Pesticides in Western Canadian Mountain Air and Soil

Aug 1, 2007 - Department of Chemistry and Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, T...
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Environ. Sci. Technol. 2007, 41, 6020-6025

Pesticides in Western Canadian Mountain Air and Soil GILLIAN L. DALY,† YING D. LEI,† CAMILLA TEIXEIRA,‡ DEREK C. G. MUIR,‡ AND F R A N K W A N I A * ,† Department of Chemistry and Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4, and Aquatic Ecosystem Protection Research Division, Environment Canada, 867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6

The distribution of organochlorine pesticides (OCP; in past and current use) in the mountains of western Canada was determined by sampling air, soil, and lichen along three elevational transects in 2003-2004. Two transects west of the Continental Divide were located in Mount Revelstoke and Yoho National Park, while the Observation Peak transect in Banff National Park is east of the divide. XADbased passive air samplers, yielding annually averaged air concentrations, were deployed, and soils were collected at all 22 sampling sites, whereas lichen were only sampled in Revelstoke. Back trajectory analysis showed limited air mass transport from the Prairies to the east, but a high frequency of air arriving from the southwest, which includes agricultural regions in British Columbia and Washington State. Endosulfan, dieldrin, and R-hexachlorocyclohexane were the most prevalent OCPs in air and soil; hexachlorobenzene was only abundant in air; chlorothalonil, dacthal, and pentachloronitrobenzene were also consistently present. OCP air concentrations were similar across the three transects, suggesting efficient atmospheric mixing on a local and regional scale. Soil concentrations and soil/air concentration ratios of many OCPs were significantly higher west of the Continental Divide. The soil and lichen concentrations of most OCPs increased with altitude in Revelstoke, and displayed maxima at intermediate elevations at Yoho and Observation Peak. These distribution patterns can be understood as being determined by the balance between atmospheric deposition to, and retention within, the soils. Higher deposition, due to more precipitation falling at lower temperatures, likely occurs west of the divide and at higher elevations. Higher retention, due to higher soil organic matter content, is believed to occur in soils below the tree line. Highest pesticide concentrations are thus found in temperate mountain soils that are rich in organic matter and receive large amounts of cold precipitation.

Introduction Semivolatile organic compounds have the potential to undergo long-range atmospheric transport, often as a result * Corresponding author phone: (416) [email protected]. † University of Toronto Scarborough. ‡ Environment Canada. 6020

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of repeated cycles of deposition and evaporation. Some such compounds tend to preferentially deposit and accumulate in remote cold environments, both at high latitudes (1, 2) and elevations (3, 4). Temperate mountains, in particular, experience effective upslope air transport and enhanced deposition at high elevations due to low temperatures and efficient snow-scavenging (5). The presence of these compounds in alpine areas is cause for concern because of their tendency to bioaccumulate and biomagnify along aquatic and terrestrial food chains, which has been documented in the lichen-caribou-wolf food chain of Canada’s Arctic (6), in aquatic food chains in an alpine lake in the Canadian Rocky Mountains (7), and in mountain streams of southwestern British Columbia (BC), Canada (8), as well as along the ocean-salmon-bear pathway in coastal BC (9). Deliberately released into the environment and by definition toxic, semivolatile pesticides merit particular concern. Organochlorine pesticide (OCP) residues are present in air (10), snow (3, 11), vegetation (10, 12), runoff (10), aquatic amphipods (13), and lake trout (9, 14) from western Canadian mountains. This includes OCPs banned in North America for several decades (e.g., R-HCH, chlordanes, DDTs, and dieldrin) and those still in use (endosulfan) or banned recently (lindane). Although a review of levels of other current use pesticides in Canadian air and precipitation (15, 16) lists few data for the mountains of Western Canada, their atmospheric transport from intensive agricultural use areas to higher elevations has been documented for tropical and subtropical mountains (17, 18). The detection of dacthal, chlorothalonil, and chlorpyrifos in U.S. mountain (19) and Arctic snow (20) and remote Canadian lake water (21) is further indication of their potential for long-range transport. In this study, air, soil, and lichen were sampled along three transects in the mountains of western Canada and analyzed for OCPs of past and current use. The objective was to evaluate pesticide transport into the mountains of western Canada, both historically and presently. Most previous studies in the region consisted of one transect and thus lacked the ability to assess the extent of variation across the region, based on different source proximity and meteorological conditions. The current study therefore attempts to compare pesticide accumulation patterns along different mountain slopes and to relate those patterns to chemical properties, emissions, and meteorology.

Experimental Section Sampling Locations. The three transects, located in mountainous National Parks of western Canada and labeled Revelstoke (R1 to R8), Yoho (Y1 to Y6), and Observation Peak (O1 to O8), are described in detail in the Supporting Information. Figures 1 and 2 show the locations of the 22 sampling sites and Table S1 lists their elevations and coordinates. Sampling Methods. Air was sampled passively using an XAD-based method (22) that had previously been used to monitor OCPs across North America (23). Samplers were deployed in duplicate from late August 2003 to late August 2004, yielding annual average air concentrations. Along the mountain slopes, they were deployed on metal and plastic poles at heights above the maximum expected snowpack level, which in some cases is several meters (photographs in the Supporting Information). Major siting criteria were elevation, accessibility, and representativeness. Evaluation and calibration of this sampler has been described previously (22). A Riverside style auger (7 cm i.d.) was used to collect soil at each air sampling site. Shallow and rocky soils limited 10.1021/es070848o CCC: $37.00

 2007 American Chemical Society Published on Web 08/01/2007

FIGURE 1. Spatial image of sampling stations along three transects in the mountains of western Canada: Mount Revelstoke National Park and Yoho National Park, British Columbia, and Observation Peak, Banff National Park, Alberta (37).

FIGURE 2. Map of British Columbia and Alberta with the land classified according to agricultural activity (reproduced with permission from The National Atlas of Canada, 38). Airsheds for stations Y2, R5, and O4 compiled with back trajectories from August 2003 to August 2004. The area delineated by the outer lines shows where trajectories most likely passed through before arriving at the station. The area with the highest probability is delineated by the lines closer to the station. sampling to the organic-rich surface layer (ca. 5-10 cm depth). Because heterogeneity of mountaineous terrain prevents strictly randomized sampling, ten coring locations, several meters apart from each other and representative of the soil conditions in the immediate vicinity of an air sampler (within 20 m), were chosen. The soil from the ten cores was mixed in a pre-cleaned bucket and two subsamples were wrapped in baked aluminum foil, sealed in plastic bags, and stored frozen until analysis. A Park biologist collected lichen samples (Bryoria spec.) along the Revelstoke transect in summer 2004. Though collected in duplicate, these samples had to be pooled due to low sample size. The lichen were cleaned of non-lichen debris, wrapped in pre-cleaned aluminum foil, sealed in plastic bags, and stored frozen until analysis. Extraction and Quantification. The resin from most passive air samplers was extracted as described in ref 22, whereas the XAD from stations R1, R3, R4, and R7 was extracted as described in ref 24. Most analytes were quantified

by gas chromatograpy-electron capture detection as in ref 22; pentachloronitrobenzene, dacthal, chlorothalonil, chlorpyrifos, endosulfan-I, and endosulfan sulfate were quantified by gas chromatography-mass spectrometry as in ref 24. Time-averaged volumetric air concentrations in pg‚m-3 were estimated by dividing the sampler concentration (in pg‚sampler-1) by the product of the deployment period (365 d) and the sampling rate (0.5 m3‚d-1‚sampler-1) (22). The latter is quite uncertain, which propagates to a relatively high uncertainty of the annual average concentration. Soil samples were prepared and extracted as in ref 24. A description of the extraction and cleanup procedure for lichens is given in the Supporting Information. Soil and lichen extracts were analyzed for OCPs as in ref 24. Recoveries of isotopically labeled OCP standards in all soil and lichen samples ranged from 71 to 116%. Blanks for OCPs in air, soil, and lichen were low, and are given in Tables S2, S3, S5, and S6 in the Supporting Information. Concentrations in air and soil are blank corrected and reported as the average of VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Boxplot of OCP concentrations in air (ng/passive air sampler) measured in the Revelstoke (R), Yoho (Y), and Observation (O) transects. The box is defined by the 25th and 75th percentiles, whiskers mark the 10th and 90th percentiles, the median is represented by a horizontal line, the mean by a square, and outliers with a diamond. HCB, hexachlorobenzene; r/γ-HCH, r/γ-hexachlorocyclohexane; E-I/II, Endosulfan-I/II; ES, endosulfan sulfate; TC, trans-chlordane; CT, chlorothalonil; DT, dacthal; CP, chlorpyrifos; PCNB, pentachloronitrobenzene. duplicate samples. Lichen concentrations represent pooled samples and are also blank corrected. Soil Characterization. Soil moisture content, determined by drying at 75-80 °C until constant weight was achieved, varied from 10 to 55%. Organic carbon (OC) in soil, measured as described in ref 18, ranged from 1 to 65%. The Y2 sample had 65% OC, which essentially corresponds to 100% organic matter. At this sampling site, the soil was very shallow and consisted mostly of needle litter. Airshed Calculation. Five-day back trajectories arriving at the coordinates of stations Y2, O4, and R5 were calculated at 10, 100, and 200 m above ground level at 6 h intervals for each day the passive air samplers were deployed using the Canadian Meteorological Centre Trajectory Model. The more than 4000 trajectories for each of these three sites were compiled to produce back trajectory probability density maps, referred to as “airsheds”, which show the most frequent region of origin of air parcels arriving at the stations (Figure 2).

Results and Discussion Concentrations in Air. Air sampler concentrations for the OCPs in ng/sampler are reported in Tables S2 and S3 in the Supporting Information. Table S4 summarizes the median and range of volumetric air concentrations in pg/m3, and Figure 3 displays a boxplot of the OCP air concentrations. Hexachlorobenzene (HCB) is the most abundant OCP in air, followed by hexachlorocyclohexanes (HCHs), endosulfan, and DDT-related substances (DDX). Not only are the air concentrations along each transect very similar, reflected by relatively small boxes in Figure 3, but the mean air concentrations were also not significantly different across the three transects for R-HCH, γ-HCH, dieldrin, trans-chlordane, endosulfan-II, HCB, p,p′-DDT, and p,p′-DDE (by ANOVA with significance of 0.05, Table S8). This suggests efficient atmospheric mixing of OCPs within the region. The air concentrations are of a range similar to those reported 3 years earlier using the same sampling technique (annual, passive XAD-based air samplers) for four stations in the Canadian Rockies (23, 25). Air concentrations of γ-HCH, and Endosulfan I and II measured in the current study are lower than levels measured in spring/summer by high-volume air sampling in the Canadian Rockies (26). Spring/summer concentrations are expected to be higher than annual mean concentrations due to higher mean temperatures. Among the current-use OCPs, three highly chlorinated benzene derivatives (chlorothalonil, dacthal, pentachloronitrobenzene) were found consistently at concentrations 6022

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between 1 and 10 pg/m3 at all three mountains, with slightly higher levels in Revelstoke compared to the Rocky Mountains (statistically significant only for PCNB). Chlorpyrifos was also found along all three transects, but levels were low (generally below 1 pg/m3) and the CV between duplicate samplers was relatively high. The back trajectory analysis shows that air mass origin, and thus the potential impact of different pesticide use areas, differs slightly among the transects (Figure 2). Only Yoho and Observation have trajectories that reach into the Canadian prairies, whereas Revelstoke will not be impacted significantly by pesticide use east of the Continental Divide. Even for the two Rocky Mountain transects the Prairies occupy only a small part of the overall airshed. The airshed for Revelstoke reaches further west and south, and will be more strongly influenced by pesticides use in the Okanagan valley and Washington State. Intensive agriculture takes place in Washington, with high usage of chlorothalonil, chlorpyrifos, and endosulfan (27) on fruit and vegetables. The Okanagan valley is a major fruit and wine growing area in BC. Chlorothalonil is one of the top 20 most-used pesticides in Canada, and chlorpyrifos is also widely used in Canadian agriculture (28). DDT, HCHs, chlordanes, and dieldrin were used in Canada and the United States in the 1960s and 1970s to protect crops against insect damage. High soil residues of OCPs (29) and their volatilization (30) have been reported for the Fraser Valley, BC. Table S4 includes air concentrations reported for the Fraser and Okanagan valleys. The Fraser Valley air concentrations were sampled by high-volume air samplers in the summer (31) and thus will be biased to higher levels compared to the annual concentrations reported here and for the Okanagan valley. Current-use pesticides such as chlopyrifos and endosulfan were found in the agricultural regions at levels many times higher than those in the current study. Older, banned pesticides such as trans-chlordane and R-HCH were found at more comparable levels. HCB, a chemical with industrial sources and a long atmospheric residence time, has similar concentrations in the agricultural regions and the mountains of western Canada, in agreement with reports of its even distribution on a continental scale (23). Air sampling campaigns for OCPs have also been completed at high altitudes in the Californian Sierra Nevada (17), Costa Rica (18), and Europe (32). Air concentrations (measured by highvolume air samplers in summer) of chlorothalonil, chlorpyrifos, and endosulfan all exceeded 100 pg/m3 at 1920 m in the Sierra Nevada (17), suggesting efficient transport from the nearby agriculturally intensive Central Valley. Similarly, chlorothalonil and endosulfan air concentrations (passive,

FIGURE 4. Boxplot of OCP concentrations in soil (pg/g) measured in the Revelstoke (R), Yoho (Y), and Observation (O) transects (boxes left to right ) transects west to east). The box is defined by the 25th and 75th percentiles, whiskers mark the 10th and 90th percentiles, the median is represented by a horizontal line, the mean by a square, and outliers with a diamond. annual samplers) were above 100 pg/m3 at two high altitude stations in Costa Rica (18). Air concentrations (HCHs, DDX, endosulfans, and HCB) measured by high-volume air samplers throughout the year in the Central Pyrenees (2240 m) and High Tatras (1778 m, 33) were similar to levels measured in the current study, with averages below 50 pg/m3. Concentrations in Soil and Lichen. Tables S5 and S6 list all soil and lichen concentrations in pg/g. Median and range of the concentrations in soils across the three transects are summarized in Table S7. Whereas pesticide levels in air are generally similar across the mountains of western Canada (Figure 3), concentrations in soil are much lower in the Observation than in the Revelstoke and Yoho transects (Figure 4). This results in higher air/soil (A/S) concentration ratios east of the Continental Divide (Figure S1). ANOVA analyses (Table S8) show significantly different soil concentrations of dieldrin, trans-nonachlor, endosulfan-I, endosulfan-II, endosulfan-sulfate, HCB, chlorothalonil, and dacthal across the three transects. A/S concentration ratios were also significantly different across the three transects for trans-chlordane, endosulfan-II, endosulfan-sulfate, and HCB (Table S8). Lower average soil concentrations on Observation Peak may be a result of a greater proportion of samples above the tree line, where soil organic carbon content is low and soils have little potential for contaminant retention. Observation also receives less precipitation than Revelstoke, resulting in less deposition for chemicals subject to precipitation scavenging. Literature data on OCPs in high-altitude soils are limited. HCH levels measured in the current study far exceeded those measured in the Pyrenees and Tatra mountains (Table S7, 33). Conversely, HCB concentrations are lower in the mountains of western Canada than in European mountains (Table S7, 33). Higher HCH levels in the Canadian mountains are likely related to historic HCH uses in nearby agricultural areas (19) or even trans-Pacific transport of HCH from Asia. It is difficult to compare the lichen data of this study to that of other work because of limited reports for samples from high altitudes and the natural variability caused by different species, accumulation time, and water and lipid content. R-HCH concentrations are similar in lichen from Revelstoke (0.1-1.2 ng/g fresh weight, Table S5) and Ontario (0.3-2.5 ng/g fresh weight, 34). Dieldrin and endosulfan-I levels are slightly higher in Revelstoke (0.4-1.7 ng/g) than in Ontario (0.1-0.5 ng/g, 34). The Supporting Information contains a brief discussion of the relative abundance of the components of various compound groups (HCHs, DDX, endosulfan, chlordanes) in air and soil.

FIGURE 5. Air, soil, and lichen concentrations (normalized to the average) for the Revelstoke transect plotted against elevation (m) for r-HCH, HCB, heptachlor epoxide, dieldrin, endosulfan-II, and dacthal. Whiskers show the maximum and minimum measured concentration (from sample replicates), the box/circle represents the average. Whiskers are not shown if they are shorter than the box/circle. No whiskers shown for lichen because replicate samples were pooled. Air samplers deployed at the two highest sampling sites did not survive. HCB, dieldrin, dacthal, and heptachlor epoxide were not detected in lichen. Soil Concentrations, Soil Organic Carbon, and Tree Line. The three transects have variable soil organic carbon profiles with elevation (Table S1). Revelstoke has the smallest range in %OC, and soil concentrations are not correlated with %OC (except for endosulfan-sulfate, Table S9). Along the Yoho transect, the soil concentrations of R-HCH, γ-HCH, transchlordane, cis-chlordane, trans-nonachlor, endosulfan-II, endosulfan sulfate, HCB, and dacthal are correlated with %OC, with statistical significance (p < 0.05, Table S9). This is reflected in the elevational profiles (Figure S2), as Y2, with a very high OC content of 65%, has the highest soil concentrations for all of those chemicals. Only chlorothalonil is not correlated with soil OC in Yoho, which could be an indication of a local source. Dieldrin, trans-nonachlor, endosulfan-II, endosulfan sulfate, and HCB are also correlated with %OC in the Observation transect (Table S9), which has generally declining %OC with elevation. The impact of the tree line is best observed along the Observation transect, as trees extend to the top of the Revelstoke sampling range and to all but two of the Yoho sampling sites. For most chemicals, soil concentrations drop beyond the tree line (2370 m) in the Observation transect (stations O6-O8, Tables S5, Figure S3). With no trees and low %OC (top two stations have OC content of 3% and 1%), there is little ability for contaminant retention, leading to low soil concentrations. Below the tree line, the forest canopy may result in increased dry and occult deposition of pesticides. Concentrations vs Altitude. Figure 5 shows the air, soil, and lichen concentrations in the Revelstoke transect plotted against site elevation. Similar plots for Yoho and Observation are shown in Figures S2 and S3. Regression parameters for soil concentrations against elevation are given in Table S10. The soil concentrations in Yoho, normalized to the OC content, show no statistically significant increases with elevation. Neither did we observe significant correlations between soil concentrations and elevation along the Observation transect, even if concentrations are normalized to the carbon content or if stations above the tree line are excluded from the correlation (Table S10). VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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In Revelstoke, on the other hand, statistically significant (p < 0.05) increases in soil concentration with elevation exist for γ-HCH, dieldrin, endosulfan-I, endosulfan-II, and dacthal. Also, lichen concentrations of trans-nonachlor and the endosulfan species increase significantly with elevation (Table S11). These trends add to existing evidence of an increase in endosulfan concentrations with elevation in air (23, 25), snow (3), and vegetation (12) from the mountains of western Canada. The positive relationship between dieldrin soil concentrations and elevation found in Revelstoke match such a positive relationship found in amphipods (13) and snow (3) from the Canadian Rockies. HCB and heptachlor epoxide showed little change in atmospheric or soil concentrations with altitude in Revelstoke. Previous work showed no relationship between elevation and concentrations in air (25), but a positive correlation with concentrations in other media (3, 12, 13). Other media such as amphipods and vegetation are affected by different uptake processes, thus different elevation gradients can be expected. Whereas chlorothalonil was not detected in any of the soils from the Revelstoke transect, it was detected in soil above 2200 m on the Observation transect, and in soil throughout the Yoho transect up to 2561 m. This suggests either efficient transport to high elevations or efficient degradation in soil at lower elevations. Chlorothalonil had 10-fold higher soil concentrations in Yoho than in Observation, suggesting potentially a local source (e.g., the TransCanada Highway or a golf course). In California, chlorothalonil was detected at similar air concentrations at the sampling site at 1920 m in the Sierra Nevada and the site at 200 m in the Central Valley (17). Explaining Soil Concentration Variability in Mountains. Concentrations of OCPs in air that are fairly uniform across and between the three mountain transects indicate that efficient local and regional atmospheric mixing causes similar atmospheric exposure of mountain ecosystems in Western Canada to those contaminants. However, the concentrations in soils along the gradients and between the transects varied considerably and often with statistical significance, suggesting large differences in the historical net air-surface exchange along mountain slopes and between different mountains within a larger area. Part of that variability can be explained by the different retentive capacity of the soils for the pesticides which is to a large extent determined by the organic carbon content. For example, most of the soil concentration variability along the Yoho and some of the variability on the Observation transects can be attributed to the soil OC content decreasing with elevation. Also, lower median soil concentrations east of the Continental Divide are likely partly due to the higher elevation and therefore lower OC content of soil sampling sites on the Observation transect. Another part of the variability is related to the rate of wet deposition. In particular, soil concentrations that increase significantly with elevation along the Revelstoke transect can be explained by increasing deposition with elevation. The latter is a combined effect of higher precipitation rate and lower temperature at higher altitude (35). Lower temperatures results in higher scavenging efficiencies and thus higher rain and snow concentrations (3, 36). Similarly, lower soil concentrations along the Observation transect are likely due in part to lower precipitation rates east of the Continental Divide. Furthermore, whereas most of the precipitation at Revelstoke falls as snow in winter, summer rain is the dominant form of precipitation along the Observation transect in Banff National Park. This may further result in higher annual wet deposition rates west of the divide. We thus would expect highest pesticide concentrations in temperate mountain soils that receive high deposition, i.e., experience large amounts of cold precipitation, and have a high retentive capacity, i.e., have a high content of organic 6024

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matter. Often, these two factors display opposing trends with altitude in temperate mountains. Whereas the deposition rate is expected to increase with elevation (because both temperatures drop and precipitation rates increase), the soil organic matter content tends to decrease with elevation and in particular will drop strongly above the tree line. Accordingly, soils from intermediate elevations may often display the highest concentrations of persistent pesticides, and this is what is indeed observed at both Yoho (Figure S2) and Observation Peak (Figure S3). The Revelstoke transect displayed the most significant concentration gradients with altitude (Figure 5), because it is entirely below the tree line, and a smaller range of soil organic matter content should result in comparable contaminant retention at different altitudes. Under such circumstances, differences in deposition, as caused by increasing precipitation rate and increasing scavenging efficiency with altitude, are more apparent. Different pesticides differ in the efficiency by which they are scavenged by rain and snow, and also in terms of the efficiency by which they are retained and/or persist in soils. It is thus not surprising to find a variety of soil concentration trends with altitude, as they are determined by both the properties of the chemical and the characteristics of the mountain and of the sampling sites. For example, gradients will differ depending on whether the sampling transect is entirely below or above the tree line, or crosses it. Similarly, different precipitation rate changes with altitude (35) should result in different gradients on the windward and leeward side of the same mountain. Different environmental media are expected to show different trends, if the specific mechanisms of contaminant delivery are different. For example, wet deposition processes should often be dominant for soils, whereas gas absorption may be a more important uptake route for foliage samples. Ultimately, only model calculations (18) will be able to provide a full conceptual and quantitative understanding of how the interplay among chemical partitioning and degradation properties and elevational gradients in temperature, precipitation, and soil organic matter determines the variability of contaminant exposure along temperate mountain slopes.

Acknowledgments We thank Hang Xiao for help in the field, and Tom Harner, Mahiba Shoeib, Fiona Wong, and Yushan Su for advice on soil extraction. We are grateful for the assistance of Joanne Williams and Gloria Hendry from the Lake Louise, Yoho, and Kootenay Warden Service, as well as Susan Hall and Tamara Lamb from Mount Revelstoke National Park. We acknowledge funding from the Natural Sciences and Engineering Research Council and from a Canon National Parks Science Scholarship.

Supporting Information Available Tables with air and soil concentrations; figures of air/soil concentration ratios and of soil concentration profiles with altitude; details on sampling sites, lichen extraction method, and a discussion of relative abundance within compound groups. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review April 10, 2007. Revised manuscript received July 1, 2007. Accepted July 2, 2007. ES070848O

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