Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Inventory of Radionuclides in Department of Energy Cold War-Era Landfills at Lawrence Livermore National Laboratory Site 300

Victor M. Madrid¹, Michael Taffet¹, Zafer Demir¹, G. Bryant Hudson², Bradley K. Esser², and Katherine Grote^3
1 Environmental Restoration Division, Lawrence Livermore National Lab, 7000 East Ave, Livermore, CA 94550, [email protected]
² Chemical Biology & Nuclear Science Division, Lawrence Livermore National Lab, L-231, P.O. Box 808, Livermore, 94550
³ Geology, Univ of Wisconsin, Eau Claire, Eau Claire, WI 54702-4004

We conducted a subsurface inventory of tritium and uranium within landfills at Site 300, an 11 square mile explosives test site on the east side of the Central California Coast Range. The frequency of explosives tests involving tritium and depleted uranium peaked from the mid-1960s to early 1970s, when two 10-to 20-foot deep, unlined and uncapped landfills (Pits 3 and 5) received experimental debris. The landfills are incised into valley-fill alluvium and decomposed bedrock containing a shallow, ephemeral water-bearing zone that thickens and inundates landfill waste during high intensity rainfall events. The inventory was conducted for a CERCLA Remedial Investigation/Feasibility Study to characterize contaminants in soil, rock, and ground water. This inventory included 3-D visualization and property modeling to integrate a variety of independent data sets to determine the spatial relationship between residual contaminant sources and the rising and falling water table. Cone penetrometer friction and dipole-dipole resistivity data defined the depth of each pit. Refraction seismic and hydrogeologic data were used to identify the base of the shallow water-bearing zone. This basal surface was incorporated into a hydrogeologic framework model to determine the extent of pit inundation during the 1998 El Niño. Property modeling of soil and ground water chemical data was used to determine the 3-D distribution of tritium and depleted uranium remaining in the pits and surrounding soil and ground water. The highest activities of depleted uranium and tritium were co-located in samples collected near the bottom of Pit 3. The distribution of tritiogenic 3He in soil gas exhibited good spatial correlation with elevated soil moisture tritium. Determination of uranium isotopic composition by mass spectrometry allowed quantitation of depleted uranium activity in a geologic setting where high levels of natural uranium have been observed. Depleted uranium behaves non-conservatively and is restricted to the immediate vicinity of the pits, whereas tritium, a conservative tracer, has migrated thousands of feet in shallow alluvial and deeper bedrock aquifers. The preferred remedy proposed in the RI/FS includes a drainage diversion system that will divert recharge water away from the pits, minimizing pit inundation and contaminant releases during rainfall events.

Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_82518.htm). © Copyright 2005 The Geological Society of America (GSA).