1. Groundwater Use, the USGS Water Science School,2005
2. Groundwater Use for America, Westerville, OH;National Ground Water Association,2010
3. What makes water hard & how can it be improved;Water Quality Research Council;Water Rev.,1990
4. Soft Water, Hard Choice?;Burris,2004
5. Water and Air Purification Systems & Products: Residential & Commercial, Rockville, MD;Energy,2010