This digest is a publication of the Senator George J. Mitchell Center for Environmental and Watershed Research and the Maine Drinking Water Program.

ARSENIC IN MAINE GROUNDWATER
Guidance for Small Water Systems

What is arsenic and where does it come from?
How can I tell if my system has high levels of arsenic?
Why should I worry about arsenic?
How does arsenic get into drinking water?
Will drilling a new well or changing the existing well avoid an arsenic problem?
How do I remove arsenic?
Where can I go for assistance?

What is arsenic and where does it come from?
Arsenic occurs naturally in soil and bedrock, and is a common element in the earth's crust. Arsenic is usually found with other elements like sulfur, chloride, oxygen, and iron and manganese ores. Arsenic is also used in pressure-treated wood and as an ingredient in pesticides. High arsenic levels can still be found in areas of Maine where arsenic-containing pesticides were applied to apple, potato, and blueberry fields.

How can I tell if my system has high levels of arsenic?
You can not smell, taste, or see arsenic in drinking water. Laboratory testing is required to detect arsenic in water.

Why should I worry about arsenic?
Long-term exposure to low levels of arsenic in drinking water has been linked to bladder, lung, kidney and skin cancer. Arsenic can also harm the nervous system, heart and blood vessels.

As of January 2006, the maximum amount of arsenic allowed in public drinking water systems is 10 parts per billion (0.010 mg/L). In Maine, over 5% of public water systems (and as many as 25% private drinking water wells) have average arsenic concentrations above the legal limit.

Arsenic in public water supply wells, mg/L.
Source: ME Drinking Water Program

How does arsenic get into drinking water?
Arsenic is usually found in drinking water wells drilled into bedrock beneath the ground. As water moves through cracks in the bedrock, it dissolves the arsenic in the rock and carries it into the well. In the same way, arsenic in weathered rock and soil dissolves as rain percolates through the ground and into groundwater. Drilled bedrock wells are more likely to have high arsenic than dug wells or wells drilled in sand and gravel, and certain kinds of bedrock contain more arsenic than others.

The longer water sits in a well, the more time there is for arsenic to dissolve into drinking water, so wells used by homes and seasonal camps may have more arsenic than wells that serve communities. In public wells, where more water is pumped in shorter periods of time, less arsenic may be present because water moves too fast to pick up arsenic from the surrounding rock.

In general, arsenic levels will not change over time. However, the amount of arsenic in a well can vary seasonally as groundwater levels fluctuate, because water is contacting different parts of the rock.

Will drilling a new well or changing the existing well avoid an arsenic problem?
Since arsenic is usually from the bedrock below ground, a well drilled in the same area, even several hundred feet away, also has a good chance of having high arsenic levels. Making a well deeper or sealing off certain sections of a well will only work if you knew which part of the bedrock was contributing arsenic. All of these options rely on luck for avoiding an arsenic problem, and all are fairly expensive. A more reliable option with a better chance for success is removing the arsenic with water treatment.

How do I remove arsenic?
Because arsenic is dissolved in groundwater, it can't be filtered out. In order to treat for arsenic, you have to find out what form ("species") of arsenic is present. Arsenic exists in two forms depending on how much oxygen is in the water. In groundwater, where conditions are usually low-oxygen, arsenic is present as arsenite (As III). In surface water where oxygen is present, arsenic is found as arsenate (As V). Arsenate is much easier to remove, so arsenite first must be converted to arsenate by adding an oxidizing agent such as chlorine or permanganate. Once converted to arsenate, activated alumina, ion exchange, and reverse osmosis technologies can remove arsenic. Another new technology, iron-based adsorptive media, will remove both species of arsenic, but it works more effectively if all the arsenic is in the form of arsenate. Characteristics of the water can affect the success of all treatment types. High pH and certain ions can make treatment less effective and require more maintenance of the system.

Maximum arsenic concentrations in groundwater by town, mg/L.
Source: ME Drinking Water Program & ME Geological Survey
Since arsenic has been detected in groundwater throughout Maine, it is recommended that all well water be tested.

Where can I go for assistance?

• Maine Drinking Water Program
  207/287-2070
• Maine Rural Water Association
  Carlton Gardner
  207/729-6569
  cgardner@mainerwa.org
• The EPA's arsenic website has information and training materials for small water systems:
  http://www.epa.gov/safewater/arsenic.html.

Prepared by: Catherine Schmitt & John Peckenham
June 2005