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Mass balance model of source apportionment, transport and fate of PAHs in Lac Saint Louis, Quebec
Chemosphere 41 (2000) 681±692
Mass balance model of source apportionment, transport and fate of PAHs in Lac Saint Louis, Quebec Donald Mackay *, Brendan Hickie Environmental Modelling Centre, Environmental and Resource Studies, Trent University, Peterborough, Ont., Canada K9J 7B8 Received 21 May 1999; accepted 30 September 1999
Abstract A mass balance model has been developed and calibrated to describe the sources, transport and fate of seven polycyclic aromatic hydrocarbons (PAHs; anthracene, benzo(a)pyrene, benzo(b)¯uoranthene, chrysene, ¯uoranthene, phenanthrene, and pyrene) in the water and sediments of, and atmosphere over Lac Saint Louis, Quebec. The model uses speci®ed input rates from background advective ¯ows and emissions from the Alcan aluminum smelting facility at Beauharnois to deduce atmospheric concentrations and rates of wet and dry deposition to the three segment lake. Concentrations in water and sediment as well as relevant mass ¯uxes and residence times are computed and compared satisfactorily with monitoring data for ®ve of the seven PAHs. Underestimation of concentrations for anthracene and phenanthrene is attributed to unquanti®ed additional sources. The sources of the PAH burden in the lake are apportioned, and the implications of these results are discussed including likely response times to changes in loadings. It is suggested that this mass balance approach is more widely applicable to situations in which water bodies are impacted by a variety of contaminant sources. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction There is continuing concern about the presence of polycyclic aromatic hydrocarbons (PAHs) in the environment, primarily because several are believed to be human carcinogens. These substances are produced during the combustion of carbonaceous materials including wood and fuel oils, especially under conditions of limited oxygen availability. They are also emitted during aluminum smelting. Because of their ubiquitous nature it is often dicult to assign sources quantitatively to prevailing concentrations since in a speci®c air mass or lake there may be a variety of local sources and background inputs from more distant sources. One approach to apportioning sources is to compile a mass balance model which attempts to quantify all inputs and
*
Corresponding author. Tel.: +705-748-1489; fax: +705748-1569. E-mail address: [email protected] (D. Mackay).
calculates the fate of the PAHs in a speci®c region, deducing the prevailing rates of transport and transformation, and the concentrations in a variety of environmental media. Such a model can be used to demonstrate the signi®cance of each source and can guide regulatory actions. This mass balance study is of the fate of seven PAHs in the air mass above Lac Saint Louis, Quebec, and in the lake water and sediments. The major aims were to assess the relative signi®cance of industrial and background sources, estimate prevailing concentrations, and apportion them to various sources. Further, the mass balance model can provide a clear picture of the key environmental processes, show which environmental and chemical properties are the most important determinants of fate, and can be used to assess the extent and rate at which reductions in emissions will translate into reduced concentrations. The approach taken is ®rst to characterize the dimensions and properties of the water segments and the air mass above the lake, i.e., the hydrology and
0045-6535/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 4 8 6 - 5
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meteorology. Previous studies of the fate and presence of PAHs in this region are then reviewed and data tabulated on observed concentrations in air, water and sediment. Input rates of selected PAHs from the AlcanBeauharnois facility are presented for several time periods, namely, 1996, 1993±1995, 1986±1991, and 1983±1985. Input rates for the 1983±1985 period are regarded as applicable to the previous 30 yr period. The physical±chemical properties and reaction half lives of the selected PAHs are presented. The model is described, results are presented and compared with existing monitoring data, and implications of the results are discussed. 2. The Lac Saint Louis environment Lac Saint Louis is a shallow ¯uvial lake located at the con¯uence of the Ottawa and St. Lawrence Rivers and
de®nes part of the southern and western boundaries of the Island of Montreal, Quebec (Fig. 1). Studies by Rukavina et al. (1990), Pham et al. (1993), Pham and Proulx (1997), Environment Canada (1994) and others suggest that the lake is best treated as three connected segments as shown in Figs. 1 and 2. The south segment (1) receives water input from the St. Lawrence River and some input from the fraction of the Ottawa River which ¯ows to the west of Ile Perrot. Segment (2) to the north includes a small portion of Lac Deux Montagnes and receives input from the Ottawa River and exchanges water with the third segment to the east. The east segment (3) exchanges water from both segments 1 and 2 and discharges to the St. Lawrence River. The three segments are similar in area, each being about 50 km2 . Table 1 gives the ¯ow data for these segments, the values being based on the river inputs and assumed 10% back¯ows from segment 3 to 1 and 2. Advective ¯ows of air and water are illustrated in Fig. 2.
Fig. 1. Map of the Lac Saint Louis region showing the segmentation of the lake.
Fig. 2. Compartments and pathways of the Lac Saint Louis system.
D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
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Table 1 Characteristics of the tributaries and lake segments of Lac Saint Louisa Lake segments
a
South (1)
North (2)
East (3)
Lake segments Surface area (m2 ) Mean depth (m) Suspended particles (mg/l) Organic carbon content Sediment particle density Organic carbon content Vol. fraction sediment solids Sediment active depth (m) Sedimentation rate (g/m2 /d) Resuspension rate (g/m2 /d) Burial rate (g/m2 /d)
5.47 ´ 107 3.7 4.6 0.3 2 0.046 0.1 0.05 6 4 1.6
4.18 ´ 107 2.1 4.1 0.3 2 0.028 0.1 0.05 3 2 0.84
6.29 ´ 107 3 4.8 0.3 2 0.027 0.1 0.05 3 2 0.84
Inter-segment ¯ow rates (m3 /s) to 1 from to 2 from to 3 from
± 0 9493.4
336 ± 549
832.5 88.5 ±
Tributaries Flow rate (m3 /s) suspended particles (mg/l) P PAH concentration (ng/l)
St. Lawrence 8320 4.6 4.9
Ottawa 885 4.1 7.3
Chateauguay 35 7.8 42.6
St. Louis 5 18.5 85.8
Most values were derived from the St. Lawrence Vision 2000 report (Environment Canada, 1994).
Environment Canada (1994) has estimated the net annual loading of particles to the sediments of Lac Saint Louis to be between 45 000 and 130 000 t/yr, which correspond to average net deposition rates of 0.8±2.4 g/ m2 /d. According to Rukavina et al. (1990) sediment deposition rates tended to be highest in the deeper south basin (segment 1) to which we assigned a net deposition rates of 2.0 g/m2 /d with lower rates of 1.0 g/m2 /d for segments 2 and 3. These net deposition rates were used to estimate gross deposition and resuspension rates assuming that two-thirds of sedimented material is resuspended in all segments under these fairly turbulent riverine conditions.
3. PAH emissions and concentrations PAHs enter this system by three routes: 3.1. Stack emissions Direct atmospheric emission rates for the seven selected PAHs from the Alcan-Beauharnois facility are summarized for the period from 1983 to 1996 in Table 2. Total emission rates show a greater than three-fold reduction from the 1983±1985 period to 1996. The emission rates used here apply to the 1986±1991 period
Table 2 PAH emission rates (kg/h) from Alcan-Beauharnois smelter stacks from 1983 to 1996a
a
Chemical
1983±1985
1986±1991
1993±1995
1996
Anthracene Benzo(a)pyrene Chrysene Fluoranthene Phenanthrene Pyrene Benzo(b)¯uoranthene P7 PAH (kg/h) (t/yr) Other PAHs Total PAH (kg/h) (t/yr)
0.388 0.776 1.941 4.075 2.717 3.493 1.941 15.331 (134.30) 4.075 19.406 (170.00)
0.237 0.475 1.187 2.493 1.662 2.137 1.187 9.378 (82.15) 2.493 11.872 (104.00)
0.183 0.365 0.913 1.918 1.279 1.644 0.913 7.215 (63.20) 1.918 9.132 (80.00)
0.182 0.363 0.545 0.908 0.605 0.908 0.968 4.479 (39.24) 1.573 6.050 (53.00)
Data were provided by Alcan, and show the reductions in emission rates. The 1986±1991 data were used here.
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St. Lawrence, Saint Louis, Ottawa and Chateauguay Rivers entering Lac Saint Louis are 8320, 5, 885, and 35 m3 /s, respectively. The Lac Saint Louis out¯ow, referred to as ``segment 4'' in the model is thus 9245 m3 /s. Clearly the riverine advective output of PAHs from Lac Saint Louis (Table 3) exceeds the advective inputs. Smelter emission rates (Table 2) greatly exceed the advective river output suggesting that only a small fraction of the stack emissions could be deposited into Lac Saint Louis. This simple assessment does not consider degradation rates, deposition into lake sediments or other PAH sources to the atmosphere or river.
(104 t/yr total PAH). More recent emission rates available indicate values about half these quantities. 3.2. Euent discharges PAH water euent discharges from the AlcanBeauharnois facility are included in the model as part of the PAH loadings from the Saint Louis River. No other direct euent discharges to the lake are indicated. 3.3. Background advective inputs in water Background concentrations of PAHs were reported for the St. Lawrence River (at Cornwall, Ont.) and the Ottawa River by Pham et al. (1993). Concentrations in the Saint Louis and Chateauguay Rivers were measured in 1991±1992 by Environment Canada (Lemieux C., Quemarais, B., Centre St-Laurent, unpublished data). PAH concentrations near the outlet of Lac Saint Louis, but up-river from a large sewage treatment outfall were reported by Pham and Proulx (1997). Average concentrations in raw water (dissolved plus particulate) are summarized in Table 3 along with estimates of the riverine advective inputs and output of PAHs which are calculated from the concentrations and water ¯ow rates for each river. Average annual ¯ow rates for the
3.4. Background advective inputs in air An important set of input data is the concentration of the PAHs in the background air ¯owing into the air compartment. The primary wind direction is from the west and south west, from regions with relatively low population densities, but at times there are ¯ows from the north and north east directions which include the urban community of Montreal. It is thus likely that the average concentration of the PAHs in this in¯ow air lies between these ``pristine'' and ``urban'' extremes. Fortunately, air monitoring data are available from a sampling station in central Montreal (30 km northeast and
Table 3 PAH concentrations for background advective riverine inputs to model and river output and calculated advective in¯ow and out¯ow ratesa Chemical
Input St. Lawrence
a
Output St. Louis
Ottawa
Chateauguay
Weighted Total
Lac Saint Louis
Concentration in water (ng/l) Anthracene 0.14 Benzo(a)pyrene 0.11 Chrysene 0.27 Fluoranthene 0.95 Phenanthrene 1.84 Pyrene 0.7 Benzo(b)¯uoranthene 0.94 P7 PAH 4.95
0.34 7.77 23.74 17.99 2.5 11.61 22.02 85.97
0.2 0.16 0.4 1.37 2.7 1.02 1.39 7.24
0.01 0.04 0.06 6.3 6.67 29.53 0.05 42.61
0.144 0.117 0.294 1.004 1.938 0.817 0.835 5.149
0.6 0.48 1.2 4.1 8.1 3.07 4.18 21.73
Advective rates (kg/h) Anthracene Benzo(a)pyrene Chrysene Fluoranthene Phenanthrene Pyrene Benzo(b)¯uoranthene P7 PAHs
6.12 ´ 10ÿ6 1.40 ´ 10ÿ4 4.27x10ÿ4 3.24 ´ 10ÿ4 4.50 ´ 10ÿ5 2.09 ´ 10ÿ4 3.96 ´ 10ÿ4 0.00155
0.001 0.001 0.0013 0.0043 0.0086 0.0033 0.0044 0.023
1.26 ´ 10ÿ6 5.04 ´ 10ÿ6 7.56 ´ 10ÿ6 7.94 ´ 10ÿ4 8.40 ´ 10ÿ4 3.72 ´ 10ÿ3 6.30 ´ 10ÿ6 0.00537
0.0048 0.0039 0.0098 0.0334 0.0645 0.0272 0.0278 0.1714
0.0042 0.0032 0.0081 0.028 0.055 0.02 0.023 0.1415
0.0197 0.0164 0.0404 0.142 0.2731 0.1027 0.142 0.7363
River input concentrations for St. Lawrence River-Cornwall and Ottawa River were calculated from Pham et al. (1993). Data for Saint Louis and Chateauguay Rivers are unpublished values from Environment Canada (Lemieux, C., Quemarais, B.). River ouput concentrations were sampled upstream of Montreal wastewater discharge near Ile Sainte-Therese (Pham and Proulx, 1997).
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Table 4 Geometric mean concentrations of PAHs in the atmosphere (ng/m3 ) at Point Petre, Ontario and central Montreal and weighted average of 75% Point Petre and 25% Montreal data Chemical Anthracene Benzo(a)pyrene Chrysene (includes triphenylene) Fluoranthene Phenanthrene Pyrene Benzo(b)¯uoranthene (incl. (b) and (k)) P7 PAH P Total PAH ( of 28 PAHs)
Concentration in air (ng/m3 ) Point Petre
Montreal
Weighted average
0.016 0.029 0.072 0.201 0.679 0.081 0.095 1.173 1.924
2.054 0.255 1.301 4.804 22.177 2.983 1.583 36.705 50.975
0.526 0.086 0.379 1.352 6.053 0.807 0.467 10.056 14.187
downwind of Beauharnois; Fig. 1) and from a rural station located at Point Petre, Ontario (about 350 km to the southwest of Montreal on the north shore of Lake Ontario). These data were kindly provided by Dr. Tom Dann of Environment Canada for 1996, it being believed that levels have changed relatively little over the period 1980±1999. These data are given in Table 4, the values being averages of between 27 and 36 measurements taken over the course of a year. On the basis of wind direction/frequency information it was decided to estimate an annual weighted mean background air concentration as 25% of the Montreal and 75% of the Point Petre data. It should be noted that there may be a contribution to the Montreal levels from Alcan, so some ``double-counting'' may occur. The data in Table 4 suggest that concentrations in central Montreal are about a factor of 10±30 higher than those at the more pristine Point Petre. The average total suspended particulate (TSP) levels in Montreal air were 50 lg/m3 compared with 18 lg/m3 at Point Petre. These average PAH and TSP values include a small number (approximately 10%) of the values which are very high, and exceed the average by more than a factor of 4, possibly as a result of an intense but transient local source. Alcan provided a summary of B(a)P air concentration data for the period 1989±1995 from a sampling station near the south basin of Lac Saint Louis. Annual arithmetic mean concentrations showed a decline from 6.9 ng/m3 in 1989 to a low of 2.2 ng/m3 in 1993, then rising to 3.6 ng/m3 in 1995. Annual geometric mean concentrations show a more consistent trend with mean values declining 3.2-fold from 1.9 ng/m3 in 1989 to 0.6 ng/m3 in 1995. The most recent B(a)P air concentrations provided by Alcan were about three-fold higher, on average, than the measurements from central Montreal. This suggests that PAH concentrations close to the smelter and over the lake may be several fold higher than those measured in central Montreal. Obviously, there will be elevated PAH concentrations over the lake
at times when the atmospheric plume from the Beauharnois facility ¯ows in a generally north-easterly direction over the lake but no such measurements have been reported. It is likely that considerable ¯uctuations in PAH concentration occur over the lake with shifts in wind direction. 4. Physical±chemical properties Table 5 gives the selected physical±chemical properties at 25°C and estimated environmental half lives in air, water and sediment for the relevant PAHs. All values are from Mackay et al. (1992). Values were reduced by factors of 4 for vapor pressure and 2 for solubility to account for the eect of the mean annual environmental temperature of 10°C. While physical±chemical properties are considered as constants there is some degree of error associated with their measurement. A recent study by de Maagd et al. (1998) provides insight on the error associated with measurements of log Kow and water solubility for nine PAHs including ®ve of the selected compounds. For log Kow , the 95% con®dence limits were generally in the range of 0.1 to 0.2 of the mean (e.g., B(a)P log Kow 6.13, 95% con®dence limits 5.91±6.28, n 6). When expressed as log values this degree of error appears small, but the ratio of upper/lower con®dence limits for Kow is about a factor of 2. Given this level of uncertainty associated with Kow measurements, agreement within a factor of 2 between observed concentrations and model results should be considered good. Water solubilities were measured in the same study with appreciably greater consistency, with standard deviations averaging about 15% of the mean value. It transpires that the rates of reaction calculated using these half lives are relatively small compared to rates of other loss processes. Accordingly errors of a factor of 3±5 in half life have little impact on the ®nal concentrations in any of the media.
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Table 5 Physical and chemical properties of selected PAHs at 25°C and their estimated half lives (hours) in air, sediment and water; all values are from Mackay et al. (1992) Chemical Anth
B(a)P
Chry
Fluo
Phen
Pyr
B(b)F
Melting pt. (°C) Molecular wt. (g/mol) Water solubility (g/m3 ) Vapor pressure (Pa) H (Pa m3 /mol) log Kow
216.2 178.2 0.0450 0.001 3.96 4.54
175.0 252.3 0.0038 7.0 ´ 10ÿ7 0.046 6.04
255.0 228.3 0.0020 5.7 ´ 10ÿ7 0.0122 5.86
111.0 202.3 0.2600 0.001 1.04 5.22
101.0 178.2 1.1000 0.018 3.24 4.57
156.0 202.3 0.1320 0.0006 0.92 5.18
168.0 252.3 0.0015 5.0 ´ 10ÿ7 0.016 5.8
Estimated half life (hours) Air Water Sediment
55 550 17 000
170 1700 55 000
170 1700 55 000
170 1700 55 000
170 1700 55 000
170 1700 55 000
55 550 17 000
H is Henry's law constant. Kow is octanol-water partition coecient.
5. Description of the model The lake model is a three segment version of the quantitative water±air±sediment interaction (QWASI) fugacity model developed by Mackay et al. (1983) and applied to aquatic systems by Mackay (1989) and Mackay and Southwood (1992). The use of fugacity as a surrogate for concentrations has been reviewed by Paterson and Mackay (1985) and Mackay (1991). Brie¯y, fugacity, f in units of pascal, is a measure of a chemical's ``escaping tendency''. It is linearly related to concentration, C (mol/m3 ) through a proportionality constant the fugacity capacity or Z (mol/m3 Pa) which is a function of the nature of the chemical properties of the medium in which it is present and temperature. The Z value of a chemical in a compartment is deduced using a dimensionless partition coecient, K12 between two phases 1 and 2 which is essentially the ratio of Z values (i.e., Z1 /Z2 ) between the two compartments. De®nition of Z starts in the air and then progresses to other media. Rates of intermedia transport and transformation processes are evaluated by using a group of transport parameters termed D values in units of mol/h Pa. The process rates in or from a phase are expressed as the product Df (mol/h). The basic unit of the QWASI model, as shown in Fig. 3, consists of three compartments: a well-mixed sediment compartment underlying a well-mixed water column which is exposed to the atmosphere. Bulk phase Z values are calculated for the three primary media which also include the contribution of dispersed phases within each media. The air compartment is treated as an air±aerosol mixture, water as water plus suspended particles and sediment as solids plus pore water. The modelled system consists of a single atmospheric compartment and three lake segments, each of which
Fig. 3. Processes treated in the QWASI model.
contains a water and sediment phase giving a total of seven well-mixed compartments. The atmospheric compartment receives as input air in¯ow containing background concentrations of PAHs plus occasional local non-point source PAH inputs, notably the urban community of greater Montreal and from wood and fuel burning in surrounding rural areas as well as emissions from the Alcan facility at Beauharnois. Losses are by advective transport from the air mass (i.e., loss in wind), degrading reactions, and wet and dry deposition to the lake. The air±aerosol partitioning was calculated using the sub-cooled liquid vapour pressure as described by Mackay (1991) but increasing the aerosol±air partition coecient by a factor of 4 to better quantify partitioning from an intense source. The primary direction of PAH transfer is from air to water because of the low air±water partition coecients. Evaporation from the lake is assessed as a loss process in the lake model, but to eliminate excessive algebraic complexity in the model
D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
evaporative losses are not included as an input to the atmosphere. This results in a insigni®cant error because the rates are low. It had been planned originally to include a more detailed atmospheric dispersion calculation using the conventional Gaussian plume approach. This proved to be problematic because of diculties averaging the conditions over the year as a result of changes in wind direction and speed, rain and snow fall and atmospheric stability. Although the prevailing wind direction is generally from the west to south-west there are times when it is from the east or north and the plant emissions will not reach the lake at all. Gaussian plume calculations gave results which are highly speci®c to the prevailing conditions resulting in a wide variation in concentrations from hour to hour and day to day. The model suggests that the lake water, and especially the sediment respond much more slowly and it was judged adequate to do annually averaged calculations. This approach proved to be satisfactory and is, we believe, the preferred modelling approach in such situations. It is possible to assign a variability to the atmospheric concentration and explore what eect this has on the model results. The height selected for the atmospheric box was 400 m which represents a reasonable vertical extent of the plume from Alcan during its transit over the lake and is consistent with observed concentrations and emission rates as described later. Dierential mass balance equations can be written for each of the water and sediment segments, but in this case it is sucient to treat the simpler steady-state situation in which the time derivatives are set to zero and the resultant algebraic equations describe the steadystate condition which will be reached after prolonged exposure of the system to constant input conditions. The steady-state solutions are, for each water compartment, fW
EW fA D8
P
fWi Di
5 D1 D2 D0 D6 D7 DD41 D D2 D3 D4
:
1
The terms in the numerator of Eq. (1) represent the input to the water column, namely, the point source discharges (EW ), atmospheric inputs (fA D8 ) and summation P of advective ¯ows from adjacent segments ( fWi Di ). The denominator contains D values for transport processes out of the water column which are de®ned in Fig. 3. To solve this equation, the various D values are consolidated into groups: fW
P P EW fA D8 fWi Di I fWi Di : D0 DE DT
2
The term DE represents the sum of all loss terms excluding advective out¯ows of water and particles. DE
687
also includes the transfer rate to sediment by deposition (D4 ) and diusion (D5 ) multiplied by the fraction of the amount transferred which does not return from the sediment. DT is an overall loss D value from the water column and comprises out¯ows to other segments (D0 ), net loss to sediment through deposition and diusion (D4 + D5 ), evaporation into air (D7 ) as well as transformation (D6 ) and is thus (D0 + DE ). There are three equations similar in form to Eq. (1) which are solved to give fW the fugacity in water in each segment. The fugacity in the sediment fS of each lake segment can then be determined by fS
fW D4 D5 : D1 D2 D3 D4
3
All ¯uxes, concentrations, and amounts are deduced and a mass balance is compiled and checked for consistency.
6. Results and discussion The results of the model simulations given in Tables 6 and 7 are discussed in detail for one compound (B(a)P) followed by a summary for the remaining PAHs and the combined results for the seven PAHs. Initial model trials were conducted using emission data from the 1986±1991 period since most available data are from this period. The general performance of the model is then discussed. For all model trials presented, it was assumed that 50% of the Alcan-Beauharnois stack emissions entered the air compartment over Lac Saint Louis. This estimate was based on the available data on prevailing wind direction in the region. 6.1. Benzo(a)pyrene The fate of B(a)P expressed by the model as of 1986± 1991 is summarized in Fig. 4. The inputs to the atmospheric compartment are 2081 kg/yr from Alcan and an estimated 31.2 kg/yr from background sources. The latter is probably underestimated since there are no reliable data for other local non-point source inputs. The fate of these inputs is 89% loss by advection (1875 kg/ yr), 0.6% loss by reaction (11.8 kg/yr), and 10.7% loss by deposition to the lake (225.4 kg/yr). The concentration in air over the lake is estimated to be 5.16 ng/m3 of which 99% is partitioned to aerosols. This compares well with the arithmetic mean concentrations of 5.1±6.9 ng/ m3 for the period 1989±1991 provided by Alcan. Only a negligible fraction of the 225 kg/yr deposited to the lake (<1 kg/yr) evaporates back to the atmosphere. The annual depositions are: segment 1, 77.4 kg/yr; segment 2, 59.1 kg/yr; segment 3, 88.9 kg/yr. The total riverine
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Table 6 Selected results from the mass balance model Chemical Concentration Air (ng/m3 ) Water ( ng/l) Sediment (ng/g) Mass in Air (kg) Mass in Water (kg) Mass in sediment (kg) Total mass (kg) Fluxes (kg/yr) River inputs Air±water deposition Air±water absorption River out¯ow Degradation in lake Water±sediment transfer (gross) Sediment burial
Anth
B(a)P
Chry
Fluo
Phen
Pyr
B(b)F
R7 PAH
3.28 0.17±0.32 0.17±0.28 0.21 0.1 0.38 0.68
5.16 0.49±1.94 33±97 0.33 0.43 83.5 84
13.00 1.2±4.9 63±184 0.83 1.07 159 161
30.10 1.6±4.7 21±44 1.92 1.2 45.2 48
25.60 2.0±3.7 3.0±5.3 1.63 1.17 6.84 9.6
25.60 1.2±3.4 15±28 1.63 0.88 30 32
13.00 1.9±5.7 43±95 0.83 1.4 94.6 97
115.7 8.7±24.7 192±454 7.58 6.25 419 432
42 4.4 12 57 1 0.3
34.6 225 0.18 246 11 21
86 553 3 614 21 41
298 304 95 686 9 15
555 32 92 675 5 4
246 198 87 524 6 10
289 567 0.1 803 49 48
1550 1883 289 3605 102 139.3
0.02
3
6
1.7
0.3
1.2
4
16.2
98.5 85.4
97.4 84.4
95.7 54.9
76.8 14.7
97.0 52.2
96.9 64.2
92.1 56.0
Alcan contribution to loading (%) Air over lake 84.5 Lac Saint Louis 23.9
Table 7 Observed and model-predicted PAH concentrations in water and sediments of Lac Saint Louisa Chemical Anthracene Benzo(a)pyrene Chrysene Fluoranthene Phenanthrene Pyrene Benzo(b)¯uoranthene P7 PAH
Water (ng/l)
Sediments (ng/g dry wt)
Observed
Predicted
Observed
Predicted
0.6 0.48 1.2 4.1 8.1 3.1 4.2 21.8
0.17±0.32 0.5±1.9 1.2±4.9 1.7±4.8 2.0±3.7 1.2±3.4 1.9±5.7 8.7±24.7
1±5 11±43 10±43 11±46 5±15 1±24 33±91 72±230
0.17±0.28 32.9±97.4 63.2±184.6 21.2±44.0 3.0±5.3 14.6±28.4 43.6±94.7 193±454
a
Predicted concentrations are given as the range of values determined for the three lake segments. Observed water concentrations calculated from Pham and Proulx (1997) for sampling station adjacent to Ile Ste. Therese near the outlet of Lac Saint Louis. Observed sediment concentrations are the range of geometric mean concentrations for samples taken in 1984±1985 from each of the lake segments used in the model. Data are from the Environment Canada report by Champoux and Sloterdijk (1988). Geometric means are used since PAH concentrations show high spatial heterogeneity. Means are based on ®ve to eight samples per segment.
inputs are estimated to be 34.7 kg/yr with 28.9 kg/yr from the St. Lawrence, and approximately 1.2 kg/yr from the Saint Louis, 4.5 kg/yr from the Ottawa River and 0.04 kg/yr from the Chateauguay River. These amount to 13.4% of the total annual input to the lake of 259 kg/yr with atmospheric deposition accounting for the remaining 86.6% of inputs to the lake. The estimated B(a)P concentrations in the water column of the three segments range from 0.49 to 1.94 ng/ l with a combined total mass of 0.43 kg. Concentrations in the sediment range from 32.9 to 97.4 ng/g (dry wt.),
with a total mass of 83.5 kg for the three segments. The major transport and transformation processes are downriver advection (246.4 kg/yr, 95.1%), net loss to sediments (12.2 kg/yr, 4.7%) with 10.8 kg/yr loss by reaction and 3.1 kg/yr loss by burial. The fate of B(a)P emitted from the Alcan-Beauharnois facility is thus dominated by advective transport downwind with only a small fraction of the emissions which cross-over the lake actually being deposited. Of the 50% which did enter the atmosphere over the lake, 89.4% remained in the air, becoming dispersed over a
D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
689
Fig. 4. Mass balance diagram with net ¯uxes (kg/yr) for benzo(a)pyrene in Lac Saint Louis. Refer to Fig. 2 to identify the ¯uxes.
much broader range. Of the total stack emission (4162 kg/yr), 5.3% is estimated to enter the lake. A sensitivity analysis showed that the key processes for which accurate parameters are essential are, in addition to the obvious emission rates: 1. Deposition velocity of aerosol particles from the atmosphere. 2. Eective atmospheric mixing height over the lake. 3. Fraction of the emissions which ¯ow over the lake. 4. Background B(a)P concentrations in air and water including emissions from other sources. The model was run with various values for these variables to determine the sensitivity of the results to these quantities, but the general pattern of chemical fate did not change appreciably. The predicted concentrations in water are about 5 to 10-fold higher than measured concentrations entering the lake and are close to the concentration of 0.48 ng/l downstream of Lac Saint Louis (Table 7) estimated from the work by Pham and Proulx (1997). The predicted sediment concentrations are similar to the observed geometric mean concentrations from the three lake segments. The key quantities controlling the residence time of the BaP in this system are the mass in the sediment and the net ¯ux to and from the sediment, the ratio of which is the residence time, i.e., 84 kg/21 kg/yr or 4 yr. The corresponding residence time in the water is controlled by the short ¯ow residence time of the water in the system. The implication is that the sediments act as the primary repository of the BaP and will respond in a characteristic time of some 4 yr to changes in loadings. Generally similar results were obtained for the other PAHs as shown in Tables 6 and 7 which also gives data for the sum of the seven PAHs.
P 6.2. Sum of the seven PAHs ( 7 PAH) P Results for the 7 PAH are summarized in Fig. 5. The rate of emission from the Alcan-Beauharnois facility was 82262 kg/yr. The 50% of this emission estimated to pass over the lake contributed 91.8% of the input to the air compartment. Background advective input was estimated to be 3654 kg/yr based on an air concentration of 10.1 ng/m3 . The resulting average air concentration over the lake was 116 ng/m3 , about 11.5 times higher than the background concentration. Some 36.1% of the airborne PAH is bound to aerosol particles. Advection is the dominant removal mechanism from the air over the lake, accounting for 94.4% of the inputs. Deposition to the lake accounts for 4.9% of the loss (2173 kg/yr), while loss by reaction in air accounts for less than 1%. Of the inputs to Lac Saint Louis, atmospheric deposition accounts for 58.4% (2173 kg/yr) with riverine inputs contributing 41.6% (1551 kg/yr). At steady-state, the three water column segments hold a total of 6.25 kg while the sediments hold 419.5 kg. The range of predicted water concentrations for the three segments (8.6± 24.7 ng/l) are close to the downriver concentration of 21.8 ng/l (Table 6) calculated from the study by Pham and Proulx (1997). The predicted concentration range in the sediments (177±454 ng/g dry wt.) is about double the range of observed geometric mean concentrations. About 96.8% of the losses from the lake are by advection downriver. Reaction in the water column is a minor loss mechanism (0.9%). Deposition to the sediments (85 kg/ yr) represents about 2.3% of losses, of which about 19% is lost by burial and 81% by P degrading reactions. The residence time of the 7 PAHs in the sediment is 4.5 yr with a much faster response in the water column. If (as has occurred) loadings are reduced, the entire
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D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
Fig. 5. Mass balance diagram with net ¯uxes (kg/yr) for
P7
system will respond substantially within 5 yr and would approach a new steady-state condition by 10 yr. It can thus be argued that for this contaminant in this system there is little merit in considering dredging because natural remediation processes are fairly rapid and would take eect in the time frame necessary for design and implementation of a remedial dredging program. The corollary to this is that if loadings are not reduced, and dredging is done, the system will recontaminate fairly rapidly to its pre-dredged condition. 6.3. Predicted vs. observed concentrations To assess the validity of the model, predicted and observed concentrations of the chemicals of interest are compared in the various media considered. In this case adequate data exist to permit comparisons of sediment concentrations but there are few data for water or air over the lake. Benzo(a)pyrene concentrations in air from a sampling station located near the south basin of the lake were provided by Alcan, but the model was calibrated to yield equivalent air concentrations to these values (about 5 ng/m3 ) by adjustment of the mixing height of the atmosphere. Sucient information was provided to make reasonable estimates of the concentrations of the selected PAHs in water. The recent studies by Pham et al. (1993) and Pham and Proulx (1997) reported total concentrations of PAHs in the St. Lawrence River (at Cornwall), the Ottawa River, and in the St. Lawrence downriver of Lac Saint Louis (near Ile St. Therese). The latter of these sites is well situated to make comparisons with water concentrations predicted by the model. These data are summarized in Table 6. The range of predicted values either bracket or are within a factor of 3 of the measured
PAHs in Lac Saint Louis. Refer to Fig. 2 to identify the ¯uxes.
values, this being considered a satisfactory ®t. A more extensive analysis of model performance would require actual measured water concentrations at several locations within the lake. Champoux and Sloterdijk (1988) measured the concentrations of 16 PAHs, including the seven considered here, in surface sediments in 1984±1985 from 17 locations in Lac Saint Louis. Levels showed high spatial heterogeneity as demonstrated by large standard deviations and a considerable dierence between arithmetic and geometric mean concentrations (Table 7). Overall, there is good agreement between predicted and observed concentrations for ®ve of the seven PAHs and for P7 PAH, the range of predicted sediment concentrations overlap the range de®ned by the arithmetic and geometric means. The model underpredicted the sediment and water concentrations of phenanthrene and especially anthracene. The most likely explanation for these discrepancies is an underprediction of sources. Adjusting reaction half lives, deposition velocities and other parameters could not achieve the required agreement. 6.4. Sign®cance of sources Because the equations used in the model are linear and the chemical loses the ``memory'' of its source when it reaches the lake, it is possible to estimate the percentage contribution of the Alcan emissions to the air, water and sediment concentrations. This is readily done from the mass balance (Table 6). For B(a)P, for example, the inputs to the air compartment are 2081 kg/yr from Alcan and 31.2 kg/yr from other sources, thus the air concentration of 5.16 ng/m3 is 98.5% attributable to Alcan and 1.5% to other sources. Consequently, the total atmospheric deposition of 225.4 kg/yr to the lake is
D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
attributable in similar proportions. The total input of 260 kg/yr to the lake is thus apportioned as 222 kg/yr to Alcan atmospheric emissions and 38 kg/yr to other sources (34.6 kg/yr river advective input plus 3.4 kg/yr atmospheric deposition). The concentrations in water and sediment can also be attributed in the same proportions. The loading of B(a)P from the Saint Louis River, which includes the discharge from the AlcanBeauharnois facility, accounts for just 0.5% of total B(a)P loadings to the lake and 3.5% of riverine inputs. Examination of corresponding results for each PAH suggests that Alcan atmospheric emissions are responsible for 14.7±85.4% of the loadings to Lac Saint Louis. The Saint Louis River is an insigni®cant source of PAHs to the lake in all cases, contributingPbetween 0.06% and 7 0.58% of total loadings. For the PAHs, Alcan atmospheric emissions are estimated to contribute 56.0% of total loadings to the lake while the Saint Louis River contributing only 0.36% of loadings. Other riverine inputs contribute 41.5%, while background atmospheric inputs contribute the remaining 2.5%. These estimates, of course, are based on the present assumption that Alcan-Beauharnois is the only source of direct emissions and discharge into the Lac Saint Louis system and the only other sources are background advective inputs via air and water. This assumption undoubtedly overlooks the contribution of PAHs from other sources in the greater Montreal urban area including sewage treatment plant discharges, storm sewers and other urban activities. Including these other sources as inputs to the model would reduce the percentage of loadings attributed to the Alcan-Beauharnois facility. 7. Conclusions The model has successfully quanti®ed the fate of ®ve of the seven PAHs in the atmosphere over Lac Saint Louis, transport into the water and sediments, and subsequent losses by advection, degradation and burial as of 1986±1991. For two of the PAHs (anthracene and phenanthrene) the concentrations are underpredicted by factors of 2±10, most likely because there are emissions from other sources not accounted for in the mass balance. The key processes are deposition from the atmosphere to the lake and advective out¯ow in the water, but most of the mass of PAH in the system resides in the sediment where it has a residence time of some 5 yr. The model enables the concentrations to be apportioned to sources, the conclusion being that most of the PAH in the system originated from the Alcan facility at Beauharnois. Observed and predicted concentrations of the P7 PAHs lie in the range 72±454 ng/g in the sediment and 8.7±24.7 ng/l in the water, providing a basis for evaluating eects on benthic and pelagic organisms. Because of the relatively short residence time there is
691
little merit in dredging. If lower concentrations are desired the model enables the corresponding emission reductions to be deduced. Currently (1999), emissions are believed to be approximately half those used by the model, thus concentrations, masses and ¯uxes are probably correspondingly lower. It is believed that models such as this can be used to evaluate the eects of industrial, municipal and other air emissions, including background inputs and point sources on contaminant concentrations in lakes and rivers. A comprehensive air±water±sediment mass balance clearly reveals the key inputs and processes of transport and transformation and provides a sound basis for remedial planning to reduce concentrations, if they are deemed to be unacceptably high.
Acknowledgements We thank Alcan and NSERC for ®nancial support and Dr. T. Dann (Environment Canada) for kindly providing PAH air concentration data from the Montreal and Point Petre sampling stations.
References Champoux, L., Sloterdijk, H., 1988. Etude de la qualite des sediments du Lac Saint-Louis (1984±85). Rapport Technique No. 1: Geochimie et contamination. Environment Canada, Conservation et Protection, Region du Quebec. de Maagd, P.G.J., ten Hulscher, D.Th.E.M., van den Huevel, H., Opperhuizen, A., Sijm, D.T.H.M., 1998. Physicochemical properties of polycyclic aromatic hydrocarbons: aqueous solubilities, n-octanol/water partition coecients, and HenryÕs law constants. Environ. Toxicol. Chem. 17, 251±257. Environment Canada. 1994. Synthese des connaissances sur les aspects physiques et chimiques de lÕeau et des sediments du Lac Saint Louis: Rapport techniques zones dÕintervention prioritaire 5 et 6. Centre Saint-Laurent, Environment Canada, Region du Quebec. Mackay, D., 1989. An approach to modelling the long-term behaviour of an organic contaminant in a large lake: application to PCBs in Lake Ontario. J. Great Lakes Res. 15, 283±297. Mackay, D., 1991. Multimedia Environmental Models: The Fugacity Approach, Lewis Publishers, Chelsea, MI, 257 pp. Mackay, D., Joy, M., Paterson, S., 1983. A quantitative water, air, sediment interaction (QWASI) fugacity model for describing the fate of chemicals in lakes. Chemosphere 12, 981±997. Mackay, D., Shiu, W.Y., Ma, K.C., 1992. Illustrated Handbook of Physical±Chemical Properties and Environmental Fate for Organic Chemicals, vol. II. Lewis Publishers, Chelsea, MI. p. 597. Mackay, D., Southwood, J.M., 1992. Modelling the fate of organochlorine chemicals in pulp mill euents. Water. Poll. Research J. Canada 27, 509±537.
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Paterson, S., Mackay, D., 1985. The fugacity concept in environmental modelling, In: Hutzinger, O. (Ed.), The Handbook of Environmental Chemistry, vol. 2/Part C. Springer, Heidelberg, pp. 121±140. Pham, T-T., Lum, K., Lemieux, C., 1993. Sources of PAHs in the St. Lawrence River (Canada) and their relative importance. Chemosphere 27, 1137±1149.
Pham, T-T., Proulx, S., 1997. PCBs and PAHs in the Montreal urban community (Quebec, Canada) wastewater treatment plant and in the euent plume in the St. Lawrence River. Wat. Res. 31, 1887±1896. Rukavina, N.A., Mudroch, A., Joshi, S.R., 1990. The geochemistry and sedimentology of the sur®cial sediments of Lac StLouis, St. Lawrence River. Sci. Total Environ. 97/98 481±494.
Mass balance model of source apportionment, transport and fate of PAHs in Lac Saint Louis, Quebec Donald Mackay *, Brendan Hickie Environmental Modelling Centre, Environmental and Resource Studies, Trent University, Peterborough, Ont., Canada K9J 7B8 Received 21 May 1999; accepted 30 September 1999
Abstract A mass balance model has been developed and calibrated to describe the sources, transport and fate of seven polycyclic aromatic hydrocarbons (PAHs; anthracene, benzo(a)pyrene, benzo(b)¯uoranthene, chrysene, ¯uoranthene, phenanthrene, and pyrene) in the water and sediments of, and atmosphere over Lac Saint Louis, Quebec. The model uses speci®ed input rates from background advective ¯ows and emissions from the Alcan aluminum smelting facility at Beauharnois to deduce atmospheric concentrations and rates of wet and dry deposition to the three segment lake. Concentrations in water and sediment as well as relevant mass ¯uxes and residence times are computed and compared satisfactorily with monitoring data for ®ve of the seven PAHs. Underestimation of concentrations for anthracene and phenanthrene is attributed to unquanti®ed additional sources. The sources of the PAH burden in the lake are apportioned, and the implications of these results are discussed including likely response times to changes in loadings. It is suggested that this mass balance approach is more widely applicable to situations in which water bodies are impacted by a variety of contaminant sources. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction There is continuing concern about the presence of polycyclic aromatic hydrocarbons (PAHs) in the environment, primarily because several are believed to be human carcinogens. These substances are produced during the combustion of carbonaceous materials including wood and fuel oils, especially under conditions of limited oxygen availability. They are also emitted during aluminum smelting. Because of their ubiquitous nature it is often dicult to assign sources quantitatively to prevailing concentrations since in a speci®c air mass or lake there may be a variety of local sources and background inputs from more distant sources. One approach to apportioning sources is to compile a mass balance model which attempts to quantify all inputs and
*
Corresponding author. Tel.: +705-748-1489; fax: +705748-1569. E-mail address: [email protected] (D. Mackay).
calculates the fate of the PAHs in a speci®c region, deducing the prevailing rates of transport and transformation, and the concentrations in a variety of environmental media. Such a model can be used to demonstrate the signi®cance of each source and can guide regulatory actions. This mass balance study is of the fate of seven PAHs in the air mass above Lac Saint Louis, Quebec, and in the lake water and sediments. The major aims were to assess the relative signi®cance of industrial and background sources, estimate prevailing concentrations, and apportion them to various sources. Further, the mass balance model can provide a clear picture of the key environmental processes, show which environmental and chemical properties are the most important determinants of fate, and can be used to assess the extent and rate at which reductions in emissions will translate into reduced concentrations. The approach taken is ®rst to characterize the dimensions and properties of the water segments and the air mass above the lake, i.e., the hydrology and
0045-6535/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 4 8 6 - 5
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meteorology. Previous studies of the fate and presence of PAHs in this region are then reviewed and data tabulated on observed concentrations in air, water and sediment. Input rates of selected PAHs from the AlcanBeauharnois facility are presented for several time periods, namely, 1996, 1993±1995, 1986±1991, and 1983±1985. Input rates for the 1983±1985 period are regarded as applicable to the previous 30 yr period. The physical±chemical properties and reaction half lives of the selected PAHs are presented. The model is described, results are presented and compared with existing monitoring data, and implications of the results are discussed. 2. The Lac Saint Louis environment Lac Saint Louis is a shallow ¯uvial lake located at the con¯uence of the Ottawa and St. Lawrence Rivers and
de®nes part of the southern and western boundaries of the Island of Montreal, Quebec (Fig. 1). Studies by Rukavina et al. (1990), Pham et al. (1993), Pham and Proulx (1997), Environment Canada (1994) and others suggest that the lake is best treated as three connected segments as shown in Figs. 1 and 2. The south segment (1) receives water input from the St. Lawrence River and some input from the fraction of the Ottawa River which ¯ows to the west of Ile Perrot. Segment (2) to the north includes a small portion of Lac Deux Montagnes and receives input from the Ottawa River and exchanges water with the third segment to the east. The east segment (3) exchanges water from both segments 1 and 2 and discharges to the St. Lawrence River. The three segments are similar in area, each being about 50 km2 . Table 1 gives the ¯ow data for these segments, the values being based on the river inputs and assumed 10% back¯ows from segment 3 to 1 and 2. Advective ¯ows of air and water are illustrated in Fig. 2.
Fig. 1. Map of the Lac Saint Louis region showing the segmentation of the lake.
Fig. 2. Compartments and pathways of the Lac Saint Louis system.
D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
683
Table 1 Characteristics of the tributaries and lake segments of Lac Saint Louisa Lake segments
a
South (1)
North (2)
East (3)
Lake segments Surface area (m2 ) Mean depth (m) Suspended particles (mg/l) Organic carbon content Sediment particle density Organic carbon content Vol. fraction sediment solids Sediment active depth (m) Sedimentation rate (g/m2 /d) Resuspension rate (g/m2 /d) Burial rate (g/m2 /d)
5.47 ´ 107 3.7 4.6 0.3 2 0.046 0.1 0.05 6 4 1.6
4.18 ´ 107 2.1 4.1 0.3 2 0.028 0.1 0.05 3 2 0.84
6.29 ´ 107 3 4.8 0.3 2 0.027 0.1 0.05 3 2 0.84
Inter-segment ¯ow rates (m3 /s) to 1 from to 2 from to 3 from
± 0 9493.4
336 ± 549
832.5 88.5 ±
Tributaries Flow rate (m3 /s) suspended particles (mg/l) P PAH concentration (ng/l)
St. Lawrence 8320 4.6 4.9
Ottawa 885 4.1 7.3
Chateauguay 35 7.8 42.6
St. Louis 5 18.5 85.8
Most values were derived from the St. Lawrence Vision 2000 report (Environment Canada, 1994).
Environment Canada (1994) has estimated the net annual loading of particles to the sediments of Lac Saint Louis to be between 45 000 and 130 000 t/yr, which correspond to average net deposition rates of 0.8±2.4 g/ m2 /d. According to Rukavina et al. (1990) sediment deposition rates tended to be highest in the deeper south basin (segment 1) to which we assigned a net deposition rates of 2.0 g/m2 /d with lower rates of 1.0 g/m2 /d for segments 2 and 3. These net deposition rates were used to estimate gross deposition and resuspension rates assuming that two-thirds of sedimented material is resuspended in all segments under these fairly turbulent riverine conditions.
3. PAH emissions and concentrations PAHs enter this system by three routes: 3.1. Stack emissions Direct atmospheric emission rates for the seven selected PAHs from the Alcan-Beauharnois facility are summarized for the period from 1983 to 1996 in Table 2. Total emission rates show a greater than three-fold reduction from the 1983±1985 period to 1996. The emission rates used here apply to the 1986±1991 period
Table 2 PAH emission rates (kg/h) from Alcan-Beauharnois smelter stacks from 1983 to 1996a
a
Chemical
1983±1985
1986±1991
1993±1995
1996
Anthracene Benzo(a)pyrene Chrysene Fluoranthene Phenanthrene Pyrene Benzo(b)¯uoranthene P7 PAH (kg/h) (t/yr) Other PAHs Total PAH (kg/h) (t/yr)
0.388 0.776 1.941 4.075 2.717 3.493 1.941 15.331 (134.30) 4.075 19.406 (170.00)
0.237 0.475 1.187 2.493 1.662 2.137 1.187 9.378 (82.15) 2.493 11.872 (104.00)
0.183 0.365 0.913 1.918 1.279 1.644 0.913 7.215 (63.20) 1.918 9.132 (80.00)
0.182 0.363 0.545 0.908 0.605 0.908 0.968 4.479 (39.24) 1.573 6.050 (53.00)
Data were provided by Alcan, and show the reductions in emission rates. The 1986±1991 data were used here.
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St. Lawrence, Saint Louis, Ottawa and Chateauguay Rivers entering Lac Saint Louis are 8320, 5, 885, and 35 m3 /s, respectively. The Lac Saint Louis out¯ow, referred to as ``segment 4'' in the model is thus 9245 m3 /s. Clearly the riverine advective output of PAHs from Lac Saint Louis (Table 3) exceeds the advective inputs. Smelter emission rates (Table 2) greatly exceed the advective river output suggesting that only a small fraction of the stack emissions could be deposited into Lac Saint Louis. This simple assessment does not consider degradation rates, deposition into lake sediments or other PAH sources to the atmosphere or river.
(104 t/yr total PAH). More recent emission rates available indicate values about half these quantities. 3.2. Euent discharges PAH water euent discharges from the AlcanBeauharnois facility are included in the model as part of the PAH loadings from the Saint Louis River. No other direct euent discharges to the lake are indicated. 3.3. Background advective inputs in water Background concentrations of PAHs were reported for the St. Lawrence River (at Cornwall, Ont.) and the Ottawa River by Pham et al. (1993). Concentrations in the Saint Louis and Chateauguay Rivers were measured in 1991±1992 by Environment Canada (Lemieux C., Quemarais, B., Centre St-Laurent, unpublished data). PAH concentrations near the outlet of Lac Saint Louis, but up-river from a large sewage treatment outfall were reported by Pham and Proulx (1997). Average concentrations in raw water (dissolved plus particulate) are summarized in Table 3 along with estimates of the riverine advective inputs and output of PAHs which are calculated from the concentrations and water ¯ow rates for each river. Average annual ¯ow rates for the
3.4. Background advective inputs in air An important set of input data is the concentration of the PAHs in the background air ¯owing into the air compartment. The primary wind direction is from the west and south west, from regions with relatively low population densities, but at times there are ¯ows from the north and north east directions which include the urban community of Montreal. It is thus likely that the average concentration of the PAHs in this in¯ow air lies between these ``pristine'' and ``urban'' extremes. Fortunately, air monitoring data are available from a sampling station in central Montreal (30 km northeast and
Table 3 PAH concentrations for background advective riverine inputs to model and river output and calculated advective in¯ow and out¯ow ratesa Chemical
Input St. Lawrence
a
Output St. Louis
Ottawa
Chateauguay
Weighted Total
Lac Saint Louis
Concentration in water (ng/l) Anthracene 0.14 Benzo(a)pyrene 0.11 Chrysene 0.27 Fluoranthene 0.95 Phenanthrene 1.84 Pyrene 0.7 Benzo(b)¯uoranthene 0.94 P7 PAH 4.95
0.34 7.77 23.74 17.99 2.5 11.61 22.02 85.97
0.2 0.16 0.4 1.37 2.7 1.02 1.39 7.24
0.01 0.04 0.06 6.3 6.67 29.53 0.05 42.61
0.144 0.117 0.294 1.004 1.938 0.817 0.835 5.149
0.6 0.48 1.2 4.1 8.1 3.07 4.18 21.73
Advective rates (kg/h) Anthracene Benzo(a)pyrene Chrysene Fluoranthene Phenanthrene Pyrene Benzo(b)¯uoranthene P7 PAHs
6.12 ´ 10ÿ6 1.40 ´ 10ÿ4 4.27x10ÿ4 3.24 ´ 10ÿ4 4.50 ´ 10ÿ5 2.09 ´ 10ÿ4 3.96 ´ 10ÿ4 0.00155
0.001 0.001 0.0013 0.0043 0.0086 0.0033 0.0044 0.023
1.26 ´ 10ÿ6 5.04 ´ 10ÿ6 7.56 ´ 10ÿ6 7.94 ´ 10ÿ4 8.40 ´ 10ÿ4 3.72 ´ 10ÿ3 6.30 ´ 10ÿ6 0.00537
0.0048 0.0039 0.0098 0.0334 0.0645 0.0272 0.0278 0.1714
0.0042 0.0032 0.0081 0.028 0.055 0.02 0.023 0.1415
0.0197 0.0164 0.0404 0.142 0.2731 0.1027 0.142 0.7363
River input concentrations for St. Lawrence River-Cornwall and Ottawa River were calculated from Pham et al. (1993). Data for Saint Louis and Chateauguay Rivers are unpublished values from Environment Canada (Lemieux, C., Quemarais, B.). River ouput concentrations were sampled upstream of Montreal wastewater discharge near Ile Sainte-Therese (Pham and Proulx, 1997).
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Table 4 Geometric mean concentrations of PAHs in the atmosphere (ng/m3 ) at Point Petre, Ontario and central Montreal and weighted average of 75% Point Petre and 25% Montreal data Chemical Anthracene Benzo(a)pyrene Chrysene (includes triphenylene) Fluoranthene Phenanthrene Pyrene Benzo(b)¯uoranthene (incl. (b) and (k)) P7 PAH P Total PAH ( of 28 PAHs)
Concentration in air (ng/m3 ) Point Petre
Montreal
Weighted average
0.016 0.029 0.072 0.201 0.679 0.081 0.095 1.173 1.924
2.054 0.255 1.301 4.804 22.177 2.983 1.583 36.705 50.975
0.526 0.086 0.379 1.352 6.053 0.807 0.467 10.056 14.187
downwind of Beauharnois; Fig. 1) and from a rural station located at Point Petre, Ontario (about 350 km to the southwest of Montreal on the north shore of Lake Ontario). These data were kindly provided by Dr. Tom Dann of Environment Canada for 1996, it being believed that levels have changed relatively little over the period 1980±1999. These data are given in Table 4, the values being averages of between 27 and 36 measurements taken over the course of a year. On the basis of wind direction/frequency information it was decided to estimate an annual weighted mean background air concentration as 25% of the Montreal and 75% of the Point Petre data. It should be noted that there may be a contribution to the Montreal levels from Alcan, so some ``double-counting'' may occur. The data in Table 4 suggest that concentrations in central Montreal are about a factor of 10±30 higher than those at the more pristine Point Petre. The average total suspended particulate (TSP) levels in Montreal air were 50 lg/m3 compared with 18 lg/m3 at Point Petre. These average PAH and TSP values include a small number (approximately 10%) of the values which are very high, and exceed the average by more than a factor of 4, possibly as a result of an intense but transient local source. Alcan provided a summary of B(a)P air concentration data for the period 1989±1995 from a sampling station near the south basin of Lac Saint Louis. Annual arithmetic mean concentrations showed a decline from 6.9 ng/m3 in 1989 to a low of 2.2 ng/m3 in 1993, then rising to 3.6 ng/m3 in 1995. Annual geometric mean concentrations show a more consistent trend with mean values declining 3.2-fold from 1.9 ng/m3 in 1989 to 0.6 ng/m3 in 1995. The most recent B(a)P air concentrations provided by Alcan were about three-fold higher, on average, than the measurements from central Montreal. This suggests that PAH concentrations close to the smelter and over the lake may be several fold higher than those measured in central Montreal. Obviously, there will be elevated PAH concentrations over the lake
at times when the atmospheric plume from the Beauharnois facility ¯ows in a generally north-easterly direction over the lake but no such measurements have been reported. It is likely that considerable ¯uctuations in PAH concentration occur over the lake with shifts in wind direction. 4. Physical±chemical properties Table 5 gives the selected physical±chemical properties at 25°C and estimated environmental half lives in air, water and sediment for the relevant PAHs. All values are from Mackay et al. (1992). Values were reduced by factors of 4 for vapor pressure and 2 for solubility to account for the eect of the mean annual environmental temperature of 10°C. While physical±chemical properties are considered as constants there is some degree of error associated with their measurement. A recent study by de Maagd et al. (1998) provides insight on the error associated with measurements of log Kow and water solubility for nine PAHs including ®ve of the selected compounds. For log Kow , the 95% con®dence limits were generally in the range of 0.1 to 0.2 of the mean (e.g., B(a)P log Kow 6.13, 95% con®dence limits 5.91±6.28, n 6). When expressed as log values this degree of error appears small, but the ratio of upper/lower con®dence limits for Kow is about a factor of 2. Given this level of uncertainty associated with Kow measurements, agreement within a factor of 2 between observed concentrations and model results should be considered good. Water solubilities were measured in the same study with appreciably greater consistency, with standard deviations averaging about 15% of the mean value. It transpires that the rates of reaction calculated using these half lives are relatively small compared to rates of other loss processes. Accordingly errors of a factor of 3±5 in half life have little impact on the ®nal concentrations in any of the media.
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Table 5 Physical and chemical properties of selected PAHs at 25°C and their estimated half lives (hours) in air, sediment and water; all values are from Mackay et al. (1992) Chemical Anth
B(a)P
Chry
Fluo
Phen
Pyr
B(b)F
Melting pt. (°C) Molecular wt. (g/mol) Water solubility (g/m3 ) Vapor pressure (Pa) H (Pa m3 /mol) log Kow
216.2 178.2 0.0450 0.001 3.96 4.54
175.0 252.3 0.0038 7.0 ´ 10ÿ7 0.046 6.04
255.0 228.3 0.0020 5.7 ´ 10ÿ7 0.0122 5.86
111.0 202.3 0.2600 0.001 1.04 5.22
101.0 178.2 1.1000 0.018 3.24 4.57
156.0 202.3 0.1320 0.0006 0.92 5.18
168.0 252.3 0.0015 5.0 ´ 10ÿ7 0.016 5.8
Estimated half life (hours) Air Water Sediment
55 550 17 000
170 1700 55 000
170 1700 55 000
170 1700 55 000
170 1700 55 000
170 1700 55 000
55 550 17 000
H is Henry's law constant. Kow is octanol-water partition coecient.
5. Description of the model The lake model is a three segment version of the quantitative water±air±sediment interaction (QWASI) fugacity model developed by Mackay et al. (1983) and applied to aquatic systems by Mackay (1989) and Mackay and Southwood (1992). The use of fugacity as a surrogate for concentrations has been reviewed by Paterson and Mackay (1985) and Mackay (1991). Brie¯y, fugacity, f in units of pascal, is a measure of a chemical's ``escaping tendency''. It is linearly related to concentration, C (mol/m3 ) through a proportionality constant the fugacity capacity or Z (mol/m3 Pa) which is a function of the nature of the chemical properties of the medium in which it is present and temperature. The Z value of a chemical in a compartment is deduced using a dimensionless partition coecient, K12 between two phases 1 and 2 which is essentially the ratio of Z values (i.e., Z1 /Z2 ) between the two compartments. De®nition of Z starts in the air and then progresses to other media. Rates of intermedia transport and transformation processes are evaluated by using a group of transport parameters termed D values in units of mol/h Pa. The process rates in or from a phase are expressed as the product Df (mol/h). The basic unit of the QWASI model, as shown in Fig. 3, consists of three compartments: a well-mixed sediment compartment underlying a well-mixed water column which is exposed to the atmosphere. Bulk phase Z values are calculated for the three primary media which also include the contribution of dispersed phases within each media. The air compartment is treated as an air±aerosol mixture, water as water plus suspended particles and sediment as solids plus pore water. The modelled system consists of a single atmospheric compartment and three lake segments, each of which
Fig. 3. Processes treated in the QWASI model.
contains a water and sediment phase giving a total of seven well-mixed compartments. The atmospheric compartment receives as input air in¯ow containing background concentrations of PAHs plus occasional local non-point source PAH inputs, notably the urban community of greater Montreal and from wood and fuel burning in surrounding rural areas as well as emissions from the Alcan facility at Beauharnois. Losses are by advective transport from the air mass (i.e., loss in wind), degrading reactions, and wet and dry deposition to the lake. The air±aerosol partitioning was calculated using the sub-cooled liquid vapour pressure as described by Mackay (1991) but increasing the aerosol±air partition coecient by a factor of 4 to better quantify partitioning from an intense source. The primary direction of PAH transfer is from air to water because of the low air±water partition coecients. Evaporation from the lake is assessed as a loss process in the lake model, but to eliminate excessive algebraic complexity in the model
D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
evaporative losses are not included as an input to the atmosphere. This results in a insigni®cant error because the rates are low. It had been planned originally to include a more detailed atmospheric dispersion calculation using the conventional Gaussian plume approach. This proved to be problematic because of diculties averaging the conditions over the year as a result of changes in wind direction and speed, rain and snow fall and atmospheric stability. Although the prevailing wind direction is generally from the west to south-west there are times when it is from the east or north and the plant emissions will not reach the lake at all. Gaussian plume calculations gave results which are highly speci®c to the prevailing conditions resulting in a wide variation in concentrations from hour to hour and day to day. The model suggests that the lake water, and especially the sediment respond much more slowly and it was judged adequate to do annually averaged calculations. This approach proved to be satisfactory and is, we believe, the preferred modelling approach in such situations. It is possible to assign a variability to the atmospheric concentration and explore what eect this has on the model results. The height selected for the atmospheric box was 400 m which represents a reasonable vertical extent of the plume from Alcan during its transit over the lake and is consistent with observed concentrations and emission rates as described later. Dierential mass balance equations can be written for each of the water and sediment segments, but in this case it is sucient to treat the simpler steady-state situation in which the time derivatives are set to zero and the resultant algebraic equations describe the steadystate condition which will be reached after prolonged exposure of the system to constant input conditions. The steady-state solutions are, for each water compartment, fW
EW fA D8
P
fWi Di
5 D1 D2 D0 D6 D7 DD41 D D2 D3 D4
:
1
The terms in the numerator of Eq. (1) represent the input to the water column, namely, the point source discharges (EW ), atmospheric inputs (fA D8 ) and summation P of advective ¯ows from adjacent segments ( fWi Di ). The denominator contains D values for transport processes out of the water column which are de®ned in Fig. 3. To solve this equation, the various D values are consolidated into groups: fW
P P EW fA D8 fWi Di I fWi Di : D0 DE DT
2
The term DE represents the sum of all loss terms excluding advective out¯ows of water and particles. DE
687
also includes the transfer rate to sediment by deposition (D4 ) and diusion (D5 ) multiplied by the fraction of the amount transferred which does not return from the sediment. DT is an overall loss D value from the water column and comprises out¯ows to other segments (D0 ), net loss to sediment through deposition and diusion (D4 + D5 ), evaporation into air (D7 ) as well as transformation (D6 ) and is thus (D0 + DE ). There are three equations similar in form to Eq. (1) which are solved to give fW the fugacity in water in each segment. The fugacity in the sediment fS of each lake segment can then be determined by fS
fW D4 D5 : D1 D2 D3 D4
3
All ¯uxes, concentrations, and amounts are deduced and a mass balance is compiled and checked for consistency.
6. Results and discussion The results of the model simulations given in Tables 6 and 7 are discussed in detail for one compound (B(a)P) followed by a summary for the remaining PAHs and the combined results for the seven PAHs. Initial model trials were conducted using emission data from the 1986±1991 period since most available data are from this period. The general performance of the model is then discussed. For all model trials presented, it was assumed that 50% of the Alcan-Beauharnois stack emissions entered the air compartment over Lac Saint Louis. This estimate was based on the available data on prevailing wind direction in the region. 6.1. Benzo(a)pyrene The fate of B(a)P expressed by the model as of 1986± 1991 is summarized in Fig. 4. The inputs to the atmospheric compartment are 2081 kg/yr from Alcan and an estimated 31.2 kg/yr from background sources. The latter is probably underestimated since there are no reliable data for other local non-point source inputs. The fate of these inputs is 89% loss by advection (1875 kg/ yr), 0.6% loss by reaction (11.8 kg/yr), and 10.7% loss by deposition to the lake (225.4 kg/yr). The concentration in air over the lake is estimated to be 5.16 ng/m3 of which 99% is partitioned to aerosols. This compares well with the arithmetic mean concentrations of 5.1±6.9 ng/ m3 for the period 1989±1991 provided by Alcan. Only a negligible fraction of the 225 kg/yr deposited to the lake (<1 kg/yr) evaporates back to the atmosphere. The annual depositions are: segment 1, 77.4 kg/yr; segment 2, 59.1 kg/yr; segment 3, 88.9 kg/yr. The total riverine
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D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
Table 6 Selected results from the mass balance model Chemical Concentration Air (ng/m3 ) Water ( ng/l) Sediment (ng/g) Mass in Air (kg) Mass in Water (kg) Mass in sediment (kg) Total mass (kg) Fluxes (kg/yr) River inputs Air±water deposition Air±water absorption River out¯ow Degradation in lake Water±sediment transfer (gross) Sediment burial
Anth
B(a)P
Chry
Fluo
Phen
Pyr
B(b)F
R7 PAH
3.28 0.17±0.32 0.17±0.28 0.21 0.1 0.38 0.68
5.16 0.49±1.94 33±97 0.33 0.43 83.5 84
13.00 1.2±4.9 63±184 0.83 1.07 159 161
30.10 1.6±4.7 21±44 1.92 1.2 45.2 48
25.60 2.0±3.7 3.0±5.3 1.63 1.17 6.84 9.6
25.60 1.2±3.4 15±28 1.63 0.88 30 32
13.00 1.9±5.7 43±95 0.83 1.4 94.6 97
115.7 8.7±24.7 192±454 7.58 6.25 419 432
42 4.4 12 57 1 0.3
34.6 225 0.18 246 11 21
86 553 3 614 21 41
298 304 95 686 9 15
555 32 92 675 5 4
246 198 87 524 6 10
289 567 0.1 803 49 48
1550 1883 289 3605 102 139.3
0.02
3
6
1.7
0.3
1.2
4
16.2
98.5 85.4
97.4 84.4
95.7 54.9
76.8 14.7
97.0 52.2
96.9 64.2
92.1 56.0
Alcan contribution to loading (%) Air over lake 84.5 Lac Saint Louis 23.9
Table 7 Observed and model-predicted PAH concentrations in water and sediments of Lac Saint Louisa Chemical Anthracene Benzo(a)pyrene Chrysene Fluoranthene Phenanthrene Pyrene Benzo(b)¯uoranthene P7 PAH
Water (ng/l)
Sediments (ng/g dry wt)
Observed
Predicted
Observed
Predicted
0.6 0.48 1.2 4.1 8.1 3.1 4.2 21.8
0.17±0.32 0.5±1.9 1.2±4.9 1.7±4.8 2.0±3.7 1.2±3.4 1.9±5.7 8.7±24.7
1±5 11±43 10±43 11±46 5±15 1±24 33±91 72±230
0.17±0.28 32.9±97.4 63.2±184.6 21.2±44.0 3.0±5.3 14.6±28.4 43.6±94.7 193±454
a
Predicted concentrations are given as the range of values determined for the three lake segments. Observed water concentrations calculated from Pham and Proulx (1997) for sampling station adjacent to Ile Ste. Therese near the outlet of Lac Saint Louis. Observed sediment concentrations are the range of geometric mean concentrations for samples taken in 1984±1985 from each of the lake segments used in the model. Data are from the Environment Canada report by Champoux and Sloterdijk (1988). Geometric means are used since PAH concentrations show high spatial heterogeneity. Means are based on ®ve to eight samples per segment.
inputs are estimated to be 34.7 kg/yr with 28.9 kg/yr from the St. Lawrence, and approximately 1.2 kg/yr from the Saint Louis, 4.5 kg/yr from the Ottawa River and 0.04 kg/yr from the Chateauguay River. These amount to 13.4% of the total annual input to the lake of 259 kg/yr with atmospheric deposition accounting for the remaining 86.6% of inputs to the lake. The estimated B(a)P concentrations in the water column of the three segments range from 0.49 to 1.94 ng/ l with a combined total mass of 0.43 kg. Concentrations in the sediment range from 32.9 to 97.4 ng/g (dry wt.),
with a total mass of 83.5 kg for the three segments. The major transport and transformation processes are downriver advection (246.4 kg/yr, 95.1%), net loss to sediments (12.2 kg/yr, 4.7%) with 10.8 kg/yr loss by reaction and 3.1 kg/yr loss by burial. The fate of B(a)P emitted from the Alcan-Beauharnois facility is thus dominated by advective transport downwind with only a small fraction of the emissions which cross-over the lake actually being deposited. Of the 50% which did enter the atmosphere over the lake, 89.4% remained in the air, becoming dispersed over a
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Fig. 4. Mass balance diagram with net ¯uxes (kg/yr) for benzo(a)pyrene in Lac Saint Louis. Refer to Fig. 2 to identify the ¯uxes.
much broader range. Of the total stack emission (4162 kg/yr), 5.3% is estimated to enter the lake. A sensitivity analysis showed that the key processes for which accurate parameters are essential are, in addition to the obvious emission rates: 1. Deposition velocity of aerosol particles from the atmosphere. 2. Eective atmospheric mixing height over the lake. 3. Fraction of the emissions which ¯ow over the lake. 4. Background B(a)P concentrations in air and water including emissions from other sources. The model was run with various values for these variables to determine the sensitivity of the results to these quantities, but the general pattern of chemical fate did not change appreciably. The predicted concentrations in water are about 5 to 10-fold higher than measured concentrations entering the lake and are close to the concentration of 0.48 ng/l downstream of Lac Saint Louis (Table 7) estimated from the work by Pham and Proulx (1997). The predicted sediment concentrations are similar to the observed geometric mean concentrations from the three lake segments. The key quantities controlling the residence time of the BaP in this system are the mass in the sediment and the net ¯ux to and from the sediment, the ratio of which is the residence time, i.e., 84 kg/21 kg/yr or 4 yr. The corresponding residence time in the water is controlled by the short ¯ow residence time of the water in the system. The implication is that the sediments act as the primary repository of the BaP and will respond in a characteristic time of some 4 yr to changes in loadings. Generally similar results were obtained for the other PAHs as shown in Tables 6 and 7 which also gives data for the sum of the seven PAHs.
P 6.2. Sum of the seven PAHs ( 7 PAH) P Results for the 7 PAH are summarized in Fig. 5. The rate of emission from the Alcan-Beauharnois facility was 82262 kg/yr. The 50% of this emission estimated to pass over the lake contributed 91.8% of the input to the air compartment. Background advective input was estimated to be 3654 kg/yr based on an air concentration of 10.1 ng/m3 . The resulting average air concentration over the lake was 116 ng/m3 , about 11.5 times higher than the background concentration. Some 36.1% of the airborne PAH is bound to aerosol particles. Advection is the dominant removal mechanism from the air over the lake, accounting for 94.4% of the inputs. Deposition to the lake accounts for 4.9% of the loss (2173 kg/yr), while loss by reaction in air accounts for less than 1%. Of the inputs to Lac Saint Louis, atmospheric deposition accounts for 58.4% (2173 kg/yr) with riverine inputs contributing 41.6% (1551 kg/yr). At steady-state, the three water column segments hold a total of 6.25 kg while the sediments hold 419.5 kg. The range of predicted water concentrations for the three segments (8.6± 24.7 ng/l) are close to the downriver concentration of 21.8 ng/l (Table 6) calculated from the study by Pham and Proulx (1997). The predicted concentration range in the sediments (177±454 ng/g dry wt.) is about double the range of observed geometric mean concentrations. About 96.8% of the losses from the lake are by advection downriver. Reaction in the water column is a minor loss mechanism (0.9%). Deposition to the sediments (85 kg/ yr) represents about 2.3% of losses, of which about 19% is lost by burial and 81% by P degrading reactions. The residence time of the 7 PAHs in the sediment is 4.5 yr with a much faster response in the water column. If (as has occurred) loadings are reduced, the entire
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D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
Fig. 5. Mass balance diagram with net ¯uxes (kg/yr) for
P7
system will respond substantially within 5 yr and would approach a new steady-state condition by 10 yr. It can thus be argued that for this contaminant in this system there is little merit in considering dredging because natural remediation processes are fairly rapid and would take eect in the time frame necessary for design and implementation of a remedial dredging program. The corollary to this is that if loadings are not reduced, and dredging is done, the system will recontaminate fairly rapidly to its pre-dredged condition. 6.3. Predicted vs. observed concentrations To assess the validity of the model, predicted and observed concentrations of the chemicals of interest are compared in the various media considered. In this case adequate data exist to permit comparisons of sediment concentrations but there are few data for water or air over the lake. Benzo(a)pyrene concentrations in air from a sampling station located near the south basin of the lake were provided by Alcan, but the model was calibrated to yield equivalent air concentrations to these values (about 5 ng/m3 ) by adjustment of the mixing height of the atmosphere. Sucient information was provided to make reasonable estimates of the concentrations of the selected PAHs in water. The recent studies by Pham et al. (1993) and Pham and Proulx (1997) reported total concentrations of PAHs in the St. Lawrence River (at Cornwall), the Ottawa River, and in the St. Lawrence downriver of Lac Saint Louis (near Ile St. Therese). The latter of these sites is well situated to make comparisons with water concentrations predicted by the model. These data are summarized in Table 6. The range of predicted values either bracket or are within a factor of 3 of the measured
PAHs in Lac Saint Louis. Refer to Fig. 2 to identify the ¯uxes.
values, this being considered a satisfactory ®t. A more extensive analysis of model performance would require actual measured water concentrations at several locations within the lake. Champoux and Sloterdijk (1988) measured the concentrations of 16 PAHs, including the seven considered here, in surface sediments in 1984±1985 from 17 locations in Lac Saint Louis. Levels showed high spatial heterogeneity as demonstrated by large standard deviations and a considerable dierence between arithmetic and geometric mean concentrations (Table 7). Overall, there is good agreement between predicted and observed concentrations for ®ve of the seven PAHs and for P7 PAH, the range of predicted sediment concentrations overlap the range de®ned by the arithmetic and geometric means. The model underpredicted the sediment and water concentrations of phenanthrene and especially anthracene. The most likely explanation for these discrepancies is an underprediction of sources. Adjusting reaction half lives, deposition velocities and other parameters could not achieve the required agreement. 6.4. Sign®cance of sources Because the equations used in the model are linear and the chemical loses the ``memory'' of its source when it reaches the lake, it is possible to estimate the percentage contribution of the Alcan emissions to the air, water and sediment concentrations. This is readily done from the mass balance (Table 6). For B(a)P, for example, the inputs to the air compartment are 2081 kg/yr from Alcan and 31.2 kg/yr from other sources, thus the air concentration of 5.16 ng/m3 is 98.5% attributable to Alcan and 1.5% to other sources. Consequently, the total atmospheric deposition of 225.4 kg/yr to the lake is
D. Mackay, B. Hickie / Chemosphere 41 (2000) 681±692
attributable in similar proportions. The total input of 260 kg/yr to the lake is thus apportioned as 222 kg/yr to Alcan atmospheric emissions and 38 kg/yr to other sources (34.6 kg/yr river advective input plus 3.4 kg/yr atmospheric deposition). The concentrations in water and sediment can also be attributed in the same proportions. The loading of B(a)P from the Saint Louis River, which includes the discharge from the AlcanBeauharnois facility, accounts for just 0.5% of total B(a)P loadings to the lake and 3.5% of riverine inputs. Examination of corresponding results for each PAH suggests that Alcan atmospheric emissions are responsible for 14.7±85.4% of the loadings to Lac Saint Louis. The Saint Louis River is an insigni®cant source of PAHs to the lake in all cases, contributingPbetween 0.06% and 7 0.58% of total loadings. For the PAHs, Alcan atmospheric emissions are estimated to contribute 56.0% of total loadings to the lake while the Saint Louis River contributing only 0.36% of loadings. Other riverine inputs contribute 41.5%, while background atmospheric inputs contribute the remaining 2.5%. These estimates, of course, are based on the present assumption that Alcan-Beauharnois is the only source of direct emissions and discharge into the Lac Saint Louis system and the only other sources are background advective inputs via air and water. This assumption undoubtedly overlooks the contribution of PAHs from other sources in the greater Montreal urban area including sewage treatment plant discharges, storm sewers and other urban activities. Including these other sources as inputs to the model would reduce the percentage of loadings attributed to the Alcan-Beauharnois facility. 7. Conclusions The model has successfully quanti®ed the fate of ®ve of the seven PAHs in the atmosphere over Lac Saint Louis, transport into the water and sediments, and subsequent losses by advection, degradation and burial as of 1986±1991. For two of the PAHs (anthracene and phenanthrene) the concentrations are underpredicted by factors of 2±10, most likely because there are emissions from other sources not accounted for in the mass balance. The key processes are deposition from the atmosphere to the lake and advective out¯ow in the water, but most of the mass of PAH in the system resides in the sediment where it has a residence time of some 5 yr. The model enables the concentrations to be apportioned to sources, the conclusion being that most of the PAH in the system originated from the Alcan facility at Beauharnois. Observed and predicted concentrations of the P7 PAHs lie in the range 72±454 ng/g in the sediment and 8.7±24.7 ng/l in the water, providing a basis for evaluating eects on benthic and pelagic organisms. Because of the relatively short residence time there is
691
little merit in dredging. If lower concentrations are desired the model enables the corresponding emission reductions to be deduced. Currently (1999), emissions are believed to be approximately half those used by the model, thus concentrations, masses and ¯uxes are probably correspondingly lower. It is believed that models such as this can be used to evaluate the eects of industrial, municipal and other air emissions, including background inputs and point sources on contaminant concentrations in lakes and rivers. A comprehensive air±water±sediment mass balance clearly reveals the key inputs and processes of transport and transformation and provides a sound basis for remedial planning to reduce concentrations, if they are deemed to be unacceptably high.
Acknowledgements We thank Alcan and NSERC for ®nancial support and Dr. T. Dann (Environment Canada) for kindly providing PAH air concentration data from the Montreal and Point Petre sampling stations.
References Champoux, L., Sloterdijk, H., 1988. Etude de la qualite des sediments du Lac Saint-Louis (1984±85). Rapport Technique No. 1: Geochimie et contamination. Environment Canada, Conservation et Protection, Region du Quebec. de Maagd, P.G.J., ten Hulscher, D.Th.E.M., van den Huevel, H., Opperhuizen, A., Sijm, D.T.H.M., 1998. Physicochemical properties of polycyclic aromatic hydrocarbons: aqueous solubilities, n-octanol/water partition coecients, and HenryÕs law constants. Environ. Toxicol. Chem. 17, 251±257. Environment Canada. 1994. Synthese des connaissances sur les aspects physiques et chimiques de lÕeau et des sediments du Lac Saint Louis: Rapport techniques zones dÕintervention prioritaire 5 et 6. Centre Saint-Laurent, Environment Canada, Region du Quebec. Mackay, D., 1989. An approach to modelling the long-term behaviour of an organic contaminant in a large lake: application to PCBs in Lake Ontario. J. Great Lakes Res. 15, 283±297. Mackay, D., 1991. Multimedia Environmental Models: The Fugacity Approach, Lewis Publishers, Chelsea, MI, 257 pp. Mackay, D., Joy, M., Paterson, S., 1983. A quantitative water, air, sediment interaction (QWASI) fugacity model for describing the fate of chemicals in lakes. Chemosphere 12, 981±997. Mackay, D., Shiu, W.Y., Ma, K.C., 1992. Illustrated Handbook of Physical±Chemical Properties and Environmental Fate for Organic Chemicals, vol. II. Lewis Publishers, Chelsea, MI. p. 597. Mackay, D., Southwood, J.M., 1992. Modelling the fate of organochlorine chemicals in pulp mill euents. Water. Poll. Research J. Canada 27, 509±537.
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Paterson, S., Mackay, D., 1985. The fugacity concept in environmental modelling, In: Hutzinger, O. (Ed.), The Handbook of Environmental Chemistry, vol. 2/Part C. Springer, Heidelberg, pp. 121±140. Pham, T-T., Lum, K., Lemieux, C., 1993. Sources of PAHs in the St. Lawrence River (Canada) and their relative importance. Chemosphere 27, 1137±1149.
Pham, T-T., Proulx, S., 1997. PCBs and PAHs in the Montreal urban community (Quebec, Canada) wastewater treatment plant and in the euent plume in the St. Lawrence River. Wat. Res. 31, 1887±1896. Rukavina, N.A., Mudroch, A., Joshi, S.R., 1990. The geochemistry and sedimentology of the sur®cial sediments of Lac StLouis, St. Lawrence River. Sci. Total Environ. 97/98 481±494.