High resolution imaging of vadose zone transport using surface and crosswell ground penetrating radar methods
To effectively clean up many contaminated sites there is a need for information on heterogeneities at scales ranging from one centimeter to tens of meters, as these features can alter contaminant transport significantly. At the Department of Energy's Hanford, Washington site heterogeneities of interest can range from localized phenomena such as silt or gravel lenses, fractures, clastic dikes, to large-scale lithologic discontinuities. In the vadose zone it is critical to understand the parameters controlling flow. These features have been suspected of leading to funneling and fingering, additional physical mechanisms that could alter and possibly accelerate the transport of contaminants to underlying groundwater. For example, it has been observed from the studies to date that over relatively short distances there are heterogeneities in the physical structure of the porous medium and structural differences between repacked soil cores and the field site from which the materials initially came (Raymond and Shdo, 1966). Analysis of cores taken from the vadose zone (i.e., soil surface to water table) has been useful in identifying localized zones of contamination. Unfortunately, these analyses are sparse (limited to a few boreholes) and extremely expensive. The high levels of radioactivity at many of the contaminated sites increase drilling and sample costs and analysis time. Cost of drilling and core analysis for the SX tank farm has exceeded $1M per borehole (50 meter deep) for sampling. The inability to track highly mobile species through the vadose zone highlights an important need: the need for methods to describe the complete vadose zone plume and to determine processes controlling accelerated contamination of groundwater at Hanford. A combination of surface and crosswell (i.e. borehole) geophysical measurements is one means to provide this information. The main questions addressed with the radar methods in this study are: (1) What parts of the vadose zone-groundwater system control flow geometry? (2) What physical properties or mechanisms control flow and transport in unconsolidated soils of the vadose zone? (3) What is the optimum suite of field tests to provide information for predicting flow and transport behavior? (4) How can the information obtained during site characterization be used for building confidence in predictive numerical models? Fully developed, application of geophysics should enable location of contaminant distributions. Questions addressed in this study were the sensitivity, resolution, and accuracy of the geophysical methods in order to derive the spatial and temporal distribution of properties controlling transport and contaminant distribution between and away from boreholes and the surface. Implicit in this activity is that geophysical methods will be used to extrapolate and extend measurements made at the point scale to the volumetric scale. Overall there are two broad hypotheses being addressed in the geophysical work: (1) Geophysical methods can identify physical and chemical heterogeneity controlling contaminant transport at a meaningful scale. (2) Geophysical methods have the sensitivity to directly or indirectly detect the location of introduced fluids and/or contaminants at relevant concentrations (i.e. the subsurface has been altered from its natural state enough to create anomalies that can be detected in terms of a combination of mechanical, electrical, and thermal effects).
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director, Office of Science. Biological and Environmental Research (US)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 815524
- Report Number(s):
- LBNL-53151; R&D Project: 467201; TRN: US200319%%319
- Resource Relation:
- Other Information: PBD: 5 Nov 2002
- Country of Publication:
- United States
- Language:
- English
Similar Records
High-Resolution Characterization of Comingled Contaminants in the Deep Vadose Zone, Hanford Site, Washington - 17387
Vadose zone transport field study: Detailed test plan for simulated leak tests