Abstract
Northwestern Indiana is one of the most heavily industrialized and largest steel-producing areas in the United States. High temperature processes, such as fossil-fuel combustion and steel production, release contaminants to the atmosphere that may result in wet deposition being a major contrib-utor to major ion and trace-metal loadings in northwestern Indiana and Lake Michigan. A wet-deposition collection site was established at the Gary (Indiana) Regional Airport in June 1992 to monitor the chemical quality of wet deposition. Weekly samples were collected at this site from June 30, 1992, through August 31, 1993, and were analyzed for pH, specific conductance, and selected major ions and trace metals. Forty-eight samples collected during the study were of sufficient volumes for at least some of the determinations to be performed. Median constituent concentrations were determined for samples collected during warm weather (April 1 through October 31) and during cold weather (November 1 through March 31). These median concentrations then were substituted for missing values from samples collected during the same periods with insufficient volumes for analysis of all the con- stituents of interest. Constituent concentrations were converted to weekly loadings. Two values were calculated to provide a range for the weekly loading for samples with measured concentrations of constituents less than the method reporting limit. The minimum weekly loading was computed by substituting zero for the constituent concentration, and the maximum weekly loading was computed by substituting the method reporting limit for the concentration. If, for a constituent, all of the sample concentrations measured were greater than the method reporting limit, a single annual loading value was computed. Ranges for the annual loadings for samples collected at the Gary (Indiana) Regional Airport, calculated by summing the minimum and maximum weekly loadings, were determined for sulfate (26.0 to 26.4 kilograms per hectare), copper (9.9 to 11 grams per hectare), and lead (10 to 11 grams per hectare). A single annual loading value was computed for manganese (69 grams per hectare) and zinc (24 grams per hectare). These annual loadings could be used to assist in estimating the contribution of wet deposition to the total annual constituent loadings in the Grand Calumet River in northwestern Indiana.
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