United States Nuclear Regulatory Commission - Protecting People and the Environment

ISCORS Assessment of Radioactivity in Sewage Sludge: Radiological Survey Results and Analysis (NUREG-1775)

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Date Published: November 2003

Prepared by Andrea R. Jones

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From 1998 to 2000, through the Interagency Steering Committee on Radiation Standards (ISCORS), the U.S. Nuclear Regulatory Commission (NRC) and the U.S. Environmental Protection Agency (EPA) conducted a joint survey (hereafter referred to as the ISCORS sewer survey) to collect information on radioactivity in sewage sludge and ash from sewage treatment plants referred to in the industry as publicly owned treatment works (POTWs).

The objectives of the survey were to (1) obtain national estimates of high-probability occurrences of elevated levels of radioactive materials in sludge and ash at POTWs, (2) estimate how much radioactive contamination comes from NRC and Agreement State licensees and how much from naturally occurring radioactivity, and (3) support rulemaking decisions by NRC and EPA.

The voluntary survey had two components: a questionnaire and a program for sampling and analyzing sewage sludge and incinerator ash. Questionnaires were sent to 631 POTWs, requesting information regarding wastewater sources, wastewater and sludge treatment processes, and sewage sludge disposal practices. Using the information from the 420 returned questionnaires, NRC and EPA selected 313 POTWs to be sampled. The selection emphasized POTWs with the greatest potential to receive waste from licensees and in areas with higher levels of naturally occurring radioactive material (NORM). Altogether 311 sewage sludge samples and 35 ash samples were taken. Approximately half of the samples were analyzed by the U.S. Department of Energy's Oak Ridge Institute for Science and Education (ORISE) in Oak Ridge, Tennessee, under contract to NRC, and the remainder were analyzed by the EPA's National Air and Radiation Environmental Laboratory (NAREL) in Montgomery, Alabama.

The results of the analyses revealed that samples primarily contained NORM such as radium. With the exception of NORM, most other samples were at or near the limit of detection. Based on the results obtained, ISCORS found that the levels are generally comparable to what is found in other media (e.g. soil and fertilizer). The specific results of the analyses and data obtained by the questionnaire are discussed in detail in this report.

The survey and the sampling and analysis program results respond, in part, to the recommendations of the 1994 GAO report (GAO/RCED-94-133) that the NRC estimate the occurrence of elevated levels of radioactive materials at POTWs. The Subcommittee is preparing several supplemental reports (both to be published in final form after public review and comment). The first is a dose assessment report that describes exposure scenarios for sewage sludge processing, use, and disposal. The parameters used in the dose modeling for the scenarios and the dose conversion factors calculated for each radionuclide detected are included. A second report provides recommendations for POTW operators on determining sources of radioactivity at POTWs, describe sampling and analysis procedures, and suggest alternative courses of action if circumstances (e.g. location in a high NORM area) or actual measurements indicate that a problem may exist. The reports and laboratory data used in the sewage sludge analyses can be publicly accessed at http://www.iscors.org/.

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