Stream chloride concentrations as a function of land use: a comparison of an agricultural watershed to an urban agricultural watershed

The concentration of chloride (Cl⁻) in streams in northern regions has increased as a result of applications of deicers. This study focused on quantifying the relationship between land use and stream Cl⁻ concentrations. The study area comprises two adjacent watersheds in central Illinois, with similar geology and climate but different land uses (agricultural and urban). GIS analysis delineated watershed land use and calculated road surface areas. Stream water samples were collected and analyzed for anionic composition. During the winter months, streams dominated with urban land use experienced a 20-fold increase in Cl⁻ concentrations (range between 36 and 1350 mg L⁻¹); Cl⁻ concentrations in agricultural dominated streams also increased, but the increase was smaller (3X) and concentrations remained low (between 11 and 58 mg L⁻¹). As road salts are not the sole source of Cl⁻ in a stream, Cl⁻ and bromide (Br⁻) mass ratios (Cl/Br) and Cl⁻ and sodium (Na) molar ratios ([Cl]/[Na]) were used to identify potential sources of Cl⁻. The ratios indicate urbanized watersheds were impacted by road salts; agricultural watersheds ratios indicate other anthropogenic sources. A nonlinear relationship between urban land use and stream Cl⁻ concentrations indicates urban land use as low as 23% results in elevated Cl⁻ concentrations (greater than 150 mg L⁻¹) in stream waters.