Arsenic treatment and whole house filtration

Arsenic in Prescott area well water occurs naturally from rock formations rather than from contamination, and it cannot be tasted, smelled, or seen. The EPA limit for public systems is 10 micrograms per liter. Treatment options are adsorptive media, reverse osmosis, and anion exchange. Which one fits depends on your arsenic level, which chemical form it is in, and whether you treat the whole house or just the kitchen tap.

Why this is a local issue

Arsenic here is geological. It leaches from the rock the groundwater moves through, which is why two wells a mile apart can return very different numbers, and why there is no way to reason your way to your own result.

A 2026 Arizona State University study of 47 private wells in Yavapai County found 11% showed arsenic contamination. Arizona health agencies have run separate health consultations on arsenic in private wells in parts of the county, including the Paulden area.

That is enough to justify testing. It is not enough to assume your well has a problem. See what is actually in Yavapai County well water.

The form of arsenic changes the treatment

This is the part that gets skipped, and it is why some systems underperform.

Arsenic occurs in two forms in groundwater. Arsenic V, sometimes written As(5), is the oxidized form and is comparatively easy to remove. Arsenic III, or As(3), is the reduced form, and it is harder to capture because it carries no charge at typical water pH.

Well water frequently contains As(3), because water underground has little contact with oxygen. That has a direct consequence: several treatment methods need an oxidation step ahead of them to convert As(3) into As(5) before removal.

A quote that does not mention which form your water contains is not a designed system.

Treatment options compared

Method	Removes	Notes
Adsorptive media	Both forms, better on As(5)	Media captures arsenic and is replaced periodically. Some iron oxide media produce no liquid waste and need no backwashing. Pre-oxidation improves efficiency
Reverse osmosis	As(5) well; not effective on As(3) without pre-oxidation	Usually point-of-use at a tap. Produces a reject stream, which can be a large share of processed water
Anion exchange	As(5) only, so pre-oxidation is required	Whole house scale, low maintenance. Only suitable for low TDS water, and not used above roughly 100 ppb arsenic
Distillation	Both forms	Effective but slow and energy hungry. Rarely the practical household answer

Reverse osmosis at the kitchen sink is the common starting point for drinking and cooking water, because ingestion is the exposure route that matters and treating 20 gallons a day costs far less than treating everything the house uses.

Point of use versus whole house

Point of use treats one tap, normally the kitchen. Lower cost, smaller system, less water wasted. It handles drinking and cooking, which is where arsenic exposure actually occurs.

Point of entry, meaning whole house, treats everything. Higher cost and larger equipment. It makes sense when there are multiple drinking points, when you want treated water for a garden or animals, or when other contaminants in the same water justify a whole house train anyway.

Neither answer is automatically right. It depends on your numbers and your household.

Whole house filtration more broadly

Arsenic is rarely the only thing on a well test. The same 2026 ASU sample found 10% turbidity, 10% sulfate, 10% fluoride, and 45% exceeding hardness limits.

A whole house setup usually stacks in order:

1. Sediment filtration first, protecting everything downstream
2. Oxidation if iron, manganese, sulfur, or As(3) is present
3. Specific contaminant treatment, such as adsorptive media for arsenic
4. Softening for hardness, after the things that would foul the resin
5. Point of use polishing at the kitchen for drinking water

Order matters. A softener placed ahead of iron removal gets fouled. Media placed ahead of sediment filtration gets blinded.

Test after you treat, as well as before

Treatment equipment degrades. Media exhausts, membranes foul, and a system quietly stops performing without any visible sign, exactly like the original problem.

Testing treated water at least once a year is the only way to confirm the system is still holding arsenic below 10 parts per billion. Put it on a calendar. See well water testing.

Next step

Get a lab test first. Once you have numbers, we will connect you with an independent local contractor who installs water treatment in the Prescott area. Start here.