Chloride Signature And Transport In An Urban-Agricultural Watershed

Abstract

Manual and high-frequency observations (n = 525) of chloride (Cl-), bromide (Br-), nitrate (NO3-N), sodium (Na +), calcium (Ca 2+), and potassium (K +) in stream and tile-drain waters were conducted in an urban-agricultural watershed (10% urban/impervious, 87% agriculture) to investigate the importance of stormflow to Cltransport and to explore potential differences in the signature of Cloriginating from an urban source as compared to an agricultural source. The study was undertaken in Evergreen Lake Watershed (ELW) located in central Illinois. Samples were collected on a weekly interval from February 2018 to February 2019 at three station along Six Mile Creek (SMC), the main drainage of ELW. All storm events were sampled at high-frequency at the most downstream station while select storm events were sampled at an upstream station to compare how export changes along the stream. Mann-Whitney U test show ratios of Clto Br-(p = 0.045), NO3-N (p < 0.0001), Ca 2+ (p < 0.0001), and Na + (p < 0.0001) to be statistically significantly different between urban and agricultural waters. Clratios indicate that road salt is the dominant source of Clin ELW while KCl fertilizer is a secondary source. Total Clexport during the study period was 7.77x10 5 kg. Storm events are vital to Clexport in ELW, accounting for 57.5% of total Clload during only 19% of the study period. The importance of storm events varies seasonally with winter and spring storms accounting for nearly half of total Clexport, while summer and fall storm event account for only 10% of total export. Results imply two periods of Clflushing in ELW. The first is associated with flushing of road salt from impervious surfaces following the cold season and second is associated with flushing salt build up after the dry season. When road salt is present on watershed surfaces increased discharge always corresponds with increased Clexport. During storm events when road salt is not present on watershed surfaces increased in discharge corresponds with increased Clexport for lower discharges (Q < 4 m 3 /s). At higher discharges the relationship between discharge and Clexport reaches an asymptote (~ 0.2 Kg/s) where further increases in discharge do not correspond with increased Clexport. Overall, urban, tile, and stream waters exhibit some degree of salinization in ELW with respect to Clas almost all sample concentrations were elevated above calculated background threshold concentration of 18 mg/l. These results highlight the dramatic influence that deicing operations on only a small portion of a watershed can have on the Cldynamics of the entire system. Road salt appears to be able to salinize the dominantly agricultural ELW raising Clconcentrations to levels hazardous to ecosystems and water supplies (>100 mg/l), although the exact extent of salinization in ELW remains unknown and is likely confined to surface and ground water in areas where deicing operations occur and that receive road salt runoff. Fertilizer inputs should also not be ignored as this work and others show th...

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

“…The data show Cl − concentration decreased along SMC as tile inputs dilute higher Cl − concentration water exiting the headwaters of SMC. Load calculations elucidated that Cl − export in SMC increased downstream, indicating the presence of additional Cl − sources to SMC along the reach (Oberhelman, 2019). Peterson and Benning (2013) reported dilution of Cl − concentration but increased Cl − mass flux downstream in a similar urban‐agricultural watershed, supporting the conclusion of sustained Cl − input along SMC.…”