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

Increasingly, studies have shown that urban streams have relatively high major ion concentrations compared to natural reference streams due largely to anthropogenic inputs (e.g. road salts, fertilizer, concrete). These studies have documented the differences between urban and natural streams, but decadal trends in major ion concentrations have been sparsely investigated. Anion data from 1999-2014 was analyzed from 4 watersheds within the Baltimore Ecosystem Study’s (BES) archived water sample data using the USGS tool Weighted Regressions on Time, Discharge, and Season (WRTDS) to determine decadal trends. Archived samples were subsampled in order to analyze cation concentrations for the same time period. The 4 watersheds used in this study lie on a forested to urban gradient in Baltimore, MD that is underlain by felsic bedrock. Within the watersheds, land use has remained relatively stable throughout the study period, with impervious surface cover changing <1% during this period. Anion (Cl^- and SO₄^2-) and cation (Ca²⁺, Mg²⁺, Na⁺) concentrations were much higher in the urban watersheds compared to the forested watershed (Cl^- by 50x, SO₄^2- by 12x, Na⁺ by 15x, Ca²⁺ by 50x, and Mg²⁺ by 25x). Na⁺/Cl^- ratios, which are used as an indicator of cation exchange processes within watersheds, were close to 1 in the forested watershed, but decreased drastically at even 1% impervious surface cover. Concentrations of both anions (Cl^-) and cations (Ca²⁺, Mg²⁺, Na⁺) in the urban watersheds have increased during the study period, while only sulfate (SO₄^2-) decreased during the study period. The observed changes indicate that impervious surface cover combined with application of road salts, have increased anion (Cl^-, SO₄^2-) and cation (Ca²⁺, Mg²⁺, Na⁺) concentrations. Further, the use of road salts within the study’s watersheds have altered cation exchange composition, leading to Na⁺/Cl^- ratios differing between the forested watershed and the watersheds containing any impervious surface cover.