Los Alamos Women in Science
Northern Chapter of the New Mexico Network for
Women in Science and Engineering

Tuesday, June 13, 2000
11:30 am - 1:30 pm, Otowi cafeteria side room C, LANL

TALK at 12:00:
Alyssa Olson, EES-5, LANL
"Modeling Flow and Radionuclide Transport
at the Western Pahute Mesa Area of the Nevada Test Site"


At Pahute Mesa on the Nevada Test Site, a small amount of plutonium was recently detected 1.3 km from its source, the BENHAM underground nuclear test. This observation is critical because it illustrates a gap in our current understanding of the mobility of plutonium in groundwater, previously considered immobile because of its chemical properties. The plutonium was found in two aquifers, a lava formation at about the same elevation as the detonation point and a welded tuff formation 600 m above the detonation point, separated by a low permeability zeolitized tuff. In both locations, the plutonium was associated with colloid particles, particles 50 to 500 uM in size. To study and understand the conditions that led to this colloid-facilitated transport of plutonium, we have developed a modeling strategy that consists of the subregional groundwater flow system and reactive transport processes associated with fractured rock. We are developing a subregional-scale (8 km by 10.4 km) flow model to investigate the influence of complex geologic features and hydrologic parameters on the magnitude and direction of ground water flow through the areas between the BENHAM source and wells in which plutonium was observed. Within the subregional-scale domain, we have embedded a site-scale(2.6 km by 3.2 km) domain containing the BEHNAM test area and the observation site, where the spatial distribution of hydrogeologic properties such as permeability and porosity can be varied at the 50 m scale. This high resolution domain helps us to evaluate the impact of spatial variability in the transport models. Further, at an even finer scale near the source, we are evaluating thermal convective transport as the major mechanism for the movement of plutonium up and out of the nuclear test chimney/cavity system into the two separate aquifers. This mechanism is critical for explaining how plutonium was found above a low permeability tuff aquitard. Once the plutonium has reached the welded tuff and the lava aquifers, chemical modeling of diffusion, sorption and plutonium-colloid interactions are considered. We are integrating all of these components into a total system analysis tool for future risk assessment by the DOE and the state of Nevada.

Nonmembers Welcome!  

Sideroom C is reserved from 11:30 to 1:30 for people who like to eat earlier or later, and informal discussion. You are welcome to come for just the talk at noon, and questions are encouraged!

For driving directions to Otowi Cafeteria (TA-3, building 261) at LANL, see: http://www.lanl.gov/tools/maps
(direct link to map showing the Otowi Building: http://nis-www.lanl.gov/cgi-bin/wthi?ta=3&bldg=261).




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LAWIS Lunch-Time Talk Organizer: Huyen Dinh, dinh@lanl.gov, 665-0737
LAWIS home page: http://home.lanl.gov/lawis
NMNWSE home page: http://www.nmnwse.org