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Chloride balance of an urban basin in the Chicago area


  • Gerald M. Wulkowicz,

  • Z. A. Saleem


During the relatively mild winter of 1972–1973, 20,260 t (metric tons) of NaCl and 40 t of CaCl2 were spread on roads within the Salt Creek basin of the Chicago metropolitan area. Over 600 water samples were collected from 11 creek sites within the basin during the salting season. Chloride concentration in the water varied from 35 to 1530 mg/1 during the sampling period. Road salt chloride content of the water at any time is dependent on (1) temperature and colligative properties of NaCl, (2) duration of precipitation events, and (3) dilution capacity of the stream. Models are presented for three subsections of the basin based on the relationship between chloride concentration and discharge of the creek during nonsalting periods. A fourth subsection could not be modeled because of insufficient data. Monthly chloride budgets for all four subsections from November 1 through April 30 show that between 55 and 72% of the road salt chloride was removed by the creek from the various subsections. Changes in the percentage of chloride removed correlate well with the following indicators of the degree of urbanization: (1) percentage of area as streets, (2) highway density, (3) population density, and (4) road salt application per unit area. Sixty-two percent of the chloride was removed by the creek from the basin during the first 6 months of the study period. Further sampling after April 30 but before salting commenced in December 1973 indicates that an additional 10% of road salt chloride was removed from the basin by the creek during the second 6-month period. A portion of the remaining chloride was removed through the groundwater, and the rest was retained in the basin because of lag mechanisms.

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