When conducting risk-assessment studies for commercial nuclear facilities, the U.S. Nuclear Regulatory Commission (NRC) must be able to assess the movement of radioactive material in the environment. So Agricultural Research Service soil scientists Timothy Gish, Yakov Pachepsky, and Andrey Guber partnered with NRC researchers to evaluate and improve the accuracy of subsurface contaminant transport models.
The team set up their study at the ARS Beltsville Agricultural Research Center in Beltsville, Maryland, at a site that is equipped with remote sensing gear and other instrumentation for monitoring a range of geophysical and biophysical processes. This site—the Optimizing Production Inputs for Economic and Environmental Enhancement (OPE3) study area—was set up in 1998 to study major environmental and economic issues facing U.S. agriculture.
The researchers wanted to assess how the vadose zone—the zone between the soil surface and the groundwater zone—affects contaminant transport. They focused on subsurface structural features, processes, and events in the vadose zone that could drastically change the fate and transport of pollutants in a contaminant plume.
Over 2 years, the team added several nontoxic chemical tracers to irrigation water and used 12 site wells to monitor levels of those tracers at 3 different depths in the soil. Surface runoff, soil moisture profiles, soil water potential, groundwater levels, and weather variables were also monitored.
The researchers compared the field data they collected on water flow and tracer concentrations with results from model simulations. Then they applied a range of abstraction techniques to models of varying complexity to further pinpoint conditions that could significantly affect the movement of water—and contaminants—below the soil surface.
For instance, they found that tracer transport in soils and shallow groundwater could be strongly affected by gaps in the vadose zone’s restrictive fine-material layers. The complex topography in this layer could cause preferential flow and transport along pathways in low parts of the surface.
Similar dynamics could direct preferential flow around natural capillary barriers and funnel subsurface flow through coarse-textured soils sandwiched between finer layers. In addition, continuous voids in fine-grained sections could also prompt rapid flow conditions.
NRC staff will be able to use the refined models to estimate pollutant transport scenarios for risk-assessment studies of nuclear facilities.—By Ann Perry, Agricultural Research Service Information Staff.
Yakov A. Pachepsky is with the USDA-ARS Environmental Microbial and Food Safety Laboratory, 10300 Baltimore Ave., Bldg. 173, Room 203, Beltsville, MD 20705-2350; (301) 504-7468.
"Partnering With the NRC" was published in the April 2010 issue of Agricultural Research magazine.