CHLORIDE SIGNATURE AND TRANSPORT IN AN URBAN-AGRICULTURAL STREAM

Freshwater quality in snowbelt regions is threatened by salinization. Represented as rising chloride (Cl^-) concentration, salinization is linked to a dramatic increase in deicing salt application over the past 70 years. The threat posed by salinization to aquatic ecosystems and drinking water supplies make the study of Cl^- dynamics imperative. Manual and high frequency sampling of a low order urban-agricultural stream and surrounding field tiles located in central Illinois was conducted to investigate the importance of stormflow to Cl^- transport and to explore potential differences in the signature of Cl^- originating from an urban source as compared to an agricultural source. Weekly sample collection at three points along the stream began in February 2018 and will conclude in February 2019; event sampling during storms supplemented the weekly sampling data. Anion data from weekly samples indicate that waters are impacted by road salt in the winter (Feb-Mar) where Cl^- concentrations range from 100-350 mg/l while Cl^- levels decline to 30-120 mg/l in the spring (Mar-Jun) and summer (Jun-Aug). Waters from the urban headwaters of the stream exhibited a higher Cl^‑ concentration (10-60 mg/l) except during the month of February. High frequency storm samples illustrate a more complex Cl^- dynamic during the sample interval. Cl^- concentrations spike during storms in late March and mid-April (>200 and 100 mg/l respectively) after the first storm flush and weather conditions that warrant salt application. The spikes may represent continued flushing of Cl^- through the system. Cl^-/NO₃-N ratios along with Cl/Na ratios close to 1 support road salt as the source of Cl^- in this stream. Agricultural waters from three different tiles exhibit a much more variable Cl^-/NO₃-N ratio. Waters from one sampled tile plots along the vector that indicates the expected influence of nitrogen and anhydrous ammonia fertilizers, while two other tiles plot along the animal waste and septic effluent vector with elevated Cl^- concentrations. Variable chloride concentrations observed in agricultural input to this stream represent a potential secondary chloride source that could contribute to salinization in agricultural or multi-land use watersheds.