Consider getting a MIOX purifier from Mountain Safety Research to kill the Giardia in your well. http://www.msrcorp.com/miox.asp This little device can be "clicked up" to treat an unlimited amount of water and kills Giardia after four hours. Find the source of contamination and ask the County Extension Agent to refer you for a free consultation on putting a rim around the top of your well so runoff, which may include deer or other infested animal feces, from flowing into your well. They will also check your septic system to see if there is cross-contamination. God bless!