High-Frequency Data Reveals Deicing Salt Application Causes Elevated and Increasing Conductivity and Chloride along with Widespread Exceedances of Chloride Water Quality Criteria in Urban Eastern US Streams

Observed increases in specific conductance (SC) of streams in North America, Europe, Oceania, etc. represents a threat to aquatic ecosystem health. While elevated SC is often associated with urbanization, the drivers and magnitude of SC increases within and across regions need to be better quantified. We use a dataset containing nearly 30 million high-frequency observations and ~6400 discrete observations collected by the US Geological Survey (USGS) to investigate SC and modeled chloride concentrations [Cl] at 93 sites in the eastern United States (US). Sites are located in three regions: Southeast, Mid-Atlantic, and New England. Our data provides robust quantitative evidence that urbanization consistently elevates SC, particularly in the Mid-Atlantic and New England where deicing salts are widely applied. Within each region, SC and [Cl] significantly increase with impervious surface cover (ISC) with higher correlation and larger increases for a given level of ISC for the Mid-Atlantic and New England, e.g., SC at 10% ISC is 75, 284, and 439 μS/cm in the Southeast, Mid-Atlantic, and New England, respectively, with [Cl] of 4.4, 48, and 104 mg/L. High-frequency data are essential for identifying exceedances of the US Environmental Protection Agency Cl criteria for aquatic life, which are evaluated over periods of four-day (chronic, 230 mg/L) or one-hour (acute, 860 mg/L). In the Mid-Atlantic and New England, ubiquitous and frequent exceedances of the EPA chloride criteria occurred at sites with more than 9-10% ISC and/or with median [Cl] greater than 30-70 mg/L. Changes in [Cl] over time were evaluated at sites with ≥5 years of high-frequency data. In the Mid-Atlantic and New England, [Cl] is increasing in all streams with the fastest increases at the sites with the highest [Cl]. By contrast, no increases in [Cl] were observed in the Southeast. Increases in [Cl] at urban sites with already high [Cl] strongly indicates a reservoir of Cl in groundwater and soil from legacy and contemporary deicer applications is driving changes in stream chemistry and will continue to have effects for many years to come. Our results have broad implications for understanding the effects of urbanization on freshwater ecosystems and water quality, given the sensitivity of many aquatic organisms to incremental changes in SC in Eastern US streams.