Tritium Transport from a Solid Radioactive Waste Burial Ground Whose Heterogeneous Conductivity Field is Based on Site Lithologic Data

L. L. Hamm, G. P. Flach, M. K Harris, P. A. Thayer, and J. S. Haselow

Variability in hydraulic conductivity can dominate regional scale groundwater (GW) flow patterns and dispersive transport. An approach for generating heterogeneous conductivity fields from sediment lithologic descriptions is employed. The approach involves creating a fine-scale representation of mud (silt+clay) fraction using "stratified" interpolation that is translated into horizontal and vertical conductivities using direct correlations. The fine scale conductivity fields are then scaled-up to a coarser grid for use in GW modeling. The approach is demonstrated using a 3-D model for tritium migration from a Savannah River Site solid radioactive waste burial ground. The site area contains a significant number of core logs for defining lithology, has complex vertical heterogeneity in the upper aquifer units, numerous monitoring wells for hydraulic heads, and several tritium sources (ideal tracer) for assessing the model's accuracy. This study demonstrates that detailed lithologic information can be successfully incorporated directly into the models. A more defensible method of addressing aquifer heterogeneities (than the traditional layered approach) is achieved because the model more closely reflects the actual lithologic data. For the burial ground field-observed preferential pathways for tritium transport occur without the need to resort to the artificial creation of locally high conductivity zones. A comparison to measurements is presented for tritium breakthrough at the down-gradient seepage face of Fourmile Branch Creek.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California