Iodine-131

Although drinking water levels of iodine-131 pose no risk to public health in Philadelphia, PWD continually monitors and investigates challenges to our water quality.

About Iodine-131

What is Iodine-131 (I-131)?
Iodine-131 is a radioactive form of iodine. When certain atoms disintegrate, they release a type of energy called ionizing radiation. This energy can travel as either electromagnetic waves (i.e., gamma or X-rays) or as particles (i.e., alpha, beta or neutrons). The atoms that emit radiation, such as radioactive iodine, cesium, and plutonium, are called radionuclides. Iodine-131 emits beta and gamma radiation.

Where does I-131 come from?
Iodine-131 is widely used in the medical field for the diagnosis and treatment of thyroid disease and is also a byproduct of nuclear energy production. I-131 and elements like this come from different sources, but most frequently are linked to people who have received medical treatments. Whatever medication is not absorbed by the body is flushed into the sewer system. While wastewater treatment plants do a great job of removing contaminants, detectable levels can remain. In the scientific literature, there are many references to wastewater treatment plant discharge serving as a pathway of I-131 to surface waters, and the results of PWD sampling support these findings.

Is I-131 only found in water in and around Philadelphia?
There are numerous studies confirming that trace amounts of I-131 are present in waterways around the world. PWD’s sampling, in conjunction with sampling efforts by the PADEP, has confirmed that I-131 is also present in Philadelphia waters. Although the presence of I-131 in drinking water sources has been confirmed elsewhere in the country and around the world, PWD has decided to take a proactive approach to better understand the sources and levels of I-131 in our area.

Is I-131 always in the water?
Occurrence of I-131 in the watershed is highly episodic. One person receiving I-131 treatment will potentially excrete high enough levels of I-131 to be measurable in the watershed.

How long does I-131 stay in the environment?
Iodine-131 is a short-lived radioactive element with a half-life of 8 days, meaning every 8 days it loses half of its radioactivity. Unlike some radioactive chemicals, it does not persist in the environment for a long time.

Health and Safety

Are there I-131 standards for drinking water safety?
The Environmental Protection Agency (EPA) drinking water standard, known as the Maximum Contaminant Level (MCL), for I-131 is 3 pCi/L and is based on a long term average of I-131 levels, not a single day sample. From a health perspective, the MCL is developed assuming a risk factor of 1/1,000,000 based on long-term consumption over 70 years of 2 liters per day of contaminated water.

Is Philadelphia’s drinking water safe?
Philadelphia’s drinking water is safe. Average I-131 levels in PWD drinking water are well below the conservative EPA MCL of 3 pCi/L. In fact, an infant would have to drink nearly 1900 eightounce glasses of water containing acceptable levels of I-131 in a 24-hour period to experience the same amount of exposure we experience on a daily basis from natural sources in our environment.

How has PWD confirmed that the drinking water is safe in regard to levels of I-131?
Following the earthquake and tsunami that severely damaged Japan’s Fukushima nuclear power plant in the spring of 2011, the EPA and the Pennsylvania Department of Environmental Protection (PADEP) increased monitoring of radioactive contaminants, including I-131, in both surface water and treated drinking water in Pennsylvania. Upon reviewing the monitoring results from both agencies, PWD identified the need to implement a sampling program to better characterize sources and levels of I-131 in Philadelphia’s drinking water supplies, the Schuylkill and Delaware rivers.

PWD commenced a multi-phased sampling program for I-131 in April 2011. The sampling program consists of routine, weekly sampling of Philadelphia’s drinking source waters and drinking water. Since April 2011, PWD has collected hundreds of samples. PWD has thus far sampled in the Delaware River, Schuylkill River, and Wissahickon Creek. The sampling so far indicates that there are detectable levels of I-131 in all three bodies of water; however, the drinking water levels of I-131 pose no risk to public health. PWD also continues to work with the EPA and PADEP radiation experts to better understand potential sources of I-131 which may be impacting the watershed.

What We're Doing

How often is the water monitored? At what locations?
PWD monitors its drinking water every day at its three water treatment plants and at points throughout its 3,000 mile delivery system. Our water is consistently better than Safe Drinking Water Act (SDWA) standards and meets all heath standards. Iodine-131 sampling is not required under the SDWA. However, PWD has been working with the EPA since 1989 on a voluntary basis to provide samples of river water and drinking water for analysis of various radiological elements.

PWD also continuously monitors water quality throughout the Schuylkill and Delaware River watersheds. PWD has a nationally recognized Source Water Protection Program, which embodies the department’s multi-barrier approach to ensuring the safety and quality of Philadelphia’s drinking water. Philadelphia’s Source Water Program staff work closely with the department’s treatment plant managers and operators to anticipate and respond to emergencies and challenges to conventional treatment techniques.

Can drinking water treatment plants remove I-131?
PWD samples indicate such low levels of I-131 in drinking water that the effectiveness of drinking water treatment technologies at removing I-131 is inconclusive. As a precautionary measure, PWD applied activated carbon at the Queen Lane water treatment plant during April 2011. The application of activated carbon was discontinued in early May due to the consistently low levels of I-131 observed in source and treated drinking water samples.

What is RadNet?
RadNet is a national radiological surveillance program, managed by the EPA, which monitors environmental radioactivity in the United States in order to provide baseline data during routine conditions and provide data for assessing public exposure and environmental impacts resulting from nuclear emergencies and large scale natural disasters. RadNet collects and analyzes data on radionuclides in air, rainwater, surface water, milk and drinking water samples.

Should I be concerned about the EPA RadNet data, published in April 2011, showing Philadelphia drinking water results among the highest in the nation for I-131 levels?
The RadNet results for Philadelphia drinking water indicate there is no risk to public health. However, PWD has not ignored the recent upward trend in I-131 levels. In response to recent data, PWD reviewed the RadNet database in consult with EPA and PADEP and developed a Radionuclides Joint Action Plan (PADEP, EPA, PWD) in April 2011. As mentioned above, PWD is conducting a multi-phased watershed sampling and assessment program for I-131 with the goal of characterizing I-131 levels in the Schuylkill River watershed.

How do the levels of I-131 found during April 2011 compare to past measurements?
Through the RadNet sampling program, EPA detected low levels of I-131 in a number of drinking water samples before and since the Japanese nuclear incident. The EPA sample results for I-131 published in the April 2011 RadNet posting are unrelated to radiation from Japan and other nuclear-power sources in the Philadelphia area.

Are there elevated levels of any other kind of radioactive particle in Philadelphia drinking water (I have seen cesium mentioned)?
None have been identified. The Safe Drinking Water Act requires water utilities to sample for radionuclides every 6-9 years. All radionuclides monitored in Philadelphia’s drinking water have been below the regulatory standards. PWD will continue sampling for a variety of radionuclides now and into the future to ensure that this is the case.

Where can I get the most up-to-date information on this issue?
PWD will regularly update this web page as new data is collected and analyzed and additional questions are brought to our attention.

Additional Information

March 28, 2012 Iodine-131 Panel Discussion Presentations:

FAQ: Radionuclides in Drinking Water (Water Research Foundation)

Fact Sheet: Radiation Exposure (American Nuclear Society)

Article: Radiological Lab Results Don't Have To Be Confusing (American Water Works Association)

Article: Cancer patients' urine suspected in Wissahickon iodine-131 levels (Philadelphia Inquirer)

Article: Radioactive Iodine in Philadelphia Water Tied to Thyroid Patients (Philadelphia Inquirer)

Article: Radionuclide Discharges to Sewer - An Investigation (Environment Agency)

Article: Concentrations of Iodine-131 Released from a Hospital into a Municipal Sewer (RSO Magazine)