Mechanism of plutonium transport in a shallow aquifer in Mortandad canyon, Los Alamos National Laboratory, New Mexico

Potential environmental effects of radioactive isotopes are of great public concern. Fortunately most long-lived isotopes have strong affinities for solids and limited mobility under natural conditions. It recently has been proposed that some isotopes may form colloids that move with groundwater. The detection of plutonium in groundwater some 3400 m down Mortandad Canyon at the Los Alamos National Laboratory is cited widely as an example of such transport. The current work re-examines data from this canyon to evaluate the significance of such transport. Pu-239 entering the canyon increased sharply in the early 1980s. Routine monitoring during this period shows that isotopically distinct plutonium appeared in one downgradient well before it appeared in wells closer to the source. If this is ignored, plutonium moved at least twice as fast as groundwater flow and easily outdistanced a tritium peak. Isotopically heavy plutonium arrived simultaneously in surface alluvium and groundwater, and the isotopic composition of plutonium in alluvium and groundwater are identical. Plutonium clearly did not move down-canyon via groundwater. The potential for plutonium movement through groundwater on colloids may be overstated at this and other sites.