GoldSim modelers at Neptune and Company have devised a variety of models to aid
in environmental decision making (see
www.neptuneandco.com/goldsim). One such model is a contaminant
transport and regulatory compliance model of a radioactive waste disposal site
at the Nevada Test Site (NTS), near Mercury, northwest of Las Vegas, Nevada. In
addition to the classical waterborne transport of radionuclides, Neptune's
model includes transport of subsurface materials to the ground surface by
burrowing animals (of widely divergent taxa), and the transport of
contamination to the surface by plants. Since the desert at the NTS is so dry,
biotic transport mechanisms are likely to play a critical role in the movement
of contaminants from waste forms at depth to the accessible environment. The
GCD wastes are deeply buried (21 to 36 m below the ground surface), and
contaminants are hypothesized to move upward from that depth to near the ground
surface in water, advecting with and diffusing within a small amount of
interstitial water. The upward flux is inferred from the observed strong
gradient of interstitial water potentials from depth to the ground surface.
Once the contaminants are within reach of plant roots and burrowing animals,
they can be brought to the surface and pose a potential threat to human health
through exposure to surface soils. The modeled subsurface consists of a
vertically-oriented cylinder of waste and alluvial overburden nested inside
another cylinder of surrounding alluvium.
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image)
Both the interior cylinder and the annular ring are subdivided into several
cells, in order to model contaminant transport by advection of water, diffusion
in the water phase, advection of alluvium by burrowing animals, and
plant-induced transport.
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image)
Contamination brought to the surface by plants is modeled not by moving any
contaminated medium, but rather by the simple addition to surface cells (and
complementary subtraction from subsurface cells) of predetermined amounts of
each contaminant. This transfer of contamination is implemented using a GoldSim
Consequence element, once the appropriate amounts of contamination have been
calculated based on plant uptake factors and productivity rates. Neptune and
Company anticipates adding to this model various dose assessment methodologies,
corresponding to different regulatory drivers, based on surface soil and water
contaminant concentrations. The model is a management system tool that will
support effective long-term management of the NTS low-level radioactive waste
disposal facilities.
