Abstract

Between 1945 and 1952 the development of nuclear weapons at Los Alamos National Laboratory, New Mexico, resulted in the disposal of plutonium into the alluvium of nearby Acid and (to a lesser degree) DP Canyons. Previous research has identified some of this material in the Rio Grande and defined its distribution in the regional river system. The purpose of this paper is to explore the connection between the disposal sites and the main river, a 20 km link formed by the fluvial system of Acid, Pueblo, DP, and Los Alamos Canyons. Empirical data from 15 yr of annual sediment sampling throughout the canyon system has produced 458 observations of plutonium concentration in fluvial sediments. These data show that, overall, mean plutonium concentrations in fluvial sediment decline from 10 000 fCi/g near the disposal area to 100 fCi/g at the confluence of the canyon system and the Rio Grande. In finer detail, the concentrations fluctuate with downstream distance depending on the trap efficiency of various reaches, as controlled by hydraulic conditions. Temporal data from sites repeatedly sampled show the passage of waves of contaminated sediment through the canyon system. Field mapping identified 108 deposits of sediment, including active bed load, flood plains, bars, channel fills, and slack-water deposits. Volumes of sediment in these deposits (calculated from field measurements of the dimensions of the features), combined with the mean concentration values, produced a first approximation of the amount of plutonium in each deposit. The geographic distribution of deposits and plutonium is clustered: of the 1000 mCi of plutonium in the canyon system, 78% is in lower Pueblo Canyon, 18% is in lower Los Alamos Canyon, and the remainder is in the upper reaches of the system. Simulations using a computer model for water, sediment, and plutonium routing in the canyon system show that discharges as large as the 25 yr event would fail to develop enough transport capacity to completely remove the contaminated sediments from Pueblo Canyon. Lesser flows would move some materials to the Rio Grande by remobilization of stored sediments. The simulations also show that the deposits and their contaminants have a predictable geography because they occur where stream power is low, hydraulic resistance is high, and the geologic and/or geomorphic conditions provide enough space for storage.

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