Arsenic in Well Water: Understanding the Risks and Treatment Solutions

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Arsenic in Well Water: Understanding the Risks and Treatment Solutions

Arsenic is a naturally occurring element that can silently contaminate private well water across the United States. Invisible, tasteless, and odorless, arsenic often goes undetected until a proper water test reveals its presence. For homeowners who rely on private wells, understanding how arsenic gets into groundwater—and how to treat it—is critical for our health and safety.

Arsenic in well water

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natural occurence of arsenic in water
Why Arsenic Occurs Naturally in Groundwater

Arsenic enters well water naturally through the erosion of rocks and soil that contain arsenic-bearing minerals. Over time, rainwater and groundwater movement dissolve these minerals, allowing arsenic to leach into aquifers and private wells. No pollution or human activity is required—this is a geological occurrence most commonly found in areas with high natural mineral content, such as parts of the                                       Midwest, Southwest, and Northeast United States.

🟊 Additional factors that influence arsenic levels in well water include:

  • Geologic formations rich in arsenopyrite or sulfide ores
  • Low-oxygen (anaerobic) groundwater environments
  • Natural fluctuations in pH and redox potential
  • Seasonal influx of rainwater and snow melt into the aquifer

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EPA Arsenic Standard for Drinking Water

EPA Arsenic Standard for Drinking Water

The EPA's Maximum Contaminant Level (MCL) for arsenic in drinking water is: 10 parts per billion (ppb)

Levels above this standard increase the risk of long-term health problems and are unsafe for consumption, especially for children and pregnant women.  

EPA - Drinking Water Arsenic Rule

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Health risks of arsenic in drinking water

Health Risks of Long-Term Arsenic Exposure:

Arsenic is a known carcinogen, and exposure through drinking water can cause severe and sometimes irreversible health damage.



These symptoms can easily be mistaken for other conditions, which makes regular testing crucial.

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Arsenic Types in Well Water: As V vs. As III

Arsenic Types in Well Water: As V vs. As III

There are two primary forms of arsenic found in groundwater:



1. Arsenic V (As5 or arsenate)
2. Arsenic III (As3 or arsenite)

  • Found in oxidizing (oxygen-rich) environments

  • Less toxic and easier to remove using adsorption or anion exchange

  • Found in reducing (oxygen-poor) environments

  • More toxic and more difficult to remove

  • Must be oxidized to As5 before filtration

Converting all arsenic in the water to AS V is essential in choosing/sizing the right treatment system.

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Whole House Treatment  (Point of Entry)

To protect your entire household from arsenic exposure, a point-of-entry (POE) Whole House AS treatment system is recommended. Having a whole house filtration system to oxidize arsenic and iron, with the proper filtration media to remove arsenic and iron, is the best way to ensure your family has arsenic free water for the whole house

Step 1: Oxidation to Convert As3 to As5

Arsenic 3 must be converted to arsenic 5 before it can be effectively removed. This is achieved through:

  • Aeration tanks (inject oxygen to oxidize arsenic)

  • Chemical oxidants like chlorine, potassium permanganate, or hydrogen peroxide

  • Catalytic filtration media (e.g., manganese dioxide-based filters)

Once oxidized, As3 converts to As5, it then bonds with oxidized iron, making it filterable.


Step 2: Filtration of Arsenic and Iron

Once converted to As5 and combined with oxidized iron, the contaminants can be filtered using arsenic-specific filtration media, such as:

  • Iron oxide-based adsorptive media (Aldex CR 26, Bayoxide E33, Katalox Light)

  • Activated alumina

  • Specific Anion exchange resins

  • Greensand Plus (if iron and manganese are also present)

These media effectively remove both arsenic and iron from incoming water at POE. 


Whole House Iron & Arsenic Filtration System

  • Iron + Arsenic Filtration Tank (specialized filtration media)

  • Backwashing system to flush and clean media 

  • Air Draw - To restore head of air in the tank to oxidize iron, convert AS and slow rinse of media.

  • Always confirm filter media compatibility with your water’s pH, iron concentration, and arsenic levels.

  • Arsenic Removal Media - CR26, Katalox Light, and AD Edge Bayoxide 33




Water Chemistry Parameters for Arsenic  & Iron Removal

The most commonly used whole house Arsenic/Iron filtration system are aeration/oxidation filtration systems, as shown in the diagram above. The application of any arsenic removal system is dependent on several parameters in well water. 100ppm of dissolved oxygen levels in the water, PH of the water must be 6.8 or higher, and Total Alkalinity of the water supply.  When the water supply does not meet these parameters, chemical oxidation/injection systems are the preferred application method used to oxidize the arsenic and iron and then filter it through a whole house backwashing filtration system.  

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Point-of-Use (POU) - Drinking Water Solutions

Even with whole-house systems, it’s smart to add a final barrier at the kitchen sink. A reverse osmosis drinking water system is the best option to assure the final barrier to protect your drinking water is clean, and safe. Reverse osmosis systems can be installed under the sink or installed in the basement with lines run up to your sink, refrigerator, ice maker, pot filler, and other water appliances.  

Best Point-of-Use (POU) Options for Arsenic removal:

  • Reverse Osmosis (RO) Systems – Effective for both As3 and As5 after oxidation

  • Reverse Osmosis (RO) Systems - Such as the one shown above can have an As3 filter installed in the middle filter housing to aid in removing all valences of arsenic.  

  • Certified under-sink water filters – Look for NSF/ANSI 53 or 58 certifications for arsenic removal





If you have a private well, testing for arsenic should be a top priority. Because of its invisibility and toxicity, arsenic is one of the most dangerous, yet overlooked, contaminants in rural drinking water supplies. Yet there are many affordable and practical water treatment options to remove arsenic for your whole house and/or drinking water needs.  Don't overlook your your health and wellness. A complete well water analysis, including arsenic, should be done annually. There are certified water testing labs near you. They can even come out and take the sample themselves to save you time and give you peace of mind. 


🔑 Key Takeaways:

  • Arsenic enters groundwater naturally through geologic erosion and arsenic levels will fluctuate.

  • The EPA MCL for arsenic is 10 ppb. Drinking water standards.

  • As3 is more dangerous and harder to remove than As5 and must be oxidized to be removed.

  • Aeration + specialized filtration is required for whole-house protection

  • Reverse osmosis offers a final safeguard at the point of use

Protect your home and family with the right water treatment system, and test your well water annually.



I wish you, Good days and Good water!

The Water Softener Blog

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