In the State of Oregon, it is required by law to test well water used for domestic purposes prior to selling a property.
"In any transaction for the sale or exchange of real estate that includes a well that supplies ground water for domestic purposes, the seller of the real estate shall, upon accepting an offer to purchase that real estate, have the well tested for arsenic, nitrates and total coliform bacteria. The Oregon Health Authority also may, by rule, require additional tests for specific contaminants in specific areas of public health concern. The seller shall submit the results of the tests required under this section to the authority and to the buyer within 90 days of receiving the results of the tests... The test is valid for one year."
The State of Oregon requires all testing be done by an accredited laboratory.
Periodic Well Water Testing
Oregon Health Authority recommendations for yearly testing
Arsenic should be tested at least once in the life of a well.
"Some areas in Oregon are particularly susceptible and certain counties are encouraged to routinely check their wells' arsenic levels. If arsenic is present in an initial test (above 8ppb), you should retest. Upon the retest, If high concentrations continue, a treatment system should be installed. With a treatment system, testing should be done each year at point of use and every three years at the well head. " - Oregon Public Health