39-1 Assessing Organic Geochemistry and Disinfection Byproduct Formation Potential Across Catskill Watersheds: Implications for New York City Drinking Water

Session: Functions of River Corridors, Floodplains and Wetlands

Presenting Author:

Kevin Ryan

Authors:

Ryan, Kevin A¹, Bates, Madelyne², Black, Gabrielle³, Chowdhury, Maanal⁴, Doolittle, Ally⁵, Hladik, Michelle⁶, Olsson, Ann⁷, Rienzo, Angela⁸, Reimer, Benjamin⁹, Scott, Durelle¹⁰, Wardinski, Katherine¹¹

(1) U.S. Geological Survey, New York Water Science Center, Troy, , (2) Contractor to the U.S. Geological Survey, New York Water Science Center, Troy, , (3) United States Geological Survey, California Water Science Center, Sacramento, , (4) Contractor to the U.S. Geological Survey, New York Water Science Center, Troy, , (5) U.S. Geological Survey, New York Water Science Center, Troy, , (6) U.S. Geological Survey, California Water Science Center, Sacramento, , (7) U.S. Geological Survey, New York Water Science Center, Troy, , (8) U.S. Geological Survey, New York Water Science Center, Troy, , (9) Contractor to the U.S. Geological Survey, New York Water Science Center, Troy, , (10) Virginia Tech, Department of Biological Systems Engineering, Blacksburg, , (11) Virginia Tech, Department of Biological Systems Engineering, Blacksburg, ,

Abstract:

The Catskill region of New York State supplies the majority of New York City’s unfiltered drinking water through six reservoirs and their watersheds. These watersheds span topographic gradients, land cover types, and hydrologic settings, which influence the mobilization and transformation of dissolved organic matter (DOM), a key biogeochemical reactant within aquatic ecosystems. DOM plays a central role in river corridor function, influencing nutrient cycling, microbial activity, and water quality. Of particular concern for drinking water management is the formation of disinfection byproducts (DBPs), which occur when disinfectants such as chlorine react with DOM during water treatment. DBPs such as trihalomethanes (THMs) and haloacetic acids (HAAs) are regulated due to their human health risks.

 

This study investigated the spatial variability of DOM composition and DBP formation potential (DBPfp) across Catskill watersheds, with a focus on the role of soils and watershed physiography in shaping DOM and DBP metrics. Water samples from 30 streams and rivers were collected approximately every six weeks in 2025 during baseflow conditions and analyzed for water quality indicators including dissolved organic carbon (DOC). The influence of soil organic matter was assessed using water-extractable organic matter (WEOM) leached from 62 soil samples representing organic and mineral horizons across varied riparian positions and vegetation cover. DOM composition was assessed via UV–visible spectroscopy for all samples and molecular metrics of WEOM were assessed via high-resolution mass spectrometry. DBPfp was assessed via standardized chlorination incubations followed by quantification of regulated THMs and HAAs.

 

Initial results indicate DBPfp increased linearly with dissolved organic carbon (DOC) across all samples where baseflow DOC concentrations (1–2 mg-C/L) were shown to exceed regulatory thresholds for total THMs and HAAs in finished drinking water. However, DBPfp normalized to DOC revealed that DOM composition also plays a critical role. Spectroscopic and molecular metrics of DOM aromaticity were also correlated with DBPfp, which further demonstrates the importance of this role. Most soil-derived DOM contributed to DBP precursors, however, organic horizons with higher WEOM concentrations yielded the highest THM formation potential. The combined spatial heterogeneity in DOM properties and DBPfp across physiographic gradients in watersheds and soils indicates that riparian zones with abundant, leachable organic matter may disproportionately influence DBP formation risks in downstream drinking water supplies subject to ongoing environmental change.

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