Arsenic in Well Water

Most people have heard about the toxicity of arsenic. Arsenic was widely used for poisoning rats and other pests, and it was a popular “poison of choice” in mystery novels. Because of this, many people associate it with “poison”. Indeed, overexposure to arsenic is hazardous to your health.

Arsenic poisoning can cause a number of different ailments, including both acute and chronic (long-term) symptoms. Some of the long-term symptoms include increased risk of cancer (lung, bladder and skin), high blood pressure, heart disease, and discoloration and thickening of the skin, and there is evidence that overexposure to the chemical may lower the IQ of children.

The U.S. Environmental Protection Agency (USEPA) has set a maximum contaminant level (MCL) for arsenic of 10 micrograms per liter (µg/L or parts per billion). A MCL is the maximum concentration of a contaminant that is legally allowed in public drinking water systems under the Safe Drinking Water Act. The long-term health risks associated with concentrations above the MCL are considered to be unacceptable.

The USEPA set a maximum contaminant level goal (MCLG) for arsenic of zero, meaning that the target concentration for arsenic in drinking water supplies should be zero. Health risks associated with this dangerous element are lowest when there is no detectable arsenic in the water supply.

History of Arsenic

Although the use of arsenic in industry and agriculture is on the decline, arsenic contamination of drinking water can result from contact with soil affected by human activities. It has been used in wood preservatives, pesticides, metal alloys and semiconductors, as well as in animal feeds and medical treatments.

Because the chemical has been so widely used, arsenic-contamination is widespread in soil. Water infiltrating through contaminated ground can pick up arsenic and carry it into aquifers as it migrates deeper underground. Because of this, arsenic concentrations exceeding the USEPA’s MCL of 10 micrograms per liter are not uncommon in water supplies.

Naturally-occurring arsenic

What most people don’t know is that arsenic is naturally-occurring in rocks, soils and sediments, and unsafe levels can exist in well water even without contamination due to human activities. A variety of geologic conditions can cause high concentrations of arsenic in well water, such as:

• The presence of minerals that are high in arsenic
• The presence of geothermal waters
• Intense evaporation in arid climates
• Chemical conditions in the aquifer that favor high arsenic concentrations

However, in the case of arsenic, “natural” does not mean “good for you”. On the contrary, arsenic concentrations above the MCL and originating from natural sources are a common problem in the U.S. and in many areas of the world. For example, a recent study by the U.S. Geological Survey reported that more than 10 percent of wells in a certain geologic province of New England have arsenic concentrations exceeding the MCL [1].

Click map to enlarge
Map summarizing current data on the occurrence of arsenic in well water in the United States.

Arsenic in water is odorless and tasteless, so detection requires chemical testing. Concerned well owners should have their water tested by a certified drinking water laboratory. If unsafe levels are in your well water, then you have a number of possible options:

• Connect to a municipal water supply
• Modify your well to draw water from an aquifer with lower levels of arsenic
• Use bottled water for drinking and cooking
• Treat your drinking water to remove it

How to Remove Arsenic From Water

Arsenic can be removed from drinking water by reverse osmosis or by adsorption filters designed for arsenic removal. Reverse osmosis works by forcing the water through a membrane that allows water molecules to pass through but blocks larger ions, such as ones associated with iron, lead and arsenic. In a home setting, reverse osmosis systems are usually small systems (called point-of-use systems) located near the kitchen sink.

Reverse osmosis systems are cost-effective, but they can only produce a few gallons of treated water each day. Depending on the chemistry of the water, a reverse osmosis system may also require pretreatment to remove iron or manganese or to oxidize the arsenic (if a reduced form of arsenic - As(III) - is present in the water). Furthermore, the taste of the water will be affected by the removal of the minerals.

Adsorption filters work by removing arsenic molecules by adsorbing them onto mineral particles or resins. This takes place in filter cartridges that may be part of point-of-use systems (near the kitchen sink) or point-of-entry systems that treat all water entering the house. These filters must be replaced on a regular basis, and their performance can be affected by the presence of other minerals in the water. Adsorption filters may be used in combination with reverse osmosis systems for enhanced effectiveness.

Follow this link to arsenic removal systems.

References

[1] https://www.usgs.gov/mission-areas/water-resources/science/arsenic-and-drinking-water