Environ. Sci. Technol. 2000, 34, 2909-2912
Regional Distribution of Trifluoroacetate in Surface Waters Downwind of Urban Areas in Northern California, U.S.A. THOMAS M. CAHILL* AND JAMES N. SEIBER Center for Environmental Sciences and Engineering/ Mailstop 199, University of Nevada, Reno, Nevada 89557
Trifluoroacetate (TFA), a breakdown product of the hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), has been found at higher concentrations in surface waters near urban areas compared to globally remote sites, but the scale of the urban enrichment, namely local or regional, is unknown. To determine the scale of urban enrichment of TFA in Northern California, a series of streams were sampled in 1998 along a transect upwind and downwind of the San Francisco and Sacramento metropolitan areas. In addition, 17 remote sites were sampled in British Columbia and the Yukon Territory, Canada and in Alaska, U.S.A. to determine the baseline TFA concentrations in Northern Hemisphere surface waters. The results showed elevated TFA concentrations in surface waters around and immediately downwind of urban areas. The enrichment was approximately 5-6 times higher than the concentrations in upwind sites. The northern remote sites showed a median TFA concentration of 21 ng/L, which was statistically indistinguishable from the upwind coastal sites of the Californian transect. The mechanism for the urban enrichment was unknown, but it may have been the result of additional sources of TFA other than the HFC/ HCFCs or faster formation of TFA due to higher HFC/ HCFC and hydroxyl radical concentrations.
Introduction In the 1980s, the chlorofluorocarbons (CFCs) were implicated in the thinning of the earth’s stratospheric ozone layer, and hence they were slowly phased out and replaced by the hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) during the 1990s. Unlike the CFCs, which are saturated with halogens and thus relatively unreactive, the HFCs and HCFCs contain one or more hydrogen atoms that are vulnerable to abstraction by hydroxyl radicals which makes the HFC/HCFCs susceptible to degradation in the troposphere. One of the products formed from HFC/HCFC degradation is trifluoroacetic acid and its corresponding acetate anion (TFA, CF3COO-) (1). TFA has a high water solubility, low pKa, and low Henry’s law constant, thus TFA is predominately removed from the atmosphere through wet deposition (2, 3) where it can potentially impact aquatic ecosystems. TFA is both exceptionally stable and mildly phytotoxic (4), so there is concern that TFA may accumulate * Corresponding author address: Canadian Environmental Modelling Centre, Trent University, 1600 West Bank Drive, Peterborough, Ontario, Canada K9J 7B8; phone: (705)748-1005; fax: (705)748-1080. 10.1021/es991435t CCC: $19.00 Published on Web 06/10/2000
2000 American Chemical Society
in certain terminal water bodies to the point that toxic concentrations may be achieved (5, 6). The atmospheric lifetimes of the HFC/HCFCs range from 1.5 to 40 years (1), so they will globally circulate. Observed atmospheric concentrations of HFCs in globally remote sites showed increasing concentrations of the HFC/HCFCs during the 1990s (7-9). Based on the global circulation of the precursor compounds, Kotamarthi et al. (3) predicted that TFA production and washout should create rainfall concentrations of TFA in the range of 0.12 µg/L by 2010, although concentrations as high as 0.45 µg/L are possible in parts of Europe and North America. Environmental samples, both rain and surface waters, from Germany already exceed the predicted global rainfall concentrations, while the few remote sites sampled showed significantly lower TFA concentrations (10). While observed TFA concentrations in urbanized areas were higher than globally remote sites, the scarcity of reported data prevents the determination of the scale, namely local or regional, of the TFA enrichment around and downwind of metropolitan areas. To determine the distribution of TFA in the environment resulting from urban areas, surface waters were sampled along a transect from the clean upwind coastal areas of California, through the San Francisco and Sacramento metropolitan areas, and into the relatively pristine Sierra Nevada Mountains downwind of the metropolitan areas. To ensure that the coastal samples represent background conditions, samples were collected in Alaska, Yukon Territory, and British Columbia to determine background concentrations in North America. Combined, these samples helped to determine the relative scale, namely local, regional, or global, of TFA enrichment.
Experimental Methods Environmental samples were collected in two sets during 1998 to determine the distribution of TFA in the environment. The first set of samples was collected along a transect upwind and downwind of the San Francisco and Sacramento metropolitan areas to test for local and regional urban enrichment of TFA. The transect was divided into six segments representing different regions (Figure 1). The first segment consisted of seven streams along the coast of Northern California. The prevailing northwesterly winds brings a clean air mass from the Pacific Ocean (11), hence these sites should not be impacted by the urban plume of the San Francisco Bay Area. The second segment was a set of streams (n ) 8) immediately downwind of the San Francisco Bay area. The third segment was composed of streams (n ) 7) in the Sierra Nevada foothills below 500 m in elevation. These sites are downwind of the San Francisco Bay Area and the Central Valley metropolitan areas such as Sacramento (11). The last three segments represent the west slope of the Sierra Nevada above 1600 m elevation (n ) 10), the Tahoe Basin (n ) 12), and the east slope of the Sierra Nevada (n ) 10). To reduce temporal variation in the data set, all the samples along the transect were collected during an 8 day period between 08/23/98 and 08/30/98. No significant rain events occurred within a week of sample collection that might cause mixing or dilution effects. The second set of samples was collected to determine the baseline concentration of TFA in the remote areas of the Northern Hemisphere. These samples consisted of 17 sites in British Colombia and Yukon Territory in Canada and Alaska, U.S.A. The sampling sites ranged from near Lytton, BC (50.2N 121.6W) to Fairbanks, AK (64.8N 147.7W). The sites were remote from urban areas with two exceptions, VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Locations of sampling sites, with each segment marked by a different symbol, and predominant spring and summertime surface wind flow patterns, marked by arrows (adapted from ref 11). In contrast, the wintertime meteorology, which is when the majority of the wet deposition occurs, is dominated by northerly wind (25% of the time), southerly wind (22%), and stagnation (18%), which do not favor transport into the Sierra Nevada Mountains. which were the Chena River in Fairbanks, AK and the Yukon River in Whitehorse, YT. All these samples were collected between 7/23/98 and 7/30/98. In order for the sampling sites along the Californian transect to be representative of local TFA deposition, they were chosen based on the following criteria. The sites had to be small flowing streams that originated within the transect segment. Small streams have a rapid turnover rate, and they lack the complex mixing issues involved with lakes. In addition, the sampling sites could not be downstream of a large lake or reservoir that might cause dilution or mixing effects. The streams within a segment needed to be in different watersheds, thus the sites were independent of each other. Although the streams sampled within each segment varied in size, a similar distribution of stream sizes was sampled in each transect segment to control for stream volume. Small watersheds lacking industrial sites or heavy urban populations were selected to ensure that the predominate, if not only, source of TFA would be through atmospheric deposition. While previous studies have shown that urban wastewater discharges tend not to contain significant TFA contamination (12), urban water discharge was a possible confounding factor that was avoided. Sample site criteria for the northern samples were relaxed in that larger rivers and two small lakes were sampled in addition to streams. At each sampling site along the transect, two separate samples were collected into 1-L glass bottles by submerging the bottle under the surface of the water. The sample bottles, precleaned in the laboratory, were additionally washed 3 times with site water prior to collection of the sample. The bottle caps were lined with aluminum foil to prevent potential contamination from the bottle caps. Two sample bottles were tested for TFA contamination by filling them with 18 MΩresistance Nanopure water and subsequently analyzing the water. No detectable TFA was observed in these blanks. The samples from the northern sites consisted of only one 60 mL bottle of water from each site. The samples were analyzed for TFA using a headspace gas chromatography (HS-GC) method detailed in Cahill et al. (13). Both of the field replicates collected from each stream along the Californian transect were analyzed separately, while the northern samples had only a single bottle of water that was analyzed. Fifteen milliliters of sample water, to which 1 mL of 0.5 M (NH4)2CO3 was added, was evaporated in a headspace vial to dryness using an oven. An additional 15 mL of sample water and 1 mL (NH4)2CO3 were added to the same vial and evaporated to dryness again in order to increase 2910
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effective sample volume to 30 mL. After the sample had been evaporated, 4 mL of a 25% methanol/75% 9 M sulfuric acid solution was added to the vial. The vial was subsequently capped and heated to extract and derivatize the TFA to methyl-TFA which was then analyzed using a headspace gas chromatograph equipped with an electron capture detector. Method blanks, consisting of Nanopure water, run along with the samples showed no detectable TFA. Statistical analyses of the data sets were conducted by Minitab statistical software version 12 for Windows. Since some of the data sets were not normally distributed as determined by an Anderson-Darling normality test (P < 0.05), a nonparametric Mann-Whitney test with R ) 0.05 was used to test for differences between data sets.
Results The transect results clearly show enrichment of TFA in local water systems near urban areas (Figure 2). The coastal sites showed consistent and low concentrations of TFA with a median concentration of 24.2 ng/L. In contrast, sites immediately downwind of the San Francisco Bay Area had significantly higher TFA concentrations with a median concentration of 144 ng/L (Mann-Whitney, P < 0.001). There was a high degree of variability between streams, although the TFA concentrations in the streams appeared to be independent of the stream size. The concentrations were lower (Mann-Whitney, P ) 0.04) in the foothills of the Sierra Nevada with a median concentration of 95.4 ng/L. As the transect proceeded into the high elevations of the Sierra Nevada, the TFA concentrations dropped in the last three segments with median values of 24.1, 27.8, and 21.5 ng/L, respectively. These segments were not statistically different from each other (P > 0.05), and the combined Sierra sample set was indistinguishable (P > 0.05) from the coastal segment. The differences between field replicates from the same stream averaged 11.6%, although this was elevated by samples near the MQL of 8 ng/L. Streams with concentrations greater than 30 ng/L had better precision with an average difference between replicates of 6.6%. The samples collected in Alaska and Canada showed low and fairly consistent concentrations of TFA with a median value of 21.1 ng/L. The concentrations typically ranged from
