1. Nuclear and Radiochemistry Group, Los Alamos National Laboratory, P.O. Box 1663, MS J514, Los Alamos, New Mexico 87545, Earth and Environmental Sciences Division (EES-14), Los Alamos National Laboratory, P.O. Box 1663, MS J966, Los Alamos, New Mexico 87545, and Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720