Chloride - Understanding Lake Data

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• CHEMICAL PROPERTIES - Page 11

• Chloride

The presence of chloride (Cl-) where it does not occur naturally indicates possible water pollution. Chloride does not affect plant and algae growth and is not toxic to aquatic organisms at most of the levels found in Wisconsin. Chloride is not common in Wiscons soils, rocks or minerals, except in areas with limestone deposits. Figure 7 shows the geographic distribution of chloride in Wisconsin lakes.

FIGURE 7. Generalized distibution gradients of chloride in the surface waters of Wisconsin Lakes. (Adapted from Lillie and Mason, 1983(Exit DNR))
Note:This figure's key is corrected from the printed book

Sources of chloride include septic systems (chloride values of 50 to 100 mg/l are common in septic tank effluent), animal waste, potash fertilizer (potash = potassium chloride), and drainage from road-salting chemicals. Increases in chloride, either seasonally or over time, can mean that one or more of these sources is affecting the lake.

An increase in chloride from human or animal waste suggests that other nutrients are also entering the lake. Higher chloride concentrations from spring to fall may be the effect of lawn fertilizer runoff or septic systems during heavy use by summer residents. Higher values in spring after the snow melts may signify runoff from drainage basins or highways as a major source of chloride. Since lakes vary in their natural chloride content, it is important to have background data or a long term database to document changes.

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For more information on this topic, contact:

James Vennie
Watershed Management
(608) 266-2212

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