Chloride naturally occurs in freshwater streams at low concentrations. Concentrations of chloride in rainwater are 0 to 2 mg/L and average concentrations of chloride in freshwater lakes and streams are 0 to 100 mg/L (Goldman and Horne 1983); concentrations in unpolluted streams generally contain less than 20 mg/L. Salts containing chloride are often used for deicing roads in winter; road-salt use in the United States amounts to between 8 and 12 million tons annually (Kunze and Sroka, 2004).

Chloride concentrations measured in Phase II stream sites ranged from 30.2 to 971 mg/L, with a median concentration of 146 mg/L. Highest median concentrations were observed in streams whose drainage areas are small (less than 19 mi²) and highly urban (greater than 80 percent), such as Underwood Creek (295 mg/L), Honey Creek (260 mg/L), Lincoln Creek (246 mg/L), and Root River at Grange Avenue (246 mg/L)(table 1A, fig.2, fig. 8). Chloride concentrations indicated a positive relation with increasing urban land use (fig. 9). Lowest median concentrations were observed in streams whose drainage areas are large (greater than 600 mi²) and more heterogeneous in land use: Milwaukee River at Mouth (54.9 mg/L), Milwaukee River at Milwaukee (91.9 mg/L), and Milwaukee River near Cedarburg (75.9 mg/L). Forty-three samples (from 12 stream sites) had concentrations above the USEPA national chronic freshwater-quality criterion of 230 mg/L for non-priority pollutants (U.S. Environmental Protection Agency, 2006d). One sample collected from Root River at Grange Avenue in February, 2004, had a concentration above the USEPA national acute freshwater-quality criterion of 860 mg/L for non-priority pollutants (U.S. Environmental Protection Agency, 2006d).

Figure 8. Distribution of chloride concentrations, by site, in the Milwaukee Metropolitan Sewerage District planning area, Wis. Water-quality criteria lines represent the U.S. Environmental Protection Agency national chronic and acute freshwater-quality criteria for non-priority pollutants for chloride in surface water (230 mg/L and 860 mg/L, respectively) (U.S. Environmental Protection Agency, 2006e). Site abbreviations listed in table 1.

Figure 9. Median chloride concentrations plotted against percent urban land use in drainage basins for 15 stream sites in the Milwaukee Metropolitan Sewerage District planning area, Wis. Site abbreviations listed in table 1.

Stream sites indicated no consistent response in chloride concentration with respect to flow (fig. 10A). With respect to seasonal response, median chloride concentrations were highest during the winter (482 mg/L); median concentrations ranged from 115 to 175 mg/L for remaining seasons (fig. 10B).

Figure 10. Distributions of chloride concentrations for stream and harbor samples, Milwaukee Metropolitan Sewerage District planning area, Wis. Stream-sample distributions are grouped by flow (A), season (B), and flow and season combined (C). Harbor-sample distributions are grouped by season only (D) (no harbor samples were collected in winter). Water-quality criteria lines represent the U.S. Environmental Protection Agency national chronic and acute freshwater-quality criteria for non-priority pollutants for chloride in surface water (230 mg/L and 860 mg/L, respectively) (U.S. Environmental Protection Agency, 2006e).

Data were available for Phase I and Phase II chloride concentration comparisons at eight sites (appendix 5). At seven of those sites, median chloride concentrations measured in Phase II were notably higher than those measured in Phase I, with percent differences ranging from 29 percent at Root River at Grange Avenue to 289 percent at Kinnickinnic River. This increase likely was due to the lack of Phase I data collected during winter months, whereas winter and snowmelt sample collection was part of Phase II. Only the Root River near Franklin site indicated no notable difference.

Median chloride concentrations measured in harbor samples ranged from 10.6 to 98.9 mg/L. Harbor samples had notably lower median concentrations (19.3 mg/L) than stream sites (146 mg/L). Inner-harbor sites (26.1–35.4 mg/L) had higher median concentrations than outer-harbor sites (11.4–11.9 mg/L)(fig. 8). Harbor samples collected in spring had the highest median concentration (36.2 mg/L) and the highest degree of variability (fig. 10d). Samples collected during summer and autumn had lower median concentrations (14.6 and 17.7 mg/L, respectively) than those collected during spring (36.2 mg/L). No winter samples were collected at harbor sites.

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Loads, Yields, and Volumetrically Weighted Concentrations of Chloride

Chloride loads for June through September 2004 ranged from 858 tons at Kinnickinnic River to 12,900 tons at Milwaukee River at Milwaukee (table 11). VW concentrations ranged from 41 mg/L at Milwaukee River near Cedarburg to 101 mg/L at Menomonee River at Wauwatosa. The average daily chloride yield ranged from 0.15 tons/mi²/d at the Milwaukee River near Cedarburg and Milwaukee River at Milwaukee sites to 0.37 tons/mi²/d at the Kinnickinnic River site. In water year 2005, chloride loads ranged from 5,940 tons at Kinnickinnic River to 32,700 tons at Milwaukee River at Milwaukee. VW concentrations ranged from 69 mg/L at Milwaukee River near Cedarburg to 349 mg/L at Kinnickinnic River. VW chloride concentrations of 245 mg/L at Menomonee River at Wauwatosa and 349 mg/L at Kinnickinnic River were above the USEPA national chronic freshwater-quality criterion of 230 mg/L for non-priority pollutants, but below the USEPA national acute freshwater-quality criterion of 860 mg/L for non-priority pollutants (U.S. Environmental Protection Agency, 2006d). The average daily yields ranged from 0.10 tons/mi²/d at Milwaukee River near Cedarburg to 0.87 tons/mi²/d at Kinnickinnic River.

Table 11. Annual chloride load and yield, average daily chloride yield, total water flow, and volumetric total chloride concentrations for four Phase II sites in the Milwaukee Metropolitan Sewerage District planning area, Wis.

[tons/mi², tons per square mile; tons/mi²/d, tons per square mile per day; ft³/s, cubic foot per second; mg/L, milligram per liter; values in bold are for complete water years]

There was a seasonal pattern in chloride loads in water year 2005 with generally higher loads being recorded in January through March (table 12). The greatest monthly loads of chloride for data collected in 2004 were in June, likely a result of the high stream discharge that month (Milwaukee River near Cedarburg had 357 percent greater runoff than normal).

Table 12. Monthly and annual chloride loads for four Phase II sites in the Milwaukee Metropolitan Sewerage District planning area, Wis., for water years 2004 and 2005.

[WY, water year]

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