West Beach Landfill: Alameda Point, California
GoldSim modelers at Neptune and Company, Inc. 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 Installation Restoration 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, Inc. has performed a Remedial Investigation (RI) of the site. This GoldSim model was developed as part of that RI.
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.
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 by 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 alluvium (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.
Neptune and Company, Inc. would like to obtain site-specific data in order 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 that have been used in models developed after the WBL model.