June 2009

Subsurface flow

Tracking contamination from reservation to river

Divining just how contaminants such as radioactive particles behave as they move from soils to groundwater in complex subterranean worlds has long challenged computer modelers and their simulations.

With the help of high-performance computers, however, national laboratory and university researchers are working together to stretch models of how reactive chemicals move through porous surfaces. Their work may help the Department of Energy in its cleanup of wastes left behind by nuclear materials production.

For Peter Lichtner, a physicist at Los Alamos National Laboratory (LANL), and Glenn Hammond, a computational geohydrologist at Pacific Northwest National Laboratory (PNNL), the target is a complex real-world problem: contaminants in sediment at the Hanford Nuclear Reservation in Washington state. Their main tool is PFLOTRAN – what they and other modelers call “a parallel multiphase and multicomponent reactive geochemical transport code.”

Lichtner is principal investigator for the project, which is part of the groundwater science application area in the Department of Energy’s Scientific Discovery through Advanced Computing (SciDAC) program. His team’s work is creating a better understanding of what happens as water moves underground, carrying contaminants that react and change.

A groundwater plume at the site carries uranium into the nearby Columbia River. The project focuses on a subsection of the reservation, the Hanford 300 site, where nuclear reactor fuel was made and nuclear materials were disposed from World War II until before the site’s last reactor shut down in 1987.

The model predicts moving contaminants’ behavior, in three dimensions, over periods ranging from hours to years.

A variety of chemicals from water to bicarbonate and uranium are part of the scenario, moving through sediment that contains everything from fine sand to inches-long cobbles under various saturation conditions.

Detailed modeling of contaminant leaching from the site requires computer processor cores – and plenty of them.

“In these groundwater models, you have to deal with three-dimensional problems, so you need a lot of (computer) nodes” to break down and solve the complex equations that go into them, Lichtner says. “To do that on a single processor you’d either run out of memory or time. It would take months or years.”

In the early 1990s, Westinghouse Hanford, the site manager at the time, used a simpler but then-current model for flow. It predicted uranium would clear from Hanford 300 in 10 to 15 years. Almost 20 years later, contamination levels are nearly unchanged. Adding details – the site’s underground layers, heterogeneities of the surface, the complexity of the chemical types, or species, underground and other factors – requires more and more equations and thus more and more computational power.

River stages: Modeling a moving target

It is not just the complicated nature of the underlying surface and the many chemical processes that make this a challenging model to build. The movement of the Columbia River through the site also is complex.

Says Hammond, “If you look at the plot of the river stage, which is the elevation of the river, over a year, it’s all over the place.”

PFLOTRAN’s model covers an area 1,350 by 2,500 by 20 meters aligned with the Columbia River. The actual Hanford 300 area, near the south end of the reservation, is made up of a 1.35 square-kilometer industrial site and about 2.6 square kilometers of surrounding land.