Chloride source delineation in an urban‐agricultural watershed: Deicing agents versus agricultural contributions

Analyses ( n = 525) of chloride (Cl − ), bromide (Br − ), nitrate as nitrogen (NO 3 ‐N), sodium (Na + ), calcium (Ca 2+ ) and potassium (K + ) in stream water, tile‐drain water and groundwater were conducted in an urban‐agricultural watershed (10% urban/impervious, 87% agriculture) to explore potential differences in the signature of Cl − originating from an urban source as compared with an agricultural source. Only during winter recharge events did measured Cl − concentrations exceed the 230 mg/L chronic threshold. At base flow, nearly all surface water and tile water samples had Cl − concentrations above the calculated background threshold of 18 mg/L. Mann–Whitney U tests revealed ratios of Cl − to Br − ( p = .045), to NO 3 ‐N ( p  < .0001), to Ca 2+ ( p  < .0001), and to Na + ( p  < .0001) to be significantly different between urban and agricultural waters. While Cl − ratios indicate that road salt was the dominant source of Cl − in the watershed, potassium chloride fertilizer contributed as an important secondary source. Deicing in watersheds where urban land use is minimal had a profound impact on Cl − dynamics; however, agricultural practices contributed Cl − year‐round, elevating stream base flow Cl − concentrations above the background level.