ROAD SALT AND CONCRETE AS SIGNIFICANT CONTRIBUTORS TO HIGH ION CONCENTRATIONS & HIGH SPECIFIC CONDUCTANCE IN URBAN STREAMS

MOORE, Joel, Urban Environmental Biogeochemistry Laboratory, Towson University, 8000 York Rd, Towson, MD 21252; Dept. of Physics, Astronomy, & Geosciences, Towson University, 8000 York Road, Towson, MD 21252, MUELLER, Corey, Dept of Physics, Astronomy, & Geosciences`, Towson University, 8000 York Road, Towson, MD 21252; Urban Environmental Biogeochemistry Laboratory, Towson University, 8000 York Road, Towson, MD 21252 and WOODWARD, Gregory, Urban Environmental Biogeochemistry Laboratory, Towson University, 8000 York Rd, Towson, MD 21252; Dept. of Physics, Astronomy, and Geosciences, Towson University, 8000 York Rd, Towson, MD 21252, joelmoore6@gmail.com

The percentage of precipitation that enters streams as runoff is much higher in urban watersheds than forested watersheds. That high percentage of runoff in urban watersheds might be expected to result in relatively dilute waters, however the opposite is true. Urban streams have been consistently found to have high concentrations of major ions (Ca²⁺, K⁺, Mg²⁺, Na⁺, SO₄^2–, and HCO₃^–/alkalinity) compared to forested streams. High concentrations of, or altered ratios in, major ions can negatively impact aquatic organisms and water quality for human consumption and indirectly impact aquatic organisms through increased specific conductance. The sources of dissolved ions are frequently unclear due to the complexity of urban systems. This study will more quantitatively establish the concentrations and sources of ions to urban streams through collection and analysis of samples from several watersheds across a rural to urban gradient. The study watersheds contain felsic silicate bedrock with no carbonate bedrock or wastewater treatment plants, which simplifies the identification and quantification of major ion sources to urban streams. Concentrations of all major ions increase by 10–100x as the percentage of urban land cover increases and pH generally increases as well. In the forested watershed, HCO₃^– and Na⁺ are the most common ions and the average Na/Cl ratio is 1.17; by contrast, in the most urban watershed, Cl^– and Na⁺ are the most common ions and the average Na/Cl ratio is 0.61. Cl^– and Na⁺ concentrations are more variable in urban streams than Ca²⁺ or HCO₃^–. Since Ca²⁺ and HCO₃^– concentrations increase with urban land cover, the most urban streams are at chemical equilibrium with respect to calcite even though no carbonate bedrock is present in any of the watersheds; concrete is likely the main source of these ions. Our initial results indicate that de-icing salt (NaCl) and concrete are the two biggest contributors of major ions as urban land cover increases.

Session No. 8

T32. Urban Geochemistry

Sunday, 1 November 2015: 8:00 AM-12:00 PM

Room 308 (Baltimore Convention Center)

Geological Society of America Abstracts with Programs. Vol. 47, No. 7, p.41

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2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

ROAD SALT AND CONCRETE AS SIGNIFICANT CONTRIBUTORS TO HIGH ION CONCENTRATIONS & HIGH SPECIFIC CONDUCTANCE IN URBAN STREAMS