--> Stratigraphic and Sedimentologic Controls on Groundwater Flow in Late Neogene Sediments at the Hanford Site, South-Central Washington, by K. A. Lindsey, A. G. Law, K. R. Fecht, and S. P. Reidel; #90986 (1994).
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Abstract: Stratigraphic and Sedimentologic Controls on Groundwater Flow in Late Neogene Sediments at the Hanford Site, South-Central Washington

Kevin A. Lindsey, Al G. Law, Karl R. Fecht, S. P. Reidel

The U.S. Department of Energy Hanford Site is situated in the Pasco basin, a structural basin formed in Miocene continental Previous HitfloodNext Hit basalts and intercalated continental sediments. Sediments overlying the basalt comprise the suprabasalt geohydrologic system at the Hanford Site. This system contains radioactive and chemical contamination from previous waste-disposal activities. The saturated part of the suprabasalt geohydrologic system is dominated by late Miocene to middle Pliocene alluvial-lacustrine deposits of the Ringold Formation. Pleistocene cataclysmic Previous HitfloodNext Hit deposits of the Hanford formation and related Pliocene-Pleistocene alluvium dominate the unsaturated part of this system.

Groundwater flow generally is controlled by the textural characteristics and distribution of gravel-rich facies deposited in Ringold fluvial systems. Paleogeographic reconstructions indicate these fluvial systems were strongly influenced by syndepositional deformation. Consequently, fluvial tracts follow the structural grain of the Pasco basin, forming preferred flow paths that typically follow synclinal axes. Basinwide lacustrine fines and paleosol systems separate these tracts into a series of semi-confined aquifer zones that generally are isolated from near-surface recharge and contamination.

Hanford formation deposits form lithologic zones that fine away from Previous HitfloodNext Hit channelways. Stratigraphic-sedimentologic heterogeneities within and between these zones result in complex vadose zone flow paths marked by rapid downwards movement and variable lateral spreading. Significant groundwater mounds and perched water is uncommon in open-framework gravels deposited in high-energy main Previous HitfloodNext Hit tracts. Perched water is common in silt-rich Hanford formation strata deposited in Previous HitfloodTop backwaters and Pliocene-Pleistocene alluvium. However, lateral spreading is limited by lateral discontinuities such as scour cut-and-fill structures. Numerous clastic dikes also provide vertical conduits deeper into the subsurface.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994