Groundwater containing legacy contaminants (pollutants that remain after their sources have been controlled) moves through aquifers in response to the hydraulic gradient. As the groundwater moves, contaminants accumulate on solids (e.g., soil, alluvium, and rock). Clean groundwater entering the aquifer upgradient of the site is contaminated by “bleed back” from the solids phases in the soil or rock. Accurate cleanup times are difficult to predict because of this contaminant desorption effect.

Groundwater is often monitored at contaminated sites, but solids phases typically are not. To understand groundwater plume migration, scientists need to understand both the chemistry of the groundwater and the solids matrix and the connection between them. An analogy can be made to human physiology: to understand a disease, a doctor needs to also understand how pathogens are transported through the bloodstream and how they interact with different parts of the body. Understanding the interaction between the water phase and the solids phases in an aquifer is important to estimating plume migration rates and cleanup times.

This 3-D representation shows a high degree of spatial variation in the distribution of
“weakly held” uranium at a GJO test facility. The red and orange data points in the center
portion of some of the borings indicate a zone of high concentration of uranium.
 
Core samples collected at the drill site are labeled with description information,
placed in bags, and transported to a laboratory.

 

The U.S. Department of Energy Office of Legacy Management has a testing center at the Grand Junction site in Colorado, used to investigate the connection between contaminated groundwater and solids phases in the aquifer. A uranium plume was formed by water seepage from a legacy uranium test mill. Twenty-two spots were drilled on a 4-acre section of the uranium plume for direct access to the solids. All drillings passed through the alluvium and entered the bedrock beneath. Drilling cores were split into 366 samples, each representing 1 foot of rock layers.

A variety of tests were conducted. Uranium was drawn out of each sample using several different liquid extractants to help determine how tightly the uranium was bound to the sediment. Because uranium interacts differently with different minerals, it is useful to determine ties between elements and specific mineral phases. Mapping these associations helps scientists understand the potential for uptake and release of uranium. Petrography is used to establish the physical makeup and mineralogic properties of a sample. Fission tracks are used to associate the distribution of uranium with these properties. Column tests were used to characterize uranium release from the sediment samples in flowing water.

Information from these studies is still being analyzed.

Early findings include:

  • Uranium desorption—in part—is rate limited, as determined by varying groundwater flow rates.
  • Much of the uranium is associated with the fine-grained soil combination that binds and coats sedimentary grains. This matrix has a reddish-orange color that suggests the presence of iron oxides.

 

In this fission-track map (top) and its associated photomicrograph
(bottom), the arrows point to an area of oxidized matrix material
with a high density of fission tracks. The high-density fission tracks
indicate high uranium concentrations. The correlation of track
density to petrographic analysis suggests that uranium is sorbed
to iron-oxide-rich matrix material. This information can be used in
groundwater models.