High-Frequency Data Reveal Deicing Salts Drive Elevated Specific Conductance and Chloride along with Pervasive and Frequent Exceedances of the U.S. Environmental Protection Agency Aquatic Life Criteria for Chloride in Urban Streams

Increasing specific conductance (SC) and chloride concentrations Cl negatively affect many stream ecosystems. We characterized spatial variability in SC, Cl, and exceedances of Environmental Protection Agency Cl criteria using nearly 30 million high-frequency observations (2-15 min intervals) for SC and modeled Cl from 93 sites across three regions in the eastern United States: Southeast, Mid-Atlantic, and New England. SC and Cl increase substantially from south to north and within regions with impervious surface cover (ISC). In the Southeast, Cl weakly correlates with ISC, no Cl exceedances occur, and Cl concentrations are constant with time. In the Mid-Atlantic and New England, Cl and Cl exceedances strongly correlate with ISC. Cl criteria are frequently exceeded at sites with greater than 9-10% ISC and median Cl higher than 30-80 mg/L. Tens to hundreds of Cl exceedances observed annually at most of these sites help explain previous research where stream ecosystems showed changes at (primarily nonwinter) Cl as low as 30-40 mg/L. Mid-Atlantic chronic Cl exceedances occur primarily in December-March. In New England, exceedances are common in nonwinter months. Cl is increasing at nearly all Mid-Atlantic and New England sites with the largest increases at sites with higher Cl.