INCREASING MAJOR ION CONCENTRATIONS AND CHANGING ION RATIOS IN URBAN STREAMS OVER A 15-YEAR PERIOD IN BALTIMORE, MD

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). While previous work has documented the differences in major ion chemistry between natural and urban streams, investigation of ion concentration changes in urban streams on the decadal scale is sparse. We used anion data from 1999–2012 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. The forested watershed had constant Cl^- (~2.5 mg/L) concentrations while median annual concentrations increased in the two urban watersheds from 29 to 84 and 73 to 110 mg/L. SO₄^2- concentrations decreased somewhat in all watersheds. 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 urban land use 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.