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Subsurface Biogeochemical Research Program

(SC GG 5.21.1)
Develop predictive model for contaminant transport that incorporates complex biology, hydrology, and chemistry of the subsurface. Validate model through field

Fourth Quarter Results


From: Jack Parker [mailto:parkerjc@ornl.gov]
Sent: Tuesday, September 19, 2006 3:52 PM
To: Kuperberg, Michael
Cc: Gary Jacobs; Scott Brooks; zhangf@ornl.gov; tim.scheibe@pnl.gov; yilin.fang@pnl.gov; Dave Watson
Subject: RE: Q4 quarter measure for ERSD (now CESD)

Mike,
Here is the final Milestone report.
Regards, Jack

New information on biogeochemistry, groundwater and subsurface media from the Oak Ridge Field Research Center (FRC) was utilized to update site-wide and plot-scale 3-D flow and transport models as reported for the second quarter of FY06. The present report documents comparisons between field data from the FRC and predictions from the updated models. The FRC is a focal point for SBR field research on natural and stimulated biologically-mediated attenuation of metals and radionuclides. Information from these studies on the interactions among biological, chemical and physical processes in a real-world setting with complex hydrogeology and contaminant characteristics promises to significantly improve DOE’s ability to effectively manage legacy waste sites. Computer models are important tools for interpreting field and laboratory data from this complex system to understand the nature of process interactions, to help guide ongoing and future research efforts, and to enable predictions of plume-scale behavior in response to various remediation/management strategies. Field-scale modeling efforts are being conducted by ORNL, PNNL, Oregon State University and Stanford University researchers on experimental plots within the FRC and at a larger scale that encompasses all of the field plots to better understand their interactions and large-scale behavior. These efforts have utilized the code HYDROGEOCHEM Version 5, which simulates three-dimensional transient density-dependent, fully-anisotropic saturated and unsaturated water flow, dissolved transport, and complex equilibrium- kinetically-limited biogeochemical reactions. A site-wide 3-D flow and transport model was developed encompassing an area of approximately 280 acres that includes FRC Areas 1, 2 and 3, the former S3 Ponds and the Bear Creek watershed from its head waters to the NT2 tributary. Refinements in the model were completed to incorporate new data and improvements in the conceptual model of the site. The refinements include:

  • Modify the gravel fill zone that extends from slightly west of Area 3 to Bear Creek to incorporate information from new soil borings that indicate the fill directly overlays saprolite locally and is more extensive than previously understood. This may significantly affect shallow groundwater flow and contaminant transport towards Bear Creek.

  • Modify the areal extent of the rock-saprolite transition zone based on new drilling results and geophysical testing, which indicate the zone is not as extensive as previously assumed.

  • Incorporate the permeable barrier trench in FRC Area 2 in the model.

  • Add a “potential high permeability zone” in the bedrock inferred from recent geophysical testing.

Locations of the refined features are shown in Figure 1. The refined site-wide FRC model is discretized into 8 layers of 23,967 elements and 10 layers of 13,680 nodes. Four types of bedrock, including a “potential high permeability zone” identified by geophysical testing, are overlain by saprolite, gravel fill, a permeable trench and a rock saprolite “transition” zone (Figure 2). Nonlinear optimization was performed to recalibrate a steady-state groundwater flow model to stream gauging data and water levels from 74 wells. Rock and saprolite were modeled as anisotropic media with a maximum permeability along strike, minimum permeability in the cross-bed direction and intermediate permeability along the dip direction. Field-scale dispersivities and effective porosities were manually calibrated to measured nitrate concentrations from 19 wells within and near the dissolved plume by simulating non-reactive nitrate plume evolution from the S3 Ponds considering density-dependent flow. Sensitivity of groundwater flow model predictions to refinements implemented in model in comparison to measured water levels are illustrated in Figure 3 and final model comparisons with observed water levels and groundwater nitrate concentrations are shown in Figure 4. Measured and simulated dissolved phase concentrations for a north-south cross-section through the former S3 Ponds (Figure 5) indicate that the model is able to accurately predict vertical plume migration driven by large dissolved-phase density gradients. A high resolution model was also developed for Area 2 to help design and interpret field-scale experiments in this area (Figure 6). The 800 m3 model domain encompasses the disturbed fill, coarse gravel, and intact saprolite zones. The model considers pulsed injection of tracers and ethanol in three wells (FW213, FW212, FW214) and simulates 94 chemical species, 8 biomass populations, 58 equilibrium reactions, 77 kinetic reactions, and 37 terminal electron accepting reactions. Reactions and reaction coefficients and aquifer properties were based on data from field and laboratory studies. For the experimental design, only the first seven days of system response were simulated, under the assumption that the injection strategy could and would be modified following initial observations if necessary. Field measurements of bromide, nitrate, sulfate and ethanol in wells MLS-A, MLS-B, FW216 which are 2.5 m downstream of the injection location and in FW202 located 7.5 m downgradient are generally well-predicted by the model, indicating that the timing and rates of utilization of the various terminal electron acceptors are reasonably approximated by the models.  Model deviations occur for bromide at MLSA-4 and MLSA-5, suggesting that the flow pathway is not accurately described. This likely reflects uncertainty in the aquifer permeability distribution rather than improper modeling of the reaction system and rates. The injection system was operated as initially designed for approximately 11 months, and we are now performing longer-term simulations for comparison with field observations.  Figure 8 shows results of simulations of the first two months of biostimulation compared with field data, for sampling location MLSB-4.  The most interesting feature of this plot is that the simulation model predicts a loss of reductive capacity shortly after the end of the first week (about 200 hours), leading to a simulated rebound of nitrate, sulfate, and uranium concentrations.  In the field, however, nitrate levels remained negligible, and sulfate and uranium concentrations continued to decrease through the two-month period and longer (with the exception of some short periods of high rainfall during which oxic water entered the system).  This significant difference clearly demonstrates that the model does not account for one or more important factors that lead to the feasibility of long-term reduction under low-level electron donor delivery.   This in turn points out an area in which greater understanding of physical, biological, and geochemical processes is needed, illustrating the potential interplay between modeling and experimentation in developing scientific knowledge.Prepared by: Jack Parker (ORNL) with contributions from Tim Scheibe (PNNL), Fan Zhang (ORNL), Yilin Fang (PNNL), Wiwat Kamolpornwijit (PNNL), Eric Roden (Univ. Wisconsin), Dave Watson (ORNL), and Scott Brooks (ORNL) 

figure 1
Figure 1.  Site-wide model refinements implemented in FY06.
figure 2
Figure 2. Site-wide model domain discretization and distribution of material types.
figure 3
Figure 3.  Sensitivity of groundwater flow model predictions to refinements implemented in model in comparison to measured water levels.
figure 4
Figure 4. Comparison of field observations and refined model predictions of total water head (left) and nitrate concentrations (right) for the site-wide model.
figure 5
Figure 5. Simulated (top) versus measured (bottom) dissolved nitrate concentrations for site-wide model in vertical north-south vertical section through the former S3 Ponds.
figure 6
Figure 6. Model domain for high-resolution Area 2 model.
figure 7
Figure 7. Comparisons of Area 2 short-term (one week) model predictions with observations.
figure 8
Figure 8. Comparison of Area 2 long-term (2 months) model predictions with observations.

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