J-Field Confined Aquifer Modeling

Argonne National Laboratory

Introduction

As described in the Overview, a model was designed to evaluate worst-case contaminant transport along an assumed direct flowpath from the confined aquifer beneath the Toxic Burn Pits (TBP) area of J-Field to a hypothetical pumping well. Details of the modeling effort are contained in the Full Document. Below are some of the key features of the model's design and film loops showing the results.

Model Setup and Assumptions

The current modeling effort focused on TCE as the main contaminant of concern. Modeling relies on an extremely conservative approach (i.e. promoting the spread of contaminants) in order to provide a worst-case scenario of contaminant transport to a hypothetical receptor. A 100-year period was simulated with results saved at 10-year intervals. Key conservative assumptions include:

• A continuous source of TCE equal to the maximum historical aqueous concentration in the confined aquifer of 1,600 ppb
• An assumed flowpath directly from J-Field to the nearest hypothetical pumping well about four miles away. The existence of such a flowpath is purely a conservative assumption, as complex hydrogeologic conditions are present between the confined, Pleistocene valley-fill aquifer beneath J-Field and the surrounding Cretaceous sand/clay units. Whether flow from beneath J-Field would ultimately travel east toward Kent Co., southwest toward Sparrows Point, or in any other direction loses importance in light of the setup and results of this modeling effort.
• Conservative retardation relying on parameter values at the low end of the range of literature values
• Assumed highly transmissive aquifer relying on hydraulic conductivity at the high end of measured values
• Remediation by natural attenuation is assumed, based on recent data collected by Target. First-order degradation is assumed, with the highest field-measured half-life from the literature (4.5 years) assumed for TCE.

Results

Color Contoured Results

This is an eye-catching visualization of how the conservative model setup results in the TCE plume extending away from J-Field, but stabilizing and decaying in the presence of anaerobic aquifer conditions. Note that at time = 0.00, the solution is at 10 years (3650 days), at time = 0.10, the solution is at 20 years (7300 days), etc.

Labeled Contour Results

These are the same results, but the labeled contours give far better resolution of the leading edge of the plume. Note that despite the extremely conservative aspects of the model, the 5 ppb contour stabilizes beneath the bay in 50 years.

Conclusions

Despite the conservative flowpath and input values, the modeling indicates that the TCE source at the TBP area does not pose a risk to well water users in the region. Although TCE produces decay products that are not included in the present modeling effort, these compounds are assumed to also be degraded in the anaerobic confined aquifer. Other natural attenuation processes that play a major role in attenuating an actual plume include dilution and dispersion. These factors were purposefully ignored by the model in order for the simulation to promote the growth of a plume toward a receptor in a worst-cast scenario.