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Road salt applied to prevent icy conditions in winter does not remain confined to pavement; much of it washes into nearby rivers and streams as snow melts. Once dissolved, chloride ions persist in waterways far longer than many other pollutants, since they do not break down chemically or settle out like sediment. Studies of urbanized watersheds have found chloride concentrations remaining elevated well into summer months, long after the last application of salt, indicating that groundwater slowly releases stored chloride back into surface water over an extended period. Aquatic species with narrow tolerances for salinity are particularly affected, and some municipalities have begun testing alternative de-icers, though most alternatives remain more expensive than traditional rock salt, slowing widespread adoption.