Arsenic III is one of the most soluble and prevalent forms of arsenic in groundwater and is often written interchangeably as arsenite, arsenic 3, or “trivalent arsenic”.

Arsenite = arsenic 3 = trivalent arsenic

Arsenic III is a form of inorganic arsenic, which is known to be highly toxic in comparison to organic arsenic, which is considered relatively harmless.

Arsenic III is the most toxic and bioreactive form of inorganic arsenic in drinking water. Unfortunately, it’s also particularly difficult to filter out.

You’re more likely to find arsenite in reduced well waters rather than city water. Note: most city or community water systems will treat or mix water to remove total arsenic to at least below the EPA MCL of 10 parts per billion (PPB). However, if water is treated with free chlorine then you probably don’t have high levels of arsenic III in your tap because chlorine rapidly oxidizes arsenic III to arsenic V.

The only way to know your arsenic 3 concentration (and your arsenic 5 concentration) is to run a water test in an environmental testing lab.

Arsenic III is much more difficult to remove than arsenic V by both conventional and unconventional treatment methods. There are two ways to treat arsenite in tap water:

The surest way to remove arsenite from your water is by applying a pre-treatment method that oxidizes arsenic III to arsenic V. Following such pre-treatment, you can then effectively remove (both forms of) arsenic with reverse osmosis or certain types of filtration.

Another way to remove arsenic III from your tap is by distillation, which requires no pretreatment. Distillation treatment systems remove nearly all metals by heating the water, collecting the steam and cooling it back to precipitation as “treated” clean water.