EPA has developed three different radiation and radionuclide concentration "benchmarks" to help users interpret their RadNet search results. These benchmarks are the regulatory-based concentration such as the drinking water maximum contaminant level (MCL), the target risk concentration, which is based on EPA's cancer risk range, and the minimum detectable concentrations (MDCs). For a variety of reasons described below, no benchmarks are available for some search results, while several benchmarks are available for others.
Users are encouraged to compare their search results with these benchmarks. Note, however, that there are two important points to keep in mind regarding these benchmarks and comparisons:
- Results that exceed the benchmarks do not necessarily indicate a health risk, but may warrant more extensive characterization and detailed assessment.
- These benchmarks are not legally binding and are provided for reference only. The benchmarks were developed to provide context for RadNet data and should not be used for any other purposes.
The following sections describe each of the three types of benchmarks compiled for the RadNet data.
Regulatory-based Concentrations — Maximum Contaminant Levels (MCLs) for Water
The Maximum Contaminant Level (MCL)-based benchmark is the maximum permissible level of a contaminant in water that is delivered to any user of a public water system. EPA is required to set MCL's for certain drinking water contaminants under the Safe Drinking Water Act (SDWA). MCL-based benchmarks are available for most of the RadNet analytes found in drinking water; most analytes lacking MCL benchmarks still have target upper and lower risk range concentrations associated with them for comparison to the search results. Technically, the MCL is applicable to drinking water only and not to surface water, but the MCL-based benchmark is used here for surface water as a matter of convenience because of the highly site-specific nature of the water quality standards available for surface water. See the EPA website on water quality standards for more information. For more information on radionuclide MCLs, see the EPA drinking water website for radionuclides.
Target Risk Concentrations
EPA has calculated target risk benchmark concentrations for most combinations of RadNet media (air, water, milk, and precipitation) and radionuclides or radiation types. These risk concentrations reflect EPA's process for determining the level of contamination considered "safe or acceptable", consistent with the National Oil and Hazardous Substances Pollution Contingency Plan (NCP) at 40 CFR 300.430(e)(2)(i)(A)(2). As a result of exposure to a particular contaminant, the target risk benchmarks are determined based on all of the following three factors:
- An individual lifetime risk of no greater than about 1 in 10,000 of contracting fatal cancer,
- The lifetime risk for the majority of people within a given area, of less than 1 in 1,000,000 of contracting fatal cancer, and
- A small total estimated number of additional cases of death or disease as a result of exposure to a contaminant.
Thus, two target risk concentrations have been calculated for each radionuclide, one corresponding to the upper end of this risk range (1 in 10,000) and the other corresponding to the lower end (1 in 1,000,000).
Minimum Detectable Concentrations
The average minimum detectable concentration (MDC) is an indication of the ability of the instrumentation to detect the analyte. If the analyte is present below this level of detection, it may be reported as a non-detect (ND). The MDC is the minimum concentration that gives a 95% probability of detection when the detection criteria are chosen to give only a 5% probability of false detection in a blank sample. The average MDC is calculated by averaging all of the MDCs from the search results. For results lacking a sample-specific MDC, the average MDC is estimated using the Combined Standard Uncertainty (CSU) according to the following equation: MDC=1.645*CSU. The CSU is the estimated standard deviation of the result. As with the beta trigger levels, the MDCs generally are relatively low and are not based on health risk. Thus, results that exceed these levels should not necessarily be interpreted as significant. Users should examine the other available benchmarks for the given samples to obtain a better understanding of the significance of results that exceed the average MDC. For more information on these minimum detectable concentrations, see the RadNet Search User Guide.
Screening Levels for Performing Additional Analysis
These screening levels for gross radioactivity are simply guidelines EPA's National Air and Radiation Environmental Laboratory (NAREL) uses to decide whether or not to determine the identity and activity of individual alpha, beta, and gamma emitters in the sample, they do not correspond to any dose or risk level. Gross alpha and gross beta levels are difficult to associate with either a regulatory-based concentration or a target risk concentration because they typically represent more than one radionuclide. The unique characteristics of the emissions from the various radionuclides can result in different risks for the same alpha and/or beta activity levels. Users should examine the results of the additional (gamma or radiochemical) analyses, if available to obtain a better understanding of the significance of the radiation that exceeds these trigger levels. NAREL requires characterization of samples having alpha and/or beta activities that exceed the following screening levels:
Beta Activity in Air
Continuously operating samplers collect airborne particulates on filters that are collected twice weekly and sent to NAREL for analysis. A gross beta analysis is performed on each air filter, a gamma analysis is performed if the beta activity is greater than 1 pCi per cubic meter. In addition, annual station by station composites of the air particulates filters are analyzed for plutonium (Pu-238, combined Pu-239/Pu-240) and uranium (U-234, U-235, U-238).
Beta Activity in Precipitation
All stations routinely submit precipitation samples as rainfall, snow or sleet occurs. The precipitation samples are composited at NAREL into single monthly samples for each station. Each month that precipitation occurs, an aliquot of the composited sample is analyzed for gross beta, and gamma emitting nuclides. Additional analyses are performed on precipitation samples with gross beta activities of greater than 16 pCi/L.
Alpha Activity in Drinking Water
Samples of drinking water at the tap, are obtained quarterly and sent to NAREL for analysis. Analyses include gross alpha, radium-226 if the gross alpha exceeds 2 pCi/L, and radium-228 if the radium-226 falls between 3 and 5 pCi/l. Additionally, for stations that demonstrate gross alpha levels greater than 2 pCi/L, an annual composite is analyzed for plutonium-238, combined plutonium-239 and 240, and uranium-234, 235, and 238. Other analyses routinely performed on these samples include: gross beta, strontium-90, and gamma on annual composites; tritium on a quarterly basis; and iodine-131 on one quarterly sample per year for each station.
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