Two alternative designs for the demonstration emplacement of a viscous liquid barrier (VLB) at the Brookhaven National Laboratory (BNL), Long Island, New York, are investigated by means of numerical simulation. The application of the VLB technology at the BNL site involved a surface-modified colloidal silica (CS), which gels upon addition of an appropriate electrolyte. Lance injection was used for the CS barrier emplacement. The lance injections occur in three stages: primary, secondary, and tertiary. The geometry of the barrier is based on the wedge model. The first design is based on optimization principles and determines the parameters that maximize uniformity and minimize permeability by minimizing an appropriate objective function while meeting the design criteria. These include a maximum hydraulic conductivity of 10-7 cm/s and a minimum thickness of 1 m. The second design aims to meet the same criteria and reflects standard chemical grouting practices. The combined effects of the key design parameters (i.e., lance spacing, injection location and spacing, gel time, injection rate, and volume) on the barrier permeability are studied. The optimization-based design is shown to have a significantly better performance than the standard engineering design. The interpenetration of adjacent CS bulbs appears to be of critical importance in meeting the barrier specifications. The three-dimensional simulations show that the barrier performance depends heavily on the path by which the final state is achieved. The in situ field measurements of the barrier permeability are consistent with, and appear to validate, the model predictions. ¿ 1999 American Geophysical Union