Environ. Sci. Technol. 2008, 42, 8668–8674
Changes in Aluminum Concentrations and Speciation in Lakes Across the Northeastern U.S. Following Reductions in Acidic Deposition RICHARD A. F. WARBY,* CHRIS E. JOHNSON, AND CHARLES T. DRISCOLL Department of Civil and Environmental Engineering, 151 Link Hall, Syracuse University, Syracuse, New York, 13244
Received April 23, 2008. Revised manuscript received July 13, 2008. Accepted July 28, 2008.
We surveyed 113 lakes in the northeastern U.S. in 2001 that had previously been sampled in 1986 to evaluate the effects of reductions in acidic deposition on the concentrations and speciation of aluminum (Al). We found ubiquitous decreases intheconcentrationsoftotalAlandinorganicmonomericaluminum (Ali) across the region. Median total Al decreased from 1.45 to 1.01 µmol L-1 across the region, with the largest decrease in the Adirondacks (4.60 µmol L-1 to 2.59 µmol L-1). Organic monomeric aluminum (Alo) also decreased region-wide and in all the subregions except the Adirondacks. The speciation of Ali shifted from largely Al-F complexes in 1986 to largely Al-OH complexes in 2001 in ponds whose concentrations were above the detection limit (>0.7 µmol L-1). In 2001, only seven lakes studied, representing a population of 130 lakes in the region, had Ali concentrations above a toxic limit of 2 µmol L-1 compared with 20 sample lakes, representing 449 lakes, in 1986.Thus,weestimatethatmorethan300lakesinthenortheastern United States no longer have summer Ali concentrations at levels considered harmful to aquatic biota.
Introduction The Clean Air Act Amendments (CAAA) of 1970 and 1990 in the U.S. (1), and similar legislation in Europe and Canada, have controlled emissions of sulfur dioxide (SO2) and, to a lesser extent, nitrous oxides (NOx). These controls have decreased sulfate (SO42-) and hydrogen ion (H+) deposition across these regions (2-4). In response research has shifted from documenting the effects of acidic deposition to understanding the recovery of aquatic and terrestrial ecosystems (2-7). Varying degrees of chemical recovery of surface water in the U.S, Canada, and Europe have been reported (2-7). The effects of acidic deposition are greatest in areas characterized by thin, base-poor soils with base saturation less than 10-15% (8) and soil solutions with pH 4 ha. In phase II of the Eastern Lakes Survey, these lakes were resampled during the spring, summer, and fall of 1986 to assess seasonal variability in water quality (16). Since discharge is not monitored at most of the DDRP sites, precipitation amount was used as a surrogate for hydrologic flow conditions, which are important in controlling the acid-base chemistry of surface waters (6). Further details are provided in the Supporting Information. During the summer of 2001 (May 28th to August 4th) we collected water samples from 130 of the 145 original DDRP lakes. The remaining lakes were not sampled due to inaccessibility and/or refusal of access. The DDRP did not consider the speciation of Al, but the ELS-II survey did. Therefore, our study only considers the 113 lakes common to both surveys (our survey in 2001 and the ELS-II survey in 1986). Furthermore, we only used the summer data from the ELS-II survey as the resurvey in 2001 was also conducted during the summer. Lake waters in this region typically exhibit their highest ANC and lowest Al concentrations during the summer (17). The northeastern U.S. was subdivided into five subregions (Figure S1 in the Supporting Information): the Adirondacks (ADR; N ) 20), Catskills and Poconos (CATPOC; N ) 17), Central New England (CNE; N ) 26), Southern New England (SNE; N ) 21), and Maine (N ) 29). 10.1021/es801125d CCC: $40.75
2008 American Chemical Society
Published on Web 11/06/2008
Chemical Methods. The samples were collected at the surface of the water near the middle of the ponds using an inflatable raft. The pH of the water samples was determined within 8 h of collection using a combination glass electrode and a portable pH-meter. The samples were neither acidified nor filtered in the field. The water samples were stored at 4 °C before analysis at Syracuse University. Water samples were analyzed for all major solutes and Al species using standard methods. The analytical methods used were virtually identical to the original ELS-II (see the Supporting Information). Computational Methods. The speciation of Ali for both surveys was calculated using the chemical equilibrium program MINEQL+ (Version 4.0, Environmental Research Software, 1998). The aqueous species considered and the thermodynamic data are described in Driscoll and Postek (10), and the nomenclature for Al fractions and species are provided in Table S1 in the Supporting Information. For these calculations, the pH was fixed at the measured value. The speciation of Ali is only reported for the 23 ponds which had Alm concentrations above the analytical detection limit (>0.7 µmol L-1) in both 1986 and 2001. The uncertainty associated with chemical equilibrium constants and input values used to determine Al speciation and solubility are detailed in refs 18and 19. Since neither the 1986 nor the 2001 data were normally distributed, nonparametric statistics were used for data analysis. Medians rather than means were used to report central tendency. The Wilcoxon matched pairs test was used to determine the significance of the changes reported, with p < 0.05 indicated as significant.
Results and Discussion Fractionation of Total Al. We found statistically significant decreases in the concentrations of total Al and Ali across the whole region and in the Adirondack and Southern New England subregions (Table 1). The Ali concentration also decreased significantly in the Central New England and Maine subregions. Monomeric organic Al (Alo) decreased regionwide and in all the subregions except for the Adirondacks where median Alo increased from 0.23 µmol L-1 to 0.95 µmol L-1 (Table 1). Median total Al decreased from 1.45 to 1.01 µmol L-1 across the region, with the largest decrease in the Adirondacks, where median total Al decreased from 4.60 µmol L-1 to 2.59 µmol L-1 (Table 1). While Ali exhibited the largest concentration decrease in the Adirondacks (-1.26 µmol L-1; Table 1), the largest proportional decreases were observed in CNE and Maine, where Ali decreased from ∼35% to ∼2% of total Al, and from ∼30% of total Al to 2 µmol L-1 in 1986 and 2001 year All Ponds Alm >0.7 µmol L-1
1986 2001
Ali >2 µmol L-1
1986 2001
no. of ponds
Whole Region
Adirondacks
CATPOC
CNE
SNE
Maine
n N
113 3666
20 527
17 679
26 873
21 590
29 997
n N n N
61 1790 26 597
16 388 13 268
6 183 3 86
16 490 4 103
8 193 4 81
15 536 2 59
n N n N
20 449 7 130
10 208 5 90
3 86 2 40
3 86 0 0
3 66 0 0
1 29 0 0
draining a spruce plantation in Mid-Wales. He found that Al3+, Al-F, Al-Si, and Alo complexes were the predominant forms of Al, with Al-SO4 and Al-OH-F less abundant. Unlike in our study he found that Al-Si comprised about 20% of total dissolved Al at pH >5, and concluded that these complexes were an important part of the total dissolved Al concentrations. Like Al-F, concentrations of Al-OH-F species decreased significantly between 1986 and 2001. For the 23 ponds considered for the speciation of Ali, the median concentration of Alo increased from 0.33 µmol L-1 to 1.12 µmol L-1, despite a decrease in DOC from 318 µmol C L-1 to 289 µmol C L-1 (Table S2 in the Supporting Information). These waters typically had greater concentrations of both Al and DOC than the full set of 113 lakes. Driscoll and Postek (10) found that the ratio of Alo:DOC across the ELS-II sites increased with decreases in pH and increases in Ali. They hypothesized that this pattern could be due to increased availability of aqueous Al to participate in complexation with organic ligands. They also suggested that there might be an increased affinity of Al for naturally occurring ligands with decreases in pH. However, our data suggest that increases in pH result in an increased affinity for Al complexation with organic ligands (see the Supporting Information), especially since both aqueous Al and DOC concentrations decreased over the study period (Table 2; Table S2 in the Supporting Information). This pattern is consistent with the increased deprotonation of organic acids with increasing pH. This phenomenon was not observed when all 113 sites were included (i.e., typically higher pH waters). The speciation of Al in the 35 lakes that had Alm above detection in 1986 but below detection in 2001 was also investigated (see the Supporting Information). These lakes were characterized by low Al3+ concentrations, whereas Al-OH and Al-F complexes dominated the Ali fraction. Since the median change in Alm between the 1986 and 2001 survey was 1.28 µmol L-1 (Table 1), it is not surprising that these 35 ponds were below detection in 2001 given their median Alm concentration of 1.22 µmol L-1 in 1986. Implications for Application of Biogeochemical Models. Biogeochemical models are important tools to facilitate understanding of the acid-base status of soil and surface waters, and predict watershed response to changes in acidic deposition. The model of acidification of groundwater in catchments (MAGIC), a widely used biogeochemical model, assumes that the concentration of Al3+ in solution is regulated by the solubility of gibbsite (Al(OH)3) (31). The model then calculates the other species of Ali in the surface waters using thermodynamic relationships. The main concerns with this modeling approach are that surface waters are often highly undersaturated with respect to gibbsite, and that there is a 8672
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wide range of solubility products (*Kso) that can be used for gibbsite (10). While in 1986, surface waters were undersaturated with respect to the solubility of natural gibbsite at pH values 0.7 µmol L-1). By 2001 only 26 of the sampled lakes, representing 597, had detectable concentrations of Alm (Table 4). Thus, as a result of the improved water quality following reduced acidic deposition, the number of lakes with detectable concentrations of Alm during summer has declined by about two-thirds. Ecological Implications. The decrease in the number of ponds that had Ali above 2 µmol L-1 between 1986 and 2001 represents summer conditions. Investigators have found concentrations of Al in the Adirondacks to be greatest during spring snowmelt (17). Bailey et al. (27) found that total concentrations of organic Al at Cone Pond, New Hampshire, was highest in the summer due to the mobilization of DOC from wetlands during the growing season. Guibaud and Gauthier (24) who studied Al speciation of two locations along the Vienne River in France also found strong seasonality of Ali speciation. They concluded that potentially toxic concentrations of Al (Al3+, AlOH2+, and Al(OH)2+) occurred mostly during the summer months. Palmer et al. (32) observed decreases in Ali in
streamwater at the Hubbard Brook Experimental Forest in New Hampshire between 1982 and 1998. They predicted that Ali will fall below the toxic threshold for fish (2 µmol L-1) by about 2010, but they noted that episodic acidification of the streams will likely continue to result in increased fish mortality, as fish survival is likely linked to peak Al conditions rather than average conditions. Baldigo et al. (11) found that while brook trout mortality in the southwestern Adirondacks was highly variable, it had not changed appreciably between studies conducted in the 1980s and 1990s and their study between 2001 and 2003. Considering the seasonality exhibited by concentrations of Ali in different regions, our population estimates of the number of lakes in both studies experiencing potentially toxic Ali concentrations are likely underestimates since water chemistry conditions typically show the highest ANC and lowest Ali in the summer. Furthermore, lakes exhibit net deposition of Al to sediments (10) so Al concentrations may be higher in tributary water. Both of these factors may adversely affect fish response. Nevertheless, in this study, since both surveys were conducted at a similar time of year, the data suggest a significant decrease in the number of ponds experiencing toxic conditions of Ali between 1986 and 2001.
Acknowledgments We thank the W.M. Keck Foundation for their financial support, without which this project would not have been possible. We thank Mario Montesdeoca, Mary Margaret Koppers, and the laboratory staff at Syracuse University for their help in analysis of the samples. We also thank the 10 members of the field crew who helped with the collection of the samples in 2001.
Supporting Information Available Additional tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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