Characterization of selected radionuclides in sediment and surface water in Standley Lake, Great Western Reservoir, and Mower Reservoir, Jefferson County, Colorado, 1992

Abstract

Median 239>240pu concentrations in lakesediment grab samples from Standley Lake, Great Western Reservoir, and Mower Reservoir were 0.037, 0.105, and 0.351 picocuries per gram (pCi/g), and the differences between the reservoirs were statistically significant at p<0.05. The highest 239'240pu concentrations were measured in lake-sediment grab samples collected from reservoirs closest to Rocky Flats Environmental Technology Site, which is consistent with the spatial pattern of 239>240pu concentrations identified in soil surrounding the site in previous studies. The maximum concentration of 239>240pu dissolved in lake water was 0.009 picocuries per liter, well below limits suggested by the Colorado Department of Public Health and Environment. Dissolved gross alpha and uranium isotope concentrations were below National Drinking Water Standards in all water samples. There was no statistically significant difference between 239>240pu concentrations in lakesediment grab samples collected from Standley Lake in 1983-84 and in 1992; however, there was a small, but statistically significant difference at Great Western Reservoir (p<0.05). Mean 239'240Pu concentrations at Great Western Reservoir were 0.140 pCi/g lower in 1992, and median concentrations were 0.040 pCi/g lower. One possible explanation for the difference in concentrations in lake-bottom-sediment grab samples is that new sediments with relatively low 239'240pu concentrations may have buried older sediments containing higher concentrations of 239'240Pu; the grab-sampling technique collects only the top 2 to 4 inches of sediment from the lake bottom. In lake-bottom-sediment cores, trends in 239'240Pu concentrations with depth in 1992 were Abstract similar to trends identified in the 1983-84 study. Maximum 239'240pu concentrations occurred at depths ranging from 13 to 31 inches below the sediment-water interface at most sites. There was a small, but statistically significant (p<0.05) difference in 239'240pu concentrations in co-located lake-bottom-sediment cores collected in 1983-84 and in 1992. Measured concentrations tended to be higher in 1983-84 than in 1992. The median difference between data sets was 0.050 pCi/g, and differences tended to increase with concentration; in samples with concentrations above 1.5 pCi/L, concentrations were 10 to 30 percent higher in 1983 84 than in 1992. The differences in concentrations could be attributable to spatial variations in sediment and 239'240pu deposition. INTRODUCTION Standiey Lake, Great Western Reservoir, and Mower Reservoir are man-made water bodies used for domestic water supply and irrigation in northeastern Jefferson County, Colorado (fig. 1). These reservoirs (hereinafter referred to as lakes) are located 1.5 to 3.3 mi east of the Rocky Flats Environmental Technology Site (RFETS, formerly known as the Rocky Flats Plant), which is about 16 mi northwest of Denver, Colorado. The RFETS, which is owned by the U.S. Department of Energy (DOE) and operated by contractors, was built in 1951 and has been used for plutonium ...