Department of Energy Computational Science Graduate Fellowship

STOMPing Ground

By William J. Cannon

IF YOU LIKE A GOOD MISNOMER, you need look no further than the Scenic Highway site outside Baton Rouge, Louisiana. There, spread over 17 acres of graded dirt, are dozens of recovery wells. Down the road a mile and a half, at a place called Brooklawn, are more wells on 60 acres — 165 wells at the two sites, about a mile from the Mississippi River. Take in the view, but don’t drink the water.

Scenic and Brooklawn comprise the EPA’s Petro Processors Superfund Site where, in the 1960s and ’70s, for 13 and 11 years respectively, a “disposal company used to pour organic waste from petroleum companies into large ponds and trenches,” says Mark White, senior research engineer at the Department of Energy’s Pacific Northwest National Laboratory.

White is code author and custodian of STOMP, which stands for “subsurface transport over multiple phases.” STOMP parcels up a piece of ground into three-dimensional cubes, then simulates underground flow and transport of very bad things, environmental nightmares both happening and waiting to happen. White and his STOMP team at PNNL’s Hydrology Group try to get on the case before the nightmare has happened; their intervention can lead to action that can prevent a given pollutant from reaching groundwater.

At Scenic and Brooklawn, the nightmare was happening. A site-restoration company had cleaned up the surface and, as common sense might dictate, drilled wells from which to pump out the nasty stuff that had seeped downward. White’s word for that stuff sounds like “Dean Apples” — DNAPLs, denser-than-water nonaqueous (oily) phase liquids (NAPL), among the many forms of pollutants covered in the transport equations of STOMP.

In this context, the important thing about a DNAPL is that it sinks in water. Mart Oostrom, a senior research engineer in White’s group, ran a STOMP simulation for Scenic only to discover that the army of pumps, which had cost a small fortune to install and to operate and were intended to suspend the organic pollutants, actually made things worse. The model, also applied for Brooklawn, showed “that as millions of gallons of water flowed upward, the water table dropped,” White says. “The DNAPLs migrated farther down into the system and toward the drinking-water aquifer. They shut down the perimeter wells.”

As a result, recovered groundwater continues to be treated to remove hazardous liquids and other contaminants. Unlike its namesake, the Scenic simulation and others turn out to be rather elegant. STOMP solves a series of equations that describe the physical properties in the hydrology system under investigation. The program can be tailored to track the migration of materials through water, brine, ice, oily liquid pollutants, through just about any fluids that might fill underground pores, thanks to a so-called variable source code that enables anyone using it to dial in the desired governing equations to be solved: water mass, air mass, dissolved-oil mass, oil mass, salt mass and heat. Equations solve problems specific to transport of chemicals in solution, radioactive decay and chemical reactions. At the end of this process is a time-lapse picture of underground plumes of pollutants expanding and receding, depending on how the invasive chemicals interact with the soils and elements in the ground, under any conditions the modeler can conceive of. “We have put the simulator through a rigorous verification procedure against analytical solutions, laboratory experiments and field demonstrations,” White says. “I want to emphasize that the nature of this work is collaborative. Mart and I ponder why the numerical simulations don’t agree with the experimental observations. Numerical simulation solves a collection of mathematical equations in an attempt to describe physical processes.