and semivolatile organic pollutants that are known to biomagnify through the food chain. Bioconcentration factors for musk xylene and musk ketone correlate well with their octanol-water partition coefficient (Kow) (5), while observed bioconcentration factors for AHTN and HHCB tend to be lower than predicted from Kow (2).

Two nitro musks (musk xylene and musk ketone) and a polycyclic musk (AHTN) have been shown to demonstrate estrogenic activity in an assay using human breast cancer cells (6). Although a causal relationship has not been established, musk xylene and musk ketone concentrations in womens blood have been correlated to several different clinical parameters of the endocrine system, including higher rates of miscarriage in women with higher musk xylene concentrations (7).Twopolycyclic musks, HHCB and AHTN, have weak estrogenic potency in humans; however, this effect is too weak to induce an estrogenic effect at current exposure levels (8). Musk xylene, musk ketone, HHCB, AHTN, ATII, AHMI, and ADBI have been found in human adipose tissue and breast milk (9-12).

Musk xylene and musk ketone were first identified in the aquatic environment in 1981 (13). In field studies on surface waters in Europe, these compounds have been shown to bioaccumulate in aquatic ecosystems (1, 5, 14-16). They have also been found in suspended (17, 18) and surficial (16, 19) sediments. Studies in Europe have identified wastewater treatment plant discharges as an important source of these compounds to surface water (17, 19-24).

FIGURE 1. Structures of polycyclic musks and nitro musks.

There have been few studies on synthetic musks in North America. Musk xylene, musk ketone, HHCB, and AHTN were measured in aquatic biota in Canadian surface water (25). These compounds have also been measured in wastewater treatment plant effluent in Canada (26) and the United States (24, 27). These compounds appear to be consistent components of contemporary municipal waste discharge. In a study of 10 typical wastewater treatment plants in the United States, Simonich et al. (24) found that the concentrations of HHCB and AHTN varied by less than a factor of 2 and that concentrations of musk xylene and musk ketone varied by a factor of 10 between treatment plants. Osemwengie and Steinberg measured several synthetic musks downstream of a wastewater treatment plant (WWTP) in the southwestern Two nitro musks (musk xylene and musk ketone) and a polycyclic musk (AHTN) have been shown to demonstrate estrogenic activity in an assay using human breast cancer cells (6). Although a causal relationship has not been established, musk xylene and musk ketone concentrations in womens blood have been correlated to several different clinical parameters of the endocrine system, including higher rates of miscarriage in women with higher musk xylene concentrations (7).Twopolycyclic musks, HHCB and AHTN, have weak estrogenic potency in humans; however, this effect is too weak to induce an estrogenic effect at current exposure levels (8). Musk xylene, musk ketone, HHCB, AHTN, ATII, AHMI, and ADBI have been found in human adipose tissue and breast milk (9-12). Musk xylene and musk ketone were first identified in the aquatic environment in 1981 (13). In field studies on surface waters in Europe, these compounds have been shown to bioaccumulate in aquatic ecosystems (1, 5, 14-16). They have also been found in suspended (17, 18) and surficial (16, 19) sediments. Studies in Europe have identified wastewater treatment plant discharges as an important source of these compounds to surface water (17, 19-24).  There have been few studies on synthetic musks in North America. Musk xylene, musk ketone, HHCB, and AHTN were measured in aquatic biota in Canadian surface water (25). These compounds have also been measured in wastewater treatment plant effluent in Canada (26) and the United States (24, 27). These compounds appear to be consistent components of contemporary municipal waste discharge. In a study of 10 typical wastewater treatment plants in the United States, Simonich et al. (24) found that the concentrations of HHCB and AHTN varied by less than a factor of 2 and that concentrations of musk xylene and musk ketone varied by a factor of 10 between treatment plants. Osemwengie and Steinberg measured several synthetic musks downstream of a wastewater treatment plant (WWTP) in the southwestern United States (28). The concentrations of HHCB, AHTN, and musk xylene reported in this study were 1-2 orders of magnitude lower than reported for wastewater effluent. WWTP sludge has also been identified as a potential source of these compounds to the terrestrial environment (29-31).

TABLE 1. Synthetic Musks and Selected Properties

		  CAS	
		  Registry		  S  	   H  	    log		vapor
compound 	  No. 		mol wt 	(mg/L) (Pa m3/mol)  Kow 	pressure (Pa)
HHCB, galaxolide  1222-05-5 	258.4 	1.75 	11.3 a 	    5.9 a 	0.073
AHTN, tonalide 	  1506-02-1 	258.4 	1.25 	12.5 a 	    5.7 a 	0.068
musk xylene 	  81-15-2 	297.2 	0.49 	0.018 b     4.9 b 	0.00003
musk ketone 	  81-14-1 	294.3 	1.9 	0.0061 b    4.3 b 	0.00004
ATII, traseolide  68140-48-7 	258.4 	0.085 	85.1 c 	    8.1 c 	1.2
ADBI, celestolide 13171-00-1 	244.3 	0.015 	1801c 	    6.6 c 	0.020
AHMI, phantolide  15323-35-0 	244.3 	0.027 	646c 	    6.7 c 	0.024
DPMI, cashmeran   33704-61-9 	206.3 	0.17 	9.9 c 	    4.9 c 	5.2

a Ref 41. b Ref 39. c Ref 40.

Little is known about the levels of synthetic musks in air. Kalleborn et al. report atmospheric concentrations of HHCB, AHTN, ATII, musk xylene, and musk ketone for Norway (32). The atmospheric lifetime for HHCB has been evaluated in a chamber study to be 5.3 h (33). From the latter study, it was concluded that the potential for long-range atmospheric transport of HHCB is small. In this study, two nitro musks (musk xylene and musk ketone) and six polycyclic musks (HHCB, AHTN, ADBI, ATII, AHMI, and DPMI) were examined in Milwaukee, WI, air and Lake Michigan air and water. The objectives of this study were to report atmospheric musk concentrations in an urban and in remote locations in the United States; to report musk concentrations in Lake Michigan water; and to evaluate the loading of synthetic musk fragrances to Lake Michigan. To our knowledge, this is the first report of these compounds in air and natural water of North America.

Materials and Methods

Chemicals. All solvents used were Fisher Scientific Pesticide Grade (Fair Lawn, NJ).HHCB(1,3,4,6,7,8-hexahydro-4,6,6,7,8-hexamethylcyclopenta--2-benzopyran), AHTN (7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronaphthalene), ADBI (4-acetyl-1,1-dimethyl-6-tert-butylindan), AHMI (6-acetyl-1,1,2,3,3,5-hexamethylindan), DPMI (6,7-dihydro-1,1,2,3,3-pentamethyl-4(5H)-indanone), and ATII (5-acetyl-1,1,2,6-tetramethyl-3-isopropylindan) were obtained from Promochem (Teddington, UK). Musk xylene (1-tert-butyl-3,5-dimethyl-2,4,6-trinitrobenzene) and musk ketone (4-tertbutyl-3,5-dinitro-2,6-dimethylacetophenone) were obtained from Sigma-Aldrich (St. Louis, MO). Pentachloronitrobenzene was obtained from Chem Service (West Chester, PA). Deuterated polycylic aromatic hydrocarbons used as a recovery surrogate standard (fluoranthene-d10) and an internal standard (pyrene-d10) were obtained from Cambridge Isotope Laboratories (Andover, MA).

Air and Water Sample Collection. Details of the air and water sample collection and extraction have been described elsewhere (34, 35). Briefly, high-volume air samples were collected at the Great Lakes Water Institute, near the shore of Lake Michigan and south of downtown Milwaukee, WI, during June 2001 and over Lake Michigan aboard the U.S. EPA R/V Lake Guardian during June 1999 and May 2000. The on-shore samples were collected on the roof of the Institute, approximately 20mabove water level. The over lake samples were collected about 7 m above the lake surface. The onshore air sample volumes ranged from 57 to 298m3 and were collected during 4-h intervals during the day and 12-h overnight. The over-water air sample volumes ranged from 130 to 601m3 and were collected for 12-24 h. The air samples consisted of an operationally defined airborne particulate phase collected on a glass fiber filter and a gas phase collected on XAD-2 resin. Water samples were collected on Lake Michigan during June 1999 and May 2000 aboard the U.S. EPA R/V Lake Guardian. These water samples consisted of an operationally defined water particulate phase collected on glass fiber filters and a dissolved phase collected on XAD-2 resin. A total of 1064-1600 L of water was pulled through the glass fiber filters, and 100 or 200 L of filtered water was pulled through the XAD-2 resin. All XAD-2 resin was extracted for 24 h with methanol, acetone, dichloromethane, and hexane prior to sampling. Glass fiber filters were combusted at 450 C for at least 4 h prior to sampling.

Sample Extraction and Cleanup. After being sampled, the XAD-2 resin and glass fiber filters used in air sampling were extracted separately with acetone/hexane for 24 h. A total of 416 ng of fluoranthene-d10 surrogate standard was added to each sample in 100 L of hexane prior to extraction. The XAD resin and glass fiber filters used in water sampling were rinsed with acetone to remove water and then extracted with acetone/hexane. The acetone/hexane extract was combined with the rinse acetone prior to sample cleanup.

Cleanup of all samples was performed using 3% deactivated silica gel as has been described elsewhere (34, 36). Three sample fractions result from this separation (hexane, dichloromethane/hexane, and methanol). Musk xylene and fluoranthene-d10 elute in the dichloromethane/hexane fraction, and the remaining musk compounds elute in the methanol fraction. The extracts were reduced to 100 L using N2, and then 100 L of internal standard was added prior to GC/MS analysis. Pentachloronitrobenzene was used as an internal standard for the methanol fraction, and pyrened10 was used for the dichloromethane/hexane fraction.

Analytical Methods. All samples were analyzed by an HP6890 gas chromatograph coupled to an HP5973 mass selective detector using selected ion monitoring (SIM) and electron impact mode. A 30-m 5% phenyl methyl siloxane capillary column was used (HP-5MS; 250 m i.d., 0.25 m film thickness). Samples were injected splitless with an inlet temperature and pressure of 250 C and 11.8 psi, respectively. A constant 1.2 mL/min column flow rate was used. The initial oven temperature (90 C) was held for 2 min. This was followed by a 10 C/min ramp to 150 C, then 1.5 C/min to 170 C, and finally 30 C/min to 300 C. The final temperature (300 C) was held for 7 min. The mass spectrometer quadrupole and source temperatures were 150 and 230 C, respectively. The electron energy was 70 eV. The internal standard method was used for quantification of all compounds. The quantification and confirmation ions for each compound are listed in Table 2.

The HHCB standard used was a technical mixture containing 74% HHCB. This reported purity was based on a GC/FID analysis by the distributor of the technical mixture. With the GC program used in this study, two coeluting peaks (Figure 2) account for  approximately74%of the TIC response. The mass spectra are the same for each peak. Mass spectra for these coeluting peaks and additional components of the technical HHCB mixture is provided in the Supporting Information. This double peak has been observed previously and was attributed to an isomer of HHCB (37). The sum of both peaks was used in this analysis and has been reported as HHCB.

All compounds were baseline separated except for HHCB and ATII. Example chromatograms are provided in the Supporting Information. The major ion of HHCB and ATII is present in the mass spectrum of the other (Figure 3); however, the relative contribution of each ion in the mass spectrum of the other is very small. The quantification ion for ATII was 215; the relative abundance of this ion in the HHCB mass spectra was 4%. The quantification ion for HHCB was 243; the relative abundance of this ion in the ATII mass spectra was less than 2%. HHCB is present in much larger quantities than ATII in environmental samples; consequently, the potential interference by ATII in the identification and quantification of HHCB is insignificant. The potential interference by HHCB to the ATII mass spectrum is also small because of the very small relative abundance of ion 215 in the HHCB spectrum. Detection of ATII was based on the abundance of ion 215 being greater than2%of the abundance of ion 258. In any case, concentrations reported for ATII are somewhat uncertain and represent an upper limit.

TABLE 2. Quantification and Confirmation Ions

		quantification	 confirmation 
compound	ion 		 ion
HHCB 		243 		 258
AHTN		243 		 258
musk xylene 	282 	  	 297
musk ketone 	279 		 294
ATII 		215
ADBI 		229 		 244
AHMI 		229 		 244
DPMI 		191 		 206

TABLE 3. Average Sample to Blank Mass Ratios

		`			airborne			water
		        gas             particulate	dissolved	particulate
compound 	Milwaukee over lake 	Milwaukee	lake 		lake
HHCB 		 9.2 	    7.9 	2.8 		 7.9 		1.5
AHTN 		77 	   11 		2.6 		 3.8 		5.1
musk xylene 	20 	   15 		0.9 		 6.4 		1.1
musk ketone 	 3.4 	    1.7 	1.6 		 2.8 		2.4
ATII 		19 	   14 		0.8 		 4.8 		0.2
ADBI 		24 	   14 		0.7 		 2.2 		0.05
AHMI 		18 	   17 		0.9 		17 		0.1
DPMI 		 0 	    0 		0 		 0 		0

FIGURE 2. HHCB and AHTN chromatogram ( m/ z) 243). The double peak present in the standard and sample is due to coelution of HHCB and an HHCB isomer.

FIGURE 3. Partial mass spectra for ATII (a) and HHCB (b). Ions 215 and 243 appear in both spectra. The relative abundance of ion 215 in the HHCB mass spectrum is less than 2%. The relative abundance of ion 243 in the ATII mass spectrum is about 4%.

 

Results

Quality Assurance. Quality assurance was met by quantification of surrogate standards added to every sample and comparison of the sample masses to those in field blanks. The average recovery of fluoranthene-d10 was 86 27%, 76 12%, 79 27%, 56 34%, and 59 34% in the Milwaukee gas-phase, Milwaukee airborne particulate-phase, Lake Michigan gas-phase, Lake Michigan dissolved-phase, and Lake Michigan water particulate-phase samples, respectively. Musk xylene sample masses were corrected by the fluoranthene recovery-d12. All other masses were corrected using average recoveries from spike experiments.

Table 3 lists average sample to blank mass ratios. For AHTN, HHCB, ATII, ADBI, AHMI, and musk xylene, significantly higher masses were found in the gas-phase samples as compared to the field blanks (p < 0.07). These ratios were calculated from the average mass found in field blanks and the average sample mass collected in each media. It is apparent that the field blanks and samples were similar in all media for musk ketone. All of the airborne particulate phase samples are similar to the field blanks. The dissolved phase shows the most variability in sample to blank ratios between compounds. Additional information about the sample and field blank masses is provided in the Supporting Information. Samples were not blank corrected, and all samples were used in the calculation of the average concentrations reported. The percentage of samples exceeding the field blank values at the 95% confidence level for each media is listed with each average concentration.

Atmospheric Concentrations. Synthetic musk fragrances are found primarily (more than 80%) in the gas phase. All of the musks were found in the air except DPMI. Average gas phase concentrations in Milwaukee and over Lake Michigan are shown in Table 4. The polycyclic musk concentrations were significantly higher (p < 0.007) in Milwaukee than over the lake, with HHCB present in the highest concentrations in Milwaukee and over the lake. HHCB, AHTN, and musk ketone concentrations in Milwaukee were higher than those reported in Norway, the only previous report for these compounds in ambient air. ATII concentrations were higher in Milwaukee and about the same over the lake as those in Norway. The nitro musk concentrations in Milwaukee and over the lake were similar. Additionally, the musk xylene and musk ketone concentrations in Milwaukee and over the lake were similar to those in Norway.

The airborne particulate-phase concentrations and 95% confidence intervals measured in Milwaukee are shown in Table 5. Fewer compounds were measured above the field blanks at the 95% confidence level in the airborne particulate phase than the gas phase. The average airborne particulate-phase sample masses for musk xylene, musk ketone, ATII, ADBI, and AHMI were similar to the field blank masses. Musk ketone was found in the largest concentration; however, musk ketone was very high in the airborne particulate-phase blanks, so these high values may be due to contamination. Musk xylene had the largest fraction in the airborne particulate-phase. Because of the small concentration of airborne particulates over Lake Michigan and the small contribution of the airborne particulate phase to the overall atmospheric concentration of synthetic musk fragrances, they were not measured over the lake.

TABLE 4. 95% Confidence Intervals of Gas-Phase Musk Concentrations

		                       gas-phase concentration (ng/m3)           .
		Milwaukee		over lake		Kjeller, Norway b
compound	N=26 a			N=11 a			N= 5 a
HHCB 		4.11.4 (100) 		1.10.6 (100) 		0.140.06 (100)
AHTN 		2.51.0 (100) 		0.490.3 (100) 		0.0520.020 (100)
musk xylene 	0.0320.025 (96) 	0.0140.006 (100) 	0.0460.023 (100)
musk ketone 	0.0930.048 (42) 	0.130.14 (18) 		0.0150.025 (100)
ATII 		0.170.048 (81) 	0.0400.022 (100) 	0.0120.007 (100)
ADBI 		0.190.052 (96) 	0.0420.023 (100) 	na c
AHMI 		0.240.085 (96) 	0.0390.018 (100) 	na
DPMI 		ND (0) 			ND (0) 			na

a The percentage of samples exceeding the field blank values at the 95% confidence
  level is in parentheses. b Ref 32. c na, not applicable.

TABLE 5. 95% Confidence Intervals of Airborne Particulate-Phase Concentrations

		airborne		airborne 
		particulate-phase	particulate-phase 	fraction in 
compound	concn (ng/m3) 		concn (ng/mg)		airborne particulate
		N=25 c			N=25 c			phase (%)
HHCB 		0.550.27 (56) 		1.30.50 (56) 		13
AHTN 		0.420.20 (52) 		1.40.70 (52) 		17
musk xylene a 	0.0100.005 (17) 	0.0400.024 (17) 	31 d
musk ketone b 	13.97.1 (40) 		51.225.3 (40) 		na e
ATII 		0.0340.014 (8) 	0.130.07 (8) 		19 d
ADBI 		0.0300.014 (4) 	0.110.05 (4) 		16 d
AHMI 		0.0330.016 (16) 	0.110.05 (16) 		13 d
DPMI 		nd (0) 			nd (0) 			na

a N) 24 for musk xylene. 
b The musk ketone airborne particulate concentrations were affected by 
  contamination as indicated by high masses in the field blanks. 
c The percentage of samples exceeding the field blank values at the 95% 
  confidence level is in parentheses. 
d Because the sample masses are similar to the field blank masses in the 
  airborne particulate phase for these   compounds, this fraction represents
  the upper limit in the airborne particulate phase. 
e na, not applicable; nd, not determined.

TABLE 6. 95% Confidence Intervals of Lake Michigan Water Concentrations

				dissolved-phase concn (ng/L)
		fraction in			  typical
		dissolved	Lake Michigan	  WWTP
compound	phase (%)	N= 13 b		  effluent
HHCB 		99 		4.72.5 (92) 	  1640(N= 12) c
AHTN 		97 		1.00.8 (85) 	  1150(N= 12) c
musk xylene a 	94 		0.0490.021 (100)   40(N= 12) c
musk ketone 	95 		0.0810.052 (38)    40(N= 12) c
ATII 		100 		0.110.10 (69) 	   310(N= 25) d
ADBI 		100 		0.0290.023 (38)   110(N)= 30) d
AHMI 		100 		0.520.20 (92) 	   270(N= 12) d
DPMI 		nd e 		nd (0) 			na

a N= 14 for musk xylene. 
b The percentage of samples exceeding the field blank values at 
  the 95% confidence level is in parentheses. 
c U.S., all reported treatment types ( 24). 
d Germany ( 19). 
e nd, not determined; 
na, not applicable.

Water Concentrations. Synthetic musk fragrances are found primarily (more than 90%) in the dissolved phase in Lake Michigan. Musk xylene was measured above the field blanks at the 95% confidence level in all of the dissolved-phase samples. HHCB and AHMI were measured above the field blanks at the95%confidence level in92%of the samples; AHTN was above the field blanks in 85% of the samples. HHCB and AHTN were found in the highest concentrations. The dissolved concentrations in Lake Michigan reported here are less than currently available predicted no effect concentrations (PNEC) for AHTN, HHCB, musk xylene, and musk ketone (38, 39). ATII, ADBI, and AHMI were not detected in any water particulate-phase samples. Musk xylene was detected in all the water particulate-phase samples. HHCB, AHTN, and musk ketone were detected in 15%, 54%, and 38% of the water particulate-phase samples, respectively. DPMI was not found in any dissolved or water particulate-phase samples.

The musk concentrations found in Lake Michigan were several orders of magnitude lower than concentrations reported in wastewater treatment plant effluent (Table 6). The concentrations of HHCB, AHTN, musk xylene, and musk ketone in wastewater treatment plant effluent are an average from 12 treatment plants in the United States (24, 27). Values for the other musks were not available for U.S. treatment plants, so the ATII, ADBI, and AHMI values in Table 6 are from treatment plants in Germany(19).DPMI concentrations in wastewater treatment plant effluent have not been reported.

Discussion

To assess the state of synthetic musks in Lake Michigan, four processes affecting concentrations of these compounds in the lake were quantified: gaseous atmospheric deposition, discharge from wastewater treatment plants, volatilization, and outflow through the Straits of Mackinac. Atmospheric loading from airborne particulate deposition and precipitation is neglected because of the relatively low airborne particulate-phase musk concentrations found in Milwaukee and the low airborne particulate concentrations over Lake Michigan. Losses due to sedimentation are considered negligible due to the small percentage found in the water particulate phase. Biodegradation, photolysis, and other reaction rates for synthetic musks in a lake environment are unknown and not included.

Gaseous Atmospheric Deposition and Volatilization. The gross atmospheric deposition flux (Fd) to Lake Michigan was estimated using

where kol is the overall mass transfer coefficient, Ca is the average gas-phase concentration over the lake, H is the Henrys law constant (39-41), and A is the surface area of Lake Michigan (57 800 km2) (42).

FIGURE 4. Loadings (atmospheric deposition and wastewater treatment plant discharge) and losses (volatilization and outflow) of synthetic musk fragrances to Lake Michigan.

The gross volatilization flux (Fv) from Lake Michigan was estimated using

where Cw is the average dissolved-phase concentration. The overall mass transfer coefficient was determined as described elsewhere (43, 44) assuming 20 C and a 5 m/s wind speed. It ranged from 1.2 10-6 m/d for musk xylene to 0.39 m/d for ADBI.

Estimation of Wastewater Treatment Discharge. The input to the lake from wastewater treatment plants (FWWTP) was determined from the total volumetric flow rate of wastewater into Lake Michigan (QWWTP) and published synthetic musk concentrations in wastewater treatment plant effluent (CWWTP):

Wastewater treatment plant effluent is discharged into Lake Michigan from Wisconsin, Michigan, and Indiana. The average flow of wastewater treatment plant effluent into Lake Michigan each day in 2001, as reported to the U.S. EPA under the National Pollutant Discharge Elimination System (NPDES), including both direct discharge into the lake and discharge into tributaries that feed into the lake, was 2.67 106 ( 5.3 105 m3/d (about 700 MGD) (45).

Flow Through the Straits of Mackinac. Losses of musks due to flow out of the lake (Fout) were estimated from the average flow through Straits of Mackinac (Qout ) 2520m3/s) (42) and the average concentration of each musk measured in the lake (C):

The daily fluxes to and from the lake for each compound are summarized in Figure 4. On an annual basis ~12 kg of HHCB and ~5 kg of AHTN are atmospherically deposited into Lake Michigan. During this same time ~235 kg of HHC Band ~55 kg of AHTN volatilize from the lake. Assuming the typical musk concentrations in WWTP effluent listed in Table 6, ~1597 kg of HHCB and ~1120 kg of AHTN are discharged into the lake each year from wastewater treatment plants. The contributions from each of these fluxes are compared in Figure 4. The input from wastewater treatment plants for all compounds is orders of magnitude higher than from atmospheric deposition. HHCB, with about 1% of the total input coming from the atmosphere, has the highest atmospheric contribution to the lake. The net air-water exchange is out of the lake for the polycylic musks and into the lake for the nitro musks. 

FIGURE 5. Comparison of measured and estimated steady-state synthetic musk concentrations.

Steady-State Musk Concentration in Lake Michigan.

Assuming the four processes described above control the concentration of musks within Lake Michigan and the loading fluxes estimated above, the steady-state concentration (Css) of each compound in the lake was determined:

A comparison of the steady-state and measured concentrations is shown in Figure 5. The concentrations of the most prevalent synthetic musks (HHCB, AHTN, AHMI) in the lake are within an order of magnitude of the steady-state values and less than the steady-state concentrations except for AHMI and ADBI. The wastewater treatment plant effluent concentrations for AHMI and ADBI were from Germany, which could explain this discrepancy. The predicted steady-state values may be too high due to losses that are unknown and cannot currently be included in the mass balance: photodegradation, microbial degradation, and permanent loss to the sediments.

Acknowledgments

Funding for this work was provided in part by the Center for Global and Regional Environmental Research, Environmental Health Sciences Research Center (an NIEHS center) and the Center for Health Effects of Environmental Contamination. David M. Wethington and Sondra M. Miller were responsible for sample collection and extraction in Milwaukee and Lake Michigan, respectively. We would like to thank Sreedevi Yedavalli and Matt Gluckman (U.S. EPA, Region 5) for assistance determining wastewater treatment plant discharges to Lake Michigan.

Supporting Information Available

A TIC and mass spectra for the HHCB technical mixture and an example chromatogram of a sample and field blank; 95% confidence intervals of all sample and field blank masses for each chemical. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review July 15, 2003. Revised manuscript received

October 10, 2003. Accepted October 23, 2003.

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