Contaminant Transport Modeling in Alluvial Channels at Los Alamos National Laboratory, New Mexico, USA

We describe contaminant transport modeling activities for reaches of Pueblo Canyon, near Los Alamos National Laboratory (LANL), state of New Mexico, USA, during the summer rainy season of 2002; 2 years after the Cerro Grande fire. The REACH6 model is a distributed, dynamic model which uses sediment discharge and stream channel characterization data to compute transport of sediment-associated contaminants, i.e. heavy metals such as plutonium-239, 240 (Pu-239, 240 or sometimes simply Pu). We integrated the application of the US Army Corps of Engineers models for hydrology, hydraulics, and sediment transport with the REACH6 model. We compared data from suspended sediment and contaminant concentration measurements in Pueblo Canyon with corresponding calculations from the REACH6 Model. For a flood event on June 22, 2002, the measurements indicated that during the suspended sediment sampling period about 14.4 mCi of Pu was transported out of lower Pueblo Canyon. The REACH6 prediction for the same sampling period was a transport of 23.4 mCi or some 62% more than the value of 14.4 mCi. Given the uncertainties in flow and sediment monitoring and the fact that the model includes bed load transport as well as suspended sediment transport, these prediction results are a good match with the measured values. Even so, data from more events are needed to evaluate better the uncertainty associated with these predictions. The contaminant transport model provides LANL with opportunities to develop state-of-the-art adaptive management and decision support tools for its environmental programs. However, these programs must adapt their network design and operation; reach sampling, and reporting procedures to take advantage of these opportunities. The adaptations must include more frequent documentation of contaminant inventories in the channel systems and additional monitoring of flow, sediment and contaminants.