THREE-DIMENSIONAL TRANSIENT INVERSE CALIBRATION OF A SITE-WIDE GROUNDWATER FLOW MODEL USING A FACIES-BASED APPROACH AT THE HANFORD SITE, SOUTHEAST WASHINGTON

A three-dimensional transient inverse calibration has been performed using UCODE, a universal inverse modeling code developed jointly by the U.S. Geological Survey and the International Groundwater Modeling Center at the Colorado School of Mines. The transient inverse calibration evaluates hydraulic impacts to the unconfined aquifer system of Hanford waste management operations between 1943 and present and uses approximately 70,000 water level measurements made at the Hanford Site since the mid-1940s. This effort is part of an overall effort to strengthen the technical defensibility of the groundwater flow and transport model at the Hanford Site and to develop a more robust capability to incorporate uncertainty related to alternative conceptual models into future predictions.

The focus of the current inverse modeling effort has been to implement and evaluate a facies-based approach for representing the hydraulic conductivity distributions in the Hanford and middle Ringold formations, two major hydrostratigraphic units within the unconfined aquifer system. Other modifications that were incorporated into this effort include 1) improved approaches for handling run-on recharge from upland areas (Cold Creek, Dry Creek, Rattlesnake Springs) based on watershed modeling results, 2) improved approach for representing artificial discharges from site operations, and 3) minor changes to the geologic conceptual model.

An overall comparison of residual error statistics generally indicates an improvement in model fit for the facies-based model relative to a previously inversed model. The current model represents a first attempt to fully incorporate the facies-based approach for representing the hydrogeologic structure of the model, and further refinement of this distribution. We anticipate additional