Under the U.S. Navy’s Environmental Pollution Abatement Ashore Program, and in conjunction with the Naval Facilities Engineering Service Center and Encapco Technologies LLC, the Desert Research Institute (DRI) tested the effectiveness of an organic-based emulsion for stabilizing and chemically binding soil particles and contaminants. The emulsion, developed by Encapco Technologies LLC, and was sprayed onto test plots at the Nevada Test Site as well as the Yuma Proving Ground in Arizona.
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| View of the PI-SWERL blade (a) and how the instrument would be placed on the ground for taking measurements (b). |
Nevada Test Site
At the Nevada Test Site (NTS), the emulsion was tested for its effectiveness in preventing wind suspension of soils contaminated with 239+240Pu from past nuclear testing. Pu soil contamination is often associated with respirable size soil particles (PM 10 or less) and poses an inhalation risk. The Portable in- situ wind erosion laboratory (PI-SWERL), developed and patented at DRI (Etyemezian et al., 2003), was a key field instrument in testing the effectiveness of the emulsion. The PI-SWERL uses a rotating circular blade inside the cylindrical instrument placed flues with the ground to induce wind shear at the soil surface and simulate stresses caused by winds.
The PI-SWERL provides an index of erodibility through the relationship between the revolutions per minute (RPMs) to PM 10 emissions. This measurement, using an empirical calibration, can be translated into a shear stress or friction velocity vs. PM 10 emissions curve, similar to those obtained from straight-line wind tunnel measurements. An advantage of the PI-SWERL is the relative speed with which measurements can be performed. An average test during this study required less than 15 minutes.
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Comparison of dust emissions from an emulsion treated versus untreated plot at the Nevada Test Site. (Click on image to enlarge ) |
At the NTS test plots, the effectiveness of different concentrations of the emulsion in preventing PM 10 emissions was tested 3, 20, 40, and 60 weeks after application of the emulsion in May 2004 using the PI-SWERL. PM 10 emissions on controls plots were measured too. During each test, the PI-SWERL RPM’s were gradually increased to simulate increasing wind speed. A meteorological tower was also set up at the “Smoky Fan” site to collect data on temperature, wind, and precipitation. At friction velocities corresponding nominally to sustained wind speeds around 6.5 m/s and higher, the Encapco application suppressed erodible dust emissions compared to the untreated plots for as long as one year after application, although the most significant suppression occurred in the first four months.
Yuma Proving Ground
In applying a treatment agent over large areas, stabilization of contaminants cannot be the only consideration. For example, if an emulsion were to decrease soil hydraulic conductivity (K s) dramatically, negative effects could include 1) decreases in water infiltration that could prevent the germination of seeds, decreasing the natural ability of vegetation to stabilize soils; and 2) rapid runoff that could erode, for example, desert pavements that are critical landscape features for soil stability in many arid lands in the southwestern U.S. At the Yuma Proving Ground in Arizona (YPG), the effects of the emulsion on hydraulic properties of the soil were a major focus of the research.
Why was YPG chosen as a study site? At YPG as well as other U.S. Department of Defense facilities in the southwestern U.S., depleted uranium (DU) oxide used in hardened penetrator munitions testing exists as a surface soil contaminant. Depleted uranium is composed primarily of a 238U oxide. It is both radioactive and chemically toxic. Stabilization of DU using an organic emulsion with a chelating agent could reduce the transport of DU from air and water pathways, the latter including overland flow and transport of the DU in both soluble and insoluble forms.
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“Disturbed” test plot on the desert pavement at the Yuma Proving Ground. |
Similar to the NTS, the studies at YPG were performed on test plots where the emulsion was applied in different concentrations. Experiments were conducted at two types of surfaces at YPG: an “older,” desert pavement-armored surface which sloped down to a desert wash with adjoining, “younger” alluvial surfaces. On the “old” surface, 5 x 5 meter plots were established that included controls plots, “disturbed” and undisturbed plots. The old surface was disturbed by raking the desert pavement off the plots. Similar emulsion treatment and control plots, disturbed and undisturbed, were also established on the “younger” alluvial surface along the wash.
To measure changes in hydraulic conductivity, tension infiltrometers (TI) tests were conducted. All TIs were equipped with pressure transducers and dataloggers so that water intake rates could be monitored every 15 seconds. Two analytical methods were used to solve for K s. The semi-empirical, nonlinear least-squares regression routine for Wooding’s Analysis and a numerical inversion method using the HYDRUS-2D model. Portable rainfall simulators were also used to evaluate how the emulsion affected runoff at the sites.
Properties of the soil were measured in October 2004 prior to the application of the emulsion in December of the same year. Researchers returned in March and June 2005, and January 2006 to compare result to the October 2004 measurements. All emulsion-treated plots on the old and young surfaces showed significant decreases in K s after the emulsion was applied. Nonetheless, even with the lower Ks, no significant differences were seen on surface runoff potential or the soil water conditions like water content or soil temperature. The field results indicated that the emulsion did hve a significant short-term impact on the structure of the plant communities, but no long-term effects were seen. The overall impact of the emulsion on hydraulic properties diminished after 6 months, with the decrease in K s appearing to have persisted slightly longer on the young surface. However, just as at the NTS, higher-than-average precipitation occurred during the study period and may have reduced the length of time when the emulsion affected the soil hydraulic properties.
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Changes in Ks over time after the emulsion was applied on the young and old surface at the Yuma Proving Ground. By June of 2005, values had returned to normal conditions. |
Future Use of Emulsions for Soil Stabilization
While the emulsion showed promise at the NTS in preventing suspension of contaminated soil at reducing inhalation risk, the effectiveness was about the same for both the more concentrated and the more dilute emulsion. Understanding at what concentrations it remains effective could help to make it more cost effective to use. Testing the emulsion at lower concentrations may also help to determine if dilute forms of it can stabilize contaminants such as DU and Pu and minimize impacts on soil hydraulic properties at the same time. The time of the year when an emulsion is applied may need to be considered as well, such as avoiding periods of the year when water infiltration is important for germination of plant seedlings.
New Research: Contaminated Soil and Fire
Increases in fire frequency because of invasive plant species and possibly climate change are now well-documented and visible phenomena in much of the western United States. However, relatively little research has been conducted on the consequences of increased fire on the stability of soil contamination. Possible implications are more frequent suspension of contaminants because of reduced plant cover as well as changes in soil properties (e.g., breakdown of contaminant particles into smaller, more respirable sizes). Coupled with fires is a greater potential for water erosion and transport of contaminated soil after a site burns. Both of these phenomena could be important considerations in long-term stewardship of contaminated sites. This is a focus of new research by DRI beginning in 2006.
References
Etyemezian, V., S. Ahonen, J. Gillies, K. Kuhns, H. Moosmuller, G. Nikolich and M. Pitchford. 2003. PI-SWERL: A New Technique for Measuring Wind Blown Dust Emission Potential. NARSTO Workshop on Innovative Methods for Emission-Inventory Development and Evaluation, Austin, TX.
Young, M.H. T.G. Caldwell, D.G. Meadows, D.S. Shafer, S Zitzer, J.J. Miller, E.V. McDonald, J.O. Goreham, V. Etyemezian, G. Nikolich. 2006. DRI Technical Report: Results of Uranium Oxide Soil Stabilization Study, Yuma Proving Ground, AZ.