Retrieved 2012-03-03. ↑ dwi.gov.uk ↑ anglianwater.co.uk ↑ Section 36 "Hardness" https://www.epa.ie/pubs/advice/water/quality/Water_Quality.pdf ↑ Wilson, Amber; Parrott, Kathleen; Ross, Blake (June 1999). "Household Water Quality – Water Hardness". Retrieved 2009-04-26. ↑ Briggs, J.C., and Ficke, J.F.; Quality of Rivers of the United States, 1975 Water Year -- Based on the National Stream Quality Accounting Network (NASQAN): U.S. Geological Survey Open-File Report 78-200, 436 p.