Use of Aerial Radiological Surveys to Identify Low-Level NORM Associated with a Remediation Site - 22384

The U.S. Army Corps of Engineers (USACE) conducted an aerial survey of the vicinity of the former Middlesex Sampling Plant (MSP) in Middlesex, NJ to identify properties potentially contaminated with uranium- and thorium-series radionuclides related to the operations at the Site. Proper processing of the gamma spectroscopic data was essential to minimizing the Minimum Detectable Concentration (MDC) in spite of low count rates due to higher-than-expected altitudes, variations in altitude and atmospheric densities during different periods of measurement, and variations in background due to underlying geologic formations and diurnal radon emanation. The MSP was an industrial operation located in Middlesex, NJ, that assayed uranium and thorium ores between 1943 and 1955 for the Manhattan Engineer District. In 1976, the MSP Formerly Utilized Sites Remedial Action Program (FUSRAP) Site (Site) was designated for remedial action which included excavation and offsite disposal of radiologically contaminated soils with activities greater than 5 picocuries per gram (pCi/g) of radium-226 (Ra-226) greater than background. Background for the MSP is approximately 1 pCi/g for background. Therefore, the cleanup criterion is approximately 6 pCi/g. In 1978, an aerial radiological survey was conducted over a 2.8 kilometer (km) by 3.2 km area surrounding the MSP. Several locations, designated as Vicinity Properties (VP) were identified as potentially having elevated radiation levels. In 2021, the USACE conducted an updated airborne survey for the Site, over a survey area approximately four times the extent of the 1978 survey. While aerial radiological surveys are a useful tool for quickly characterizing radiation levels in large areas, for many remediation sites, such as this Site, the activity limits are relatively low and can be hard to distinguish from background. Several factors affect the sensitivity of an aerial survey including the aircraft altitude, which has one of the largest effects, the number of detector systems, and the dwell time for each measurement. For the MSP several methods to increase the sensitivity of the survey were evaluated. According to International Atomic Energy Agency (IAEA), for an infinite slab source, the gamma signal decreases by approximately one-half for every 91 meters of height (300 feet). Originally, the survey was planned for 91 meters above ground level (AGL), which would have resulted in an MDC of approximately 5 pCi/g of Ra-226 over a 929 square meter area, which is the average size of a residential lot in Middlesex. However, due to the population density of the survey area, the Federal Aviation Administration specified the lowest altitude that the airplane could operate was 152 meters AGL. In addition, due the presence of a large ridge, the altitude of the plane varied widely in the northern portion of the survey area where the aircraft had to traverse the ridge. This increased the apparent pre-survey calculated MDC to approximately 24 pCi/g of Ra-226 over a 929 square meter area. For this survey the number of detector systems and dwell time could not be altered. To increase the sensitivity of the survey, the following adaptations to the survey methodology, as well as the data processing were evaluated. 1. Coadding: The line spacing for the survey was decreased to allow for coaddition of the measurement points. Typically, a line spacing between 76 meters and 152 meters is used to provide 100 percent aerial coverage based on the solid angle subtended by the types of large scintillation packs used. For this survey, which requires greater sensitivity, a line spacing of 15 meters was used. After normalizing for background and altitude, the count rates for nine points within a 33 m radius of each individual data point were coadded. This methodology reduced the calculated MDC from 24 to 17 pCi/g. While coaddition resulted in a decrease in the MDC, the MDC was still significantly greater than the cleanup criterion of 6 pCi/g (i.e., 5 pCi/g above background). Furthermore, it was found that even after a mathematical altitude correction was implemented, high winds and terrain issues caused a lack of precision in the count rates due to varying altitudes from flight to flight. 2. k-values: A statistic, termed the k-value, which is unitless, represents a ratio of one energy region over another energy region. The k-value was calculated for each survey point by dividing the net region of interest (ROI) counts for the photopeak of interest (e.g., the 1764 keV peak for Bi-214) by the net comparison ROI counts (780 to 960 keV). The comparison ROIs are based on energy ranges recommended by United States Department of Energy (USDOE) for their aerial surveys. Because both the net ROI and comparison ROI are collected at the same altitude, use of the k value greatly reduces the uncertainty associated with varying altitudes, terrain and weather conditions (i.e., atmospheric density). This methodology reduced the calculated MDC from 24 to about 12 pCi/g, which is still not within the cleanup criterion objective. 3. Coadding k-values: By applying the above two techniques together, the k-values for each data point were coadded (actually co-averaged) by averaging the k-values for each data point with those of its 14 closest neighbors. This methodology reduced the calculated MDC from 24 to 7 pCi/g, resulting in a value which is near the cleanup criterion objective of 6 pCi/g. Use of k-values to increase sensitivity of an aerial survey significantly reduces uncertainty associated with altitude correction. This leads to better sensitivity and better data to help guide ground-based field work. Groups performing environmental remediation of NORM contaminated sites may benefit from this paper and presentation. (authors)