Strong Seasonality in the Response of Streams to Declining Acidic Deposition—the Latest on the Chemical Recovery of Adirondack Streams

Acidic deposition effects on surface waters have been well documented in North America, and several decades of declining sulfur deposition along with more recent declines in nitrogen deposition have shifted the focus to recovery. Partial chemical recovery of surface waters has been recorded in impacted areas such as the Adirondack region of NY, but most information on surface-water recovery in the northeastern U.S. has been collected from lakes. Because lakes can provide in-lake acid-buffering that doesn't occur in streams, these water bodies may not follow the same recovery trajectory. To provide up-to-date information on how stream chemistry is responding to declining deposition, 64 Adirondack streams were sampled in 2004-05 during summer base flow and spring snowmelt, and again in 2014-15 during the same seasons and flow conditions. Two tributaries of Buck Creek, in the same region, also provided biweekly and event chemistry data from 1998 to 2015. Decreases in SO4 concentrations were observed in nearly all streams and were likely to be driving the decreases also observed in base-flow concentrations of inorganic monomeric Al, the form toxic to aquatic biota (P <0.01). Changes over time showed a strong seasonal dependency, with increases in pH and base-cation surplus (BCS) in the summer (P<0.01), but decreases in pH (P <0.01) and no change (P >0.1) in BCS during snowmelt. Despite decreasing atmospheric N deposition throughout the study period, concentrations of NO3 showed no difference between 2004-05 and 2014-15 on an annual basis (P >0.1), although concentrations were higher during snowmelt (P <0.05) and lower during summer (P <0.05) in 2004-05 than in 2014-15. Severe episodic acidification occurred throughout the record at Buck Creek during elevated flows, regardless of season. As a result of base flow recovery and continued high-flow acidification driven by NO3 release, wide fluctuations in stream chemistry continue to occur.