GoldSim Modeling of the
West Beach Landfill
Alameda Point, California
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Background
GoldSim modelers at Neptune and Company have devised a variety of models
to aid in environmental decision making. One such model is a contaminant
transport model of a landfill at the west end of Alameda Point,
operated by the former Naval Air Station Alameda. West Beach Landfill (WBL)
was constructed by building a seawall in the shallow San Francisco Bay,
delineating an area of the bay to be filled in with wastes. The enclosed
area is connected to the Bay via a culvert which penetrates the seawall.
The intention was apparently to completely fill in the area behind the
seawall, but operation of the landfill was halted before this could be
accomplished. The waste, much of which directly lies in the waters of the
Bay, was covered with a thin layer of soil.
The site, named IR Site 2, has
remained essentially untouched since that time, and now supports a
variety of biota, including some endangered species such as the Least Tern.
The site includes two unfilled wetlands, Southern Pond and Northern Pond.
Southern Pond receives most of its water from runoff of precipitation
falling on the landfill, and the water level changes with the amount of
precipitation in the area. The Northern Pond (see photo), on the other
hand, is in tidal connection with San Francisco Bay via the culvert
(see photo), and the water level and salinity vary with the tides.
One plan for the disposition of this site is to allow it to become a
National Wildlife Refuge. In order to determine the feasibility of
this plan, Neptune and Company has performed a Remedial Investigation
of the site. This GoldSim model was developed as part of that RI.
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Conceptual Model of Contaminant Transport
The conceptual model of contaminant transport at the WBL includes
leaching of contaminants by water from precipitation percolating
through the thin cover as well as by tidally-influenced waters
inundating the lower part of the waste pile adjacent to Northern Pond.
Airborne transport is considered to be negligible in this wet environment.
In addition to waterborne transport, contaminants (if they exist) are
expected to be mobilized by biotic processes. Plants, including
grasses, forbs, and shrubs, can bring contaminants to the surface by
uptake in the root systems, fixing them in the aboveground parts of
the plant, and shedding those plant parts as litter on the ground
surface. Animals, represented by squirrels, moles, gophers, and
worms, can transport bulk materials through the excavation
of burrows, where materials are assumed to be brought directly to
the surface from various depth layers, and the burrows are also
subject to collapse so that the mass balance of material is preserved.
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The GoldSim Model
The GoldSim model of the WBL (click on image to get a full-sized
screen shot) consists of several soil/waste layer cells, with a
soil layer on top, three layers of variably-saturated waste, and
a bottom layer of saturated wastes. This is essentially a 1-D model
since few data are available regarding the spatial distribution
of wastes.
Contamination brought to the surface by plants is modeled by moving a
medium called "Plant", a technique that dates from modeling with
the predecessor to GoldSim: RIP. (This technique is rather convoluted,
and has since been replaced by a more efficient mechanism.)
The rates of transfer of different chemical constituents by plants is
based on plant uptake factors and productivity rates.
Transport by animals is done simply creating an advective connection
between upper subsurface cells and the topmost surface soil cell
in both the inner cylinder and the surrounding lateral ring. These
advective connections move bulk allivium (part dirt and part water).
Complementary connections are made to account for burrow collapse,
with materials cascading from the topmost cell to the one below,
to the one below that, and so on.
A more complete writeup on the GoldSim model for West Beach Landfill
can be downloaded here
as a 16 MB pdf file.
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Future Work
Neptune and Company would like to obtain site-specific data from which
to construct stochastic parameter distributions, such as those representing
biotic behavior. The largest source of uncertainty, however, is probably
the nature and extent of the waste constituents. The model could benefit
greatly from adequate waste characterization, including the spatial
distribution of wastes. We would also like to update the modeling of
biotic transport with computationally more efficient techniques which
have been used in models developed after the WBL model.
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Go to Neptune's GoldSim page
Send comments regarding these pages to
John Tauxe
Last modified: 12 March 2002
