Freshwater rivers and lakes naturally contain low concentrations of chloride. Freshwater generally has chloride concentrations below 100 mg/L.

What is it?

Chloride is a naturally occurring ion found dissolved in water. Ions are atoms or molecules with a positive or negative charge. Chloride is often measured as an indication of salinity, or the saltiness of water. However, other ions also affect salinity, such as calcium, magnesium, sodium, potassium, carbonate and bicarbonate.

Chloride in water can come from natural sources (such as the weathering of rocks and soils) as well as from human activity (such as road salts, industrial and municipal effluent, and agricultural inputs).

Why does it matter?

Too much chloride in freshwater can be toxic to plants and animals. Chloride does not break down. So, if there is a steady flow of chloride into a river or lake and not enough water to dilute the chloride, it will be harder for plants, bugs and fish that are adapted to freshwater conditions to survive.

High chloride levels in freshwater are a growing concern in areas where road salts are applied in the winter to encourage snow and ice melt.

How is it measured?

Chloride is usually measured by sending a water sample to a lab for analysis. It is typically reported in milligrams per liter (mg/L).

In lakes, chloride levels can change at different water depths. So, it is a good idea to measure chloride at multiple depths.

Related to chloride: calcium, magnesium, potassium, sodium, bicarbonate, carbonate, sulfate.