Monitoring of groundwater across the Hanford Site (Figure 1.1) has characterized a number of radioactive contaminant plumes that emanate from various operational areas (Hartman and Dresel 1997). The most widespread plumes are from groundwater contaminated by tritium and iodine-129. Smaller plumes of strontium-90, technetium-99, and plutonium contain concentrations exceeding U.S. Environmental Protection Agency (EPA) and State of Washington interim drinking-water standards (DWS). Uranium concentrations are also found at levels greater than those in the proposed DWS. In recent years, concentrations of cesium-137 and cobalt-60 have also been found at a few locations at levels that equal or exceed the DWS.
The Hanford Groundwater Project (HGWP), managed by Pacific Northwest National Laboratory, is responsible for monitoring and assessing the movement of contaminants in groundwater at the Hanford Site. As part of this responsibility, the HGWP has supported the development of groundwater models to assist in identifying and quantifying existing, emerging, or potential groundwater quality problems and in assessing the potential for contaminants to migrate from the Hanford Site to the accessible environment through the groundwater pathway. A three-dimensional numerical model of groundwater flow and contaminant transport has been developed for the unconfined aquifer system. Numerical models of the groundwater flow system were previously developed and served as important tools for estimating future conditions and predicting the movement of contaminants in the unconfined aquifer. The HGWP supported the development and maintenance of a two-dimensional model (Wurstner and Devary 1993) and, more recently, a three-dimensional model (Wurstner et al. 1995).
This report describes improvements to the three-dimensional model during fiscal year (FY) 1996 and 1997 and the model's application in modeling flow and transport of selected contaminant plumes in the unconfined aquifer. The applications presented in this report consist of predicted changes in transient-flow conditions in the unconfined aquifer to the year 4000. These predicted transient-flow conditions provided the hydrologic basis for simulating migration of selected contaminants. Forecasted contaminant migration includes the transport of the tritium plume resulting from future operations of the State-Approved Land Disposal Site (SALDS) and the transport of the existing tritium, iodine-129, technetium-99, uranium, and strontium-90 plumes originating from the 200 Areas for the Hanford Site Composite Analysis.
Two-dimensional models have been used extensively at the Hanford Site and are generally adequate for predicting aquifer head changes and groundwater flow paths and travel times. The three-dimensional model described in this report was developed 1) to assist the HGWP in interpreting monitoring data; 2) to investigate contaminant mass transport issues and evaluate the future movement of contaminant plumes; and 3) to identify and quantify potential groundwater quality problems for on-site and off-site use.
The scope of activities associated with developing and applying the three-dimensional model in FY 1996 and 1997 was to
The inverse calibration procedure described in Wurstner et al. (1995) resulted in hydraulic conductivities near Gable Mountain Pond that were too high because the inverse calibration was not properly constrained with respect to observed transmissivities. Part of the effort during FY 1996 and 1997 was to recalibrate the site-wide flow model while constraining the inverse model with observed transmissivity data.
The two- and three-dimensional models were developed using the Coupled Fluid, Energy, and Solute Transport (CFEST) code (Gupta et al. 1987). Wurstner et al. (1995) describes the capabilities and approach used in the CFEST code and the code's selection for the project. The supercomputer version (CFEST-SC), developed to run on all major Unix work stations (Cole et al. 1988), was used for all flow and transport modeling prior to FY 1997. In FY1997, the refinement of this three-dimensional flow model and its application to contaminant transport in selected contaminant plumes continued with an updated version of the CFEST code called CFEST96 (Gupta1997).
Output from CFEST was graphically displayed using the Arc/Info® (2) geographic information system (GIS). The Arc/Info® GIS package was also used to store fundamental hydrogeologic data and information used to represent the three-dimensional conceptual model. The Geological Finite-Element Synthesis Tool (GEOFEST) described in Foley et al. (1995) was used to construct the three-dimensional numerical model from the GIS layers.
This report is organized as follows. Section 2.0 briefly describes other site-wide
modeling efforts that have been conducted on the Hanford Site and that are relevant
to the current model development. Section 3.0 describes the hydrogeologic and
transport framework used in the modeling. Section 4.0 describes the calibration
and application of the three-dimensional flow model. Section 5.0 describes the
application of the three-dimensional transport model in predicting future transport
of the tritium, technetium, uranium, iodine-129, and strontium-90 plumes originating
from the 200 Areas. Section 6.0 discusses the results obtained from this
modeling effort. Appendix A provides a table of artificial recharge volumes
and spring discharges used as input for the three-dimensional model. The second
table in Appendix A provides input concentrations for simulating tritium plume
transport.
