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 in the concentrations of total Al and inorganic monomeric aluminum (Al i ) 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 (Al o ) also decreased region-wide and in all the subregions except the Adirondacks. The speciation of Al i 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 Al i concentrations above a toxic limit of 2 μmol L -1 compared with 20 sample lakes, representing 449 lakes, in 1986. Thus, we estimate that more than 300 lakes in the northeastern United States no longer have summer Al i 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 (SO 2 ) and, to a lesser extent, nitrous oxides (NO x ). These controls have decreased sulfate (SO 4 2- ) 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 eco- systems (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 <4.5 (9). These effects include decreases in acid neutralizing capacity (ANC) and pH, increased concentrations of Al, and a shift in the speciation of Al from the less toxic organically bound and particulate forms to the more toxic monomeric inorganic species in surface waters (7, 10). The consequences of increases in the concentrations of inorganic monomeric Al in surface waters include the loss of species diversity and numbers of fish, invertebrates, and plankton (7). Understanding the speciation of Al is critical to assess- ments of the effects of acidification and recovery from these effects, as not all forms of Al are equally toxic and changes in the supply of complexing ligands will alter this toxicity. Aluminum forms complexes with hydroxide ions, fluoride, silica, sulfate, and organic solutes (10, 13). Monomeric Al (Alm ) is thought to be more bioavailable than other forms of Al (e.g., particulate Al, Al occluded within organic matter); with monomeric inorganic Al (Al i ) the most toxic form. Aquo (Al 3+ ) and hydroxy complexes are thought to be particularly toxic (e.g., refs 9, 10, and 14). Studies have identified that 2 μmol L -1 is a threshold above which toxic effects of Al are evident to fish and other aquatic biota (7, 11, 12). Therefore, reductions in acidic deposition may affect Al toxicity in surface waters through changes in total Al concentrations, Al speciation, or both. Unfortunately, few studies have inves- tigated the changes in the speciation of aqueous Al in response to this recovery. In this paper, we examine the regional-scale changes in the concentrations and speciation of Al between 1986 and 2001 in remote lakes impacted by acidic deposition across the northeastern U.S. We hypothesized that observed in- creases in pH would result in decreases in total Al, Alm , and Al i . We further hypothesized that the increase in pH would result in a shift in the fractionation of Al from Al i to Al o . To test these hypotheses, we sampled 113 lakes in 2001 which had been previously sampled in 1986 as part of the Eastern Lakes Survey phase II (ELS-II). Materials and Methods The National Surface Water Survey (NSWS), conducted between 1984 and 1986, included a series of synoptic surveys using a probabilistic approach. The Direct/Delayed Response Project (DDRP) was conducted in 1984 and was designed to allow the survey results to be extrapolated to the population of lakes in the region (15). The DDRP sampled 145 lake- watersheds in the northeastern U.S. region with lake surface area >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 inac- cessibility 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). * Corresponding author phone: (315) 559-7680; fax: (315) 443- 1243; e-mail: rwarby@syr.edu. Environ. Sci. Technol. 2008, 42, 8668–8674 8668 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 23, 2008 10.1021/es801125d CCC: $40.75  2008 American Chemical Society Published on Web 11/06/2008