The core panel and why
Coliform bacteria indicate whether surface contamination is reaching the water. Nitrates speak to septic or agricultural influence. Arsenic occurs naturally in some Oregon geology and is tasteless, odorless, and only findable by lab work. Together the trio screens for the big categories of trouble.
Oregon requires sellers of properties with domestic wells to test for these at transaction time and submit results to the state — but verify the testing is done properly and recently rather than assuming.
Reading results calmly
A detection is information, not doom. Bacteria issues often trace to fixable well-head or seal problems and can clear after shock chlorination and repair; other findings may point to treatment systems that thousands of rural homes run routinely.
What matters in a transaction is sequencing: test early enough that results can inform negotiation — repairs, treatment installation, credits — instead of surfacing after closing.
Local rule of thumb: Test the water like you plan to drink it for twenty years — because you do.