EPA's Maximum Contaminant Levels (MCL's) in drinking water. This list shows the amount of contaminants allowed in drinking water.
EPA's Paper on On-Site Septic and Softeners. Onsite Wastewater Treatment Systems Special Issues Fact Sheet 3 Water Softeners.
Slippery-When-Wet; Feelings you can expect from softened water.
Battelle Benefits; Comprehensive study proves efficiency benefits of softened water by Rebecca Wilhelm.