See the Water Tests page for links to information about specific contaminants, such as iron (orange stains), hydrogen sulfide (egg smell), hardness (white spots), and others.
Go to the bacteria and arsenic pages for specific treatment for those contaminants.
EPA recommends contacting these independent non-profit organizations for help in selecting a water treatment unit:
SF International (877/8-NSF-HELP) tests and certifies home water treatment units,
Water Quality Association (630/505-0160) classifies units according to the contaminants they remove as well as listing units that have earned its approval.
Water treatment units certified by these organizations will indicate certification on their packaging or labels.
A few other suggestions
Contact several different companies. Look in the yellow pages under ALL the headings that start with WATER because there are several different terms used (Water Filters, Water Purification, Water Treatment, etc.).
Get written estimates including list of parts, brand names, labor, travel, warrantee, and cost of operation and maintenance. Compare costs based on long-term operation, not initial installation costs.
Match your water condition with the treatment. Compare water test results from the treatment company with results from a certified lab. Make sure that the level or amount you are treating can be handled by the device you are buying... Don't under buy, but also, you don't need to have a lot of extra treatment capacity.
If not a health issue, base you decision to purchase a treatment system on your personal preference, not numbers. Standards and maximum contamination levels (MCLs) are set for public water systems, not private wells. If you personally would rather tolerate the taste, color, stains, etc., instead of spending the money, that is your choice.
Consider the people you will be working with. You are establishing a professional relationship that will most likely last as long as you live in this house. You may want to ask for references and ask how they were treated after the sale.
For more information contact the national Water System Council's Wellcare Hotline.
If you have complaints about a water treatment company or to check their record contact the Oregon Attorney General's Consumer Complaints Hotline at 1-877-877-9392 (toll-free in Oregon)