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

Analyses (n= 525) of chloride (Cl-), bromide (Br-), nitrate as nitrogen (NO3-N), sodium (Na+), calcium (Ca2+) 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-WhitneyUtests revealed ratios of Cl(-)to Br-(p= .045), to NO3-N (p < .0001), to Ca2+(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.