Tracking contamination
from reservation to river

(page 2 of 3)

The source of the waste is several hundred meters inland, but uranium reaches groundwater that encounters river water flowing into and out of the porous ground. The river's rise and fall does more than carry contaminants away: The daily up-and-down cycling of the river stage works like a tide, carrying materials from just above the water table into the groundwater and depositing them on the porous surface, where uranium strongly attaches to soil.

Lichtner and Hammond's model has access to 15 years of river-stage data, as well as information collected over shorter spans from the Hanford 300 site's many monitoring wells. Unfortunately, the well data was not collected over a uniform period, making it difficult to match it with river information.

"We have about a year's worth of well data that we can coordinate with the river data," Hammond says. "There are 8,760 hours in a year, and we're modeling multiple years. We're performing some calculations to see how necessary it is to have that kind of detailed information. We're doing some smoothing of the data, which allows bigger time steps."

Computational power is not the only limiting factor in improving the site model: Hourly environmental sampling is costly and prone to errors and breakdowns of the data-collecting equipment. With more work, the team hopes to work around this problem to further improve the model.

"As the model becomes more sophisticated and we apply more computational power," Hammond says, "we can actually put in some random datasets and try to make it more realistic. The model may not be perfect, but it's going to be a lot closer."

Simplification can only get one so far, however. Whereas many groundwater models approach problems with simplified assumptions, understanding the chemical and physical complexities of the Hanford site requires a full 3D model.

"Right now we're simulating about 12 (chemical species), but there's a point where you can't simplify the chemistry any further," Hammond says.

The team is trying to capture "the difference in chemistry between the river water, which should have a low carbonate concentration, and the groundwater, which is higher in carbonate. That can greatly affect the mobility of uranium. There are also differences in pH and other species as well."

The fundamental complexity of the problem requires moving beyond simplified models that can run on a workstation and into parallel computing. Developing PFLOTRAN, the code at the core of the model, advanced existing flow and transport codes to efficient parallel processing. It built on elements of PETSc, a suite of tools for parallel solution of equations for scientific applications, allowing relatively rapid development. PETSc is part of another SciDAC project, TOPS, or Towards Optimal Petascale Simulation.

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