Three-Dimensional Groundwater Models of the 300 Area at the Hanford Site, Washington State

Abstract

iii Executive SummaryThe purpose of this research is to develop and maintain numerical groundwater flow and transport models that can be used to refine the conceptual site model for groundwater beneath the 300 Area, and to assist in evaluating alternative remediation technologies focused on the 300 Area uranium plume. Groundwater flow rates and directions in the 300 Area are very dynamic because of the high hydraulic conductivities, along with the large daily, weekly, and seasonal fluctuations in the Columbia River stage. Quantifying the dynamics of groundwater flow and transport in the 300 Area aquifer will help understand the significant seasonal variability of uranium plume concentrations seen in biannual groundwater monitoring, and will help evaluate remediation options. Groundwater flow rates are very high in the upper portion of the 300 Area unconfined aquifer (within the Hanford formation), with velocities up to 10 to 15 m/d (35 to 50 ft/d) based on a tracer test and limited plume-migration data. Variability in the groundwater-flow directions is apparent from analysis of hourly and subhourly automated water-level measurements from monitoring networks established in the 300 Area. Generalized flow directions in the area between the north and south process ponds are toward the east to south, with the directions changing toward the south and west during periods of increases in the river stage (daily and seasonal).High-resolution water level and river stage data were required to simulate the dynamics of the 300 Area aquifer. Two scales of groundwater flow and transport models were developed based on the availability of high-resolution water-level monitoring data. A larger-domain model was developed that includes the 300 Area and extends north and south using data from the early 1990s water-level monitoring network. A smaller domain model was developed for a portion of the large scale model domain in the north of the 300 Area that used water-level data from another smaller monitoring well network that was established in 2004. These models focus on the highly permeable upper portion of the unconfined aquifer within the Hanford formation that has hydraulic conductivity values 2 to 3 orders of magnitude higher than the underlying Ringold Formation aquifers. These models simulate saturated and unsaturated groundwater flow and transport with the STOMP code, which was developed at Pacific Northwest National Laboratory. 1The hydrostratigraphy, topography, and bathymetry for the three-dimensional models used a consistent framework using EarthVision  software.The model domains include the lower portion of the vadose zone to encompass the range of river stage and water- The hydrostratigraphic units were determined from previously published interpretations of the 300 Area, along with data from additional wells installed since those studies. A reanalysis of some of the older geologic unit picks from well logs in the area, along with using geophysical logs, was conducted based on the detailed knowledge gained from the 300 ...

Coupling a three-dimensional subsurface flow and transport model with a land surface model to simulate stream–aquifer–land interactions (CP v1.0)

“…The Hanford Formation gravel and sand, deposited by glacial outburst floods at the end of the Pleistocene (Bjornstad, 2007), has a high average hydraulic conductivity at ∼ 3100 m day −1 (Williams et al, 2008). The fluvial deposits of the Ringold Formation have much lower hydraulic conductivity than those in the Hanford Formation but are still relatively conductive at 36 m day −1 (Williams et al, 2008). Fine-grained lacustrine Ringold silt has a much lower estimated hydraulic conductivity of 1 m day −1 .…”

“…Fine-grained lacustrine Ringold silt has a much lower estimated hydraulic conductivity of 1 m day −1 . The hydraulic conductivity of recent alluvium lining the river channel is low relative to the Hanford Formation, which tends to dampen the response of water table elevation in wells near the river when changes occur in river stage (Hammond et al, 2011;Williams et al, 2008). Overall, the Columbia River through the Hanford Reach is a prime example of a hyporheic corridor with an extensive floodplain aquifer.…”

“…Coupled with dynamic river stage variations, the resulting system is characterized by stage-driven intrusion and retreat of river water into the adjacent unconfined aquifer system. During high-stage spring runoff events, river water has been detected in monitoring wells nearly 400 m from the shoreline (Williams et al, 2008). During baseline, low-stage conditions (OctoberFebruary), the Columbia River is a gaining stream, and the aquifer pore space is occupied by groundwater.…”

“…Note that only the soil moisture and soil hydraulic properties within the top 3.8 m are transferred from PFLOTRAN to CLM4.5 to allow simulations of infiltration, evaporation, and transpiration in the next time step, as the CLM4.5 subsurface domain is limited to 3.8 m and cannot currently be easily modified. The hydrogeological properties of the Hanford and Ringold materials (Table 2) were taken from Williams et al (2008). The unsaturated hydraulic conductivity in PFLTORAN simulations was computed using the Van Genuchten water retention function (van Genuchten, 1980) and the Burdine permeability relationship (Burdine, 1953).…”

Status Report for Remediation Decision Support Project, Task 1, Activity 1.B ? Physical and Hydraulic Properties Database and Interpretation

“…Examples of some uses of these types of data are provided by Oostrom et al (2006), and Williams et al (2008).…”