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 OHA Laboratory Lists and Reporting Information. Contact Jaime Craig at 503-842-3909 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.