Since 1998 we have been investigating water and potential contaminant transport in sedimentary interbeds -- relatively thin (about 2 m) layers between much thicker layers of fractured basalt. These sedimentary interbeds have been considered the major impediment to contaminant transport between the surface and the aquifer 200 m below. Our studies include core sampling, lab measurements, numerical modeling, and transient field tests in boreholes, as well as a closely related effort: the Spreading Area Tracer Test. The main objectives are to evaluate barrier effects, and horizontal, vertical, and preferential flow possibilities in the unsaturated zone at 30 m depth and deeper. Our main collaborators are Joe Rousseau, LeRoy Knobel, and Brian Twining of the USGS INL Project.
The interbeds, depending on the magnitude and prevalence of critical hydraulic processes within them, may have the effect of (1) accelerating contaminant transport or retarding it, (2) concentrating contaminants or diluting them, and (3) determining whether the dominant movement is vertical or horizontal.
In 1999, with Larry Matson conducting the drilling and sampling operations, we successfully recovered minimally disturbed core samples from a sedimentary interbed 40 to 50 m deep near the Radioactive Waste Management Complex (RWMC). We measured field water contents and matric pressures on these cores using new tensiometers and recoring techniques. The apparatus and procedures for these measurements were developed in our Menlo Park lab, and applied to the freshly obtained samples in the USGS Core Library Lab on the INL site.
On 18 core samples recovered near the RWMC we have made a complete set of retention, saturated and unsaturated conductivity (K), and porosity measurements. Much of this work applies the Steady State Centrifuge (SSC) method. On a total of 40 samples we measured particle-size distributions and other bulk media properties useful in characterizing this interbed. The results clearly demonstrate a layered structure within the interbed that is likely to substantially affect water and contaminant transport.
Since 1999 we have been investigating the nature of the sedimentary intbeds near the Idaho Nuclear Technology and Engineering Center (INTEC), northeast of the RWMC. The subsurface at this location is much more complex than that near the RWMC with many more basalt flow units and thin sedimentary interbeds. In 2001, a new field-scale facility called the Vadose Zone Research Park was established near the INTEC for the study of the movement of water and solutes through the unsaturated zone.
The data from this study are being used in the development of a property-transfer model which will allow the prediction of hydraulic properties based on more easily-measured properties. The results thus far are promising for the possibility of developing INL-specific correlations of hydraulic properties and particle size distributions. This may permit particle-size measurements to suffice in other areas of the INL where interbed hydraulic properties are important but previously unknown.
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