Hanford Site History
The Hanford site encompasses 560 square miles
within the Columbia River Basin in southeastern Washington. Hanford originally
focused on plutonium production but has now shifted to environmental restoration
and managing wastes generated by past operations.
At the current time Hanford also conducts advanced research and development for advanced reactors, energy technologies and waste disposal practices.
Numerical information was available
in the form of boreholes which indicated the depth to the water table and
the measured mount of tritium contamination, measured in nano Curies. The
basic objective was to use the existing information to develop a preliminary
site characterization model of the tritium plume. This would provide a better
understanding of the distribution and location of the contaminant as well
as create a visual representation of the subsurface. In addition, such
characterization could provide an objective, defensible modeling method that
could be used in optimizing future sampling programs and set the initial
guidelines for remediation planning.
Site Characteristics
The Hanford site is in flat terrain underlain
by a complex stratigraphy of alluvium and gravel, with granite bedrock.
Topography slopes gradually southwest from 230 to 120 meters adjoining the
river. Below surface the water table gradient slopes gradually in the same
direction from elevations of 140 to 120 meters. This southeast gradient provides
natural drainage into the river, allowing contaminant plumes to migrate down
vertically into the water table and along gradient towards the river. Numerous
monitoring wells and boreholes have been drilled to sample the location and
extent of the tritium contaminants which varies from 0 to 700 nano
Curies.
Application of 3D Geoscience
Technology
A significant problem exists in
that the extent of contamination and the amount of volume to be remediated
dramatically effects the cost of remediation. Further complicating the process
is the distribution and quantity of sample information which requires
interpretive methods to extrapolate the spatial continuity of the
contaminant.
The LYNX modeling technology was a logical choice to best characterize subsurface since it is designed specifically to deal with such spatial problems. It integrates spatial data management, geostatistical techniques, 3D modeling, volumetrics, engineering and visualization in one facility, thereby allowing precise representation and estimation of complex subsurface problems.
Site Information and Basic
Objectives
Information made available for the study included
a topographic map indicating the surface elevations over the area and a site
plan showing the location of major features such as the river, the boundaries
of the plants and generating stations.
Numerical information was available in the form of boreholes which indicated the depth to the water table and the measured mount of tritium contamination, measured in nano Curies. The basic objective was to use the existing information to develop a preliminary site characterization model of the tritium plume. This would provide a better understanding of the distribution and location of the contaminant as well as create a visual representation of the subsurface. In addition, such characterization could provide an objective, defensible modeling method that could be used in optimizing future sampling programs and set the initial guidelines for remediation planning.
Geostatistical Analysis
Initial analysis was performed on the tritium
data using basic statistics, with the intention of determining data relationships
and spatial behavior. Results indicated distinct lognormality forcing the
need for log transformations. Upon transformation, the tritium data exhibited
excellent spatial continuity up to 1500 meters, allowing a spherical
semi-variogram model to be used. Further analysis of anisotropy was performed
by selecting several different directions confirming a similar range of
influence. A secondary, indicator analysis was carried out with three cut-off
values of 1, 20, 80 nano Curies. Semi-variogram modeling confirmed the spatial
continuity ranges of tritium and the extent of the tritium plume.
Contaminant Modeling
The spherical semi-variogram model was the basis
for Kriging techniques used to estimate the contaminate distribution throughout
the study area. 3D plumes were generated thereby allowing further inspection
of plume migration, extents and behavior, making it possible to cut plans
and sections at any orientation though the plume to detail contaminant ranges.
By using enhanced visualization, 3D isosurfaces could be scrutinized showing
the effect of plume limits depending on the threshold chosen.
Characterization Summary
The use of 3D Geoscience modeling
has been used to effectively characterize the site and to provide the basis
to further planning of remediation and sampling. Although not performed in
this exercise, it is noteworthy that the development of the model not only
facilitates the quantification of volumes and a measure of the certainty
at any particular threshold or location, but it also provides the basis for
cost effective, defensible sample planning and control by being able to display
areas which have poor confidences and require additional sampling.
Finally, the development of a visual 3D model also provides an explicit visual picture of extent, degree and uncertainty, a vital tool that facilitates better analysis, understanding and expedient cleanup.