By Alycia Overbo and Sara Heger, Water Resources Center
Salt is used every day in many applications. People add salt to food, apply salt to pavement and roads after snowfall, and use salt in their water softeners. While salt is inexpensive to purchase, it can have a high environmental cost, as elevated chloride levels are toxic to many plant and aquatic species. The most commonly used salts contain chloride and research has shown that chloride levels are increasing in rivers, lakes, streams, and groundwater across North America. In Minnesota, there are 50 water bodies that exceed water quality standards for chloride and many wells have also demonstrated increasing trends for chloride, particularly the Twin Cities Metropolitan Area (TCMA). Additionally, many municipalities in the state have high chloride levels in their wastewater treatment plant effluent and reasonable potential to exceed chloride water quality standards.
While salt used in winter road maintenance is known to be a major source of chloride in TCMA, chloride contributions from other sources have not been closely examined. Use of household water softeners have been identified as an important chloride source, since water hardness across the state is generally high. Researchers at the University of Minnesota created a chloride budget for the state of Minnesota to estimate how much chloride enters the environment annually from household water softener use and other major sources. Annual chloride contributions from the following sources were estimated: household water softener use; human excreta; household appliance use; industrial discharge; chloride concentrations in drinking water; road salt use; atmospheric deposition; dust suppressant application; fertilizer application; and livestock excreta. Multiple types of data were used in the analysis, including sales records, spatial data, water quality monitoring data, and wastewater effluent monitoring data. Additionally, a survey of water conditioning professionals and plumbers was conducted to characterize water softening practices across the state.
Road salt was found to be the largest chloride source statewide, contributing over 400,000 tons of chloride annually to the environment (Figure 1). Household water softener use was found to be the largest chloride point source; other household sources contributed relatively small amounts of chloride. Chloride discharged from timer-based water softeners was particularly high; although timer-based softeners were estimated to comprise 27% of household water softeners, they contributed over 40% of chloride from water softeners due to their estimated salt use efficiency. Analysis of communities with wastewater monitoring data for chloride suggests that water softening by commercial organizations is also a substantial chloride source, although it was not included in the analysis due to lack of data. High chloride contributions were estimated from fertilizer use and livestock excreta, but since fertilizer and manure are applied over a large area statewide, they may have a lower impact on water quality than other sources.
Results from the chloride budget indicate that water softening is an important chloride source that impacts groundwater and surface water quality. Chloride reductions may be achieved through optimizing salt use for water softening, such as upgrading from timer-based softeners to demand-based softeners or setting water softeners correctly for water hardness and salt dosage. It is unknown if municipalities in Minnesota with elevated chloride in their wastewater treatment plant effluent and receiving waters can meet water quality standards solely through optimizing water softeners, but meaningful chloride reductions can be achieved through smarter softening and salt use.
Funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative‐Citizen Commission on Minnesota Resources.