Arsenic is naturally occurring and enters the water supply mostly through the weathering of rocks. Due to its natural ability to enter the water system and widespread presence, about 77 million people in the United States, and over 140 million people globally, have arsenic levels above the Maximum Contaminant Level (MCL) recommended level of 10 parts per billion (PPB).

It is a known carcinogen and can lead to a host of health effects such as skin damage, cardiovascular issues, reproductive issues, and endocrine system disruption.

Arsenic can exist in two forms–organic and inorganic.

Organic arsenic molecules are arsenic molecules attached to complicated carbon structures. It is most often found in seafood and shellfish and is generally considered non-toxic or at least considerably less toxic than its inorganic counterpart.

Inorganic arsenic molecules are simple arsenic compounds without carbon. Contrary to organic arsenic, the inorganic form is considered highly toxic and unsafe for human consumption. Inorganic arsenic naturally occurs in the environment and is commonly found in soil, sediment, and groundwater. Arsenic III and arsenic V are both forms of inorganic arsenic.

While both arsenic III and arsenic V share the fact that they are the inorganic form of arsenic, they have some very important differences.

Arsenic III and arsenic V are two different oxidation states of arsenic. The oxidation state of an atom describes how many electrons it can gain, lose, or share with other atoms when forming chemical bonds. Arsenic can take on four different oxidation forms:

The oxidation state is important in determining what kind of compounds it can form and what kind of reactions it can undergo.

For example, arsenic +3 is the only oxidation state that can form the oxyanion compound arsenate (ArO43-) which is soluble in water. The most common form of arsenic V is arsenate. It is the most soluble and prevalent form of arsenic in water.

The inherent difference in shape and charge that each different oxidation states (and their compounds) have helps determine how it can be removed from your tap water. This is why testing your water is crucial. You can only make informed treatment decisions if you know what you need to treat in the first place.

While the water source may provide some hint as to the dominant form of arsenic in your water, there are some trends. In general, arsenic V is more prevalent than arsenic III in water.

Arsenic V will usually dominate in urban water systems, especially when the water is treated with chlorine disinfection, as chlorine quickly converts most of arsenic III to arsenic V.

If you are getting your water from a private well, on the other hand, it is possible that arsenic III levels are high due to reducing conditions of groundwater.

That said, water chemistry and proper treatment is incredibly complex, so investing in testing can help you choose the best treatment unit with confidence.

While no amount of arsenic in drinking water is considered a good amount of arsenic, there is some good news if you learn that the majority of the arsenic in your water is arsenic V.

Ideally, you would want to remove all forms of arsenic from your tap, but as mentioned above, different oxidative forms of arsenic respond differently and have varied removal efficiency to various treatment technologies. While arsenic III is notoriously difficult to treat, the outlook is more promising when it comes to removing arsenic V from water.

Arsenic V can be successfully removed from drinking water by reverse osmosis, ultrafiltration, and distillation without the necessary pretreatment arsenic III needs to be successfully removed.