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This fact sheet helps water system owners and operators understand and respond to issues that may arise with arsenic

in the distribution system, or with distribution system concerns resulting from the installation of arsenic treatment.

Although arsenic is measured at the entry point to the distribution system for compliance determinations, you should be

aware that arsenic levels could increase in the distribution system at any time due to a number of factors. It is important

to remember that any process changes, including chlorination, can impact your distribution system and the water quality

at customers’ taps.

How Arsenic Can Increase in Your Distribution System

Research is still

needed to understand

what water quality

conditions cause

arsenic in pipe scales

to be re-suspended or

dissolved back into

the water. In the

meantime, if you now

have, or have had,

arsenic in your raw

water and your

distribution system

contains iron pipes or

other iron

components, realize

that you may have

arsenic buildup in

scales.

Arsenic Can Build Up on and Release in

Pipes and Storage Tanks

Public water systems with arsenic in their raw water may

find that scales on pipes and other components in their

distribution systems contain relatively high arsenic

concentrations. These arsenic-rich scales can become

dislodged and suspended in the water, and may be

ultimately delivered to consumers.

Arsenic has been shown to attach to iron in distribution

system pipes. Because iron is so effective at binding with

arsenic, corrosion deposits can have high concentrations

of arsenic solids. In a recent study, arsenic levels found in

solids that were collected after pipe sections and hydrants

were flushed were as high as 13.65 milligrams of arsenic

per gram of solid. Most of the remaining solid was

composed of iron.
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It is known that even if your water has detectable levels of

arsenic that are below the 0.010 mg/L MCL, and you have

iron pipes or components in your distribution system, your

system’s pipes may have arsenic-rich scales attached to them.

As long as the scales are not disturbed, they will remain

attached to the pipes or other distribution system components. Certain conditions, such as flushing of mains or

fire flow conditions, may result in those scales being

sloughed off and suspended in the water, releasing the arsenic. Other conditions, such as changes in water chemistry, may result in some of the arsenic dissolving back into

the water. Both of these situations could cause high arsenic levels at consumers’ taps.

Arsenic Control Measures Can Affect

Finished Water Quality

Public water systems installing arsenic treatment should

be informed about possible changes to their finished water

that may result from the arsenic treatment they install. For

example, systems may need to adjust their finished water

quality to address new concerns about corrosion. Changes

in water chemistry due to using new sources, blending

different source waters, or installing arsenic treatment are

some of the factors that can affect distribution system water

quality. In some cases, this may cause an increase in arsenic

levels in the distribution system or create simultaneous

compliance issues with other drinking water regulations.

Water systems may also find deposits of arsenicrich particles in their storage tanks or at locations

in their distribution system with low flows. If the

flow is increased or a storage tank is drawn down

to a low level, these arsenic-rich particles can get

stirred up and transported to consumers’ taps.

This situation occurs primarily when iron media

used in treatment are released into the distribution

system, or when iron particles are not properly

filtered out during iron removal treatment. If these

treatment technologies are operated correctly, this

should not be a problem for most water systems.

Is Arsenic in your Storage Tank?

Is Your Ground Water System Installing Disinfection for Pathogen Control?

Water systems that disinfect their water should be aware of the possibility of an increase in arsenic concentrations

in their distribution system, particularly if the water contains high concentrations of dissolved iron. When

chlorinated, the dissolved iron forms particles on which arsenic can accumulate. As a result, high arsenic

concentrations may occur in distribution system water even if arsenic concentrations in the raw water are

below the MCL.

This happened to a small community water system in the Midwest that began chlorinating water from a series

of wells that had raw water arsenic levels between 0.003 and 0.008 mg/L and iron concentrations up to 0.4 mg/

L. At the same time, the system installed a polyphosphate feed system for corrosion control. Soon after

chlorination began, the system received intermittent colored-water complaints from its customers with increasing

frequency across the distribution system. Samples collected from several representative locations throughout

the service area had a reddish-brown color and contained particles. A metals analysis showed high levels of

copper and iron oxides in the finished water, along with arsenic concentrations approaching 5 mg/L. Because

of the water’s colored appearance, it was considered unlikely that customers would consume the water. Doctors

and health care professionals were notified of the situation and instructed to watch for signs of arsenic poisoning.

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Switching to a New Source or Blending Sources

If you plan on switching wells or blending sources to meet

the arsenic MCL, remember that the new well’s water may

react differently in your distribution system than the water

you were using before. Be sure you understand the new

well’s water quality characteristics like pH, alkalinity, and

iron and manganese concentrations. Changes in these water

quality parameters could impact lead and copper as well as

arsenic levels, disinfection byproducts, and aesthetic

characteristics like taste, odor, and color.

f Reducing pH During Treatment

Some arsenic treatment technologies require the pH to be

reduced as a treatment step. If your system has adopted

one of these techniques, be sure your pH is raised to a level

that will not cause corrosion problems in your pipes. If

you already have a corrosion control program in place,

review whether you will need to adjust your corrosion

chemical dose in response to any change in your water

quality resulting from the installation of arsenic treatment.

Keep in mind that adjusting the pH upward for lead and

copper control may also cause arsenic to be released from

scale on pipes and components.

f Installing a Treatment Technology that Uses Iron

If you have installed an arsenic removal treatment

technology that uses iron, you should not see elevated levels

of iron in the water entering the distribution system if the

treatment technology is being operated properly. However,

if the treatment technology has been recently installed and

Researchers found that chlorinating the water caused the formation of ferrihydroxide solids. The minimal arsenic present in the groundwater was being

concentrated as it absorbed onto the solids. Copper oxide particulates also formed

and were released. To some extent, the polyphosphates served a useful role by

keeping iron in solution and counteracting the tendency for the iron oxides to

form, but additional steps were needed. For six months the system alternated

their chlorination schedule: on for one day then off two days. The system then

returned to full-time chlorination, starting with a low distribution system residual of

0.2 mg/L and gradually increasing it to 0.5 mg/L. The system continued to flush

water mains on a semi-annual schedule using a unidirectional approach. In the

last year, the system received only one colored water complaint.

For more details on this case, see “Well Water Disinfection Sparks Surprises”

by Steve Reiber and Glenn Dostal, Opflow Vol. 26 No. 3, March 2000. Hypochlorination System

operational adjustments are still being made, you may see

elevated iron levels after treatment. You may also see elevated

iron levels if you are blending with iron-rich water. In these

cases, keep in mind that arsenic adsorbs onto iron, and the

iron may deposit in your pipes and storage tanks. This

arsenic-rich iron could dislodge and be re-suspended in

the water when flows increase. If this happens, consumers

may receive pulses of water containing high levels of arsenic

and iron, and should be warned not to consume the water

if it appears rusty in color.

f Using Activated Alumina or Enhanced

Coagulation with Alum

If you plan on using activated alumina or enhanced

coagulation with alum to treat your water, consider testing

the water periodically for aluminum in the distribution

system. While not a concern in terms of health effects,

aluminum concentrations as low as 0.05 mg/L can result in

customer complaints about particles or color in their water.

How Can I know if Arsenic is a Problem in

My Distribution System?

Drinking water regulations require public water systems

to monitor for arsenic at the entry point to the distribution

system. There is no federal requirement for systems to

monitor for arsenic within the distribution system. You

may, however, want to test your distribution system water

for arsenic to be sure that the water being delivered has

arsenic levels below the MCL. If you decide to monitor

your distribution system, consider testing for arsenic at

—Continued from page 2
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Office of Water

www.epa.gov/safewater

For More Information on Managing Your

Distribution System:

Distribution Systems: A Best Practices Guide

(EPA# 816-F-06-038)

AWWA. 2004. AWWA Standard G200-04: Distribution

Systems Operation and Management. Denver, CO.

Lytle, D. A.; Sorg, T. J.; Frietch, C. 2004. Accumulation

of Arsenic in Drinking Water Distribution Systems.

Environ. Sci. Technol. 38(20); 5365-5372.

Distribution System Research – http://www.epa.gov/

nrmrl/wswrd/dw/dsr.html

 

EPA 816-F-07-005

April 2007

locations where the settling and accumulation of iron solids

or pipe scales are likely (i.e., areas with cast iron pipe, ductile

iron pipe, or galvanized iron pipe).

If your water system has installed some form of arsenic

treatment, keep in mind that the treatment you installed

may change the water quality in other ways. It might cause

the water to react differently in the distribution system.

Depending on the kind of treatment you’ve installed,

consider what distribution system problems might result.

A change in the taste, odor or appearance of the water at

customers’ taps may be the first indication of a problem.

Some water quality parameters to consider monitoring,

depending on your arsenic treatment technology, include

iron, pH, manganese, alkalinity, and aluminum.

What Should I Do if Distribution System

Arsenic Levels are High?

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Consider Notifying the Public

Even if your water system has not violated the arsenic

standard, you may want to notify consumers that you have

detected arsenic levels in the distribution system exceeding

the MCL. If you make such notice, consider using the

following standard public health effects language for

arsenic:

Some people who drink water containing

arsenic in excess of the MCL over many years

could experience skin damage or problems

with their circulatory system, and may have

an increased risk of getting cancer.

You may also want to explain how you plan to fix the

problem. For example, if you plan on cleaning or flushing

the distribution system to remove scale from the pipes,

explain briefly what you will be doing, when you’ll be doing

it, and when you expect the problem to be addressed.

f Consider Distributing EPA’s Consumer Fact

Sheet on Arsenic

EPA has developed the fact sheet Arsenic in Your Drinking

Water—Just the Facts for Consumers that explains the health

risks associated with having elevated levels of arsenic

in your drinking water. This fact sheet is available on

the web at http://www.epa.gov/safewater/arsenic/

basicinformation.html. Color copies are also available by

calling the Safe Drinking Water Hotline at 1-800-426-

4791. Consider distributing the fact sheet as part of your

public education effort if you think you have arsenic problems in your distribution system.

How Can I Prevent Arsenic

Accumulation in My Distribution

System?

There are a number of management techniques that

can be used to help keep arsenic levels low in the

distribution system. They include:

z Optimize treatment operations for turbidity

removal.

z Check finished water pH and alkalinity after

arsenic treatment is installed. If they have

changed, consider whether corrosion control

practices need to be modified.

z Adopt a unidirectional flushing program for water

mains.

z Clean and maintain your storage tank(s).

z Optimize distribution system operations to

minimize water age. This practice will prevent

sediment accumulation and water quality

deterioration.

z Operate valves and hydrants to avoid sudden

changes in flow direction or velocity. This

practice will prevent the resuspension of

sediments into the water column.

z Monitor arsenic levels at drinking water taps,

hydrants, and low flow dead-end areas.