Ongoing increases in dissolved organic carbon are sustained by decreases in ionic strength rather than decreased acidity in waters recovering from acidic deposition

Gregory B Lawrence; Karen M Roy

doi:10.1016/j.scitotenv.2020.142529

Ongoing increases in dissolved organic carbon are sustained by decreases in ionic strength rather than decreased acidity in waters recovering from acidic deposition

Sci Total Environ. 2021 Apr 20:766:142529. doi: 10.1016/j.scitotenv.2020.142529. Epub 2020 Oct 10.

Authors

Gregory B Lawrence¹, Karen M Roy²

Affiliations

¹ U.S. Geological Survey, New York Water Science Center, Troy, NY 12180, United States of America. Electronic address: glawrenc@usgs.gov.
² New York State Department of Environmental Conservation, Box 296, Ray Brook, NY 12977, United States of America. Electronic address: karen.roy@dec.ny.gov.

PMID: 33158515
DOI: 10.1016/j.scitotenv.2020.142529

Abstract

Dissolved organic carbon (DOC) has received considerable attention in freshwater research, particularly since the early 2000s when increasing trends became apparent. However, remaining questions need to be resolved to address future effects of DOC on surface waters. This study was undertaken to determine (1) the relative importance of acidity and ionic strength in driving DOC increases in waters recovering from acidification and (2) the role played by long-term acid rain effects on soil. Data obtained from temporal and spatial monitoring of 142 headwater streams throughout the Adirondack region of New York (USA) were used to evaluate chemical relationships involving DOC. Year-round monitoring of three streams of differing acidification status were combined with intermittent stream surveys during spring snowmelt throughout this 24,243 km² region that is recovering from acidification of soils and surface waters. Despite acidic deposition decreases reaching levels estimated for the early 1900s, DOC concentrations exhibited linear increases from the early 2000s through 2019. Ionic strength or conductivity showed consistent inverse relationships with DOC in all data comparisons from 2004-05 to 2018-19. In contrast, relationships between pH and DOC did not support increasing pH as an important factor in DOC increases. Inconsistent relationships between pH and DOC were due to strongly acidic organic acids that remain unprotonated throughout the pH range of these waters and limited weak-acid deprotonation below pH 6.2. Decreasing ionic strength increases DOC solubility by expanding the diffuse double layer, which fosters disaggregation of organic matter and dispersion of colloids. This affect controlled DOC solubilization below a pH of approximately 6.2. Distinguishing between ionic strength and pH effects is important because further large reductions in acidic deposition are not expected but continued soil-water dilution is likely from soil-Ca²⁺ depletion and the decreasing rate of Ca²⁺ leaching by SO₄^2- and NO₃^-, which are still being released from soil organic matter.

Keywords: Acidic deposition recovery; Acidification of surface waters; Browning of surface waters; Calcium depletion; DOC; Dissolved organic carbon.

Published by Elsevier B.V.