Date of Award:

1995

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Tom Lachmar

Abstract

The migration of radionuclides and volatile chemicals through a thick sequence of unsaturated basalts and sedimentary interbeds has been documented in the Subsurface Disposal Area (SDA) at the Radioactive Waste Management Complex (RWMC) of the Idaho National Engineering Laboratory (INEL). The three mechanisms by which this migration may have occurred are seasonal infiltration from the ground surface within the SDA; flood events in 1962, 1969, and 1982; and the lateral movement of water from the spreading areas located nearby. It is currently thought that the contaminants migrated through the unsaturated sediments and basalt flows during the three flood events.

Transducers were used to monitor water level changes in seven wells: Two completed in a perched zone associated with an interbed at 110 feet (34 meters), and five completed in a perched zone associated with an interbed at 240 feet (73 meters). The data suggest that one of the wells completed in the perched zone associated with the interbed at 110 feet receives recharge from seasonal infiltration at the ground surface. This recharge occurs primarily from February to May. Snowmelt is the major source of water for infiltration.

One water sample was taken from this well, one water sample was taken from snowmelt at the surface within the SDA, and one water sample was taken from the Big Lost River, which is the source of water diverted into the spreading areas. The three water samples were analyzed for major ions, minor ions, and trace constituents. The water sample from the well was more similar chemically to the water sample from the snowmelt than to the water sample from the Big Lost River. The chemical data support the conclusion that the snowmelt is the source of recharge to the perched zone associated with the interbed at 110 feet.

The transducer data from the five wells completed in the perched zone associated with the interbed at 240 feet indicate that one well is dry, and one well has standing water that showed virtually no change in water level. Two of the remaining wells showed seasonal recharge, probably from precipitation. These two wells, and the remaining well, showed rapid water-level rises, and declines, shortly after 157 acre-feet (194,000 m3 ) of water were diverted into the spreading areas. The perched water body associated with the interbed at 240 feet probably receives recharge from both precipitation and from water diverted into the spreading areas.

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