Measurements of actinides in soil, sediments, water and vegetation in Northern New Mexico Page: 2 of 18
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Plutonium and Uranium in Sediments of the Northern Rio Grande Valley near
Los Alamos, New Mexico
By
Bruce M. Gallaher and Deward W. Efurd
ABSTRACT
This study was undertaken during 1991 - 1998 to identify the origin of plutonium
and uranium in northern New Mexico Rio Grande and tributary stream sediments. Isotopic
fingerprinting techniques help distinguish radioactivity from Los Alamos National
Laboratory (LANL) and from global fallout or natural sources. The geographic area
covered by the study extended from the headwaters of the Rio Grande in southern Colorado
to Elephant Butte Reservoir in southern New Mexico. Over 100 samples of stream channel
and reservoir bottom sediments were analyzed for the atom ratios of plutonium and uranium
isotopes using thermal ionization mass spectrometry (TIMS). Comparison of these ratios
against those for fallout or natural sources allowed for quantification of the Laboratory
impact.
Of the seven major drainages crossing LANL, movement of LANL plutonium into
the Rio Grande can only be traced via Los Alamos Canyon. The majority of sampled
locations within and adjacent to LANL have little or no input of plutonium from the
Laboratory. Samples collected upstream and distant to LANL show an average ( s.d.)
fallout 24Pu/239Pu atom ratio of 0.169 0.012, consistent with published worldwide global
fallout values. These regional background ratios differ significantly from the 240Pu/239Pu
atom ratio of 0.015 that is representative of LANL-derived plutonium entering the Rio
Grande at Los Alamos Canyon. Mixing calculations of these sources indicate that the
largest proportion (60% to 90%) of the plutonium in the Rio Grande sediments is from
global atmospheric fallout, with an average of about 25% from the Laboratory. The LANL
plutonium is identifiable intermittently along the 35-km reach of the Rio Grande to Cochiti
Reservoir. The source of the LANL-derived plutonium in the Rio Grande was traced
primarily to pre-1960 discharges of liquid effluents into a canyon bottom at a distance
approximately 20 km upstream of the river. Plutonium levels decline exponentially with
distance downstream after mixing with cleaner sediments, yet the LANL isotopic fingerprint
remains distinct for at least 55 km from the effluent source.
Plutonium isotopes in Rio Grande and Pajarito Plateau sediments are not at levels
known to adversely affect public health. Activities of 239+240Pu within this sample set ranged
from 0.001- 0.046 pCi/g in the Rio Grande to 3.7 pCi/g near the effluent discharge point.
Levels in the Rio Grande are usually more than 1000 times lower than prescribed cleanup
standards.
.Uranium in stream and reservoir sediments is predominantly within natural
concentration ranges and is of natural uranium isotopic composition. None of the
sediments from the Rio Grande show identifiable Laboratory uranium, using the isotopic
ratios. These results suggest that the mass of Laboratory-derived uranium entering the Rio
Grande is small relative to the natural load carried with river sediments.10/10/02
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Gallaher, B. M. (Bruce M.) & Efurd, D. W. (Deward W.). Measurements of actinides in soil, sediments, water and vegetation in Northern New Mexico, article, January 1, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc935284/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.