Chronic and episodic acidification of streams along the Appalachian Trail corridor, eastern United States

Abstract

Acidic atmospheric deposition has adversely affected aquatic ecosystems globally. As emissions and deposition of sulfur (S) and nitrogen (N) have declined in recent decades across North America and Europe, ecosystem recovery is evident in many surface waters. However, persistent chronic and episodic acidification remain important concerns in vulnerable regions. We evaluated acidification in 269 headwater streams during 2010–2012 along the Appalachian Trail (AT) that transits several ecoregions and is located downwind of high levels of S and N emission sources. Discharge was estimated by matching sampled streams to those of a nearby gaged stream and assuming equivalent daily mean flow percentiles. Charge balance acid‐neutralizing capacity (ANC) values were adjusted to the 15th (Q15) and 85th flow percentiles (Q85) by applying the ANC/discharge slope among sample pairs collected at each stream. A site‐based approach was applied to streams sampled twice or more and a second regression‐based approach to streams sampled once to estimate episodic acidification magnitudes as the ANC difference from Q15 to Q85. Streams with ANC <0 μeq/L doubled from 16% to 32% as discharge increased from Q15 to Q85 according to the site‐based approach. The proportion of streams with ANC <0 μeq/L at low flow and high flow decreased from north to south. Base cation dilution explained the greatest amount of episodic acidification among streams and variation in sulfate (SO42−) concentrations was a secondary explanatory variable. Episodic SO42− patterns varied geographically with dilution dominant in northern streams underlain by soils developed in glacial sediment and increased concentrations dominant in southern streams with older, highly weathered soils. Episodic acidification increased as low‐flow ANC increased, exceeding 90 μeq/L in 25% of streams. Episodic increases in ANC were the dominant pattern in streams with low‐flow ANC values <30 μeq/L. Chronic and episodic acidification remain an ecological concern among AT streams. The approach developed here could be applied to estimate the magnitude and extent of chronic and episodic acidification in other regions recovering from decreasing levels of atmospheric S and N deposition.

Long‐Term Trends in Acid Precipitation and Watershed Elemental Export From an Alpine Catchment of the Colorado Rocky Mountains, USA

“…However, new trends are beginning to emerge, suggesting that these high elevation ecosystems are now responding to newly elevated cation deposition (e.g., NH 4 + ) and a reversal of past trends in acid deposition. Similar reversals of deposition trends and ecosystem recovery from acidification are documented in the literature, for example, in the Northeastern USA (Dethier et al, 2018; Fuss et al, 2015), as well as examples of episodic acidification (Burns et al, 2020), but the pathways of acid recovery are not well understood for the alpine ecosystems of the Rocky Mountains.…”

“…Journal of Geophysical Research: Biogeosciences (Burns et al, 2020), but the pathways of acid recovery are not well understood for the alpine ecosystems of the Rocky Mountains.…”

Acidification Of Northeastern USA Lakes From Rising Anthropogenic‐Sourced Atmospheric Carbon Dioxide and Its Effects on Aluminum Speciation

“…As such, they commonly migrate up streams from lakes to spawn. During the spring runoff, stream pH can attain values as low as 4 (Burns et al., 2019; Fakhraei & Driscoll, 2015). Many of the streams used by brook trout for spawning in northern New England and New York are also poorly buffered and can exhibit negative acid neutralizing capacities (e.g., Fuss et al., 2015; Gbondo‐Tugbawa & Driscoll, 2003; Hall et al., 1985).…”

Acidification recovery in a changing climate: Observations from thirty‐five years of stream chemistry monitoring in forested headwater catchments at the Turkey Lakes watershed, Ontario

“…The analyses presented in this study focused on annual trends and may mask seasonal and event‐scale dynamics, such as episodic acidification during spring freshet that have been observed in the northeastern United States (Burns et al, 2020; Fuss et al, 2015; Riscassi et al, 2019). Future work will try to better understand intra and inter‐ annual changes in flowpaths across the elevation gradient at TLW.…”