A three-dimensional mathematical model simulates virgin groundwater flow in the Big Horn basin, Wyoming. The computed results are compared to two published interpretations of the Tensleep Sandstone virgin potentiometric surface; both of these interpretations, Bredehoeft and Bennett, and Haun, were made from the same data set. The published maps are quite different. Bredehoeft and Bennett ignored the faults; Haun treated the faults as horizontal barriers to flow.

The hydraulic head at depth over much of the Big Horn basin is near the land surface elevation, a condition usually defined as hydrostatic. This condition indicates a high, regional-scale, vertical conductivity for the sediments in the basin. Our hypothesis to explain the high conductivity is that the faults act as vertical conduits for fluid flow. These same faults can act as either horizontal barriers to flow or nonbarriers, depending upon whether the fault zones are more permeable or less permeable than the adjoining aquifers.

A three-dimensional simulation of fluid flow in the basin indicates that either of the potentiometric interpretations, that of Bredehoeft and Bennett or that of Haun, can be reproduced. The results depend upon whether the fault zones are lateral barriers to flow. In the case where the faults are lateral barriers, the basin is broken into compartments with much of the areal head loss occurring across the fault zones.