Environ. Sci. Technol. 19Q2, 26,85-89
Sparks, D. L. Kinetics of soil chemical processes;Academic Press: New York, 1989; p 47. Hansmann, D. D.; Anderson, M. A. Environ. Sci. Technol. 1985,19,544-551. Healy, T. W.; James, R. 0.;Cooper, R. Adv. Chem. Ser. 1968, NO. 79, 62-73. James, R. 0.;Healy, T. W. J. Colloid Interface Sci. 1972, 40, 53-65. James, R. 0.;Healy, T. W. J. Colloid Interface Sci. 1972, 41,6581. Crowthier, D. L.; Dillard, J. G.; Murray, J. W. Geochim. Cosmochim. Acta 1983,47, 1399-1403. Murray, J. W.; Dillard, J. G. Geochim. Cosmochim. Acta 1979, 43, 781-787. Tewari, P. H.; Lee, W. J. Colloid Interface Sci. 1975,52, 77-88.
(31) Murray, J. W. J. Colloid Interface Sci. 1974,46,357-371. (32) Allison, J. D.; Brown, D. S.; Novo-Gradac, K. J.
MINTEQASIPRODEFAS, A geochemical assessment model for environmental systems: Version 3.0; U.S.Environmental Protection Agency: Athens, GA, 1990. (33) Brown, G. E., Jr.; Parks, G. A.; Chisholm-Brause, C. J. Chimia 1989,43, 248-256. (34) Bleam, W. F.;McBride, M. B. J. Colloid Interface Sci. 1985, 103, 124-132. (35) McBride, M. B. Soil Sci. SOC.Am. J. 1987,51,1466-1472. (36) Stone, A. T. In Rates of soil chemical processes; Sparks, D. L., Suarez, D. L., Eds. SSSA Spec. Publ., in press.
Received for review January 28, 1991. Revised manuscript received June 13, 1991. Accepted June 26, 1991.
Isotopic Composition of Sulfur in Mosses across Canada Jerome 0. Nrlagu" and Walter A. Glooschenko
National Water Research Institute, Box 5050, Burlington, Ontario L7R 4A6, Canada
rn The isotopic composition of sulfur in mosses is used as the basis for dividing the country into four source regions of atmospheric sulfur: (i) the Western region dominated by seasalt spray sulfur, (2) the Prairie region which is strongly influenced by sulfur from evaporitic dust particles, (3) Ontario and Quebec region where the sulfur is derived mostly from industrial sources, and (4) the Atlantic region with a mixture of sulfur from Atlantic seasalt sprays and the polluted air masses from continental North America. The major point sources such the smelters at Sudbury (Ontario), Rouyn-Noranda (Quebec), Thompson (Manitoba), and Flin Flon (Saskatchewan) can also be identified on the basis of their isotopic signature on the moss sulfur. The data suggest that the stable isotopic composition of sulfur in plants can be used as tracers in the study of regional and transboundary movement of pollutant sulfur.
Introduction Cryptogams (especially mosses and lichens) are very sensitive to sulfur oxides (SO,) and hence have been used in biomonitoring the degree of air pollution on both local and regional scales (1-5). Such plants are able to provide an integrated record of the intensity of air pollution because (a) they derive most of their mineral requirements along with the contaminants from the atmosphere and (b) they are efficient traps that can bioaccumulate many elements to levels well above their metabolic needs (1,6-8). In general, the relationship between the ambient concentration or deposition of SO, and the sulfur content of the moss or lichen is not straightforward. The accumulation of sulfur by any moss or lichen species can also be influenced by the duration and episodicity of sulfur fumigation, humidity and other weather conditions, rates and processes of wet/* deposition, rate and robustness of plant growth, general chemodynamics of the particular plant species, etc. These variables, by contrast, do not always induce a significant fractionation of sulfur isotopes, and the isotopic composition of sulfur in mosses should be similar to that of atmospheric sulfur oxides (9-12). Measurement of the isotopic composition of sulfur in mosses should thus represent a unique and powerful tool which has yet to be fully exploited in the study of sulfur pollution on a regional scale (13, 14). 0013-936X/92/0926-0085$03.00/0
This report presents the concentrations and isotopic composition of sulfur in mosses from bog ecosystems across Canada, with detailed sampling around two major SO, point sources (base metal smelters) at Sudbury, ON, and Rouyn-Noranda, PQ. The objectives of the study are 2-fold: (a) to demonstrate that the isotopic signature of sulfur in ambient air is similar to that in the moss samples and (b) to define any regional differences in the sources and intensity of sulfur pollution across the country.
Methodology The moss species, Sphagnum fuscum, is particularly suitable for this study. It grows on low hummocks in ombrotrophic bogs which receive all their nutrients and contaminant burdens from the atmosphere. It occurs throughout Canada and western Europe. The numerous studies using this species in biomonitoring the atmospheric deposition of metals and other contaminants show that it is an excellent accumulator of air pollutants (7). The Sphagnum spp. specimens were collected from bogs located along a transect running approximately northwest from Sudbury (7,15). In the Rouyn-Noranda region, all the samples were obtained within a 30-km radius of the copper smelter. Around these two major sources of pollutant sulfur in Canada, the occurrence of s.fuscum tends to be patchy, and samples of Sphagnum capillaceum (another moss species) and Chamaedaphne calyculata (leatherleaf)were also obtained to compare the interspecies differences in the sulfur isotope composition as well. The sampling of mosses from bogs across Canada (Figure 1) was not based on any particular grid but was determined by the occurrence of S. fuscum especially in boreal forest areas with no obvious local source of sulfur pollution. At each locality, at least four aliquot samples of the living or recently dead mosses were collected randomly from hummocks in the bog and then composited to make a volume of approximately 2.0 L. Any areas under trees or shrubs were avoided, and the collection was restricted to a depth of less than 5.0 cm. Unwanted plants and twigs were removed. About 100-200 g of the shrub leaves were also collected in the Sudbury and Rouyn-Noranda areas. Samples were kept cool in the field and stored in sealed containers upon return to the laboratory. Subsequently, the samples were dried in an oven at a temperature of 90
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 26,No. 1, 1992
85
Figure 1. Map of Canada showing the sample sites. The description of the locations is given in Table 11.
Table I. Concentrations (pg g-') and Isotopic Composition (%) of Sulfur in Mosses and Shrub around the Smelters at Sudbury (Ontario) and Rouyn-Noranda (Quebec)n
distance from smelter, km
Sphagnum capillaceum
Sphagnum fuscum conc S346
conc
634s
Chamaedaphne calyculata conc 634s
Sudburyb 18 85 85
192 192 254 254 288 288 20, NW 33,NNW 33, NNW 58, NE 20, E 22, E
770
+2.5
780
+2.2
600
+2.8
620 560 790 670
+2.2 +2.4 +2.9 +2.4
490 600
+2.1 +2.2
580
+1.1
620
+1.2
590 Rouyn-Noranda
ND
650
+1.1
950 920 1100
+1.9 +1.4 +1.2
1300 710 880 700 810 650 590
+0.67 +1.5 +1.2 +2.3 +2.9 +3.1 +3.2
520
+3.7
aSphagnum is a moss and Chamaedaphne is shrub. There is no entry in the table where a given species does not occur at a particular site. ND, not determined. bThe stations are located north of Sudbury approximately parallel to the Trans-Canada Highway between Kapuskasing and North Bay.
OC for 48 h. The dried samples were ground in a Wiley mill to 60 mesh (roughly 250 pm). The samples were digested with concentrated nitric acid and then with perchloric acid. The sulfur content of each leachate was determined gravimetrically as barium sulfate. The BaS04 was decomposed thermally (16,17) and the 34S/32Sratio in the liberated SOz was determined on a dual-collector, isotope ratio mass spectrometer (VG Micromass Model 602C). The isotope ratio results are given in the usual 634Snotation: 634S (%o) = (Rsample/Rsd - 1) X 1000 where R,, le and Rstdrefer to the 34S/32Sratios of the sample an8the standard Canyon Diablo troilite, respectively. On the basis of replicate analyses of several aliquots of the moss and lichen samples, the reproducibility of the measurements is -*15% for total sulfur and *0.5%0 for 634s.
Results and Discussion The use of Sphagnum moss to study high atmospheric burdens of sulfur has one major drawback-the species are sensitive to SO, and are often killed by the elevated doses 86
Environ. Sci. Technol., Vol. 26, No. 1, 1992
of exposure, especially close to smelters and other point sources. Exposure to sulfur pollution has been implicated in the disappearance of Sphagnum mosses in cities and around industrial areas in many parts of the world (2-4). The absence of Sphagnum mosses has been well documented around the smelters at Sudbury, ON, Rouyn-Noranda, PQ, and Flin Flon, SK (7). Thus, we are unable to determine the concentration and isotopic composition of sulfur at sites within 10-20 km radius of these Canadian smelters. Even outside the immediate vicinity of the point sources, the occurrence of mosses still tends to be patchy, reflecting, presumably, the continuing epiphyte impoverishment. Previous workers (18-20), have reported an increase in the sulfur content of lichens and other vegetation around the smelters at Sudbury. We also find that the sulfur content of Chamaedaphne spp. increases from -520 pg g-' at distances of 288 km or more to 1300 pg g-l a t a distance of 20 km from Sudbury to (Figure 2, top dramg). The sulfur content of S. fuscum decreases from -800 pg g-l at a distance of 80 km (occurrence of this species at closer distances was extremely rare) to less than 600 pg g-' at more distal locations (Table I). The sulfur concen-
-
Table 11. Concentration and Isotopic Composition of Sulfur in Moss ( S . fuscum) Samples from Different Locations in Canada"
site no.
description of location
conc, wg g-'
634S,L
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Antiquity Bary, Queen Charlotte Is., BC Prince Rupert, BC Purden Lake, BC 50 km NW, Fort St. John, BC Hythe, AB Grande Prairie, AB Debolt, AB Lac La Nonne, AB Breyrat, AB Marianna Lake, AB 25 km W, Flin Flon, SK 40 km W, Flin Flon, SK Wabowden, MB (95 km from Flin Flon) Thompson, MB Atikokan, ON Kidney Lake, ON Kinoje Lake, ON Akimiski Island, James Bay Chapleau, ON Galbraith, ON London, ON Roulier, PQ Rouyn-Noranda, PQ Rouyn-Noranda, PQ Junction, Highways 113 and 117, PQ Lac. Parent, PQ Chibougamau R. crossing, Hwy 113, PQ Chapais, PQ Riviere-du-Loup, PQ Barrington, NS
1000 1800 300 1800 2000
+16 +12 +8.2 +12 +5.8 +2.3
ND 1400 1800 1100 1100 1200 800
+7.0 +7.4 +5.4 9.3 +4.4 +4.8 +4.2 +6.1 +3.8 +11 +7.8 +4.8 +3.1 +3.8 +4.0 +1.1 +2.0 +2.5 +2.7 +4.2 +4.0 +5.0 +6.5 +6.4
ND 800 1900 500 500 1200 800 800 2800 800 900 1000 1700 600 700 500 1000 1800
The samule locations (with site numbers) are shown in Figure 1. ND, not determined.
trations in S. capillaceum are very similar to those of S. fuscum (Table I), although differences in the sulfur contents of the two species have been reported in Sweden (6). In their study of sulfur in lichens (Cladina rangiferina) of eastern Canada, Zakshek and Puckett (20) likewise found the highest concentrations (-1000 pg g-l) near Sudbury and a decrease to less than 300 pg g-l outside the zone of smelter influence. Around the Rouyn-Noranda smelter, the sulfur contents of S. capillaceum ranged from 650 to 1100 pg g-' and averaged -905 pg g-l (Table I). There were very few occurrences of S. fuscum near this smelter; the S content a t only two sites was -600 pg 8-l (Table I). The influence of the large outputs of SO, by the smelters at Sudbury on the isotopic composition of sulfur in vegetation is seen in Figure 2 and Table I. The isotopic composition of sulfur oxides released by the smelter is 0-2% while further away from the stacks, the 634S02in ambient air is 2-4%0 (21,22). The 634Svalues for Chamaedaphne decrease correspondingly from +3.7%0at a distance of -300 km to +0.67%0 at only 18 km from Sudbury (Figure 2, bottom drawing). The 634Svalues for the two moss species collected outside the 85-km radius of Sudbury are remarkably similar (Table I) and average +2.4 f 0.28%; any variation with distance from Sudbury is obscured by the obliteration of mosses closer to the smelters. The close similarity in the 634Sof the air and vegetation clearly underscores the importance of smelter emissions as the dominant source of sulfur used by plants in the Sudbury basin. In this particular region, the 634Sof plants can be regarded as a potential biomonitoring tool; previous studies had also shown that the sulfur isotope fingerprint could be used to track the reactions and deposition of the sulfur compounds released by the Sudbury smelter (21-23). The isotopic composition of S released at Rouyn-Noranda is unknown; the mean 634Svalue of +1.3 f 0.32%
1400r
-$ 1200-
*
h
8
1000'
* 600-
4
* a
200 0
50 100 150 200 250 Distance from Sudbury (km)
31
300
P
* * "
0
50
100 150 200 250 Distance from Sudbury (km)
300
Figure 2. Concentration (top) and isotopic composition (bottom) of sulfur in leatherleaf (C.ca&cukfa)as a function of distance from the smelter stacks at Sudbury, ON.
for mosses in the region (Table I) suggests that this smelter emits sulfur oxides that are slightly depleted in 34Scompared to the sulfur pollution from the Sudbury smelter stack. In their investigation of the S around the sour gas plants in Alberta, Krouse and his co-workers (9-12) likewise found that lichens and terrestrial mosses acquire 634S values similar to those found for atmospheric SO,. The sulfur contents of S. fuscum collected across Canada vary from 300 to -3000 pg g-l (Table 11). The low sulfur concentrations compared to those of cultivars and many Envlron. Sci. Technol., Vol. 26,
No. 1, 1992 87
other terrestrial plants (typically in the range of 0.1-3%; see ref 24) may be related to the high sensitivity of this moss species to sulfur pollution. In view of the wide range of ambient SO, levels to which the samples used in this study were exposed, a sulfur content in excess of 3000 pg g-' would appear to be the limiting lethal burden for S. fuscum in Canada. The suggestion is based on the fact that, around Sudbury and other point sources with elevated SO, levels, few moss specimens show sulfur contents above this limiting value of 3000 pg g-l. The exact mechanism of SO, toxicity to mosses is unknown, however, although pH is believed to play a critical role in the process (25,26). A number of studies have related the sulfur contents of mosses to regional differences in the intensities of SO, emission and deposition. Zones of cryptogram impoverishment have been closely correlated with mean winter SO2 levels in Britain and some other western European countries (2, 5 ) , and the recent decline in ambient SO, concentrations has even been accompanied by some recolonization of some urban areas by SO2-sensitivecryptogams (5). Pakarinen (6) found that the S contents of S. fuscum averaged 1150 pg g-' in southern Finland compared to 970 pg g-' in the northern part of the country. The higher values in the south were attributed to greater atmospheric sulfur deposition in this region ( 10 g ha-' year-' average) compared to the S flux ( - 5 g ha-' year-') in northern Finland. The Finnish data for moss S are comparable to the concentrations found in Canada (Table 11). Pakarinen (6) estimated that S. fuscum retains only 27% of the sulfur that is being deposited. The low retention rate may be related to the fact that wetlands with sphagnum mosses often emit large quantities of biogenic sulfur to the atmosphere (27). In a more extensive study, Malmer (8)showed that the concentrations of S in various Acutifolia species (S.fuscum, Sphagnum rubellum, and Sphagnum russowii) are positively correlated with the wet deposition of S in the Scandinavian countries. The S contents were found to be significantly higher in the southern regions, which experience higher inputs of anthropogenic sulfur compared to the northern districts. The average S concentration in the Scandinavian mosses was found to be 1340 f 1120 pg g-l (8),compared to the average Canadian data for S. fuscum of 1164 f 576 pg g-' (Table 11). Pakarinen and Gorham (28)showed that the S contents of S. fuscum in northern Minnesota, Manitoba, and Ontario varied from 600 to 1140 pg g-', with the lowest values (
