CONCRETE WEATHERING AND THE URBAN BUILT ENVIRONMENT AS A MAJOR SOURCE OF INCREASED MAJOR ION CONCENTRATIONS INCLUDING DISSOLVED INORGANIC CARBON AND WEATHERING FLUXES IN URBAN WATERSHEDS

In the last decade, several studies have documented elevated major ion concentrations in urban watersheds. Road salt is one clear contributor to elevated concentrations but concrete has also been suggested as a contributor to increased Ca²⁺ and HCO₃^– concentrations. However, in previous studies a number of factors have precluded attribution of increased Ca²⁺ and HCO₃^– to natural versus anthropogenic sources, including mixed bedrock geology (particularly carbonate rocks), point sources with unknown chemistry (such as wastewater treatment plants), and/or the lack of a forested watershed that provides a baseline for comparison. This study investigated major ion concentrations and weathering fluxes in five small watersheds in the Maryland Piedmont with 0 – 25% impervious surface cover (ISC), similar bedrock chemistry, and no major point sources. In the forested watershed (0% ISC), ion concentrations were low with the dilute waters resulting in weathering fluxes (for cations and Si) that were 80% lower than the global average. As ISC increased, the concentrations of all major ions also increased; average major ion concentrations in the most urban watershed were enriched by a factor of 25x over the forested watershed. As a result, preliminary weathering fluxes from the most urban watershed were 13x higher than the forested watershed and >2.5x the global average. These data demonstrate that weathering (or degradation) of the urban built environment, particularly due to concrete weathering and road salt addition, cause drastic changes in water chemistry and as a result, high weathering fluxes. High major ion concentrations have the potential to impact organisms in stream ecosystems through direct toxicity (e.g., Cl^–) and osmoregulatory stress and also have implications for drinking water quality. The high weathering fluxes for major ions, particularly Ca²⁺ and HCO₃^–, have important implications for carbon cycling in urban watersheds and for downstream estuarine and coastal ecosystems.