Arsenic is a classic lab-only contaminant. You cannot smell, taste, or see it, and many homeowners discover it only after a routine well test or a local advisory. Because treatment performance depends on arsenic chemistry, a basic yes-or-no screen is often only the first step.
Key Takeaways
- EPA uses a 10 ppb drinking-water standard for arsenic, and private well owners need their own testing because private wells are not routinely regulated like public systems.
- ATSDR and Penn State both emphasize using a local health department or certified laboratory for testing rather than relying on appearance or taste.
- Arsenic speciation matters. Arsenic(III) and arsenic(V) do not behave the same way in treatment, which is why some systems need pre-oxidation first.
- If arsenic is detected, many homeowners also need supporting chemistry such as iron, phosphate, pH, and sulfate checked because those factors affect treatment choice and performance.
- Retesting after installation is essential. Arsenic treatment is one of the easiest areas to misunderstand if you buy a device without confirming post-treatment performance.
A Good Testing Sequence
Start with a certified total-arsenic result. If treatment is likely, ask whether the lab or installer also needs speciation or supporting chemistry. That extra step often prevents buying the wrong media or expecting RO to solve a pretreatment issue it was never designed to handle alone.