CONCEPTUAL MODEL DEVELOPMENT AND IDENTIFICATION OF GROUNDWATER PATHWAYS FOR MONITORING SYSTEM DESIGN AT A NUCLEAR MATERIALS PROCESSING FACILITY USING 3D GEOSPATIAL MODELS

Knowledge of groundwater flow and transport pathways is essential for designing optimal monitoring systems, yet detailed pathway data are commonly not collected during initial site characterization and therefore not incorporated into early conceptual models. We present an approach for identifying site-specific groundwater pathways which involves construction of alternative conceptual 3D geohydrologic framework and property models using a former nuclear materials processing facility in Oklahoma as an example. The models formed a basis for monitoring system design at the site.

The geohydrologic framework model contains three hydrostratigrahic zones, corresponding to aquifer systems in which groundwater has been sampled since 1991. The terrace groundwater system (TGWS) aquifer (uppermost zone) is made up of terrace and alluvial deposits and a basal shale. A sandstone aquitard separates TGWS from the underlying shallow groundwater system (SGWS) aquifer (middle zone), composed of three shale units and two discontinuous sandstones. SGWS is separated from the underlying deep groundwater system (DGWS) aquifer (lowest zone) by another sandstone aquitard. Terrace and alluvial deposits form a perched aquifer and fractured shales are continuous water-bearing units.

TGWS and SGWS aquifers were contaminated during facility operation by spills and leaks of nitric acid processing solutions containing uranium ore constituents. Radioactive materials were also leached from discarded equipment and waste containers. Based on analysis of 3D models, site-specific groundwater pathways were identified. Lateral transport of uranium was indicated in TGWS along a buried erosional channel in bedrock trending south-southwest from the main processing building (MPB). Arsenic and nitrate greater than EPA MCLs also occur along this channel in TGWS and SGWS. Another pathway atop bedrock, trending west-northwest from the MPB, showed lateral migration of nitrate and arsenic in SGWS. Lateral movement of nitrate and arsenic in SGWS was indicated north and west from the largest holding pond at the site. As a result of delineation of these pathways, they were more carefully characterized by trenching and resistivity and new monitoring wells installed. The site conceptual model developed by hydrologic modelers was also modified.