INVESTIGATING DEPOSITIONAL VARIATION WITHIN THE RINGOLD FORMATION AT THE HANFORD SITE

The Miocene-Pliocene-aged Ringold Formation is a major stratigraphic unit within the unconfined suprabasalt aquifer at the U.S. Department of Energy’s Hanford Site in south-central Washington State. The Ringold Formation at the Hanford Site was deposited by the ancestral Columbia River system, and consists mainly of unconsolidated to cemented sand and granule-to-cobble-sized gravel with lesser quantities of clay and silt.

The entire Hanford Site is located within the Pasco Basin, which is divided by the Gable Mountain-Gable Butte anticline to form two smaller synclinal basins within the Site — the Cold Creek Syncline (to the south) and the Wahluke Syncline (to the north). There is some speculation that the Ringold-age fluvial and lacustrine environments in the Cold Creek Syncline may have been different from the Wahluke Syncline, resulting in somewhat different sedimentary sequences. Groundwater flow is strongly influenced by fine-grained clay and silt units, such as the lower mud subunit of the Wooded Island member recognized within the Cold Creek Syncline, and the so-called upper mud identified in portions of the Wahluke Syncline. Understanding the distribution and stratigraphic relationships between these fine-grained units is important in understanding the geologic control on groundwater flow and contaminant transport.

Detailed examination of drill logs, geologist logs, and borehole geophysical logs from 20 boreholes along a north-trending transect crossing portions of both synclines has failed to provide substantive evidence that fine-grained sediments in each syncline are representative of distinct depositional environments. Instead, as Lindsey (1995) indicates, these fine-grained units may be stratigraphically linked and representative of regional changes in the depositional environment.