Changes in stream chemistry in the east-central Adirondack region of New York State, 2010–2018

Abstract

Decreased acidic deposition occurred through the 2010s, extending the long-term decrease begun in the early 1980s. The overall decline has resulted in rates of acidic deposition in the Adirondack region that are similar to those estimated for the early 1900s. This achievement increased attention on the issue of acidic deposition recovery and raised the question of how the low deposition levels of the 2010s were affecting the Adirondack region of New York State, where past acidic deposition effects were among the highest in the United States. This report presents stream resampling results of the East-Central Adirondack Stream Survey (ECASS), the Adirondack high-elevation stream sampling program, and the first five years of stream chemistry measurements in the Boreas River Long-Term Monitoring Watersheds. In addition, a regional comparison of stream chemistry between ECASS and the Western Adirondack Stream Survey (WASS) is presented. The primary objective of these stream resampling programs was to evaluate changes in stream chemistry during the 2010s to evaluate ongoing recovery of acidic deposition within the Adirondack region. Streams in the ECASS region (19,658 km 2 ) were sampled during spring snowmelt in 2011, 2017, and 2018; summer baseflow in 2010 and 2017; and late autumn in 2011 and 2017. ECASS stream sampling results for snowmelt in 2018 are also compared with WASS stream sampling results during snowmelt of the same year to provide a spatial assessment of stream chemistry for the entire Adirondack region. In addition, high-elevation streams were sampled in 2010-2011 and 2017 during spring snowmelt and summer baseflow, and three streams in the Boreas River watershed were sampled biweekly from 2014 through 2019. Streams were selected for sampling from the approximately 200 streams sampled originally in both the ECASS and WASS. The original ECASS and WASS streams were randomly selected from the populations of accessible streams within their respective regions. For the study reported here, subsets of streams were selected from the two original groups using a random stratified design based on the stream chemistry of the original ECASS and WASS. ECASS data are presented only for streams that were sampled in each of the seasonal surveys, which was 59, 48, and 61 streams for spring snowmelt, summer baseflow, and autumn surveys, respectively. WASS data are presented for 60 streams and high-elevation data are presented for 11 streams sampled during summer baseflow and 12 streams sampled during spring snowmelt. Overall, mean values of chemical measurements did not indicate large changes between 2010-2011 and 2017-2018 in the ECASS snowmelt, summer baseflow, or autumn surveys. Minimal acidification was observed in mean values measured in any of the surveys. Mean values of acid-neutralizing capacity (ANC) remained above 80 microequivalents per liter (µeq L -1 ), mean pH remained above 6.37, and inorganic monomeric aluminum (Al i ) remained below 1.0 micromoles per liter (µmol L -1 ), with the exception of the mean value of 1.2 µmol L -1 in the 2017 autumn survey. However, in the 2017 and 2018 snowmelt samplings 15% and 10% of the streams, respectively, had Al i concentrations above 1.0 µmol L 1 , the concentration above which is toxic to fish. Based on the estimated total length of headwater streams in the ECASS region of 11,494 km, the 10% of ECASS streams that experienced harmful Al i concentrations (Al i >1.0 µmol L -1 ) during the 2018 snowmelt survey equated to a total length of 1,091 km of headwater stream length in the region. During both summer baseflow and snowmelt sampling, high-elevation streams were less acidic in 2017 than in 2010-2011. However, in 2017 the mean Al i concentration during snowmelt was 2.6 µmol L -1 , a value that reflected low to moderate toxicity to brook trout. Concentrations of Al i in 9 out of the 12 streams were above 1.5 µmol L -1 , which exceeds the value which impairs fish communities. Greater dilution of calcium (Ca 2+ ) concentrations than sulfate (SO 4 2- ) concentrations during snowmelt was a contributing factor to the more acidic stream water measured during snowmelt than summer baseflow. Comparison of 2018 ECASS and WASS snowmelt chemistry under the current low levels of acidic deposition indicated that the spatial pattern of acidification effects on stream chemistry is largely driven by inherent Ca 2+ availability tied to spatial variations in geologic factors that control acid buffering more effectively in the ECASS region than the WASS region. The higher degree of acidification in the WASS region than the ECASS region is likely to extend into the future as a result of the relatively high dissolved organic carbon (DOC) concentrations in the WASS. Pronounced increasing trends in DOC and similarly pronounced decreasing trends in Ca 2+ and nitrate (NO 3 - ) were observed in the streams of all three Boreas River watersheds. These strong increasing DOC trends occurred in streams during a period with little or no indication of further chemical recovery from acidic deposition. The strong DOC trends in Durgin Brook and Balsam Brook, which are streams that are unlikely to have ever experienced more than minimal acidification, suggest the need for further investigation to expand our understanding of the mechanisms behind the increasing DOC trends.