Conceptual groundwater transport model of Shoal test area.

The purpose of modeling the Shoal underground nuclear test area was to establish a contaminant boundary by characterizing groundwater flow and contaminant transport through three-dimensional numeric models based on site-specific hydrologic data collected during the various studies. The contaminant boundary is a model-predicted perimeter defining the eventual extent of radionuclide transport in groundwater beneath the site over the next 1000 years. DOE and NDEP will use the modeling results as the basis for managing the Shoal test area.

Data from the various studies supported the fundamental conceptual model of groundwater flowing through fractured granite toward adjoining valleys. Groundwater in the area moves mostly toward the northeast, paralleling the structural grain of the Sand Springs Range . There is also a strong component of vertical flow, driven by surface recharge. The relative proportion of lateral to vertical flow varies among the 1 000 realizations (probabilities produced by the model) driven by values for hydraulic conductivity assigned to the vertical and lateral flow categories. Embedded within this natural flow field is a region of higher hydraulic conductivity and porosity around the chimney and cavity because of rock fractures resulting from the underground nuclear test.

Maps of the model domain showing the hydraulic conductivity of the fracture zone.

Alternate models depicting behavior of the shear zone, flow conditions on the west side of the mountain range, and other aspects of boundary conditions were evaluated in conjunction with a model of the Shoal test area. The Shoal flow and transport model resulted. This model is roughly rectangular: 1588 meters wide from the northwest to southeast on the upgradient edge and 1900 meters long from the southwest to northeast. From the water table downward, the model is 1300 meters deep.

The movement or migration of particles from the test cavity was simulated through the random-walk method. The processes of sorption and matrix diffusion, which serve to retard movement of particles, were incorporated in the transport analysis. The two-well tracer test showed that sorption occurs both in the matrix and on fracture surfaces, although it was greater in the matrix. The tracer test also revealed the important role that matrix diffusion plays in the transport of radionuclides through the granite of the Shoal subsurface.

Model showing details of the shear zone. KI, KII, KIII, and KIV represent hydraulic conductivities in various parts of the shear zone. [Lower right corner: Sim. h = hydraulic head; m AMSL = meters above mean sea level].

The model was verified quantitatively and qualitatively using independent sets of data. The data indicated that the shear zone acts as a barrier to direct eastward flow; therefore, groundwater flows northeast to Fairview Valley, and the mountain range experiences low-surface recharge. A high degree of chemical variability observed in the groundwater samples taken at the Shoal test area supports the conceptual view of a relatively disconnected fracture-flow system. Stable isotopes and ¹⁴C measured in the groundwater samples from the Shoal test area indicated that the groundwater maintains very long residence times-some in excess of 10000 years. These long residence times are consistent with matrix diffusion and slow groundwater velocities depicted in the model.

Models depicting radionuclide movement in groundwater environs beneath the Shoal test area. Click graphic to open animation and click a second time to initiate.