Interim report on the modeling of the regional hydraulics of the Rustler Formation

The finite-element code ISOQUAD was used to simulate the head distribution within the two major water-bearing units of the Rustler Formation at the Waste Isolation Pilot Plant (WIPP), the Magenta and Culebra dolomites. Calculated migration of a continuously injected contaminant from an assumed smeared point source at the center of the site was observed in terms of concentration relative to the initial input. Migration rates of specific concentration fronts decreased with increasing time. The average rate of movement of the 10-3 relative concentration contour was less than 10m/y for the first 800 y in the Culebra Dolomite. By 800 y, the migration of this concentration front has essentially ceased. In the case of the Magenta Dolomite, the average rate of movement of the 10-3 relative concentration contour was 0.44 m/y for the first 676 y of continuous contaminant injection; the contaminant plume in the Magenta shows little movement thereafter. For reasons of scale, the plume calculation overestimates the actual rate of movement of contaminants. Particle velocities for selected streamlines, more characteristic of contaminant movement, are calculated. These particle velocities indicate groundwater travel times of approx. 130,000 y from the center of the WIPP site to a distance of 12.9 km (8 mi). The validity of the porous-medium approximation of the Rustler aquifers was examined by the evaluation of the drawdown portion of aquifer pump tests. Drawdown curves were successfully duplicated as a function of hydraulic conductivity, storativity, and well radius. Estimation of the effective radius examined in each well test indicates that there are no significant fluid-bearing fractures or channels within 60 m of tested wells. Because of variable test duration and in situ hydraulic properties, effective radii tested range from 37 to 1500 m (see Appendix A). 23 references