http://www.sciam.com
Scientific American site that contains articles about different water filters and how they work.

http://water.nr.state.ky.us/ww/ramp/rmhard.html
This site discusses the methodology behind hardness and a classification of water by hardness content.

http://aaawatertesting.com
An AAA Water Testing site that explains hardness, related health issues, and how home water filters work.

In addition to the three site listed above, the
idea for this project was taken from an experiment
was based on a project done through the
Department of Chemistry, Loyola College in Maryland, 2501 N. Charles St., Baltimore, MD 21210.
The title of the original experiment is
Reduction of Calcium Concentrations by the
Brita® Water Filtration System:  A Practical Experiment in Tritrimetry and Atomic Absorption Spectroscopy.
It  was written by Kimberly G. Olsen and Lisa J. Ulicny.