Baltimore Ecosystem Study Institute of Ecosystem Studies

2016 BES Annual Meeting Presentation and Poster Abstracts

Increasing major ion concentrations and changing ion ratios in urban streams over a 15-year period in Baltimore, MD
Bird, Darcy
Co-Authors: Darcy Bird 1, Peter Groffman 2, and Joel Moore 1,3 1 Environmental Science, Towson University, Towson, MD 21252, 2 Advanced Science Research Center, City University of New York, New York, NY 3 Dept of Physics, Astronomy, and

Abstract: A growing number of studies have shown that urban streams have relatively high major ion concentrations due to contributions from anthropogenic sources (e.g., road salts, fertilizer, concrete, wastewater treatment plants and other point sources). While previous work has documented the differences in major ion chemistry between forested and urban streams, investigation of ion concentration changes in urban streams on the decadal scale is sparse. We used anion data from 19992014 from 4 watersheds in the Baltimore Ecosystem Study (BES) and the new USGS water chemistry analysis tool Weighted Regressions on Time, Discharge, and Season (WRTDS) to analyze multi-year trends in ion chemistry. The 4 watersheds lie along a forested to urban gradient, are underlain by felsic bedrock, and have experienced little change in land use and land cover over the study period. These watersheds have no large point sources such as wastewater treatment plants and thus changes in ion concentrations are primarily the result of inputs from the urban built environment. The forested watershed had constant Cl- (~2.5 mg/L) concentrations while flow-normalized annual concentrations increased in the two urban watersheds from 43 to 131 and 99 to 155 mg/L, respectively. SO42- concentrations decreased somewhat in all watersheds (0.5-1 mg/L decrease). Ca2+, Mg2+, and Na+ are also elevated and increasing over time in urban watersheds. Na+/Cl- ratios at the reference site were close to 1, while developed watersheds had lower Na+/Cl- ratios, an indicator of cation exchange processes retarding Na+ movement through the watersheds. The observed changes suggest that the urban built environment, including concrete weathering and application of road salt, increased anion (Cl-) and cation (Ca2+, Mg2+, Na+) concentrations, and the use of road salts (NaCl) altered exchange processes in streams leading to different Na+/Cl- ratios in forested and urban streams.