Information on all Tillamook County Public Water Systems is available at Oregon Public Health: Drinking Water Data Online.
There are a variety of labs that do water testing; check Oregon Labs For Drinking Water and Public Testing. Contact Annette Pampush at 503-842-3902 if you feel your system may be contaminated.
Real estate transactions typically require wells to be tested for nitrate, arsenic, and total coliform. More information can be found at Well Testing & Regulations.