Trends and current status of aluminum chemistry in Adirondack headwater streams 30 Years after the Clean Air Act Amendments of 1990

Abstract Mobilization of toxic forms of aluminum (Ali) have been one of the most harmful effects of acidic deposition on aquatic ecosystems. Large decreases in acidic deposition levels have resulted in decreases in Ali concentrations in surface waters starting in the 1990s. However, recent studies indicate that fish communities are still being impaired by elevated Ali concentrations in streams, suggesting the need for a present-day assessment of the recovery status of Al chemistry in impacted regions such as the Adirondack region of New York, United States. Therefore, the recovery status of Al was assessed from long-term high-frequency monitoring of three Adirondack streams up through 2019, and multiple resampling of 127 headwater streams throughout the Adirondack region between 2004/2005 and 2018/2019. Results indicated that concentrations of Ali have continued to decrease throughout the Adirondack region, but that harmful conditions still exist in a substantial number of streams during spring snowmelt. In the western Adirondack region, 35 percent of streams during spring snowmelt, (typically the most acidic period), and 10 percent of streams during summer were still experiencing harmful concentrations of Ali (>1.0 μmol L−1) in 2018/2019. In the less-impacted east-central Adirondack region, 10 percent of streams during snowmelt, and 4 percent of streams during summer were experiencing harmful Ali concentrations in 2018. Temporal decreases in Ali concentrations were due in part to a shift in speciation from Ali to non-toxic organically complexed Al as dissolved organic carbon concentrations increased, which was also a response to decreasing acidic deposition. Increased availability of calcium resulting from acid-neutralization processes such as weathering also contributed to the decrease in Ali over the past 5–8 years. The current low levels of acidic deposition may have begun to enhance recovery by increasing the effectiveness of ambient acid-neutralization processes.