Most efforts to reduce chloride levels in Minnesota lakes and rivers have focused on salt used to keep roads and highways clear of ice.
But a new study from the University of Minnesota found that home water softeners are sending a significant amount of salt into the environment.
The increasing saltiness of lakes, streams and groundwater is causing concern across North America. Excess chloride can be toxic to fish and other aquatic life, corrode bridges and infrastructure and affect the taste of drinking water.
About 50 Minnesota water bodies have chloride levels that exceed state water quality standards.
Researchers from the University of Minnesota's Water Resources Center created a "chloride budget" to estimate how much salt enters the environment each year from various sources.
Road salt was by far the largest source of chloride statewide, contributing more than 400,000 metric tons annually to the environment.
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Household water softeners were the fourth largest source, contributing about 140,000 metric tons of salt per year.
"I think it will be surprising to a lot of people because we've been talking about road salt, road salt, road salt," said Sara Heger, a research engineer with the Water Resources Center. "A lot of people don't think about that their softener in their home is like that road salt — or that we're another source."
The study found that commercial fertilizer and manure are also major sources of salt. But because they are applied over a large area, their impact on water quality may be less, Heger said.
Salt from water softeners can enter the environment in two ways, Heger said. In homes connected to municipal sewer systems, chloride from water softeners goes down the drain and ends up at the wastewater treatment plant.
Removing chloride from water is a difficult and expensive process, so it's not cost-effective for wastewater treatment plants to do it. The state estimates about 100 Minnesota wastewater treatment plants are discharging more chloride than allowed by state standards.
In homes that bypass the water treatment system — those that have septic systems — the salty water goes into the soil and eventually travels into the groundwater.
Heger said there are steps people can take to make sure they aren't using more salt than needed. Water softeners should be serviced regularly, she said, and should be set based on the hardness level of the water, not on an automatic timer.
"If they have a water softener in their home, they really want to use the least amount of salt to remove the hardness," Heger said.